By Susannah F. Locke
In a paper published last week, MIT physicist Lorenzo Maccone hypothesizes that, yes, quantum physics is messing with our minds. The laws of physics work just as well if time is running forwards or backwards. But we all seem to experience time running in only one direction, and in the same direction as everyone else -- a mystery of physics that's yet to be solved.
So life as we observe it presents a quandary. Randomness -- or entropy -- naturally increases. A drop of food coloring in water will spread out. Heat from a pizza dissipates. But the laws of physics that govern the tiny particles that make up everything could just as well run backwards, so something doesn't jibe. Why don't we see more instances of molecules ordering themselves (sugar un-dissolving itself from my coffee, ice cubes spontaneously forming in my water) if it's just as likely according to the laws of physics?
Maccone says that we do see these events. We just don't remember them, because some quantum weirdness erases the memories. Using a bunch of math, he says that your memory and the event are in a state of quantum entanglement, and that when that state gets broken, the memory is erased.
What do you think is going on? Is time running backward sometimes? Are molecules organizing themselves behind our backs? All I know is that my brain hurts right now.
Monday, August 31, 2009
Tuesday, August 18, 2009
Researchers find ghostly images are a quantum phenomenon
Researchers develop a new setup for examining the underlying nature of ghost-imaging systems that uses entangled photon pairs as its light source.
By Matt Ford
Quantum mechanics is the source of a whole spectrum of behavior that goes against what common sense would suggest is possible. A more recent addition to the cornucopia of quantum oddities is the concept of ghost imaging. In the non-quantum world, when you take a picture of an object, the camera captures photons that have bounced off the subject of the image. Ghost imaging is the odd phenomenon whereby images are created by photons (light) that have never touched the subject during the photon's lifetime. Instead, ghost imaging measures the correlation between two light beams, where only one actually probes the object.
A debate is ongoing as to whether ghost imaging can be explained entirely by quantum physics, or if there is a classical component to it. A paper set to appear in an upcoming edition of Physical Review Letters seeks to determine whether any indication of a non-classical nature can be observed. The letter describes an experimental setup that tests ghost imaging against a form of Bell's inequality. If a proper Bell's inequality holds, then the setup can be explained using local hidden variables and may not be entirely quantum. However, if the inequality is violated, the system is quantum in nature.
The experimental setup involves a light source that emits pairs of correlated photons. These photons are spatially separated and follow two distinct paths, but are recombined at a detector at the end of the apparatus. Mathematically it can be demonstrated that a measurement of the orbital angular momentum of these photons is a direct measurement of of the amount of entanglement between them.
Given a specific set of filters, the authors claim that if a high contrast ghost image can be formed, this would demonstrate a violation of the proposed Bell inequality. That in turn would suggest that ghost imaging is a quantum phenomenon.
The authors used the difference between two measurements—the angle at which one beam hits the edge of the object being imaged, and the phase of the second beam—and plugged them into a formula that formed a Bell's inequality. If the value returned was between -2 and 2, the system would have a classical component. Instead, experimental measurements gave a value of 2.69±0.10, well above the errors that might be produced by a local-hidden-variable system. So, ghost imaging systems appear to be distinctly quantum.
As an avenue for future work, the authors consider what would happen if the experiment was repeated using a classical thermal light source instead of a pair of entangled photons. They suggest that the use of such a source would reduce the contrast to a level below a point where it would violate Bell's inequality—perhaps the system is quantum or classical, depending on the conditions under which it's observed (which sounds suspiciously quantum).
In any case, the authors conclude that the technique introduced in this paper could be used as a defining test to distinguish between the classical and quantum nature of ghost-imaging setups.
By Matt Ford
Quantum mechanics is the source of a whole spectrum of behavior that goes against what common sense would suggest is possible. A more recent addition to the cornucopia of quantum oddities is the concept of ghost imaging. In the non-quantum world, when you take a picture of an object, the camera captures photons that have bounced off the subject of the image. Ghost imaging is the odd phenomenon whereby images are created by photons (light) that have never touched the subject during the photon's lifetime. Instead, ghost imaging measures the correlation between two light beams, where only one actually probes the object.
A debate is ongoing as to whether ghost imaging can be explained entirely by quantum physics, or if there is a classical component to it. A paper set to appear in an upcoming edition of Physical Review Letters seeks to determine whether any indication of a non-classical nature can be observed. The letter describes an experimental setup that tests ghost imaging against a form of Bell's inequality. If a proper Bell's inequality holds, then the setup can be explained using local hidden variables and may not be entirely quantum. However, if the inequality is violated, the system is quantum in nature.
The experimental setup involves a light source that emits pairs of correlated photons. These photons are spatially separated and follow two distinct paths, but are recombined at a detector at the end of the apparatus. Mathematically it can be demonstrated that a measurement of the orbital angular momentum of these photons is a direct measurement of of the amount of entanglement between them.
Given a specific set of filters, the authors claim that if a high contrast ghost image can be formed, this would demonstrate a violation of the proposed Bell inequality. That in turn would suggest that ghost imaging is a quantum phenomenon.
The authors used the difference between two measurements—the angle at which one beam hits the edge of the object being imaged, and the phase of the second beam—and plugged them into a formula that formed a Bell's inequality. If the value returned was between -2 and 2, the system would have a classical component. Instead, experimental measurements gave a value of 2.69±0.10, well above the errors that might be produced by a local-hidden-variable system. So, ghost imaging systems appear to be distinctly quantum.
As an avenue for future work, the authors consider what would happen if the experiment was repeated using a classical thermal light source instead of a pair of entangled photons. They suggest that the use of such a source would reduce the contrast to a level below a point where it would violate Bell's inequality—perhaps the system is quantum or classical, depending on the conditions under which it's observed (which sounds suspiciously quantum).
In any case, the authors conclude that the technique introduced in this paper could be used as a defining test to distinguish between the classical and quantum nature of ghost-imaging setups.
Monday, August 10, 2009
Professor: Human intention can alter physical matter
By Suzanne Jacobson
During the years that William Tiller toiled in Stanford University’s department of materials science and engineering, he lived a parallel life.
He tread carefully, teaching orthodox science at one of the nation’s premier colleges, while exploring new scientific frontiers that made his colleagues uncomfortable.
Although Tiller began formulating his theories that human intention can alter physical matter during the ’60s, even today the orthodox scientific community does not readily accept the results of Tiller’s experiments. The general population, however, is intrigued. “It gives them hope,” Tiller said.
Tiller promotes psychoenergetic science, which says that human consciousness and intention can significantly affect the properties of living and non-living materials, and physical reality.
His experiments include using a special device to alter the pH level of water up or down one full unit — a 10-fold change — with no chemicals added.
In another experiment, Tiller used the same device to increase the ratio of ATP to ADP in fruit fly larvae, which turned the larvae into adults more quickly and also made them more physically fit. The biological chemicals ATP-ADP cycle to create and release energy in living things.
In a final experiment, Tiller increased the thermodynamic activity of a liver enzyme by 30 percent by exposing it to the device for 30 minutes.
The device is called an intention-host device, and it’s a small, rectangular, beige electrical box. A group of experienced meditators gather and program an intention into the box, which is then used in the experiments. Tiller says the results have been repeated around the world, and the statistical probability that the results occurred randomly is less than one in 1,000. He called all four experiments “robustly successful.”
Tiller is a soft-spoken man, lean and without eyeglasses although he is nearly 80. He moved with his wife, Jean, to Payson, in 1998 from California, which Tiller says was becoming too crowded and materialistic.
He was successful by traditional standards, a one-time department chair at Stanford. But Tiller seemingly isn’t impressed by the credential. It seems effortless.
Perhaps Tiller’s own litmus test of success is more tied to the prevalence of psychoenergetic science.
Tiller was featured in the movie, “What the Bleep Do We Know,” a cult classic documentary that interviewed scientists and mystics, and explored quantum physics, neurology and molecular biology in relation to spirituality and metaphysics.
Recently he, along with Tom Russell of Payson’s own SuperWisdom Foundation, a spiritually-inclined non-profit, began conducting discussions at various locations around town, including the library and the Payson Center for Spiritual Awareness.
Tiller’s science carries heavy spiritual implications in that one’s thoughts really can influence one’s reality.
The reluctance of Tiller’s colleagues to fully embrace his findings initially disappointed him. He thought his world-class science would convince other scientists to embrace the view that human intention can affect the physical world.
“Because they could not explain this work based on the conventional paradigm, to them it just could not exist,” Tiller said.
And so he began the lonely path of a man dedicated to a cause unaccepted by the orthodox.
“They are very wrong and they are stuck. They can’t get outside of space/time. They can’t get out of quantum (physics),” Tiller said.
Quantum physics holds when humans observe certain processes, they change. For instance, light waves change from particles to waves depending upon if somebody is watching. This is called wave-particle duality.
However, in quantum physics, this phenomenon doesn’t translate to larger particles.
Tiller believes a second level of physical reality exists in the space of the observable reality. Solid matter is actually mostly space — the space between atoms. Within that space lies the second level, which is the level Tiller believes human intention can influence.
“Since the days of Descartes, the unstated assumption of science is that no human qualities of consciousness, intention, emotion, mind or spirit can significantly influence a well-designed target experiment in physical reality,” Tiller said. “I proved that to be very wrong.”
Although he didn’t conduct the intention-host experiments until the late 1990s, Tiller’s journey began in the late 1960s at Oxford.
Tiller had received a Guggenheim Fellowship and planned a sabbatical. During daily meditations, he would contemplate “how might the universe be constructed to allow this crazy-seeming stuff to coexist with my orthodox science that I was doing with my Ph.D. students at Stanford every day.”
During the remainder of the day, Tiller would investigate the ideas that came to him through meditation. Did the insights violate any standing scientific truths? By the end of the day, he would have more questions for the next day’s meditation.
By the end of six months, he had outlined a theory that he believed to be more meaningful than the orthodox science he had practiced. Someone had to test it through rigorous scientific inquiry.
“That someone had to be me,” Tiller said he realized. He eventually relinquished the chair at Stanford, but continued to work there. He also didn’t conduct his experiments there. Ostentatious displays of unconventionality can get one fired. Look at Timothy Leary.
A philanthropist provided Tiller with $1.7 million for the experiments that began in 1997 and lasted until 2000.
In 2001, Tiller wrote “Conscious Acts of Creation: The Emergence of a New Physics,” that detailed his experiments. Orthodox scientists tend not to read it, Tiller said.
But the ideas are spreading.
The movie including Tiller, “What the Bleep Do We Know,” was originally released in one Washington movie theater in 2004. Today, the film has sold more than 1 million copies and is available worldwide.
“It’s all up to us. Collectively, we create our future,” Tiller said.
During the years that William Tiller toiled in Stanford University’s department of materials science and engineering, he lived a parallel life.
He tread carefully, teaching orthodox science at one of the nation’s premier colleges, while exploring new scientific frontiers that made his colleagues uncomfortable.
Although Tiller began formulating his theories that human intention can alter physical matter during the ’60s, even today the orthodox scientific community does not readily accept the results of Tiller’s experiments. The general population, however, is intrigued. “It gives them hope,” Tiller said.
Tiller promotes psychoenergetic science, which says that human consciousness and intention can significantly affect the properties of living and non-living materials, and physical reality.
His experiments include using a special device to alter the pH level of water up or down one full unit — a 10-fold change — with no chemicals added.
In another experiment, Tiller used the same device to increase the ratio of ATP to ADP in fruit fly larvae, which turned the larvae into adults more quickly and also made them more physically fit. The biological chemicals ATP-ADP cycle to create and release energy in living things.
In a final experiment, Tiller increased the thermodynamic activity of a liver enzyme by 30 percent by exposing it to the device for 30 minutes.
The device is called an intention-host device, and it’s a small, rectangular, beige electrical box. A group of experienced meditators gather and program an intention into the box, which is then used in the experiments. Tiller says the results have been repeated around the world, and the statistical probability that the results occurred randomly is less than one in 1,000. He called all four experiments “robustly successful.”
Tiller is a soft-spoken man, lean and without eyeglasses although he is nearly 80. He moved with his wife, Jean, to Payson, in 1998 from California, which Tiller says was becoming too crowded and materialistic.
He was successful by traditional standards, a one-time department chair at Stanford. But Tiller seemingly isn’t impressed by the credential. It seems effortless.
Perhaps Tiller’s own litmus test of success is more tied to the prevalence of psychoenergetic science.
Tiller was featured in the movie, “What the Bleep Do We Know,” a cult classic documentary that interviewed scientists and mystics, and explored quantum physics, neurology and molecular biology in relation to spirituality and metaphysics.
Recently he, along with Tom Russell of Payson’s own SuperWisdom Foundation, a spiritually-inclined non-profit, began conducting discussions at various locations around town, including the library and the Payson Center for Spiritual Awareness.
Tiller’s science carries heavy spiritual implications in that one’s thoughts really can influence one’s reality.
The reluctance of Tiller’s colleagues to fully embrace his findings initially disappointed him. He thought his world-class science would convince other scientists to embrace the view that human intention can affect the physical world.
“Because they could not explain this work based on the conventional paradigm, to them it just could not exist,” Tiller said.
And so he began the lonely path of a man dedicated to a cause unaccepted by the orthodox.
“They are very wrong and they are stuck. They can’t get outside of space/time. They can’t get out of quantum (physics),” Tiller said.
Quantum physics holds when humans observe certain processes, they change. For instance, light waves change from particles to waves depending upon if somebody is watching. This is called wave-particle duality.
However, in quantum physics, this phenomenon doesn’t translate to larger particles.
Tiller believes a second level of physical reality exists in the space of the observable reality. Solid matter is actually mostly space — the space between atoms. Within that space lies the second level, which is the level Tiller believes human intention can influence.
“Since the days of Descartes, the unstated assumption of science is that no human qualities of consciousness, intention, emotion, mind or spirit can significantly influence a well-designed target experiment in physical reality,” Tiller said. “I proved that to be very wrong.”
Although he didn’t conduct the intention-host experiments until the late 1990s, Tiller’s journey began in the late 1960s at Oxford.
Tiller had received a Guggenheim Fellowship and planned a sabbatical. During daily meditations, he would contemplate “how might the universe be constructed to allow this crazy-seeming stuff to coexist with my orthodox science that I was doing with my Ph.D. students at Stanford every day.”
During the remainder of the day, Tiller would investigate the ideas that came to him through meditation. Did the insights violate any standing scientific truths? By the end of the day, he would have more questions for the next day’s meditation.
By the end of six months, he had outlined a theory that he believed to be more meaningful than the orthodox science he had practiced. Someone had to test it through rigorous scientific inquiry.
“That someone had to be me,” Tiller said he realized. He eventually relinquished the chair at Stanford, but continued to work there. He also didn’t conduct his experiments there. Ostentatious displays of unconventionality can get one fired. Look at Timothy Leary.
A philanthropist provided Tiller with $1.7 million for the experiments that began in 1997 and lasted until 2000.
In 2001, Tiller wrote “Conscious Acts of Creation: The Emergence of a New Physics,” that detailed his experiments. Orthodox scientists tend not to read it, Tiller said.
But the ideas are spreading.
The movie including Tiller, “What the Bleep Do We Know,” was originally released in one Washington movie theater in 2004. Today, the film has sold more than 1 million copies and is available worldwide.
“It’s all up to us. Collectively, we create our future,” Tiller said.
Saturday, August 8, 2009
Quantum Mechanics and Consciousness:
A Causal Correspondence Theory
Ian J. Thompson
Physics Department, University of Surrey, Guildford GU2 5XH, U.K
October, 1990.
We may suspect that quantum mechanics and consciousness are related, but the details are not at all clear. In this paper, I suggest how the mind and brain might fit together intimately while still maintaining distinct identities. The connection is based on the correspondence of similar functions in both the mind and the quantum-mechanical brain.
Accompanying material for a talk at The Second Mind and Brain Symposium
held at the Institute of Psychiatry, Denmark Hill, London on 20th October, 1990. (pdf version)
The Mind-Brain Problem
The problem of the relation between mind and body is well known as a difficult `world knot'. Over the centuries various monistic and dualistic theories have been proposed, and the subject has had renewed interest as we try to assimilate the implications of quantum physics. These implications may make us reexamine our views of brains and bodies, but it is still not clear what consequences they have for our understanding of minds.
Earlier in the interpretation of quantum mechanics, it appeared that the problem of measurement could only be solved by introducing some basic notion of an observer, presumably a conscious observer. Many scientists and philosophers (e.g. Wigner (1962) Eccles and Popper (1977), Faber (1986), Toben (1974), Squires (1990), Donald (1990) among others) have taken this solution to indicate an essential role of consciousness in the physical world. Consciousness, to solve the measurement problem, must initiate the transition from quantum potentialities to definite actualities. It would not have to select which actuality, but merely cause some actuality to be produced. Squires and Donald accept this merely `collapse prompting' role of consciousness, and their position has the advantage of requiring no changes to quantum theory, as consciousness then makes no change to any predicted probability. Squire's conclusion, unfortunately, does not yield any functional role which may be important in mental and/or physiological processes in the brain. Donald (1990) does go on to give circumstances in which `measurement collapses' are important for the function of neural cells, but their effects are simply to reinstate the `classical' on/off nature of the sodium switches even when there are quantum uncertainties.
Since, moreover, it is possible that the `measurement problem' can be solved within physics (see e.g. Maxwell, 1988), the role of consciousness in measurement has little direct bearing on the problem of how the mind and brain function together. In order to say something relevant to the brain sciences, we have to go beyond conventional quantum mechanics (of whatever interpretation), and make new hypotheses with definite empirical content. We have to postulate that `consciousness' (whatever that may turn out to be) influences the actual outcome of neural events, and does not just observe them as if disinterestedly.
Quantum Brains
It is well known that there is a residual indeterminism of quantum mechanics in making predictions of physical processes. Quantum mechanics is also `non-local', as described for example by Bell's inequalities. If either of these two phenomena are displayed in the brain as described by quantum physics, then this may be relevant to the problem of consciousness.
Biased Probabilities:
One proposal (from Walker (1970) and later Eccles (1977)) has been that mental events influence at least some physical outcomes within the range allowed by quantum mechanics. This idea has to supplemented by an account of how these small effects are amplified to affect macroscopic outcomes, and recently possible accounts have been presented, Eccles (1989). Eccles requires that the options of a single vesicle discharging or not discharging both have non-zero quantum probabilities, so that such neural events are genuinely under-determined by quantum mechanics, and the way is open for them to be determined by the mind.
Bose-Einstein Condensates:
Another proposal by Marshall (1989) is that the mental and bodily realms derive directly from a quantum realm, as separate appearances of an underlying structure given by quantum mechanics. Since the unity of mental experience is one of its outstanding features, the proposal is that consciousness arises from a quantum mechanical structure which shows remarkably unity, namely Bose-Einstein condensates. This therefore requires not only that the brain amplifies quantum indeterminism, but also that long-range and long-time correlations of quantum phases are set up in the brain, by means for example of a structure proposed by Fröhlich.
Both these proposals go some way toward connecting the apparently disparate realms of mind and brain. From the physics side, both have quantum mechanics holding virtually unchanged. They furthermore then both require that neuro-physiological processes are in some essential way `quantum processes'. They require then that the peculiarly quantum features of matter become much more spatially spread out in the brain than is usually admitted. In the language of physics, Marshall requires that non-local correlations (of the kind described by Bell's Inequalities) are set up between parts of the brain that are very far apart on the atomic scale, and hence that the brain exhibits some of the characteristics of `quantum computers' as described by Deutsch (1985). There is a difficulty that this implies much larger quantum effects than have ever been observed in materials at room temperature. There is evidence of unusual microwave absorption, but it is not clear whether this really demonstrates quantum correlations as distinct from sets of coupled classical (non-quantum) oscillations.
Eccles' proposal is weaker than Marshall's, as he only requires that for some brief time the non-local correlations extend between the options of a single vesicle discharging or not discharging, so there can exist a coherent superposition of these two states. Eccles does not require that the two options remain coherent for any duration after the event (as Marshall would require to obtain widespread and persisting quantum phase coherences).
Both proposals are made less likely by the fact that Plank's constant, which sets the scale for quantum processes, is so small. It is not impossible that there are genuinely undetermined neural events in the brain, and/or that there are large scale quantum coherent effects, but these are difficult to set up in the noisy room-temperature non-crystalline system of the brain. Psychological ConsiderationsThere are also questions from the psychological point of view. Eccles' proposal tells us very little about the structure of the mind, apart from the fact that it must be able to predict and control the amazingly complicated system of many of the random discharges in the brain. There is little indication from what psychologists know about the mind that it has anything like the required analytical and motor capacities. The mind is best dealing with at most 5 to 8 items, and is completely swamped when it tries to understand massively parallel systems directly. A way of avoiding these difficulties will be discussed below.
Marshall's proposal seems to tell us more about the mind, namely that it arises whenever quantum mechanics gives suitable Bose-Einstein condensates. However, all psychological details of mental structure and operations would then be derived from this quantum structure. That is, he would have to claim that all human life (from mathematics and logic to arts to psychopathology) is implicit in Schrödinger's equation. Personally, I find it extremely implausible that the quantum mechanics of patterns of excitations of Bose-Einstein condensates exactly and mechanically determines the interaction patterns of ideas, images and meanings in the human mind. There are certainly similarities between the two, but these do not imply identity. To talk of similarity without giving evidence for identity leads to another approach, as we see next.
A Different Approach
Despite the apparently smallness of quantum effects in the brain, I still believe that quantum physics is important in our understanding of its function. To show this, I am going to take a different approach. I am going to assume, in a non-reductionist way, as Eccles and others do, that the mind is (in some way) distinct from the brain. But I want to then see how they could be intimately connected again. Psychologists and psychiatrists are aware of the close functional dependence of minds and brains, so that physiological changes very readily affect the mind in many ways, and mental intentions and attitudes affect both the normal and abnormal functioning of the nervous system.
I am going to take a `top-down' to the problem of interaction, and will suggest how it may usefully be organised, from the functional point of view. I am not going to assume that quantum mechanics must be unchanged; I am therefore going to follow the `spirit' if not the `letter' of quantum physics. The aim is to motivate a plausible general view which will make predictions about what goes on. Afterwards I will discuss the origins of this general view.
The guiding principle I use is that the mind and brain are similar in structure and function, and that they need each other to operate. They are not identical, but are similar in the sense of `resonating' or `corresponding' to each other from the functional point of view. In order to see how the mind and brain are similar and fit together in this general way, we do need to remember quantum physics.
Similarity of Minds and (Quantum) Brains
I am here going to draw analogies between mental and quantum phenomena. If you already know something about minds, these may help you understand quantum physics! Some of these have been pointed out by Bohm (1951, § 8.27ff) and more recently by Jahn & Dunne (1986) and Zohar (1990) (although perhaps understood in different ways: see later discussion).
Jahn and Zohar have tended to concentrate on the analogies between mental structures, and the structure of matter entailed by quantum mechanics. They look at analogies between, for example, minds and quantum systems, and then especially analogies between how minds interact and how quantum systems interact. I will be looking rather at analogies between individual mental processes within minds and individual quantum processes. I believe that only when we understand the elementary processes of both the mind and quantum matter, can we more completely understand complex systems of either kind.
In order to make analogies with quantum physics, I am going to assume something like the `propensity interpretation' of Maxwell (1988) and others (Thompson, 1988). I am also going to assume a view of mind that is in part phenomenological (after Bergson especially), and in part influenced by (a few of) the aims of artificial intelligence (AI) and the recognition of the complexity of even apparently elementary mental processes. On this basis, we can draw analogies between the fundamental processes of quantum physics and the fundamental processes of mental activity:
* Quantum objects do not evolve uniformly in time, but as a succession of `states' between intermittently `actualising' to one outcome of a certain range.
Mental entities do not evolve uniformly in time, but in a succession of `specious presents' between intermittent changes of state.
* The state of a quantum object is not a static configuration of elements, but a `state of propensity' for possible virtual proceses, possible interactions and possible actual (observable) outcomes.
The state of a mind is not static, but more a `state of propensity': it is a continual recollection of past events and continual anticipation of future possibilities.
* The state of a quantum object may be described by a wave function distributed is space, but this does not mean that the quantum object is composed of elements so distributed. Rather, it has a basic unity in that it will always act as a whole. These spatial distributions merely describe where it can actualise.
The state of a mind has a similar basic unity. The thoughts in the mind may be diverse, and each may implicitly contain diverse elements (usually the better for doing so), but if they lead to action these diverse elements were merely describing where the mind could have operated.
* The driving impetus of quantum processes are not external, but the very propensities of which quantum objects are forms. These propensities operate according to the (spatiotemporal) circumstantial relations with other objects.
The driving motivation of mental processes are not external, but the very motivational loves which underly all psychic activities. These motivations operate according to the circumstances at each time.
* Behind the apparently continuous evolution of the Schrödinger's equation, there is a progressive multitude of `virtual events' whose operation forms the potential term in Schrödinger's equation.
As a consequence of simulation efforts in AI, we know that the operation of even seemingly simple events such as the perception of solid objects must involve `behind the scenes' a great deal of information processing which is not apparent in the act of perception itself.
In view of these analogies, and also those given later, it unlikely that the mental and physical processes are independent, and merely interact with each other at certain `mental-neural' events (Eccles, 1989). We could hold that they both derive from a `quantum realm', but I hold to a more radical (but simpler) idea that the propensities for physical processes derive from mental processes, which in turn always act according to past physical events. This idea, explained further in the next section, means that the mental and physical realms are far from separable, but are intimately related in a very systematic manner. Thus, although the basic idea may not be exactly fashionable these days, it is an idea with significant predictive power and good empirical content. Nevertheless, as explained later, it has a long and venerable history, and deserves closer consideration.
How Minds and (Quantum) Brains Fit Together
In order for people to have functioning minds, their minds must at least be able to
* implement intended functions using the motor areas,
* establish permanent memories, presumably by means of permanent physiological changes,
* form perceptions using information from the visual & auditory (etc.) cortexes,
* be able to follow `internal' trains of thought/feeling/imagining without necessarily having any external effects.
One way that these requirements can be most simply accomplished is by means of the idea above, formulated in the following three principles:
I.
That the mind predisposes the physical/physiological potentialities (both deterministic or indeterministic according to quantum physics).
II.
That the `predispositional' capacities of the mind are consequentially restricted (and hence conditioned) by their actual effect.
III.
That the pattern of I and II is repeated for individual stages of more complex processes.
These principles together give what can be called `conditional forward causation', or `top-down causation'. Note that we do not have a fourth `bottom-up' principle that neural events directly cause events to occur in the mind. We do not have general matter $\rightarrow$ mind causation, although something resembling this does arise, namely `selection'. This is not causation in the primary sense of `principal causation' as `producing or generating the effect'. It is more in the secondary sense of `instrumental causation' as `providing a necessary prerequisite'.
A strong argument for these three principles is that they are already similar to what is known already to happen in quantum physics, in quantum field theory to be precise. According to that theory, there is a class of events called `virtual events' which select and predispose the ordinary quantum wave function. These virtual events operate deterministically, and describe the operation of the electric, magnetic, nuclear and gravitational forces. They are not the `final' actual events of quantum mechanics (those are the definite outcomes of events like observations). Rather, they are a `prior level' of `implicit events' whose operation is needed in order to produce the potentialities for events like observations. The principle (I) states the analogical result that mental events themselves are a `prior level' of `implicit events' whose operation is needed in order to produce the potentialities for physical events.
The argument for the principle (II) is more general. Whenever any potentiality is exercised to produce a particular outcome, future potentialities must depend on the detailed outcome. Suppose for example that at the moment, I have the potentialities of moving left or right; if I actually move left, say, then this influences (by restriction to a fixed history) of what I can do from now on. This second principle can also be seen as a `law of karma': your future life is restricted and influenced by your past actions (by selection). Physical events are in this way the necessary foundations for permanent mental history and structure.
Principle (III) has an important corollary
IIIb.
That the mind predisposes the brain to carry out those functions which `mirror' or `correspond to' the mind's own functions.
This is because mental functions involve intermediate steps, and these intermediate mental steps predispose suitable intermediate physical steps (by I), and are in turn conditioned or confirmed by them (by II). Thus the sequence of physical steps will follow the sequence of mental steps, and the overall function of the physical process will be analogous (in some sense) to the overall function of the mental process. Examples of this `mirroring' or `corresponding' have already been illustrated by the similarities between minds and brains listed in the previous section. Examples of correspondences of complex processes in the mind with those of physics will be given below.
Let us then see how these principles enable the mind and brain to function together:
* The particular functions selected by the mind to be carried out by the brain will be the establishment of spatiotemporal patterns of neural activity that may then be `decoded' in the motor cortex to lead to the desired activities, by the principle (I). Principle (IIIb) establishes a criterion for the overall functioning of this decoding.
* Permanent physiological changes lead to permanent memories in the mind, by the principle (II). (It is an empirical question, which physiological changes are relevant, but principle (IIIb) will be a guide.)
* Perceptions are formed by the sensory cortex areas deterministically forming particular patterns of neural activity, so that these physiological effects can select the subsequent perceptual content of the mind. The process here is rather subtle. The mind must have a `general disposition' to see/imagine any of its possible percepts; the role of the sensory cortex is to select the particular content, by means of principle (II). Thus we have the general psychological observation that `we see only what we are capable of and disposed to see'.
* To be able to follow `internal' trains of thought/feeling/imagining without necessarily having any external effects, the mind must be able to produce physiological effects which do not have any significant consequences. Presumably, much of the cerebral cortex can function in a `loosely connected' way, in order to provide the foundation for a set of permanent mental structures.
Mind and Brain as `Hand and Glove'
This theory of mind and brain connection establishes in intimate relation between them. It is not a relation of identity, or a relation of aspects or points of view. It is more a relation of inner and outer, or cause and effect: propensities in the brain are the causal product of mental actions. As put in by Bawden (1947), `the role of the psychical in relation to the physical (in the living organism) is essentially the relation of the potential or incipient to kinetic or overt action'.
The mind and brain fit together by approximate analogy with hand and glove, or, better, with tissue and skin. The analogy (by principle IIIb) is most precisely with the functions of tissue & skin, and not so much with their material shape. The mind provides all the directed activity of the brain, just as the tissue of the hand provides all the directed activity of the skin of the hand. When we look at the head, we only see the brain, just as we only see skin when we look at the hand. It appears that the skin of the hand does all the work, but we don't assume that that is all there is. It appears that the skin has life, but we know that all but the simplest life comes from the underlying tissue. The skin (as does the brain) has simple capacities for action and reactions, but it is a mistake to imagine that all capacities for activity and information processing belong to the skin (or the brain).
Psychology
Once we have the basic idea of functional correspondences between physical and mental systems based on their causal interrelations, then we have a fertile source for exploring the detailed structure of one using what we know about the other.
Psychology of Stages
Because there are `prior' and `post' levels of events even within physics, by the correspondence principle IIIb, there must also be prior and post levels of events within the mind. Piaget's theory of stages of development is relevant here. In table 1 the Piagetian stages have been reformulated as the acquisition of capacities for dealing with ideas on the successive layers (supplemented by a `creative stage' of Gowan (1972): see Thompson (1990) for more details). The relations between the ideas of adjacent levels is then analogous to the relation between any pair of prior and post events, according to principles I, II and III above.
Table 1: Relation between Network Layers and Piagetian Stages
During each stage, at the approximate ages shown, the child is learning to relate the concepts listed in the second column. That is, (s)he is constructing relations in a network at the given level.
Net Level Network Relations of Developed in
Piaget/ Gowan Stage during ages
5 meta-theories, paradigms creative 17-
4 plans, models, formalisms formal 12-16
3 classes, series, numbers operational 7-11
2 events, single relations, sentences preoperational (preconceptual & intuitive) 2-6
1 objects sensorimotor 0-1
0 images, motor movements (initial) -0
There appears to be a broad analogy of the `practical' layers (0, 1 and 2) with the `real' events of quantum mechanics, and of the `theoretical' layers (3, 4 and 5) with its `virtual' events. This can be spelt out in more detail, approximately as in table 2. Here, the basic idea (following principle IIIb) is that the mind at a given stage is capable of `thinking about' the corresponding features of quantum mechanics. The set of propensities/quantum systems/actual selections provides the mechanisms for the actual events of ordinary (non-relativistic) quantum mechanics, whereas the set of least-action principles/evolution equations/groups describes the basic principles and virtual events of quantum field theory.
Table 2: Relation of Network Layers to Quantum processes
Net Level Network Relations Quantum Theory
5 meta-theories, paradigms variational principles
4 plans, models, formalisms time evolution equations
3 classes, series, numbers invariance groups
2 events, single relations propensities & forces
1 objects quantum systems
0 images actual selections
Artificial Intelligence and Neural Networks
Neural networks have proved to be useful framework for formulating a wide range of information processing problems in cognitive psychology. This has lead some to postulate their sufficiency for a system of artificial intelligence. However, there are three main problems with neural networks. The first is the question of speed, the second the problem of procedures, and the third the problem of levels.
1. In order for neural networks to (approximately) solve global optimisation problems, some search procedure such as simulated annealing is required. This is notoriously slow, and, as still a procedure of `exponential complexity', will be prohibitively slow for problems of realistic complexities.
2. Neural networks have fixed connections only, and in order for them to achieve anything like translation or scale invariances for perception, then all possible translations and scales have to be `hard wired' simultaneously (Hinton et al, 1985). This could be avoided again by some `procedural' or `algorithmic' mechanisms, with `variable bindings', but this can only be implemented in neural networks by `brute force' multiple connections.
3. Neural networks seem to plausibly describe the interactions of related ideas on a given `level' of table 1. They are notoriously poor, indeed AI as a whole is notoriously poor, at modelling the connections between the separate layers (see Thompson, 1990).
It is still possible that the brain uses neural networks so that their various `final states' serve to indicate different image categories and different actions. In the present causal theory of mind-brain interaction, however, it is more likely that the mind can act by predisposing the neural nets to settle to some preferred outcome. That is, the mind determines (from the function it wishes performed) which collective state of the neural system is to be achieved. From the point of view of the brain, the neural net would appear to be the `non-deterministic automaton' of complexity theory, heading more or less quickly for the preferred solution (while thus verifying that it is a solution).
The problems of procedures and levels in neural networks are resolved by having multiple levels of simultaneous mental operation within the mind. The relations between the multiple levels then follows the principles I, II and III. Principle I means that the higher level predisposes the interactions at the lower level. Principle IIIb means that it does so in order to carry out particular functions. Principle II means that configurations at the lower level (e.g. images) select a particular perception (e.g. of an object) which would produce that image. This selection must be able to made very quickly - much more quickly than any detailed analytical algorithm could be elaborated.
New Physics
What is new physics in this proposal, is the first principle I: that the mind predisposes the physiological potentialities (whether deterministic or indeterministic according to quantum physics). This relation between mind and brain is analogous to that between virtual and real quantum events, but postulating the mind/brain relation to be of this kind has new empirical content.
The predisposition of physical propensities is of course not arbitrary. It is conditioned from the physical side by past actual events, from the mental side by the functions which may be accomplished, and from both sides by the requirement of correspondence. These constraints mean we do not have merely a `mind of the gaps' which fills in what is left undetermined by modern science. There are certainly gaps to be filled, but the present scheme will also explain what we already know, not just what we don't know.
If, for example, there is a regular sequence of actual physical events, and the predisposition is not varying, then there will be a constant production of new propensities, and hence a regular sequence of physical effects as if by a universal physical law. Thus our principles can provide a new basis for physical laws which we already know.
When there are intermediate propensities (e.g. of individual minds) then physical events do not follow the previous simple pattern. The new pattern will describe how the brain works in conjunction with the mind. Clues to this behaviour can be found by analogy with psychological processes, as discussed later.
* We saw in table 2 that the images (in the mind) correspond to actual selections (in quantum mechanics). This casts new light on our old problem of the role of consciousness in quantum measurement, and we see that perception of images does cause and correspond to the selection of actual outcomes of measurement in quantum physics. It is similar to Eccles' proposal, but now we have a better idea of the mental side of the picture. The network level 0 is the initial stage of cognitive development, and does not require any sophisticated analytical or predictive capacities: it is more the ability to `see what there is to see'. This means that the selection of outcomes of measurements does not happen at the time of the (random) event, but later, when perceptible differences have emerged, and are perceived.
* The ability of the sensorimotor mind to influence the course of events is, however, quite limited. Its role in everyday life is more to provide a source of perceptions, and implement outcomes that have already been decided. This corresponds in quantum mechanics to the small size of Planck's constant, and the rarity of quantum-random events which do have perceptible differences. Occasionally, however, there is still a need for basic decisions, and (in the mind and in quantum mechanics) it is necessary to decide between one of a small number of options. On these occasions, consciousness does limit itself to a single perception, and this does cause the selection of a particular outcome of a event left random by quantum physics.
* Most of the interesting processes in the mind and in quantum physics take place at levels 3 - 5: the `theoretical' layers as these were described earlier. Processes occur at these layers which generate in the first place the `short list' of options for the sensorimotor mind, and a great deal of detailed knowledge and derivation goes into this preparation.
* The operation of predisposition and correspondence in these `higher' levels must mean that the time-evolution equation of physics, normally taken to be Schrödinger's equation, is not fixed. We should be able to discover circumstances in which even classical systems follow a modified physical law.
`New' Metaphysics
These ideas have the possible disadvantage (or feature) that the operation of ordinary inert physical processes requires further analysis. Basically, since the propensities for physical processes derive from mental processes, all physical dispositions must derive (or have been derived from) some prior psychical level. This may sound like pan-psychism, but I am not saying that all physical processes include their own consciousnesses. There is a simpler solution, if you can accept the new metaphysics that there is some kind of Source, composed of suitable `psychic' propensities, from which everyday material propensities perpetually derive. Since the operation of this Source is always according to past physical events, we saw above that this operation amounts to the constant production of new propensities as if a `physical law' were prevailing. That is the way most scientists prefer to see the world. It is only that sometimes things are not so simple.
There may be some reaction to the apparent `dualism' in these ideas, as I have postulated minds existing separately from brains. However, this separation is only in our theory: in practice they need each other, and function together as a unified whole - as the person.
Origin of these ideas
I have presented these ideas as worth of consideration on their own, but they really have a long history in a variety of contexts. The basic idea that causation only truly works from the mind into the brain (and not vice versa) is not a popular one today, but has to be traced back to `non-standard' insights of people such as Plotinus (b. 205), Boehme (b. 1575), Swedenborg (b. 1688) and some other traditions. Swedenborg was well educated as a physicist and then physiologist, so I find his accounts the most detailed and useful. Of course, he knew nothing of quantum mechanics (only Newtonian mechanics), so I have had to `re-apply' his principles in the light of what we now know about the physical world. He, however, has the clearest presentation of the idea of `conditional forward causation' (he calls it `influx into uses'), and he gives the most complete account of the `correspondences' which exist between mental and bodily things. For a brief summary of his ideas, see Thompson (1989).
Conclusion
In order to understand how the mind and brain function together, it is not enough for there to be gaps in our physical theories. We also need to have a unified picture of both minds and brains. In this paper I have tried to outline such a unified approach. These suggestions require some departure from what is commonly accepted in the physical sciences, but we can still learn a great deal from what has already been discovered both there and in the psychological sciences.
* Bawden, H.H. (1947) `The Psychical as a Biological Directive', Philos. Science, XIV pp. 56 - 67.
* Bohm, D. (1951) `Quantum Theory', Prentice-Hall.
* Bohm, D. (1980) `Wholeness and the Implicate Order', Routledge and Kegan-Paul.
* Deutsch, D. (1985) `Quantum Theory, the Church-Turing principle and the universal quantum computer', Proc. R. Soc. Lond, A 400, pp. 91 - 117.
* Donald, M.J. (1990) `Quantum Theory and the Brain', Proc. R. Soc. Lond, A 427, pp. 43 - 93.
* Eccles, J. and K.R. Popper, (1977) `The Self and Its Brain', Springer.
* Eccles, J. (1989) `Evolution of the Brain: Creation of the Self', Routledge.
* Faber, R.J. (1986) `Clockwork Garden'.
* Gowan, J.C. (1972) `Development of the Creative Individual', Knapp.
* Hinton, G.E. and K.J. Lang (1985) `Shape Recognition and Illusory Conjunctions', Proceedings of IJCAI-85, pp. 252 - 259.
* Jahn, R.G and B.J. Dunne (1986) `On the Quantum Mechanics of Consciousness, with Application to Anomalous Phenomena', Foundations of Physics, 16 pp. 721 - 772.
* Marshall, I.N. (1989) `Consciousness and Bose-Einstein Condensates', New Ideas in Psychology, 7 pp. 73 - 85.
* Maxwell, N. (1988) `Quantum Propensiton Theory: A testable resolution of the wave-particle dilemma', The British Journal for the Philosophy of Science, 39, pp. 1 - 50.
* Squires, E.J. (1990) `An Attempt to Understand the Many-worlds Interpretation of Quantum Theory', in Quantum Theory without Reduction, ed. Cini and Levy-Blond, Adam Hilger, pp. 151 - 160.
* Thompson, I.J. (1988) `Real Dispositions in the Physical World', The British Journal for the Philosophy of Science, 39, pp. 67 - 79 (Html version).
* Thompson, I.J. (1989) `Swedenborg and Modern Science', Scientific and Medical Network Newsletter, 26, pp. 3 - 8 (Html version).
* Thompson, I.J. (1990) `Layers of Semantic Nets and Developmental Psychology', unpublished (Html version).
* Toben, B. (1974) `Space-Time and Beyond', Dutton.
* Walker, E.H. (1970) `The Nature of Consciousness', Mathematical Biosciences, 7, pp. 131 - 178.
* Wigner, E. (1962) `Remarks on the Mind-Body Question', pp. 284 - 302 in The Scientist Speculates, I.J. Good, ed: Basic Books, N.Y.
* Zohar, D. (1990) `The Quantum Self', Bloomsbury.
Ian J. Thompson
Physics Department, University of Surrey, Guildford GU2 5XH, U.K
October, 1990.
We may suspect that quantum mechanics and consciousness are related, but the details are not at all clear. In this paper, I suggest how the mind and brain might fit together intimately while still maintaining distinct identities. The connection is based on the correspondence of similar functions in both the mind and the quantum-mechanical brain.
Accompanying material for a talk at The Second Mind and Brain Symposium
held at the Institute of Psychiatry, Denmark Hill, London on 20th October, 1990. (pdf version)
The Mind-Brain Problem
The problem of the relation between mind and body is well known as a difficult `world knot'. Over the centuries various monistic and dualistic theories have been proposed, and the subject has had renewed interest as we try to assimilate the implications of quantum physics. These implications may make us reexamine our views of brains and bodies, but it is still not clear what consequences they have for our understanding of minds.
Earlier in the interpretation of quantum mechanics, it appeared that the problem of measurement could only be solved by introducing some basic notion of an observer, presumably a conscious observer. Many scientists and philosophers (e.g. Wigner (1962) Eccles and Popper (1977), Faber (1986), Toben (1974), Squires (1990), Donald (1990) among others) have taken this solution to indicate an essential role of consciousness in the physical world. Consciousness, to solve the measurement problem, must initiate the transition from quantum potentialities to definite actualities. It would not have to select which actuality, but merely cause some actuality to be produced. Squires and Donald accept this merely `collapse prompting' role of consciousness, and their position has the advantage of requiring no changes to quantum theory, as consciousness then makes no change to any predicted probability. Squire's conclusion, unfortunately, does not yield any functional role which may be important in mental and/or physiological processes in the brain. Donald (1990) does go on to give circumstances in which `measurement collapses' are important for the function of neural cells, but their effects are simply to reinstate the `classical' on/off nature of the sodium switches even when there are quantum uncertainties.
Since, moreover, it is possible that the `measurement problem' can be solved within physics (see e.g. Maxwell, 1988), the role of consciousness in measurement has little direct bearing on the problem of how the mind and brain function together. In order to say something relevant to the brain sciences, we have to go beyond conventional quantum mechanics (of whatever interpretation), and make new hypotheses with definite empirical content. We have to postulate that `consciousness' (whatever that may turn out to be) influences the actual outcome of neural events, and does not just observe them as if disinterestedly.
Quantum Brains
It is well known that there is a residual indeterminism of quantum mechanics in making predictions of physical processes. Quantum mechanics is also `non-local', as described for example by Bell's inequalities. If either of these two phenomena are displayed in the brain as described by quantum physics, then this may be relevant to the problem of consciousness.
Biased Probabilities:
One proposal (from Walker (1970) and later Eccles (1977)) has been that mental events influence at least some physical outcomes within the range allowed by quantum mechanics. This idea has to supplemented by an account of how these small effects are amplified to affect macroscopic outcomes, and recently possible accounts have been presented, Eccles (1989). Eccles requires that the options of a single vesicle discharging or not discharging both have non-zero quantum probabilities, so that such neural events are genuinely under-determined by quantum mechanics, and the way is open for them to be determined by the mind.
Bose-Einstein Condensates:
Another proposal by Marshall (1989) is that the mental and bodily realms derive directly from a quantum realm, as separate appearances of an underlying structure given by quantum mechanics. Since the unity of mental experience is one of its outstanding features, the proposal is that consciousness arises from a quantum mechanical structure which shows remarkably unity, namely Bose-Einstein condensates. This therefore requires not only that the brain amplifies quantum indeterminism, but also that long-range and long-time correlations of quantum phases are set up in the brain, by means for example of a structure proposed by Fröhlich.
Both these proposals go some way toward connecting the apparently disparate realms of mind and brain. From the physics side, both have quantum mechanics holding virtually unchanged. They furthermore then both require that neuro-physiological processes are in some essential way `quantum processes'. They require then that the peculiarly quantum features of matter become much more spatially spread out in the brain than is usually admitted. In the language of physics, Marshall requires that non-local correlations (of the kind described by Bell's Inequalities) are set up between parts of the brain that are very far apart on the atomic scale, and hence that the brain exhibits some of the characteristics of `quantum computers' as described by Deutsch (1985). There is a difficulty that this implies much larger quantum effects than have ever been observed in materials at room temperature. There is evidence of unusual microwave absorption, but it is not clear whether this really demonstrates quantum correlations as distinct from sets of coupled classical (non-quantum) oscillations.
Eccles' proposal is weaker than Marshall's, as he only requires that for some brief time the non-local correlations extend between the options of a single vesicle discharging or not discharging, so there can exist a coherent superposition of these two states. Eccles does not require that the two options remain coherent for any duration after the event (as Marshall would require to obtain widespread and persisting quantum phase coherences).
Both proposals are made less likely by the fact that Plank's constant, which sets the scale for quantum processes, is so small. It is not impossible that there are genuinely undetermined neural events in the brain, and/or that there are large scale quantum coherent effects, but these are difficult to set up in the noisy room-temperature non-crystalline system of the brain. Psychological ConsiderationsThere are also questions from the psychological point of view. Eccles' proposal tells us very little about the structure of the mind, apart from the fact that it must be able to predict and control the amazingly complicated system of many of the random discharges in the brain. There is little indication from what psychologists know about the mind that it has anything like the required analytical and motor capacities. The mind is best dealing with at most 5 to 8 items, and is completely swamped when it tries to understand massively parallel systems directly. A way of avoiding these difficulties will be discussed below.
Marshall's proposal seems to tell us more about the mind, namely that it arises whenever quantum mechanics gives suitable Bose-Einstein condensates. However, all psychological details of mental structure and operations would then be derived from this quantum structure. That is, he would have to claim that all human life (from mathematics and logic to arts to psychopathology) is implicit in Schrödinger's equation. Personally, I find it extremely implausible that the quantum mechanics of patterns of excitations of Bose-Einstein condensates exactly and mechanically determines the interaction patterns of ideas, images and meanings in the human mind. There are certainly similarities between the two, but these do not imply identity. To talk of similarity without giving evidence for identity leads to another approach, as we see next.
A Different Approach
Despite the apparently smallness of quantum effects in the brain, I still believe that quantum physics is important in our understanding of its function. To show this, I am going to take a different approach. I am going to assume, in a non-reductionist way, as Eccles and others do, that the mind is (in some way) distinct from the brain. But I want to then see how they could be intimately connected again. Psychologists and psychiatrists are aware of the close functional dependence of minds and brains, so that physiological changes very readily affect the mind in many ways, and mental intentions and attitudes affect both the normal and abnormal functioning of the nervous system.
I am going to take a `top-down' to the problem of interaction, and will suggest how it may usefully be organised, from the functional point of view. I am not going to assume that quantum mechanics must be unchanged; I am therefore going to follow the `spirit' if not the `letter' of quantum physics. The aim is to motivate a plausible general view which will make predictions about what goes on. Afterwards I will discuss the origins of this general view.
The guiding principle I use is that the mind and brain are similar in structure and function, and that they need each other to operate. They are not identical, but are similar in the sense of `resonating' or `corresponding' to each other from the functional point of view. In order to see how the mind and brain are similar and fit together in this general way, we do need to remember quantum physics.
Similarity of Minds and (Quantum) Brains
I am here going to draw analogies between mental and quantum phenomena. If you already know something about minds, these may help you understand quantum physics! Some of these have been pointed out by Bohm (1951, § 8.27ff) and more recently by Jahn & Dunne (1986) and Zohar (1990) (although perhaps understood in different ways: see later discussion).
Jahn and Zohar have tended to concentrate on the analogies between mental structures, and the structure of matter entailed by quantum mechanics. They look at analogies between, for example, minds and quantum systems, and then especially analogies between how minds interact and how quantum systems interact. I will be looking rather at analogies between individual mental processes within minds and individual quantum processes. I believe that only when we understand the elementary processes of both the mind and quantum matter, can we more completely understand complex systems of either kind.
In order to make analogies with quantum physics, I am going to assume something like the `propensity interpretation' of Maxwell (1988) and others (Thompson, 1988). I am also going to assume a view of mind that is in part phenomenological (after Bergson especially), and in part influenced by (a few of) the aims of artificial intelligence (AI) and the recognition of the complexity of even apparently elementary mental processes. On this basis, we can draw analogies between the fundamental processes of quantum physics and the fundamental processes of mental activity:
* Quantum objects do not evolve uniformly in time, but as a succession of `states' between intermittently `actualising' to one outcome of a certain range.
Mental entities do not evolve uniformly in time, but in a succession of `specious presents' between intermittent changes of state.
* The state of a quantum object is not a static configuration of elements, but a `state of propensity' for possible virtual proceses, possible interactions and possible actual (observable) outcomes.
The state of a mind is not static, but more a `state of propensity': it is a continual recollection of past events and continual anticipation of future possibilities.
* The state of a quantum object may be described by a wave function distributed is space, but this does not mean that the quantum object is composed of elements so distributed. Rather, it has a basic unity in that it will always act as a whole. These spatial distributions merely describe where it can actualise.
The state of a mind has a similar basic unity. The thoughts in the mind may be diverse, and each may implicitly contain diverse elements (usually the better for doing so), but if they lead to action these diverse elements were merely describing where the mind could have operated.
* The driving impetus of quantum processes are not external, but the very propensities of which quantum objects are forms. These propensities operate according to the (spatiotemporal) circumstantial relations with other objects.
The driving motivation of mental processes are not external, but the very motivational loves which underly all psychic activities. These motivations operate according to the circumstances at each time.
* Behind the apparently continuous evolution of the Schrödinger's equation, there is a progressive multitude of `virtual events' whose operation forms the potential term in Schrödinger's equation.
As a consequence of simulation efforts in AI, we know that the operation of even seemingly simple events such as the perception of solid objects must involve `behind the scenes' a great deal of information processing which is not apparent in the act of perception itself.
In view of these analogies, and also those given later, it unlikely that the mental and physical processes are independent, and merely interact with each other at certain `mental-neural' events (Eccles, 1989). We could hold that they both derive from a `quantum realm', but I hold to a more radical (but simpler) idea that the propensities for physical processes derive from mental processes, which in turn always act according to past physical events. This idea, explained further in the next section, means that the mental and physical realms are far from separable, but are intimately related in a very systematic manner. Thus, although the basic idea may not be exactly fashionable these days, it is an idea with significant predictive power and good empirical content. Nevertheless, as explained later, it has a long and venerable history, and deserves closer consideration.
How Minds and (Quantum) Brains Fit Together
In order for people to have functioning minds, their minds must at least be able to
* implement intended functions using the motor areas,
* establish permanent memories, presumably by means of permanent physiological changes,
* form perceptions using information from the visual & auditory (etc.) cortexes,
* be able to follow `internal' trains of thought/feeling/imagining without necessarily having any external effects.
One way that these requirements can be most simply accomplished is by means of the idea above, formulated in the following three principles:
I.
That the mind predisposes the physical/physiological potentialities (both deterministic or indeterministic according to quantum physics).
II.
That the `predispositional' capacities of the mind are consequentially restricted (and hence conditioned) by their actual effect.
III.
That the pattern of I and II is repeated for individual stages of more complex processes.
These principles together give what can be called `conditional forward causation', or `top-down causation'. Note that we do not have a fourth `bottom-up' principle that neural events directly cause events to occur in the mind. We do not have general matter $\rightarrow$ mind causation, although something resembling this does arise, namely `selection'. This is not causation in the primary sense of `principal causation' as `producing or generating the effect'. It is more in the secondary sense of `instrumental causation' as `providing a necessary prerequisite'.
A strong argument for these three principles is that they are already similar to what is known already to happen in quantum physics, in quantum field theory to be precise. According to that theory, there is a class of events called `virtual events' which select and predispose the ordinary quantum wave function. These virtual events operate deterministically, and describe the operation of the electric, magnetic, nuclear and gravitational forces. They are not the `final' actual events of quantum mechanics (those are the definite outcomes of events like observations). Rather, they are a `prior level' of `implicit events' whose operation is needed in order to produce the potentialities for events like observations. The principle (I) states the analogical result that mental events themselves are a `prior level' of `implicit events' whose operation is needed in order to produce the potentialities for physical events.
The argument for the principle (II) is more general. Whenever any potentiality is exercised to produce a particular outcome, future potentialities must depend on the detailed outcome. Suppose for example that at the moment, I have the potentialities of moving left or right; if I actually move left, say, then this influences (by restriction to a fixed history) of what I can do from now on. This second principle can also be seen as a `law of karma': your future life is restricted and influenced by your past actions (by selection). Physical events are in this way the necessary foundations for permanent mental history and structure.
Principle (III) has an important corollary
IIIb.
That the mind predisposes the brain to carry out those functions which `mirror' or `correspond to' the mind's own functions.
This is because mental functions involve intermediate steps, and these intermediate mental steps predispose suitable intermediate physical steps (by I), and are in turn conditioned or confirmed by them (by II). Thus the sequence of physical steps will follow the sequence of mental steps, and the overall function of the physical process will be analogous (in some sense) to the overall function of the mental process. Examples of this `mirroring' or `corresponding' have already been illustrated by the similarities between minds and brains listed in the previous section. Examples of correspondences of complex processes in the mind with those of physics will be given below.
Let us then see how these principles enable the mind and brain to function together:
* The particular functions selected by the mind to be carried out by the brain will be the establishment of spatiotemporal patterns of neural activity that may then be `decoded' in the motor cortex to lead to the desired activities, by the principle (I). Principle (IIIb) establishes a criterion for the overall functioning of this decoding.
* Permanent physiological changes lead to permanent memories in the mind, by the principle (II). (It is an empirical question, which physiological changes are relevant, but principle (IIIb) will be a guide.)
* Perceptions are formed by the sensory cortex areas deterministically forming particular patterns of neural activity, so that these physiological effects can select the subsequent perceptual content of the mind. The process here is rather subtle. The mind must have a `general disposition' to see/imagine any of its possible percepts; the role of the sensory cortex is to select the particular content, by means of principle (II). Thus we have the general psychological observation that `we see only what we are capable of and disposed to see'.
* To be able to follow `internal' trains of thought/feeling/imagining without necessarily having any external effects, the mind must be able to produce physiological effects which do not have any significant consequences. Presumably, much of the cerebral cortex can function in a `loosely connected' way, in order to provide the foundation for a set of permanent mental structures.
Mind and Brain as `Hand and Glove'
This theory of mind and brain connection establishes in intimate relation between them. It is not a relation of identity, or a relation of aspects or points of view. It is more a relation of inner and outer, or cause and effect: propensities in the brain are the causal product of mental actions. As put in by Bawden (1947), `the role of the psychical in relation to the physical (in the living organism) is essentially the relation of the potential or incipient to kinetic or overt action'.
The mind and brain fit together by approximate analogy with hand and glove, or, better, with tissue and skin. The analogy (by principle IIIb) is most precisely with the functions of tissue & skin, and not so much with their material shape. The mind provides all the directed activity of the brain, just as the tissue of the hand provides all the directed activity of the skin of the hand. When we look at the head, we only see the brain, just as we only see skin when we look at the hand. It appears that the skin of the hand does all the work, but we don't assume that that is all there is. It appears that the skin has life, but we know that all but the simplest life comes from the underlying tissue. The skin (as does the brain) has simple capacities for action and reactions, but it is a mistake to imagine that all capacities for activity and information processing belong to the skin (or the brain).
Psychology
Once we have the basic idea of functional correspondences between physical and mental systems based on their causal interrelations, then we have a fertile source for exploring the detailed structure of one using what we know about the other.
Psychology of Stages
Because there are `prior' and `post' levels of events even within physics, by the correspondence principle IIIb, there must also be prior and post levels of events within the mind. Piaget's theory of stages of development is relevant here. In table 1 the Piagetian stages have been reformulated as the acquisition of capacities for dealing with ideas on the successive layers (supplemented by a `creative stage' of Gowan (1972): see Thompson (1990) for more details). The relations between the ideas of adjacent levels is then analogous to the relation between any pair of prior and post events, according to principles I, II and III above.
Table 1: Relation between Network Layers and Piagetian Stages
During each stage, at the approximate ages shown, the child is learning to relate the concepts listed in the second column. That is, (s)he is constructing relations in a network at the given level.
Net Level Network Relations of Developed in
Piaget/ Gowan Stage during ages
5 meta-theories, paradigms creative 17-
4 plans, models, formalisms formal 12-16
3 classes, series, numbers operational 7-11
2 events, single relations, sentences preoperational (preconceptual & intuitive) 2-6
1 objects sensorimotor 0-1
0 images, motor movements (initial) -0
There appears to be a broad analogy of the `practical' layers (0, 1 and 2) with the `real' events of quantum mechanics, and of the `theoretical' layers (3, 4 and 5) with its `virtual' events. This can be spelt out in more detail, approximately as in table 2. Here, the basic idea (following principle IIIb) is that the mind at a given stage is capable of `thinking about' the corresponding features of quantum mechanics. The set of propensities/quantum systems/actual selections provides the mechanisms for the actual events of ordinary (non-relativistic) quantum mechanics, whereas the set of least-action principles/evolution equations/groups describes the basic principles and virtual events of quantum field theory.
Table 2: Relation of Network Layers to Quantum processes
Net Level Network Relations Quantum Theory
5 meta-theories, paradigms variational principles
4 plans, models, formalisms time evolution equations
3 classes, series, numbers invariance groups
2 events, single relations propensities & forces
1 objects quantum systems
0 images actual selections
Artificial Intelligence and Neural Networks
Neural networks have proved to be useful framework for formulating a wide range of information processing problems in cognitive psychology. This has lead some to postulate their sufficiency for a system of artificial intelligence. However, there are three main problems with neural networks. The first is the question of speed, the second the problem of procedures, and the third the problem of levels.
1. In order for neural networks to (approximately) solve global optimisation problems, some search procedure such as simulated annealing is required. This is notoriously slow, and, as still a procedure of `exponential complexity', will be prohibitively slow for problems of realistic complexities.
2. Neural networks have fixed connections only, and in order for them to achieve anything like translation or scale invariances for perception, then all possible translations and scales have to be `hard wired' simultaneously (Hinton et al, 1985). This could be avoided again by some `procedural' or `algorithmic' mechanisms, with `variable bindings', but this can only be implemented in neural networks by `brute force' multiple connections.
3. Neural networks seem to plausibly describe the interactions of related ideas on a given `level' of table 1. They are notoriously poor, indeed AI as a whole is notoriously poor, at modelling the connections between the separate layers (see Thompson, 1990).
It is still possible that the brain uses neural networks so that their various `final states' serve to indicate different image categories and different actions. In the present causal theory of mind-brain interaction, however, it is more likely that the mind can act by predisposing the neural nets to settle to some preferred outcome. That is, the mind determines (from the function it wishes performed) which collective state of the neural system is to be achieved. From the point of view of the brain, the neural net would appear to be the `non-deterministic automaton' of complexity theory, heading more or less quickly for the preferred solution (while thus verifying that it is a solution).
The problems of procedures and levels in neural networks are resolved by having multiple levels of simultaneous mental operation within the mind. The relations between the multiple levels then follows the principles I, II and III. Principle I means that the higher level predisposes the interactions at the lower level. Principle IIIb means that it does so in order to carry out particular functions. Principle II means that configurations at the lower level (e.g. images) select a particular perception (e.g. of an object) which would produce that image. This selection must be able to made very quickly - much more quickly than any detailed analytical algorithm could be elaborated.
New Physics
What is new physics in this proposal, is the first principle I: that the mind predisposes the physiological potentialities (whether deterministic or indeterministic according to quantum physics). This relation between mind and brain is analogous to that between virtual and real quantum events, but postulating the mind/brain relation to be of this kind has new empirical content.
The predisposition of physical propensities is of course not arbitrary. It is conditioned from the physical side by past actual events, from the mental side by the functions which may be accomplished, and from both sides by the requirement of correspondence. These constraints mean we do not have merely a `mind of the gaps' which fills in what is left undetermined by modern science. There are certainly gaps to be filled, but the present scheme will also explain what we already know, not just what we don't know.
If, for example, there is a regular sequence of actual physical events, and the predisposition is not varying, then there will be a constant production of new propensities, and hence a regular sequence of physical effects as if by a universal physical law. Thus our principles can provide a new basis for physical laws which we already know.
When there are intermediate propensities (e.g. of individual minds) then physical events do not follow the previous simple pattern. The new pattern will describe how the brain works in conjunction with the mind. Clues to this behaviour can be found by analogy with psychological processes, as discussed later.
* We saw in table 2 that the images (in the mind) correspond to actual selections (in quantum mechanics). This casts new light on our old problem of the role of consciousness in quantum measurement, and we see that perception of images does cause and correspond to the selection of actual outcomes of measurement in quantum physics. It is similar to Eccles' proposal, but now we have a better idea of the mental side of the picture. The network level 0 is the initial stage of cognitive development, and does not require any sophisticated analytical or predictive capacities: it is more the ability to `see what there is to see'. This means that the selection of outcomes of measurements does not happen at the time of the (random) event, but later, when perceptible differences have emerged, and are perceived.
* The ability of the sensorimotor mind to influence the course of events is, however, quite limited. Its role in everyday life is more to provide a source of perceptions, and implement outcomes that have already been decided. This corresponds in quantum mechanics to the small size of Planck's constant, and the rarity of quantum-random events which do have perceptible differences. Occasionally, however, there is still a need for basic decisions, and (in the mind and in quantum mechanics) it is necessary to decide between one of a small number of options. On these occasions, consciousness does limit itself to a single perception, and this does cause the selection of a particular outcome of a event left random by quantum physics.
* Most of the interesting processes in the mind and in quantum physics take place at levels 3 - 5: the `theoretical' layers as these were described earlier. Processes occur at these layers which generate in the first place the `short list' of options for the sensorimotor mind, and a great deal of detailed knowledge and derivation goes into this preparation.
* The operation of predisposition and correspondence in these `higher' levels must mean that the time-evolution equation of physics, normally taken to be Schrödinger's equation, is not fixed. We should be able to discover circumstances in which even classical systems follow a modified physical law.
`New' Metaphysics
These ideas have the possible disadvantage (or feature) that the operation of ordinary inert physical processes requires further analysis. Basically, since the propensities for physical processes derive from mental processes, all physical dispositions must derive (or have been derived from) some prior psychical level. This may sound like pan-psychism, but I am not saying that all physical processes include their own consciousnesses. There is a simpler solution, if you can accept the new metaphysics that there is some kind of Source, composed of suitable `psychic' propensities, from which everyday material propensities perpetually derive. Since the operation of this Source is always according to past physical events, we saw above that this operation amounts to the constant production of new propensities as if a `physical law' were prevailing. That is the way most scientists prefer to see the world. It is only that sometimes things are not so simple.
There may be some reaction to the apparent `dualism' in these ideas, as I have postulated minds existing separately from brains. However, this separation is only in our theory: in practice they need each other, and function together as a unified whole - as the person.
Origin of these ideas
I have presented these ideas as worth of consideration on their own, but they really have a long history in a variety of contexts. The basic idea that causation only truly works from the mind into the brain (and not vice versa) is not a popular one today, but has to be traced back to `non-standard' insights of people such as Plotinus (b. 205), Boehme (b. 1575), Swedenborg (b. 1688) and some other traditions. Swedenborg was well educated as a physicist and then physiologist, so I find his accounts the most detailed and useful. Of course, he knew nothing of quantum mechanics (only Newtonian mechanics), so I have had to `re-apply' his principles in the light of what we now know about the physical world. He, however, has the clearest presentation of the idea of `conditional forward causation' (he calls it `influx into uses'), and he gives the most complete account of the `correspondences' which exist between mental and bodily things. For a brief summary of his ideas, see Thompson (1989).
Conclusion
In order to understand how the mind and brain function together, it is not enough for there to be gaps in our physical theories. We also need to have a unified picture of both minds and brains. In this paper I have tried to outline such a unified approach. These suggestions require some departure from what is commonly accepted in the physical sciences, but we can still learn a great deal from what has already been discovered both there and in the psychological sciences.
* Bawden, H.H. (1947) `The Psychical as a Biological Directive', Philos. Science, XIV pp. 56 - 67.
* Bohm, D. (1951) `Quantum Theory', Prentice-Hall.
* Bohm, D. (1980) `Wholeness and the Implicate Order', Routledge and Kegan-Paul.
* Deutsch, D. (1985) `Quantum Theory, the Church-Turing principle and the universal quantum computer', Proc. R. Soc. Lond, A 400, pp. 91 - 117.
* Donald, M.J. (1990) `Quantum Theory and the Brain', Proc. R. Soc. Lond, A 427, pp. 43 - 93.
* Eccles, J. and K.R. Popper, (1977) `The Self and Its Brain', Springer.
* Eccles, J. (1989) `Evolution of the Brain: Creation of the Self', Routledge.
* Faber, R.J. (1986) `Clockwork Garden'.
* Gowan, J.C. (1972) `Development of the Creative Individual', Knapp.
* Hinton, G.E. and K.J. Lang (1985) `Shape Recognition and Illusory Conjunctions', Proceedings of IJCAI-85, pp. 252 - 259.
* Jahn, R.G and B.J. Dunne (1986) `On the Quantum Mechanics of Consciousness, with Application to Anomalous Phenomena', Foundations of Physics, 16 pp. 721 - 772.
* Marshall, I.N. (1989) `Consciousness and Bose-Einstein Condensates', New Ideas in Psychology, 7 pp. 73 - 85.
* Maxwell, N. (1988) `Quantum Propensiton Theory: A testable resolution of the wave-particle dilemma', The British Journal for the Philosophy of Science, 39, pp. 1 - 50.
* Squires, E.J. (1990) `An Attempt to Understand the Many-worlds Interpretation of Quantum Theory', in Quantum Theory without Reduction, ed. Cini and Levy-Blond, Adam Hilger, pp. 151 - 160.
* Thompson, I.J. (1988) `Real Dispositions in the Physical World', The British Journal for the Philosophy of Science, 39, pp. 67 - 79 (Html version).
* Thompson, I.J. (1989) `Swedenborg and Modern Science', Scientific and Medical Network Newsletter, 26, pp. 3 - 8 (Html version).
* Thompson, I.J. (1990) `Layers of Semantic Nets and Developmental Psychology', unpublished (Html version).
* Toben, B. (1974) `Space-Time and Beyond', Dutton.
* Walker, E.H. (1970) `The Nature of Consciousness', Mathematical Biosciences, 7, pp. 131 - 178.
* Wigner, E. (1962) `Remarks on the Mind-Body Question', pp. 284 - 302 in The Scientist Speculates, I.J. Good, ed: Basic Books, N.Y.
* Zohar, D. (1990) `The Quantum Self', Bloomsbury.
Saturday, August 1, 2009
Deepening the Quantum Mysteries
by John Gribbin
The "central mystery" of quantum physics just got more mysterious. Experimenters from the United States and Austria have got together to provide a new demonstration of how light going through a "double slit" experiment seems to know before it sets out in its journey exactly what kind of traps have been set for it along the way.
This is a variation on the Young's slit experiment, familiar from school laboratory demonstrations of the wave nature of light. When a beam of monochromatic light is shone through two narrow holes in a screen, the light spreading out from the two holes interferes, just like ripples interfering on the surface of a pond, to produce a characteristic pattern on a second screen.
The mystery is that light can also be described as a stream of particles, called photons. The light source in a Young's slit experiment can be turned down to the point where it consists of individual photons going through the experiment, one after the other. If the spots of light made by individual photons arriving at the second screen (actually a photoelectric detector) are added together, they still form an interference pattern, as if each photon goes through both holes and interferes with itself on the way through the experiment. It was Richard Feynman who described this as "the central mystery" of quantum theory, and then corrected himself, saying that in fact it is "the only mystery". If you understood this, you would understand quantum physics -- but as Feynman also said, "nobody understands quantum mechanics" (The Character of Physical Law, BBC Publications, 1965).
The new demonstration of how incomprehensible the quantum world is has been made by Raymond Chiao, of the University of California, Berkeley, Paul Kwiat, of the University of Innsbruck, and Aephraim Steinberg, of the US National Institute of Standards and Technology, in Maryland. Their results were presented at a meeting in Nathiagali, Pakistan.
In fact, the team has carried out several tests of the stranger predictions of quantum theory, but the most dramatic is what they call the "quantum eraser". In this variation on the Young's slit theme, the experiment is first set up in the usual way, and run to produce interference. Quantum theory says that the reason why interference can occur, even if light is a stream of photons, is that there is no way to find out, even in principle, which photon went through which slit. The "indeterminacy" allows fringes to appear.
But then Chiao and his colleagues ran the same experiment with polarising filters in front of each of the two slits. Any photon going one way would become "labelled" with left-handed circular polarization, while any photon going through the other slit is labelled with right-handed circular polarization. In this version of the experiment, it is possible in principle to tell which slit any particular photon arriving at the second screen went through. Sure enough, the interference pattern vanishes -- even though nobody ever actually looks to see which photon went through which slit.
Now comes the new trick -- the eraser. A third polarising filter is placed between the two slits and the second screen, to scramble up (or erase) the information about which photon went through which hole. Now, once again, it is impossible to tell which path any particular photon arriving at the second screen took through the experiment. And, sure enough, the interference pattern reappears!
The strange thing is that interference depends on "single photons" going through both slits "at once", but undetected. So how does a single photon arriving at the first screen know how it ought to behave in order to match the presence or absence of the erasing filter on the other side of the slits?
All of these experiments were carried out using beams of individual photons, and there is no way in which the results can be explained by using classical physics. They lay bare the mysteriousness of quantum mechanics in all its glory, and in particular demonstrate its "non local" nature -- the way in which a photon starting out on its journey behaves in a different way for each experimental setup, as if it knew in advance what kind of experiment it was about to go through.
Don't worry if you don't understand this. Richard Feynman didn't, and he warned "do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will go 'down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."
More atoms that communicate faster than light
PHYSICISTS still struggling to come to terms with experiments which show instantaneous communication between quantum particles under special circumstances are now faced with another puzzle. Correcting a mistake made by Enrico Fermi more than sixty years ago, Gerhard Hegerfeldt, of the University of Gttingen, has shown that in theory any pair of atoms can communicate faster than light.
The now-familiar puzzle of what are called "non local" interactions develops from theoretical work by John Bell, of CERN, in the 1960s and experiments by Alain Aspect in Paris in the 1980s. Together, these show that a pair of photons ejected in opposite directions from an atom remain somehow entangled, as if they were one particle. Measuring the state of one of the photons instantaneously affects the state of the other one, wherever it may be. Now, it seems that even atoms which have never come into contact (from the perspective of classical Newtonian physics) are entangled in a similar way.
The calculation Fermi carried out in 1932, in the early days of quantum mechanics, concerned the response of one atom to radiation emitted by another atom of the same kind, some distance away. If the second atom is in an excited state, sooner or later it will emit radiation, falling back to its ground state. This radiation will have exactly the right frequency to excite the second atom (this is one of the principles underlying the way atoms are "pumped" into an excited state to make a laser).
Common sense tells us that the first atom cannot be excited until a finite time after the second atom decays -- until there has been time for radiation travelling at the speed of light to cross the gap. That is the result Fermi found. But it now turns out that he made a mistake in his calculation. Probably because the mistaken conclusion matched common sense, it took a long time for this to come to light. But Hegerfeldt's correct version of the calculation now makes it clear that there is a small chance that the first atom will be excited as soon as the second atom decays (Physical Review Letters, vol 72 p 596). As with all such quantum puzzles, this is only the beginning of the story; now, the experts have to explain what this mathematical result means. The best interpretation of the evidence so far seems to be that we should not think of any object, not even a single atom, as an "isolated system".
Because particles must also be considered as waves (one of the basic tenets of quantum mechanics), the individual particles in the atom are spread out, and there is a finite (though small) chance of finding them anywhere in the Universe. So the wave functions of the electrons in the first atom overlap with those of the electrons in the second atom. They are entangled, like the two photons produced in the Aspect experiment, and when an electron in one atom jumps down an energy level that can instantaneously make its counterpart in the other atom jump up by the same amount.
Solving the quantum mysteries
John Gribbin
For seventy years, physicists have worried about what quantum mechanics means. They can use quantum physics, to be sure; witness the successful designs of lasers and computer microchips, and the understanding of molecules that makes genetic engineering possible. But the equations that are a routine part of this kind of work contain one embarrassing feature. The say, according to the standard interpretation (the Copenhagen interpretation), that nothing is real unless you look at it, that an electron (say) exists only as a wave of probability, called a wave function, which collapses into reality when it is measured, and promptly dissolves into unreality when you stop looking at it. We are no further advanced philosophically, on this picture, than the image of the tree in the quad which disappears when nobody is looking at it.
In fact, few physicists worry about such things. Most of them prefer to ignore them, in the hope they will go away. But now there is another interpretation, which solves all of the quantum mysteries. The snag is, those physicists may not like this one, either, because it involves signals that travel backwards in time.
The archetypal example of the quantum mysteries is the "experiment with two holes", where the measured position of a single electron that passes through two holes in a screen can only be explained in terms of the wave function travelling through both holes at once and interfering with itself. But perhaps you've heard that one already, so here is a rather sideways look at the whole business of collapsing wave functions, a thought experiment which says that the lack of an observation can make the wave function of a system collapse. This wonderful example of the strangeness of the quantum world dates back to the early 1950s, and is known as "Renninger's negative- result experiment", after the German physicist Mauritius Renninger who first thought of it. It is one of the easiest examples of quantum strangeness to understand -- but not to explain.
Imagine that we have a source which will emit a single quantum particle in a random direction (ordinary radioactive nuclei do exactly this, so there is nothing special about the source). This source is in the middle of a large hollow sphere, and the inner surface of the sphere is lined with material that will give a flash at the point where the particle hits it. The accepted quantum description of what happens when the source emits a particle is that a quantum probability wave spreads out evenly in all directions around the source, since there is an equal probability for the particle being emitted in any direction. When the probability wave reaches the inner surface of the spherical shell, there is just one flash of light as the wave collapses to a single point. The particle is only "real" when it is being observed -- when it makes the flash of light -- not while it is travelling from the source to the sphere.
So far, simple enough. But now imagine that half way between the source and the sphere there is a hemispherical shield, which blocks off exactly half of the outer sphere from the field of view of the source. Like the outer sphere, this inner hemispherical shell is lined with scintillating material that will flash when it is struck by a particle from the source. Now what happens when the source emits a particle? We are not interested in exactly where on the outer or inner spheres the particle makes a flash of light, only in which of the two spheres it strikes. Either the particle strikes the inner sphere and makes it flash, or it strikes the outer sphere and makes it flash. There is an equal probability of either outcome of the experiment. Now, suppose that the source is once again triggered into emitting a particle. Once again, standard quantum theory describes this as an expanding spherical shell of probability, moving out evenly in all directions. We wait for a time longer than the time needed for it to reach the inner hemisphere, but too short for it to have reached the outer sphere, and see no flash on the inner sphere. So we know that the final state of the experiment will involve a flash on the outer sphere -- the particle must have been emitted in the wrong direction to strike the inner hemisphere. From a 50:50 probability of the flash occurring either on the hemisphere or on the outer sphere, the quantum wave function has collapsed into a 100 per cent certainty that the flash will occur on the outer sphere. But this has happened without the observer actually "observing" anything at all! It is purely a result of a change in the observer's knowledge about what is going on in the experiment. It requires an observer intelligent enough to infer what is happening, and what would have happened if the particle had been heading towards the inner hemisphere (so a cat, for example, clearly would not be intelligent enough to cause this particular collapse of a wave function). Under these circumstances, the absence of an observation can collapse the quantum wave function as effectively as an actual observation can. At least, so says the Copenhagen interpretation.
This central role for the observer -- not just any observer, but an intelligent observer -- lies at the heart of the standard Copenhagen interpretation of quantum mechanics.
But a giant leap in what might be called quantum philosophy has recently been taken by the American physicist John Cramer. He has taken a new look at the wave equations of quantum mechanics -- the famous Schrdinger equation, and the equations describing the probability waves, which travel, like photons, at the speed of light. What Cramer has pointed out is that the equations actually have two sets of solutions, one equivalent to a positive wave flowing into the future (a "retarded" wave), and the other describing a negative wave flowing into the past (an "advanced" wave). As all physicists learn at university (and most promptly forget) the full version of the wave equation has two sets of solutions -- one corresponding to the familiar simple Schrdinger equation, and the other to a kind of mirror image Schrdinger equation describing the flow of negative energy into the past.
The proper mathematical description of the wave function actually includes a mixture of both ordinary ("real") numbers and imaginary numbers -- those numbers involving i, the square root of minus one. Such a mixture is called a complex variable, for obvious reasons; it is written down as a real part plus (or minus) an imaginary part. The probability calculations needed to work out the chance of finding an electron (say) in a particular place at a particular time actually depend on calculating the square of the complex number corresponding to that particular state of the electron.
But calculating the square of a complex variable does not simply mean multiplying it by itself. Instead, you have to make another variable, a mirror image version called the complex conjugate, by changing the sign in front of the imaginary part -- if it was + it becomes -, and vice versa. The two complex numbers are then multiplied together to give the probability. For equations that describe how a system changes as time passes, this process of changing the sign of the imaginary part and finding the complex conjugate is equivalent to reversing the direction of time!
The basic probability equation, developed by Max Born back in 1926, itself contains an explicit reference to the nature of time, and to the possibility of two kinds of Schrdinger equations, one describing waves that move forward in time and the other representing waves that move backward in time.
The remarkable implication is that ever since 1926, every time a physicist has taken the complex conjugate of the simple Schrdinger equation and combined it with this equation to calculate a quantum probability, he or she has actually been taking account of the influence of waves that travel backwards in time, without knowing it. There is no problem at all with the mathematics of Cramer's interpretation of quantum mechanics, because the mathematics, right down to Schrdinger's equation, is exactly the same as in the standard Copenhagen interpretation. The difference is, literally, only in the interpretation -- Cramer accepts that the wave flowing backward in time is real, and should be taken seriously, not ignored. The way Cramer describes a typical quantum "transaction" is in terms of a particle "shaking hands" with another particle somewhere else in space and time. One of the difficulties with any such description in ordinary language is how to treat interactions that are going both ways in time simultaneously, and are therefore occurring instantaneously as far as clocks in the everyday world are concerned. Cramer does this by effectively standing outside of time, and using the semantic device of a description in terms of some kind of pseudotime. This is no more than a semantic device -- but it certainly helps to get the picture straight.
It works like this. When an electron vibrates, on this picture, it attempts to radiate by producing a field which is a time-symmetric mixture of a retarded wave propagating into the future and an advanced wave propagating into the past. As a first step in getting a picture of what happens, ignore the advanced wave and follow the story of the retarded wave. This heads off into the future until it encounters an electron which can absorb the energy being carried by the field. The process of absorption involves making the electron that is doing the absorbing vibrate, and this vibration produces a new retarded field which exactly cancels out the first retarded field. So in the future of the absorber, the net effect is that there is no retarded field. But the absorber also produces a negative energy advanced wave travelling backwards in time to the emitter, down the track of the original retarded wave. At the emitter, this advanced wave is absorbed, making the original electron recoil in such a way that it radiates a second advanced wave back into the past. This "new" advanced wave exactly cancels out the "original" advanced wave, so that there is no effective radiation going back in the past before the moment when the original emission occurred. All that is left is a double wave linking the emitter and the absorber, made up half of a retarded wave carrying positive energy into the future and half of an advanced wave carrying negative energy into the past (in the direction of negative time). Because two negatives make a positive, this advanced wave adds to the original retarded wave as if it too were a retarded wave travelling from the emitter to the absorber. In Cramer's words:
The emitter can be considered to produce an "offer" wave which travels to the absorber. The absorber then returns a "confirmation" wave to the emitter, and the transaction is completed with a "handshake" across spacetime.
But this is only the sequence of events from the point of view of pseudotime. In reality, the process is atemporal; it happens all at once. This is because, as Einstein explained with his special theory of relativity, signals that travel at the speed of light take no time at all to complete any journey -- in effect, for light signals every point in the Universe is next door to every other point in the Universe. Whether the signals are travelling backwards or forwards in time doesn't matter, since they take zero time (in their own frame of reference), and +0 is the same as -0 -- and all the quantum probability waves do travel at the speed of light.
The situation is more complicated in three dimensions, but the conclusions are exactly the same. This interpretation makes no predictions that are different from those of conventional quantum mechanics, but it provides a conceptual model which helps many people to think clearly about what is going on in the quantum world. It means that when an electron is faced with a choice of two holes to go through, the offer goes through both but the handshake only comes back through one, so it knows where to go; and in Renninger's experiment, the particle setting out from the radioactive nucleus has already made its handshake and knows which hemisphere it will end up on. There is no more mystery about the quantum mysteries at all -- provided you can live with waves that go backwards in time.
Photons faster than light
Nothing can travel faster than light -- unless it is a quantum particle "tunneling" through a barrier that, according to good old Newtonian physics, it should not be able to penetrate at all. Physicists have puzzled for decades over how long this mysterious tunneling process takes, but they need puzzle now longer, for it has been measured. And, sure enough, it takes place faster than light.
Quantum tunneling is of more than just esoteric interest. The phenomenon is related to quantum uncertainty, and to wave-particle duality. When two quantum particles, such as two protons, come close to one another, but do not actually touch, the uncertainty in their positions allows their quantum waves to overlap to some extent. As a result, they may "tunnel through" the gap between them, and interact. This is exactly what happens inside the Sun and stars -- protons which are kept at a distance from one another by the repulsion of their positive charge can still fuse together because of tunneling. And that nuclear fusion is what keeps the interior of the Sun hot, and makes its surface shine. Without tunneling, we would not be here.
Raymond Chaio, of the University of California, Berkeley, and his colleagues have actually been measuring a different, but related, kind of tunneling. They have devised an experiment in which two photons (particles of light) are produced simultaneously in a source, and travel on parallel paths. One photon goes straight to a detector; the other is confronted by a barrier which would reflect the light of the photons obeyed the laws of classical, "Newtonian" physics. But according to quantum theory there is a high probability that some of the photons arriving at the mirror will tunnel straight through, and go on their way to the detector.
Sure enough, that is what happens. The barrier is 1.1 micrometers thick, so anything travelling through it at the speed of light would take 3.6 femtoseconds (3.6 thousand million millionths of a second) on the journey. But the new experiment is so sophisticated that it can compare the arrival times of pairs of photons, one of which has gone past the barrier and one through it, and shows that the one which goes through the barrier arrives first. It tunnelled through the barrier faster than the speed of light, in less than 3.6 femtoseconds. As the researchers put it, "it is as though the particle 'skipped' the bulk of the barrier". But don't ask them, or anyone else, what it means -- in the words of Richard Feynman, "nobody understands quantum mechanics".
Uncertainty rules in the quantum world
IN SCENES reminiscent of the great debates between Niels Bohr and Albert Einstein in the 1930s, fundamental quantum physics has been tested in a series of new "thought experiments", and has passed those tests with flying colours.
The standard interpretation of quantum physics, known as the Copenhagen Interpretation, was established largely through the efforts of Bohr at the beginning of the 1930s. The strange features of the quantum world are founded on what Bohr dubbed complementarity -- the way an entity such as an electron can behave either as a wave or as a particle -- and on Werner Heisenberg's principle of uncertainty, which says that a quantum entity such as an electron does not possess both a position and a momentum simultaneously.
As Richard Feynman was fond of pointing out, the strangeness of the quantum world is encapsulated in "the experiment with two holes". If electrons (or photons, the "particles of light") are fired one at a time through a standard Young's double slit type of experiment and arrive at a detector screen on the other side, they leave the "gun" on one side of the experiment as particles, and arrive at the screen on the other side of the two slits as particles, each making a single spot on the screen. But somehow they pass through the two slits in between as waves, interfering with one another (even though they pass through one at a time!) so that the pattern built up on the screen by the accumulation of spots is an interference pattern.
This wave-particle duality is linked with the uncertainty principle. "Waviness" is a property associated with momentum -- a typical wave is spread out, so it has no definite location in space, but it does have a direction in which it is going. By contrast, a particle can have a precisely defined position. Heisenberg found that the quantum equations imply a strict tradeoff between the two complementary properties. If the position of a quantum entity is precisely defined (for example, when it hits a detector screen), its waviness is suppressed; but if it is allowed to give full expression to its wave nature, the particle aspect vanishes.
In practical terms, this means that we can never measure both momentum and position precisely, at the same time, for any particle. Einstein argued that this was simply a reflection of our clumsiness. Measuring the position of an electron, say, would involve bouncing light off it, and the very act of bouncing light off the electron would make it recoil, changing its momentum (and, indeed, its position). He thought that there were real little particles, like tiny billiard balls, involved in quantum interactions, and that the appearance of fuzziness and interference in experiments was a result of the deficiencies of the experiments.
Bohr argued that the fuzziness was an intrinsic feature of the quantum world, and that within the limits set by Heisenberg's uncertainty principle an electron itself does not "know" both where it is and where it is going. Over several years, beginning at the end of the 1920s, Einstein tried to dream up idealised thought experiments which could in principle measure both the position and the momentum of a particle such as an electron at the same time, thereby refuting Heisenberg. Each time, Bohr found a flaw in Einstein's argument, proving that the experiment could not work as Einstein had thought, even in principle. Bohr's success in this debate with Einstein was a major reason why the Copenhagen Interpretation became the established way of thinking about the quantum world. For sixty years, it seemed there was nothing to add to the debate. Then, in 1991 Marlan Scully, of the University of New Mexico, and colleagues claimed that they had found a way to carry out the kind of measurement Einstein had sought for in vain (Nature, vol 351 p 111). The essence of their argument (see Jim Baggott, "Beating the uncertainty principle", New Scientist, 15 February 1992) was that atoms could be sent through a double slit experiment in an excited state. Behind each slot there would be a detector known as a micromaser cavity, and the experiment would be timed so that each atom emitted a photon as it passed through the appropriate cavity. This would "switch on" one of the detectors, showing which slit the atom passed through, but would leave the atom free to carry on and make its mark on the final detector screen.
In such experiments without the cavities, atoms have been shown to behave like waves when passing through the two slits, creating an interference pattern. But if it is possible to detect which slit each atom passes through, without disturbing the flight of the atom, it would surely be impossible to produce interference. That requires something going through both slits at once. The presence of the cavities would make the interference pattern vanish, as if by magic, demonstrating the absurdity of the Copenhagen Interpretation. But now it seems that things are not that simple. Pippa Storey and colleagues at the University of Auckland have shown that even in this kind of idealised experiment that atoms are disturbed by the presence of the cavities, and in just the right way to "wash out" the interference pattern. They point out (Nature, vol 367 p 626) that although the ejection of a photon by the excited atom need not affect its forward momentum through the slit(s), there is always some uncertainty in the amount of sideways momentum imparted to the atom by the kick of the departing photon.
The situation is complicated by the prediction, in line with quantum uncertainty, that instead of a single photon being emitted cleanly from the atom, the atom can be involved in interactions with "virtual" photons, which emerge briefly from the vacuum (out of "nothing at all") before disappearing. But the conclusion is that provided the cavities are narrower than the separation between the two slits in the experiment (which they have to be if they are to tell us which slit each atom passes through), the interference pattern is destroyed. And the washing out of the interference pattern can even be understood by treating the atoms as waves. Interference can only occur if the waves passing through the two slits remain in phase, but "Because the atom's motion is primarily longitudinal," say the New Zealand team, "a transverse momentum kick will simply change its direction slightly . . . the displacement . . . is effectively the familiar phenomenon of refraction: a position-dependent change in the phase of a wave results in a change in the direction of propagation". The loss of interference from a double slit in the presence of cavity detectors is caused by momentum kicks, which are themselves of a size determined by the uncertainty principle. Einstein, no doubt, would have taken this as philosophically (if unbelievingly) as he took his other setbacks; but the ghosts of Bohr and Heisenberg must be smiling.
Molecules make quantum waves
THE WEIRDNESS of the quantum world has taken another step towards the everyday world as a result of experiments which show iodine molecules behaving as waves in interference experiments. This brings properties often regarded as unique to subatomic particles out into the open on much larger scales.
It is a fundamental feature of quantum mechanics that entities described by the quantum equations are not simply particles or waves, but exhibit a mixture of wave and particle properties. Light, for example, will behave as a wave in interference experiments, with two sets of waves interacting with one another to form a new pattern, just as ripples on a pond (or in your bath) interact with one another. On the other hand, in other experiments light will behave as a stream of tiny particles, called photons.
Wave-particle duality was first discovered to be a feature of light in the early part of this century (Albert Einstein's Nobel Prize was awarded for his proof that photons exist). In the 1920s, researchers found that electrons, traditionally regarded as particles, could behave as waves in experiments where an electron beam is diffracted from a crystal lattice -- indeed, in one of the nicest examples of wave- particle duality, the physicist J. J. Thomson {ED: NB always "J J", never referred to by name} received a Nobel Prize for discovering that the electron is a particle, while his son George received a Nobel Prize for proving that the electron is a wave.
Moving up the mass scale, first neutrons (each nearly 2,000 times the mass of an electron) and then beams of atoms and molecules were shown to diffract like waves when passed through small apertures. Over the past ten years or so, the wave-particle duality has been demonstrated ever more clearly. Not just diffraction (in which one beam, or wave, bends as it passes an obstruction) but interference (in which two beams or waves interact with one another) has been demonstrated both for electrons and atoms. Now, a team of researchers at the University of Paris-North, at Villetaneuse in France, has done the trick with molecules.
In the traditional version of the interference experiment with light, two beams of light are generated by passing light from a single source through two slits in a screen. Then, the two beams are allowed to interfere, producing a characteristic stripey pattern of light and shade. The new experiment is conceptually similar, but instead of passing through holes in a screen the iodine molecules (I2, which each have a mass about 254 times that of a neutron) interact with laser beams. The first interaction, with a pair of laser beams, puts each molecule into what is known as a "superposition of states", effectively two wave packets marching side by side. A second pair of laser beams recombines the wave packets to make "particles". At least, that is the theory. What happens in practice? After they have passed through the laser beams, the iodine molecules arrive at a detector. The distribution of the molecules arriving at the detector does not resemble the pattern you would expect if they were a stream of particles travelling through the experiment, but exactly matches the stripey pattern of peaks and troughs corresponding to interference by waves (Physics Letters A, vol 188 p 187). These are the heaviest "particles" which have ever demonstrated their wave "character" directly in experiments.
The "central mystery" of quantum physics just got more mysterious. Experimenters from the United States and Austria have got together to provide a new demonstration of how light going through a "double slit" experiment seems to know before it sets out in its journey exactly what kind of traps have been set for it along the way.
This is a variation on the Young's slit experiment, familiar from school laboratory demonstrations of the wave nature of light. When a beam of monochromatic light is shone through two narrow holes in a screen, the light spreading out from the two holes interferes, just like ripples interfering on the surface of a pond, to produce a characteristic pattern on a second screen.
The mystery is that light can also be described as a stream of particles, called photons. The light source in a Young's slit experiment can be turned down to the point where it consists of individual photons going through the experiment, one after the other. If the spots of light made by individual photons arriving at the second screen (actually a photoelectric detector) are added together, they still form an interference pattern, as if each photon goes through both holes and interferes with itself on the way through the experiment. It was Richard Feynman who described this as "the central mystery" of quantum theory, and then corrected himself, saying that in fact it is "the only mystery". If you understood this, you would understand quantum physics -- but as Feynman also said, "nobody understands quantum mechanics" (The Character of Physical Law, BBC Publications, 1965).
The new demonstration of how incomprehensible the quantum world is has been made by Raymond Chiao, of the University of California, Berkeley, Paul Kwiat, of the University of Innsbruck, and Aephraim Steinberg, of the US National Institute of Standards and Technology, in Maryland. Their results were presented at a meeting in Nathiagali, Pakistan.
In fact, the team has carried out several tests of the stranger predictions of quantum theory, but the most dramatic is what they call the "quantum eraser". In this variation on the Young's slit theme, the experiment is first set up in the usual way, and run to produce interference. Quantum theory says that the reason why interference can occur, even if light is a stream of photons, is that there is no way to find out, even in principle, which photon went through which slit. The "indeterminacy" allows fringes to appear.
But then Chiao and his colleagues ran the same experiment with polarising filters in front of each of the two slits. Any photon going one way would become "labelled" with left-handed circular polarization, while any photon going through the other slit is labelled with right-handed circular polarization. In this version of the experiment, it is possible in principle to tell which slit any particular photon arriving at the second screen went through. Sure enough, the interference pattern vanishes -- even though nobody ever actually looks to see which photon went through which slit.
Now comes the new trick -- the eraser. A third polarising filter is placed between the two slits and the second screen, to scramble up (or erase) the information about which photon went through which hole. Now, once again, it is impossible to tell which path any particular photon arriving at the second screen took through the experiment. And, sure enough, the interference pattern reappears!
The strange thing is that interference depends on "single photons" going through both slits "at once", but undetected. So how does a single photon arriving at the first screen know how it ought to behave in order to match the presence or absence of the erasing filter on the other side of the slits?
All of these experiments were carried out using beams of individual photons, and there is no way in which the results can be explained by using classical physics. They lay bare the mysteriousness of quantum mechanics in all its glory, and in particular demonstrate its "non local" nature -- the way in which a photon starting out on its journey behaves in a different way for each experimental setup, as if it knew in advance what kind of experiment it was about to go through.
Don't worry if you don't understand this. Richard Feynman didn't, and he warned "do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will go 'down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."
More atoms that communicate faster than light
PHYSICISTS still struggling to come to terms with experiments which show instantaneous communication between quantum particles under special circumstances are now faced with another puzzle. Correcting a mistake made by Enrico Fermi more than sixty years ago, Gerhard Hegerfeldt, of the University of Gttingen, has shown that in theory any pair of atoms can communicate faster than light.
The now-familiar puzzle of what are called "non local" interactions develops from theoretical work by John Bell, of CERN, in the 1960s and experiments by Alain Aspect in Paris in the 1980s. Together, these show that a pair of photons ejected in opposite directions from an atom remain somehow entangled, as if they were one particle. Measuring the state of one of the photons instantaneously affects the state of the other one, wherever it may be. Now, it seems that even atoms which have never come into contact (from the perspective of classical Newtonian physics) are entangled in a similar way.
The calculation Fermi carried out in 1932, in the early days of quantum mechanics, concerned the response of one atom to radiation emitted by another atom of the same kind, some distance away. If the second atom is in an excited state, sooner or later it will emit radiation, falling back to its ground state. This radiation will have exactly the right frequency to excite the second atom (this is one of the principles underlying the way atoms are "pumped" into an excited state to make a laser).
Common sense tells us that the first atom cannot be excited until a finite time after the second atom decays -- until there has been time for radiation travelling at the speed of light to cross the gap. That is the result Fermi found. But it now turns out that he made a mistake in his calculation. Probably because the mistaken conclusion matched common sense, it took a long time for this to come to light. But Hegerfeldt's correct version of the calculation now makes it clear that there is a small chance that the first atom will be excited as soon as the second atom decays (Physical Review Letters, vol 72 p 596). As with all such quantum puzzles, this is only the beginning of the story; now, the experts have to explain what this mathematical result means. The best interpretation of the evidence so far seems to be that we should not think of any object, not even a single atom, as an "isolated system".
Because particles must also be considered as waves (one of the basic tenets of quantum mechanics), the individual particles in the atom are spread out, and there is a finite (though small) chance of finding them anywhere in the Universe. So the wave functions of the electrons in the first atom overlap with those of the electrons in the second atom. They are entangled, like the two photons produced in the Aspect experiment, and when an electron in one atom jumps down an energy level that can instantaneously make its counterpart in the other atom jump up by the same amount.
Solving the quantum mysteries
John Gribbin
For seventy years, physicists have worried about what quantum mechanics means. They can use quantum physics, to be sure; witness the successful designs of lasers and computer microchips, and the understanding of molecules that makes genetic engineering possible. But the equations that are a routine part of this kind of work contain one embarrassing feature. The say, according to the standard interpretation (the Copenhagen interpretation), that nothing is real unless you look at it, that an electron (say) exists only as a wave of probability, called a wave function, which collapses into reality when it is measured, and promptly dissolves into unreality when you stop looking at it. We are no further advanced philosophically, on this picture, than the image of the tree in the quad which disappears when nobody is looking at it.
In fact, few physicists worry about such things. Most of them prefer to ignore them, in the hope they will go away. But now there is another interpretation, which solves all of the quantum mysteries. The snag is, those physicists may not like this one, either, because it involves signals that travel backwards in time.
The archetypal example of the quantum mysteries is the "experiment with two holes", where the measured position of a single electron that passes through two holes in a screen can only be explained in terms of the wave function travelling through both holes at once and interfering with itself. But perhaps you've heard that one already, so here is a rather sideways look at the whole business of collapsing wave functions, a thought experiment which says that the lack of an observation can make the wave function of a system collapse. This wonderful example of the strangeness of the quantum world dates back to the early 1950s, and is known as "Renninger's negative- result experiment", after the German physicist Mauritius Renninger who first thought of it. It is one of the easiest examples of quantum strangeness to understand -- but not to explain.
Imagine that we have a source which will emit a single quantum particle in a random direction (ordinary radioactive nuclei do exactly this, so there is nothing special about the source). This source is in the middle of a large hollow sphere, and the inner surface of the sphere is lined with material that will give a flash at the point where the particle hits it. The accepted quantum description of what happens when the source emits a particle is that a quantum probability wave spreads out evenly in all directions around the source, since there is an equal probability for the particle being emitted in any direction. When the probability wave reaches the inner surface of the spherical shell, there is just one flash of light as the wave collapses to a single point. The particle is only "real" when it is being observed -- when it makes the flash of light -- not while it is travelling from the source to the sphere.
So far, simple enough. But now imagine that half way between the source and the sphere there is a hemispherical shield, which blocks off exactly half of the outer sphere from the field of view of the source. Like the outer sphere, this inner hemispherical shell is lined with scintillating material that will flash when it is struck by a particle from the source. Now what happens when the source emits a particle? We are not interested in exactly where on the outer or inner spheres the particle makes a flash of light, only in which of the two spheres it strikes. Either the particle strikes the inner sphere and makes it flash, or it strikes the outer sphere and makes it flash. There is an equal probability of either outcome of the experiment. Now, suppose that the source is once again triggered into emitting a particle. Once again, standard quantum theory describes this as an expanding spherical shell of probability, moving out evenly in all directions. We wait for a time longer than the time needed for it to reach the inner hemisphere, but too short for it to have reached the outer sphere, and see no flash on the inner sphere. So we know that the final state of the experiment will involve a flash on the outer sphere -- the particle must have been emitted in the wrong direction to strike the inner hemisphere. From a 50:50 probability of the flash occurring either on the hemisphere or on the outer sphere, the quantum wave function has collapsed into a 100 per cent certainty that the flash will occur on the outer sphere. But this has happened without the observer actually "observing" anything at all! It is purely a result of a change in the observer's knowledge about what is going on in the experiment. It requires an observer intelligent enough to infer what is happening, and what would have happened if the particle had been heading towards the inner hemisphere (so a cat, for example, clearly would not be intelligent enough to cause this particular collapse of a wave function). Under these circumstances, the absence of an observation can collapse the quantum wave function as effectively as an actual observation can. At least, so says the Copenhagen interpretation.
This central role for the observer -- not just any observer, but an intelligent observer -- lies at the heart of the standard Copenhagen interpretation of quantum mechanics.
But a giant leap in what might be called quantum philosophy has recently been taken by the American physicist John Cramer. He has taken a new look at the wave equations of quantum mechanics -- the famous Schrdinger equation, and the equations describing the probability waves, which travel, like photons, at the speed of light. What Cramer has pointed out is that the equations actually have two sets of solutions, one equivalent to a positive wave flowing into the future (a "retarded" wave), and the other describing a negative wave flowing into the past (an "advanced" wave). As all physicists learn at university (and most promptly forget) the full version of the wave equation has two sets of solutions -- one corresponding to the familiar simple Schrdinger equation, and the other to a kind of mirror image Schrdinger equation describing the flow of negative energy into the past.
The proper mathematical description of the wave function actually includes a mixture of both ordinary ("real") numbers and imaginary numbers -- those numbers involving i, the square root of minus one. Such a mixture is called a complex variable, for obvious reasons; it is written down as a real part plus (or minus) an imaginary part. The probability calculations needed to work out the chance of finding an electron (say) in a particular place at a particular time actually depend on calculating the square of the complex number corresponding to that particular state of the electron.
But calculating the square of a complex variable does not simply mean multiplying it by itself. Instead, you have to make another variable, a mirror image version called the complex conjugate, by changing the sign in front of the imaginary part -- if it was + it becomes -, and vice versa. The two complex numbers are then multiplied together to give the probability. For equations that describe how a system changes as time passes, this process of changing the sign of the imaginary part and finding the complex conjugate is equivalent to reversing the direction of time!
The basic probability equation, developed by Max Born back in 1926, itself contains an explicit reference to the nature of time, and to the possibility of two kinds of Schrdinger equations, one describing waves that move forward in time and the other representing waves that move backward in time.
The remarkable implication is that ever since 1926, every time a physicist has taken the complex conjugate of the simple Schrdinger equation and combined it with this equation to calculate a quantum probability, he or she has actually been taking account of the influence of waves that travel backwards in time, without knowing it. There is no problem at all with the mathematics of Cramer's interpretation of quantum mechanics, because the mathematics, right down to Schrdinger's equation, is exactly the same as in the standard Copenhagen interpretation. The difference is, literally, only in the interpretation -- Cramer accepts that the wave flowing backward in time is real, and should be taken seriously, not ignored. The way Cramer describes a typical quantum "transaction" is in terms of a particle "shaking hands" with another particle somewhere else in space and time. One of the difficulties with any such description in ordinary language is how to treat interactions that are going both ways in time simultaneously, and are therefore occurring instantaneously as far as clocks in the everyday world are concerned. Cramer does this by effectively standing outside of time, and using the semantic device of a description in terms of some kind of pseudotime. This is no more than a semantic device -- but it certainly helps to get the picture straight.
It works like this. When an electron vibrates, on this picture, it attempts to radiate by producing a field which is a time-symmetric mixture of a retarded wave propagating into the future and an advanced wave propagating into the past. As a first step in getting a picture of what happens, ignore the advanced wave and follow the story of the retarded wave. This heads off into the future until it encounters an electron which can absorb the energy being carried by the field. The process of absorption involves making the electron that is doing the absorbing vibrate, and this vibration produces a new retarded field which exactly cancels out the first retarded field. So in the future of the absorber, the net effect is that there is no retarded field. But the absorber also produces a negative energy advanced wave travelling backwards in time to the emitter, down the track of the original retarded wave. At the emitter, this advanced wave is absorbed, making the original electron recoil in such a way that it radiates a second advanced wave back into the past. This "new" advanced wave exactly cancels out the "original" advanced wave, so that there is no effective radiation going back in the past before the moment when the original emission occurred. All that is left is a double wave linking the emitter and the absorber, made up half of a retarded wave carrying positive energy into the future and half of an advanced wave carrying negative energy into the past (in the direction of negative time). Because two negatives make a positive, this advanced wave adds to the original retarded wave as if it too were a retarded wave travelling from the emitter to the absorber. In Cramer's words:
The emitter can be considered to produce an "offer" wave which travels to the absorber. The absorber then returns a "confirmation" wave to the emitter, and the transaction is completed with a "handshake" across spacetime.
But this is only the sequence of events from the point of view of pseudotime. In reality, the process is atemporal; it happens all at once. This is because, as Einstein explained with his special theory of relativity, signals that travel at the speed of light take no time at all to complete any journey -- in effect, for light signals every point in the Universe is next door to every other point in the Universe. Whether the signals are travelling backwards or forwards in time doesn't matter, since they take zero time (in their own frame of reference), and +0 is the same as -0 -- and all the quantum probability waves do travel at the speed of light.
The situation is more complicated in three dimensions, but the conclusions are exactly the same. This interpretation makes no predictions that are different from those of conventional quantum mechanics, but it provides a conceptual model which helps many people to think clearly about what is going on in the quantum world. It means that when an electron is faced with a choice of two holes to go through, the offer goes through both but the handshake only comes back through one, so it knows where to go; and in Renninger's experiment, the particle setting out from the radioactive nucleus has already made its handshake and knows which hemisphere it will end up on. There is no more mystery about the quantum mysteries at all -- provided you can live with waves that go backwards in time.
Photons faster than light
Nothing can travel faster than light -- unless it is a quantum particle "tunneling" through a barrier that, according to good old Newtonian physics, it should not be able to penetrate at all. Physicists have puzzled for decades over how long this mysterious tunneling process takes, but they need puzzle now longer, for it has been measured. And, sure enough, it takes place faster than light.
Quantum tunneling is of more than just esoteric interest. The phenomenon is related to quantum uncertainty, and to wave-particle duality. When two quantum particles, such as two protons, come close to one another, but do not actually touch, the uncertainty in their positions allows their quantum waves to overlap to some extent. As a result, they may "tunnel through" the gap between them, and interact. This is exactly what happens inside the Sun and stars -- protons which are kept at a distance from one another by the repulsion of their positive charge can still fuse together because of tunneling. And that nuclear fusion is what keeps the interior of the Sun hot, and makes its surface shine. Without tunneling, we would not be here.
Raymond Chaio, of the University of California, Berkeley, and his colleagues have actually been measuring a different, but related, kind of tunneling. They have devised an experiment in which two photons (particles of light) are produced simultaneously in a source, and travel on parallel paths. One photon goes straight to a detector; the other is confronted by a barrier which would reflect the light of the photons obeyed the laws of classical, "Newtonian" physics. But according to quantum theory there is a high probability that some of the photons arriving at the mirror will tunnel straight through, and go on their way to the detector.
Sure enough, that is what happens. The barrier is 1.1 micrometers thick, so anything travelling through it at the speed of light would take 3.6 femtoseconds (3.6 thousand million millionths of a second) on the journey. But the new experiment is so sophisticated that it can compare the arrival times of pairs of photons, one of which has gone past the barrier and one through it, and shows that the one which goes through the barrier arrives first. It tunnelled through the barrier faster than the speed of light, in less than 3.6 femtoseconds. As the researchers put it, "it is as though the particle 'skipped' the bulk of the barrier". But don't ask them, or anyone else, what it means -- in the words of Richard Feynman, "nobody understands quantum mechanics".
Uncertainty rules in the quantum world
IN SCENES reminiscent of the great debates between Niels Bohr and Albert Einstein in the 1930s, fundamental quantum physics has been tested in a series of new "thought experiments", and has passed those tests with flying colours.
The standard interpretation of quantum physics, known as the Copenhagen Interpretation, was established largely through the efforts of Bohr at the beginning of the 1930s. The strange features of the quantum world are founded on what Bohr dubbed complementarity -- the way an entity such as an electron can behave either as a wave or as a particle -- and on Werner Heisenberg's principle of uncertainty, which says that a quantum entity such as an electron does not possess both a position and a momentum simultaneously.
As Richard Feynman was fond of pointing out, the strangeness of the quantum world is encapsulated in "the experiment with two holes". If electrons (or photons, the "particles of light") are fired one at a time through a standard Young's double slit type of experiment and arrive at a detector screen on the other side, they leave the "gun" on one side of the experiment as particles, and arrive at the screen on the other side of the two slits as particles, each making a single spot on the screen. But somehow they pass through the two slits in between as waves, interfering with one another (even though they pass through one at a time!) so that the pattern built up on the screen by the accumulation of spots is an interference pattern.
This wave-particle duality is linked with the uncertainty principle. "Waviness" is a property associated with momentum -- a typical wave is spread out, so it has no definite location in space, but it does have a direction in which it is going. By contrast, a particle can have a precisely defined position. Heisenberg found that the quantum equations imply a strict tradeoff between the two complementary properties. If the position of a quantum entity is precisely defined (for example, when it hits a detector screen), its waviness is suppressed; but if it is allowed to give full expression to its wave nature, the particle aspect vanishes.
In practical terms, this means that we can never measure both momentum and position precisely, at the same time, for any particle. Einstein argued that this was simply a reflection of our clumsiness. Measuring the position of an electron, say, would involve bouncing light off it, and the very act of bouncing light off the electron would make it recoil, changing its momentum (and, indeed, its position). He thought that there were real little particles, like tiny billiard balls, involved in quantum interactions, and that the appearance of fuzziness and interference in experiments was a result of the deficiencies of the experiments.
Bohr argued that the fuzziness was an intrinsic feature of the quantum world, and that within the limits set by Heisenberg's uncertainty principle an electron itself does not "know" both where it is and where it is going. Over several years, beginning at the end of the 1920s, Einstein tried to dream up idealised thought experiments which could in principle measure both the position and the momentum of a particle such as an electron at the same time, thereby refuting Heisenberg. Each time, Bohr found a flaw in Einstein's argument, proving that the experiment could not work as Einstein had thought, even in principle. Bohr's success in this debate with Einstein was a major reason why the Copenhagen Interpretation became the established way of thinking about the quantum world. For sixty years, it seemed there was nothing to add to the debate. Then, in 1991 Marlan Scully, of the University of New Mexico, and colleagues claimed that they had found a way to carry out the kind of measurement Einstein had sought for in vain (Nature, vol 351 p 111). The essence of their argument (see Jim Baggott, "Beating the uncertainty principle", New Scientist, 15 February 1992) was that atoms could be sent through a double slit experiment in an excited state. Behind each slot there would be a detector known as a micromaser cavity, and the experiment would be timed so that each atom emitted a photon as it passed through the appropriate cavity. This would "switch on" one of the detectors, showing which slit the atom passed through, but would leave the atom free to carry on and make its mark on the final detector screen.
In such experiments without the cavities, atoms have been shown to behave like waves when passing through the two slits, creating an interference pattern. But if it is possible to detect which slit each atom passes through, without disturbing the flight of the atom, it would surely be impossible to produce interference. That requires something going through both slits at once. The presence of the cavities would make the interference pattern vanish, as if by magic, demonstrating the absurdity of the Copenhagen Interpretation. But now it seems that things are not that simple. Pippa Storey and colleagues at the University of Auckland have shown that even in this kind of idealised experiment that atoms are disturbed by the presence of the cavities, and in just the right way to "wash out" the interference pattern. They point out (Nature, vol 367 p 626) that although the ejection of a photon by the excited atom need not affect its forward momentum through the slit(s), there is always some uncertainty in the amount of sideways momentum imparted to the atom by the kick of the departing photon.
The situation is complicated by the prediction, in line with quantum uncertainty, that instead of a single photon being emitted cleanly from the atom, the atom can be involved in interactions with "virtual" photons, which emerge briefly from the vacuum (out of "nothing at all") before disappearing. But the conclusion is that provided the cavities are narrower than the separation between the two slits in the experiment (which they have to be if they are to tell us which slit each atom passes through), the interference pattern is destroyed. And the washing out of the interference pattern can even be understood by treating the atoms as waves. Interference can only occur if the waves passing through the two slits remain in phase, but "Because the atom's motion is primarily longitudinal," say the New Zealand team, "a transverse momentum kick will simply change its direction slightly . . . the displacement . . . is effectively the familiar phenomenon of refraction: a position-dependent change in the phase of a wave results in a change in the direction of propagation". The loss of interference from a double slit in the presence of cavity detectors is caused by momentum kicks, which are themselves of a size determined by the uncertainty principle. Einstein, no doubt, would have taken this as philosophically (if unbelievingly) as he took his other setbacks; but the ghosts of Bohr and Heisenberg must be smiling.
Molecules make quantum waves
THE WEIRDNESS of the quantum world has taken another step towards the everyday world as a result of experiments which show iodine molecules behaving as waves in interference experiments. This brings properties often regarded as unique to subatomic particles out into the open on much larger scales.
It is a fundamental feature of quantum mechanics that entities described by the quantum equations are not simply particles or waves, but exhibit a mixture of wave and particle properties. Light, for example, will behave as a wave in interference experiments, with two sets of waves interacting with one another to form a new pattern, just as ripples on a pond (or in your bath) interact with one another. On the other hand, in other experiments light will behave as a stream of tiny particles, called photons.
Wave-particle duality was first discovered to be a feature of light in the early part of this century (Albert Einstein's Nobel Prize was awarded for his proof that photons exist). In the 1920s, researchers found that electrons, traditionally regarded as particles, could behave as waves in experiments where an electron beam is diffracted from a crystal lattice -- indeed, in one of the nicest examples of wave- particle duality, the physicist J. J. Thomson {ED: NB always "J J", never referred to by name} received a Nobel Prize for discovering that the electron is a particle, while his son George received a Nobel Prize for proving that the electron is a wave.
Moving up the mass scale, first neutrons (each nearly 2,000 times the mass of an electron) and then beams of atoms and molecules were shown to diffract like waves when passed through small apertures. Over the past ten years or so, the wave-particle duality has been demonstrated ever more clearly. Not just diffraction (in which one beam, or wave, bends as it passes an obstruction) but interference (in which two beams or waves interact with one another) has been demonstrated both for electrons and atoms. Now, a team of researchers at the University of Paris-North, at Villetaneuse in France, has done the trick with molecules.
In the traditional version of the interference experiment with light, two beams of light are generated by passing light from a single source through two slits in a screen. Then, the two beams are allowed to interfere, producing a characteristic stripey pattern of light and shade. The new experiment is conceptually similar, but instead of passing through holes in a screen the iodine molecules (I2, which each have a mass about 254 times that of a neutron) interact with laser beams. The first interaction, with a pair of laser beams, puts each molecule into what is known as a "superposition of states", effectively two wave packets marching side by side. A second pair of laser beams recombines the wave packets to make "particles". At least, that is the theory. What happens in practice? After they have passed through the laser beams, the iodine molecules arrive at a detector. The distribution of the molecules arriving at the detector does not resemble the pattern you would expect if they were a stream of particles travelling through the experiment, but exactly matches the stripey pattern of peaks and troughs corresponding to interference by waves (Physics Letters A, vol 188 p 187). These are the heaviest "particles" which have ever demonstrated their wave "character" directly in experiments.
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