CHAPTER 1: DAVID BOHM'S INTERPRETATION
OF QUANTUM THEORY

It is important to see David Bohm's interpretation of quantum theory in the context of the revolution in physics brought about by quantum mechanics at the beginning of this century. The following outline supplies the bare bones of that context which can be filled in by many fine accounts of quantum theory that have appeared in recent years. (1)

1900. In Berlin, Max Planck is struggling with the problem of black body radiation, which is the radiation that a heated, thin-walled, hollow, metal cylinder gives off through a small hole in it. Instead of this radiation being emitted in a smooth fashion, it is given off in lumps or clumps. In a moment of desperation and inspiration, Planck derives a formula which involved a very small constant he calls h that fits the data, but then he begins to struggle to understand its physical implications.

1905. In Bern, Albert Einstein, an unknown patent examiner, submits three papers to the Annalen Der Physik: the special theory of relativity, a proof for the existence of atoms based on Brownian motion, that is, the motion of small particles like grains of pollen by some unseen force, and a third paper on the photoelectric effect in which electrons are given off by a metal bombarded by light, but the emission depends not on the intensity of the light but its frequency. In this last paper Einstein extends Planck's ideas to electromagnetic radiation in order to explain this effect, and concludes that light could be considered not only a wave, as the long prevailing opinion had it, but is made up of distinct particles, or quanta, as well.

1911. Ernest Rutherford in England bombards a thin sheet of gold foil with particles and some of them bounce back, indicating that the atoms of the foil have nucleii.

1913. Neils Bohr, a young Danish physicist, tries to understand how electrons can form a stable structure with these nucleii and not spiral into them. He finds a solution based on Einstein's and Planck's work on quanta in which electrons can only take up certain orbits or states of energy.

1923. Louis de Broglie, a young physics student in Paris, reasons that if light is made up of particles, why couldn't electrons be made up of waves. This opens the door for considering the wave properties of other particles, as well. His thesis advisor, Paul Langevin, sends his work to Einstein who sees its importance.

1925. Werner Heisenberg, a young German physicist, is also apparently influenced by Einstein "who hovers over this entire subject like some sort of magisterial ghost." (2) Heisenberg decides that Bohr's picture of the orbits of the atom which had never been observed could be replaced by a purely mathematical structure. Max Born and Pascal Jordan complete his work which Born had realized made use of a branch of mathematics called matrix mechanics whose strange feature was that the result of two numbers multiplied by each other differed depending on which number was put first. For example, the position of a particle multiplied by its momentum would not be equal to its momentum multiplied by its position, but would be proportional to the constant Planck had found. Paul Dirac, a young Englishman, works out an equivalent mathematical theory. Another young physicist, a brash and aggressive friend of Heisenberg, Wolfgang Pauli, applies matrix mechanics to the light spectrum of hydrogen and comes up with the same answers that Neils Bohr had discovered.

1925-1927. Erwin Schrödinger hears of de Broglie's work through a paper of Einstein's and works out a wave theory of the atom that arrives at the same results as the work of Heisenberg, Born, Jordan and Dirac. These two different approaches were soon shown to be mathematically equivalent, but they are driven by quite different attitudes to the nature of physics, and these attitudes will play an important role in our story.

The mathematical formulation of quantum theory is falling into place, but it is being increasingly subjected to two different philosophical interpretations. Schrödinger is initially inclined to look at his wave as a matter wave, which he names after Einstein and de Broglie. They, in turn, think of this wave as a pilot wave that guides the electron. At the same time Born, Heisenberg and Bohr are veering away from trying to picture what the mathematics of quantum theory physically represents. Born has created a probabilistic interpretation of Schrödinger's waves in which the wave indicates the probability of a particle being in a certain place. Heisenberg develops his famous uncertainty principle in which the uncertainty of the position and the momentum of a particle is never zero, but always related to Planck's constant. And Bohr comes up with a principle of complementarity in which the wave and particle properties of the electron are both true, but mutually exclusive. But all these ideas, which come together to create the Copenhagen interpretation and which is going to become dominant among physicists, carry with them a considerable amount of philosophical baggage.

Heinz Pagel in his Cosmic Code portrays this dominant position with enthusiasm. Born's probabilistic interpretation of Schrödinger's waves is taken up as an indication of the of the quantum world, itself:

"This indeterminism was the first example of quantum weirdness. It implies the existence of physical events that were forever unknowable and unpredictable. Not only must human experimenters give up knowing when a particular atom is going to radiate or a particular nucleus undergo radioactive decay, but these events are even unknown in the perfect mind of God." (3)

Heisenberg's uncertainty principle arises not just from our inability to measure the properties of the electron more accurately, but from this indeterminism of the quantum world, itself. Quantum theory makes statistical predictions and rules out any subquantum or hidden variable theory that would be deterministic. The immense explanatory power of quantum theory comes "at the price of renouncing the determinism and objectivity of the natural world." (4)

What we know is what our experiments tell us, which is an inseparable mixture of our experimental methods and what we are experimenting on. We do not know what the quantum world is in itself. Bohr remarks: "It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we can say about Nature." (5) And Heisenberg comments: "Progress in science has been bought at the expense of the possibility of making the phenomena of nature immediately and directly comprehensible to our way of thought." And again, "Science sacrifices more and more the possibility of making 'living' the phenomena immediately perceptible by our senses, but only lays bare the mathematical, formal nucleus of the process." (6)

But this loss of objective reality was highly uncongenial to Einstein, Schrödinger and de Broglie. Einstein had a long running debate with Bohr and others about these matters, and resisted this interpretation to the end of his life. He wrote to Born about his probability wave, "Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the 'old one.' 1, at any rate, am convinced that He is not playing at dice." (7) De Broglie attempted a causal interpretation of wave mechanics in 1927, but it aroused considerable criticism, especially on the part of Pauli, and he dropped it.

The whole weirdness of quantum theory was summed up in the two slit experiment. In classical physics this experiment was used to demonstrate the wave theory of light. Light is directed at two narrow slits in a barrier with a screen placed behind it. If either of the slits is covered, the light shines through the other one and a line is created on the screen. But if both are open, then instead of two lines, there is a pattern of light and dark lines caused by the interference of the two waves of light that have gone through the two slits.

But what will happen if the experiment is repeated with electrons? If electrons are particles, as much evidence indicates, we should expect that the electrons shot through one slit would form a line on the screen behind it, and if the electrons are shot through both slits, two lines would be formed. In actual fact, what is found is an interference pattern like that created by the waves of light. Even if the electrons are shot one at a time through the slits, the interference pattern is formed. The crux of the mystery is in trying to explain how particles can form a wave interference pattern. In the Copenhagen interpretation, the explanation goes like this: the electron must go through either one slit or the other, but then we should not get a wave interference pattern. Further, if the electron goes through one or the other slit, it should not matter if the one It is not going through is closed. But in actual fact, if we close it, we don't get the interference pattern. So instead, we should say that we cannot talk about which slit the electron goes through until we measure it. If we measure which slit the electron goes through, then the electron acts like a particle and gives a particle pattern. If we don't measure it, the wave distribution will begin to appear. Somehow our act of measurement has forced the probability wave to collapse and the electron to appear at a distinct place. The loss of this probability wave, or wave packet, causes the loss of the interference pattern. According to this Copenhagen interpretation, what is at stake is the nature of physical reality, or classical objectivity.

"There is no meaning to the objective existence of an electron at some point in space, for example at one of the two holes, independent of actual observation. The electron seems to spring into existence as a real object only when we observe it!" (8) It is the probability wave that goes through both slits and causes the build up of the interference pattern, and somehow directs the electrons to the areas of higher probability. Richard Feynman says of the two slit experiment that it is "a phenomenon which impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery... the basic peculiarities of all quantum mechanics." (9)

John Gribbin explains the matter like this: "The electrons not only know whether or not both holes are open, they know whether or not we are watching them, and they adjust their behavior accordingly. There is no clearer example of the interaction of the observer with the experiment. When we try to look at the spread-out electron wave, it collapses into a definite particle, but when we are not looking it keeps its options open. In terms of Born's probabilities, the electron is being forced by our measurement to choose one course of action out of an array of possibilities. There is a certain probability that it could go through one hole, and an equivalent probability that it may go through the other; probability interference produces the diffraction pattern at our detector. When we detect the electron, though, it can only be in one place, and that changes the probability pattern for its future behavior-for that electron, it is now certain which hole it went through. But unless someone looks, nature herself does not know which hole the electron is going through." (10)

It is as if a myriad of ghost or potential particles become one real particle with the collapse of the wave function caused by our act of observation. "What's worse, as soon as we stop looking at the electron, or whatever we are looking at, it immediately splits up into a new array of ghost particles, each pursuing their own path of probabilities through the quantum world. Nothing is real unless we look at it, and it ceases to be real as soon as we stop looking." (11)

I don't want to belabor this view of quantum theory, but it is important to begin to fix in our minds how completely a certain philosophical view of reality has become fused with the mathematical formulation of quantum theory.

1932. John von Neumann creates a mathematical proof that purports to show that a hidden variable approach to quantum theory, i.e., that there could be some kind of subquantum level causal explanation, Is impossible. His work compounds the impression that the dominant Copenhagen interpretation goes as far as it is possible to go.

1935. Einstein continues to resist this kind of interpretation, and feels that quantum theory is incomplete. In order to demonstrate this incompleteness he creates a thought experiment, together with Boris Podolsky and Nathan Rosen. He imagines two particles coming from far away, interacting, and then moving far away from each other. He reasons that if we measure the position of the first particle we can deduce from that the position of the second particle, and if we measure the momentum of the first particle, we can deduce the momentum of the second. Therefore we can say there are elements of reality in this second particle that correspond to these deductions. And if quantum theory cannot determine both the position and the momentum of a particle, it is thus incomplete, for this second particle actually has a definite position and momentum, and the only way to avoid this conclusion would be by some "spooky action at a distance" whereby measuring the first particle actually effects the second, a possibility he does not take seriously. For Einstein, if something is real it remains real whether we measure it or not, whereas for Bohr, "A particle in reality has neither a position nor momentum. It has only the potential to manifest these complementary properties when confronted by suitable experimental apparatus." (12)

This was Einstein's last paper on the subject, but he continued to talk about it until the end of his life. In 1944 he writes to Max Born, "You believe in the God who plays dice, and I in complete law and order in a world which objectively exists, and which 1, in a wildly speculative way, am trying to capture... Even the great initial success of the quantum theory does not make me believe in the fundamental dice-game, although I am well aware that our younger colleagues interpret this as a consequence of senility. No doubt the day will come when we will see whose instinctive attitude was the correct one." (13)

1950. David Bohm has been teaching quantum theory at Princeton and has just finished writing a quantum textbook following the interpretation of Neils Bohr. It is well received, and has two distinctive qualities: more words than formulas, and a mention of the Einstein, Podolski and Rosen paper, which was not usual at the time. (14) But Bohm is dissatisfied with what he has done, and what he perceives as the unresolved issues of quantum theory preoccupy him. His life is about to change, both personally and scientifically. He has refused to testify before the House committee on unAmerican activities about his student days in Berkeley, and he is cited for contempt and suspended by the university, and discouraged from visiting. Bohm is eventually cleared of charges, but his contract is not renewed by Princeton, and he cannot find a job in the U.S. He goes to Brazil, then to Israel, and finally to England.

On the scientific front, he sends copies of his textbook to Bohr, who doesn't answer, to Pauli, who is enthusiastic, and to Einstein, who is at Princeton and who calls and invites him to visit. Einstein still feels that something is missing from quantum theory, and it should be possible to go beyond the statistical approach and create some kind of deterministic theory in which there is an objective reality independent of the observer. Bohm is strongly effected by his meeting with Einstein, sets out to look for such a theory, finds the beginning of one, and publishes two papers in 1952, von Neumann's proofs not withstanding.

Einstein, however, is not happy with his theory. It involves a new force that Bohm calls a quantum potential, which has as one of its characteristics nonlocality, and instead of breaking entirely new ground with some revolutionary formulation, it evokes ideas similar to those proposed by de Broglie more than twenty years before. Pauli criticizes it and de Broglie rallies to it, but in the world of physicists it only slowly makes headway. It goes against the prevailing mentality which either cares little for the philosophical side of quantum theory, or strongly embraces the conventional interpretation. Objections are raised that Bohm's theory produces no new empirical results, but Bohm replies that if de Broglie's ideas had prevailed in 1927, then the same objection could have been brought against anyone developing the Copenhagen interpretation later. Bohm will continue to develop and expand his ideas until the end of his life in 1992.

1964. If Bohm's 1952 papers did not make much of an impact, they did excite the interest of a young Irish physicist, John Stewart Bell. He, too, had been uneasy about the conventional interpretation, and now he saw Bohm doing what was not possible to do according to the common wisdom of physicists supported by von Neumann's mathematical proofs: creating a causal or hidden variable interpretation of quantum theory.

Bell had a deep-rooted realist streak, much like Einstein. He once said: "I think there is a real world and I think it pays very little attention to me." (15) When he looked at Bohm's papers he saw that von Neumann must somehow be wrong, and that Bohm's theory was nonlocal. It wasn't until twelve years later in 1964 that he was ready to formulate his ideas and ask himself: "Do you have to have nonlocality to get agreement with quantum mechanics?" (16) Locality meant local action between two bodies that propagated at speeds below the speed of light, and weakened as the distance between the bodies increased. Bell first refuted all the proofs like von Neumann's for the impossibility of having hidden variables. Then he asked whether there could be a local hidden variable solution which would avoid nonlocality, and he showed by his calculations it was impossible. The kind of locality found in classical physics and favored by Einstein was not compatible with quantum theory.

In a Bell type of a thought experiment, a source emits two photons that go off in opposite directions and have correlated polarizations. The photons are in what physicists call a phase-entangled state, and according to quantum theory, neither one of the photons has a definite polarization. If we set a detector in the path of either of the photons, there is a 50% chance that the photon will register as up, and a 50% chance it will register as down. But if the detectors in each direction are both set at the same angle, then the polarization of each photon is shown to be the same. When the detectors are set at a 90⁰ angle to each other, the polarizations of the particles are always opposite. But what happens if the detectors are set at some intermediate angle? This is where the possibility comes in of distinguishing a local interaction theory from quantum theory, for in a local theory we imagine that how we set one detector does not influence what happens at the other. If we set the first detector at x⁰ and the second detector at x⁰ in the opposite direction, and we assume what happens at the first detector doesn't influence what happens at the second, then if the error rate at the first detector is one out of four, and the error rate at the other detector is also one out of four, then the total error rate should be no more than two out of four. This is an example of Bell's inequality. But quantum theory predicts the error rate is three out of four. It looks like something must be wrong with our assumption that what happens at one detector does not influence what happens at the other. Quantum theory points to nonlocality. (17)

1969. John Clauser, Michael Horne, Abner Shimony and Richard Holt generalize Bell's theorem so it can be tested.

1972. Clauser does an experiment that shows that quantum theory is correct and Bell's inequality is violated.

1982. Alain Aspect at the University of Paris carries out very sophisticated experiments to close the loopholes left by earlier experiments and he, too, finds that Bell's inequality is violated. Nonlocality appears to be a feature of the universe. No doubt physicists would find many imprecisions in this all too brief history of quantum theory, but it does provide us with some background that will allow us to look deeper into the significance of David Bohm's interpretation of quantum theory and further our inquiry into the philosophically rich language that can develop in such situations. We are going to do this by examining the book that Bohm was working on, together with B.J. Hiley when he died in 1992 entitled, The Undivided Universe: An Ontological Interpretation of Quantum Theory.

Bohm and Hiley felt that an ontological interpretation of quantum theory was called for because although quantum mechanics had great operative efficacy, there remained basic questions that were unclarified: its inability to deal with individual quantum events, nonlocality, wave-particle duality and "above all, there is the inability to give a clear notion of what the reality of a quantum system could be." (18) Quantum theory "merely gives us knowledge of how our instruments will function." But says "little or nothing about reality itself." (19) Bohm and Hiley have created a theory that gives the same statistical results as the conventional interpretation, but which is, as they say, an ontological interpretation, which is "intuitively graspable" so that it is possible to see into the nature of the physical reality underlying the mathematical formalism of quantum theory, and perhaps in this way to advance quantum theory in new directions.

While much of the book is given over to demonstrating how this theory will yield all the results of the earlier interpretation, what is important for us is the sense of the ontological thirst that drives the whole process, and the picture that begins to emerge. Classical physics had implied a certain unexamined realism that allowed the physicist to go about his or her work. Particles and fields actually existed, could be known, and that knowledge didn't change them. All this was called into question with the advent of quantum mechanics, as we have seen. With the ascendancy of the Copenhagen interpretation, any quantum ontology like that proposed by Bohm and Hiley was ruled out. What we were left with was a "quantum algorithm which gives the probability of the possible results for each kind of experimental arrangement. Clearly this means that the mathematics must not be regarded as reflecting an independent quantum reality that is well defined, but rather that it constitutes in essence only knowledge about the statistics of the quantum phenomena." (20) Our authors are not satisfied with this kind of approach, for in it science doesn't deal with what is, but is limited to what is observable. There is no way to get below the surface, or in this case, below the indeterminacy that quantum phenomena present. But for Bohm just because quantum phenomena are indeterministic doesn't mean that the quantum world must be, and thus the possibility exists for a quantum ontology.

In such an ontology "the electron actually is a particle with a well defined position... which varies continuously and is causally determined" and this "particle is never separate from a new type of quantum field that fundamentally affects it." (21) This field has new features that differentiates it from the fields familiar to classical physics. This "quantum potential is independent of the strength (i.e., the intensity) of the quantum field but depends only on its form." (22) They liken it to a radio wave that guides a ship that is on automatic pilot. The energy to move the ship comes from its engine and not from the intensity of the radio wave, but the form of the wave controls the direction of the ship. In a similar way, "we may therefore propose that an electron too moves under its own energy, and that the form of the quantum wave directs the energy of the electron... Moreover, since the effect of the wave does not necessarily fall off with the distance, even remote features of the environment can profoundly affect the movement." (23)

When Bohm and Hiley consider the two slit experiments in this new way, the much proclaimed quantum weirdness dissolves. If one slit is open, the particle passes through that slit as well as its quantum wave. If both slits are open, the particle passes through one or the other, but its wave goes through both, giving rise eventually to the characteristic interference pattern.

They compare this quantum field to what they call active information in which "a form having very little energy enters Into and directs a much greater energy." The word information here is taken in its root sense "to in-form, which is actively to put form into something or to imbue something with form." (24) A radio, for example, has unformed energy coming from its power source which is informed by the radio wave so that the information in the radio wave "is potentially active everywhere, but it is actually active, only where and when it can give form to the electrical energy which, in this case, is in the radio." (25) When this kind of conception is applied to the electron in the two slit experiment, the particle is seen as having an ability to work, which is released by the active information of the quantum field, which doesn't push or pull the particle, but directs and guides its energy, which suggests "that an electron or any other elementary particle has a complex and subtle inner structure." (26)

This conception of a quantum field also points to the fact that the experiment has to be looked at as a whole. The slits themselves effect the way the waves move the particles, and the whole environment of the experiment effects how the final pattern appears on the screen. While at first glance this seems similar to Bohr's point of view, it differs markedly because in this case the wholeness involved "is open to our 'conceptual gaze' and can therefore be analyzed in thought, even if it cannot be divided in actuality without radically changing its nature." (27) Indeterminateness remains in our measurements, but that does not mean that quantum reality itself is indeterminate.

Since the strength of the quantum field does not fall off with distance, distant features of the environment can effect the particle. In a similar way, two particles can be coupled over long distances, giving rise to nonlocality. This kind of wholeness goes beyond "the actual spacial relationships" of the particles and transcends any conception of mechanism. The concept of wholeness in mechanism is concerned with the overall arrangement of the parts. "In our interpretation of the quantum theory, we see that the interaction of parts is determined by something that cannot be described solely in terms of these parts and their preassigned interrelationships... Something with this kind of dynamical significance that refers directly to the whole system is thus playing a key role in the theory. We emphasize that this is the most fundamentally new aspect of the quantum theory." (28)

Bohm and Hiley are making no claims to more precise quantum measurements because the object to be measured and the measuring instrument are still conceived to be interacting and are "'guided' by a common pool of information implying a quantum potential that connects them in a nonlocal way." (29) There is no way to predict the behavior of the particle even though this motion is determinate in itself, but their interpretation avoids the paradoxes that abound in the normal interpretation of quantum theory. Bohm and Hiley bring out the nonlocality of their quantum theory by considering the Einstein, Podolsky and Rosen thought experiment. They imagine a molecule with a total spin of zero with each of its atoms having a spin of one half. The molecule disintegrates and the atoms move far apart. The spins of the atoms should be opposite to each other. If we measure the first atom we can deduce what the spin of the second atom is, and while our measurement disturbs the first atom, according to Einstein's reasoning, it should not disturb the second atom, and so the reality of the spin of the second atom must have existed before we measured the first atom. We saw that Einstein, Podolsky and Rosen used this kind of reason to point to the incompleteness of the conventional quantum theory because in this way all the elements of spin of the second atom can be determined, which is something that the mathematical formalism of quantum theory does not allow for.

But another possibility exists. The two atoms are bound together by some unknown field so that the disturbance caused by the measurement of the first atom is communicated to the second. Considering the early work of Bohm, John Bell had asked whether "nonlocality was necessary for all possible ontological explanations of quantum mechanics." (30) The result, as we have seen, was Bell's inequality, which experiments have shown to be violated, pointing to the fact that any hidden variable must be nonlocal. In Bohm and Hiley's theory, the state of the second atom is dependent on the measurement apparatus of the first atom, and this interconnectedness is brought about by the quantum potential. The ordinary world of sense experience is a world of relatively stable structures outside of each other which locally interact, but the quantum world has "a radically different nature," for it is a world of nonlocality and indivisible wholeness. "Thus there is a kind of objective wholeness, reminiscent of the organic wholeness of a living being in which the very nature of each part depends on the whole." (31) In many cases the effect of the quantum field can be neglected "so that the classical world can be treated on its own as if it were independently existent. But according to our interpretation it is actually an abstraction from the subtle quantum world which is being taken as the ultimate ground of existence." (32)

This intuitive approach stands in strong contrast to one that starts with the mathematical formulation of quantum theory and tries to derive a physical interpretation from it. This latter approach is a reversal of the traditional role of mathematics in physics. But now very often the mathematical equations are seen as providing the most immediate contact with nature. While Bohm and Hiley agree that progress can be made from the mathematical side alone, they do not want to "regard these physical concepts as merely imaginative displays" (33) of the meaning of the equations.

This interpretation of quantum theory which can produce the results of the conventional interpretation, and thus has the status of an independent physical theory, was connected, in their minds, with more general considerations. Classical physics can be compared to an optical lens which allows the points of an object to correspond to the points of its image. The new unbroken wholeness that Bohm and Hiley champion is like a hologram in which each part contains an image of the whole object. "So in some sense, the whole object is enfolded in each part of the hologram rather than being in point-to-point correspondence." (34) The order in the hologram is thus called implicate, while the image explicate.

To illustrate this difference they imagine a device with two concentric glass cylinders, one inside the other. When the outer one is fixed, and the inner one revolves slowly, and the space between them Is filled with some viscous fluid, then an insoluble ink drop placed in the fluid will be drawn out into a long, thin thread until it is no longer visible. But if the direction is reversed, the thread and then the dot will eventually reappear. If one drop is enfolded in this way, and then another at the same point, and so on, and then the rotation of the cylinders is reversed, what we will see will look like a single drop that is appearing and disappearing. "We thus obtain an example of how form that persists in the explicate order can arise from the whole background and be sustained dynamically by a movement of enfoldment and unfoldment." (35) If a red drop and a blue drop are enfolded until they disappear and then made to reappear, we have an example that illustrates the Einstein, Podolsky, Rosen experiment, for the two explicit particles are intimately connected in the implicate order. The world of classical physics and even the movement of the particle in the quantum world belong to the explicate order, but there is also an implicate dimension to the quantum world in which active information, represented by the quantum potential, plays a fundamental role.

We are going to return later to Bohm's scientific and philosophical thoughts, but hopefully we have seen enough here to reach our first objective: important philosophical questions arise in the midst of properly scientific work. In the next two chapters we will see the same process in biology and psychology.