This is not a history book—it is a book about new ideas and progress. But sometimes there are lessons to be learned from history and from failure. Dante justified writing the Inferno, far more readable than his corresponding books describing paradise and purgatory, with the claim that exploring evil is one way to learn the path to good. At junior school, a classmate of mine once asked our highly religious headmaster why the Bible includes the Old Testament, with its descriptions of so many wicked things. He replied after some hesitation that one reason was to show the contrast between Jesus’s teachings and those of the harsher Old Testament prophets.

In something of the same spirit, we will now look at the traditional interpretations of quantum mechanics: those that originated in the first half of the 20th century, and remain (bizarrely) the best known to many science students today. I am not sneering at them, because it is easy to be wise with hindsight. But it must be said that they do not show the physics community in its best light. Please keep your skeptical instincts alert as you read on, because we are about to encounter stories of stubbornness, denial, and wishful thinking. Above all, remember that we should never believe something merely because it is advocated by someone who is very famous, or very well enshrined

in history. If we took that attitude unquestioningly, we would still be endorsing the scientific beliefs of the philosophers of ancient Greece.

These cautions given, let us now look at the views of the founding fathers of quantum, some of the greatest scientists who have ever lived.

Erwin Schrödinger developed the wave-theory formulation, which described the previously mysterious hydrogen atom with triumphant accuracy. Schrödinger’s interpretation of his wave mechanics was as simple as it was bold. His answer to the problem of wave-particle duality was that there are no particles, only waves. Just as a tsunami wave may be spread out invisibly thinly in the deep ocean, but can rise and become concentrated as it passes over shallow water, ultimately depositing most of its energy on a narrow stretch of coast, so any kind of wave can vary greatly in its physical extent. Schrödinger thought that the apparent particles of radiation and matter were merely manifestations of waves squeezed to an extreme degree—as when a water wave focused by the shape of an estuary rears up to a sudden peak and expends all its energy in knocking down a tall lighthouse, for example.

Schrödinger’s view works quite well for bound particles, such as electrons in an atom, whose behavior is described by the “time-independent” Schrödinger equation, which does not even try to answer the question of where the particle is located at any given instant. But it works much less well for particles in free space, such as an isolated proton or electron. Then the time-dependent version of the equation predicts that as long as it is not interacting with anything, the wave will continue to gradually flatten and spread out, in principle extending to infinity. Yet even a tiny observation-like interaction somewhere in this volume of space can bring an extremely pointlike electron springing into view, with dimensions that remain too small to measure—and this happens in less time than it would take light to cross the region of space where, until that moment, we thought the electron might be. As we have seen, it is hard to imagine any reasonable physical mechanism that could bring about quantum collapse in this kind of nonlocal case.

Schrödinger did not claim to have an answer to the problem, but he did make clear his contempt for the idea, which underpins several of the interpretations described below, that a system might be regarded as not having a definite state until an observation is made. If we accept that this notion makes sense in the context of microscopic systems, Schrödinger argued, then in the appropriate circumstances it would have to apply to larger systems as well—even living things such as cats. Suppose you constructed a completely observation-proof box and placed within it a cat and a sort of Russian roulette device which, as soon as the box was sealed, would fire a photon toward a polarizing filter and kill the cat if the photon happened to pass through. There is a fundamentally unpredictable 50 percent chance that the photon will pass through the filter. By the “nothing is actual until observed” argument, the cat would have no definite state until the box was opened, maybe hours later, to reveal either a dead cat with rigor mortis or a live but hungry one. Schrödinger invented his famous parable of the cat-in-a-box not to be believed, but to be disbelieved, as a reductio ad absurdum. He thought that it was manifestly ridiculous to think in the terms that the cat is neither dead nor alive until the box is opened.

Max Born had an alternative way to look at Schrödinger’s waves. He saw them as waves of probability. It will be useful to us later on to understand the modern philosophy of probability, and for this reason and for the sake of clarity, I shall extrapolate his argument into modern terminology and examples.

There is a subtle difference between probability and statistics. Consider the difference between the two following questions:

“There are 100 people in this hall. Fifty of them have had a white sticker placed on their backs. What percentage have white stickers on their backs?”

and

“I have selected at random someone from the hall who now stands

before you. What is the chance that this person’s back bears a white sticker?”

The answer to the first question is straightforward: 50 percent. The second question is trickier. After all, the person standing before you either has a sticker or doesn’t. If everyone in the audience except you can see the subject’s back, then everyone else in the room already knows that the hypothesis “There is a sticker on this person’s back” is either true with 100 percent certainty or false with 100 percent certainty. In what sense can it be correct to answer “50 percent probable”?

The matter becomes even more puzzling when you consider that the probability can change. Suppose you hesitate to answer the second question and the host goes on to say, “I will give you some further information. There are 50 women in the hall and 40 of them had white stickers placed on their backs. The remaining 10 stickers were distributed among the 50 men.”

You can see that the person before you is a woman, so it is reasonable to revise your estimate upward to 80 percent. But how can it be rational to do that? The person before you has not changed, nor has the fact that she either has a sticker on her back or has not. How can the right answer have changed?

The answer that most philosophers of mathematics would give is that probability is best thought of as a measure of ignorance. It is not rational for you to think that the physical facts of a situation change when you are given new information, but it is rational for you to take into account your reduction of ignorance. That this is not a trivial distinction is shown by the famous Monty Hall problem, in which a game show host shows you three cabinets, and gives you the following information: “One of these cabinets contains one million dollars. The other two are empty. I will ask you to choose one of the cabinets.

“Then, just to keep the audience entertained, I will open one of the other cabinets and reveal that it is empty. I will always choose an empty cabinet to open, and never your original choice. After that I will give you the opportunity either to stick with your original choice or to switch it to the remaining unopened cabinet. I will open whichever of the two cabinets you have finally chosen. If it contains the million dollars, the money is yours.”

The game starts, and you choose the left-hand cabinet. The host opens the right-hand one and shows it is empty. The million-dollar question that confronts you is, ‘Is it worth changing your choice to the middle cabinet? Or does it make no difference to your chance of winning?’

Most people (including physicists and mathematicians) reason incorrectly when they first meet this problem, along the following lines: ‘The fact of whether the middle cabinet contains the money cannot have changed as a result of all this flim-flam. Therefore, there is no rational reason to change my choice. There are two unopened cabinets; there is an equal chance that the money is in either.’

But they are profoundly mistaken. Because although the physical situation has not changed, your ignorance has reduced—and that can make it quite rational to change your choice. Your ignorance about whether the money is in your original choice of the left-hand cabinet has not changed. It is still a one-third chance, as it was at the start of the game. But your ignorance about which of the other two cabinets has the money, assuming you originally guessed wrong, has disappeared. The chance that you originally guessed wrong is two-thirds, and in that case the money must be in the middle cabinet. You double your chances of winning by switching your choice. Thus a change in your knowledge of the universe—as happens when you make a measurement of a quantum system—can revise your expectation of the probable results you will get from subsequent measurements. To a naive person this might look as though acquiring knowledge about the system actually changed the system—like guests on the Monty Hall show discovering that changing their initial choice did indeed give them a two-thirds chance of winning, and then falsely thinking that this implied that money sometimes jumped from one cabinet to another as a result of their first measurement.

Born’s approach was and is greatly respected. The rules of quantum probability are still widely referred to as Born rules. But as we have already seen, the most troubling observer effect, the EPR paradox as illustrated by the Bell-Aspect experiment and the lottery cards, cannot be explained by mere reduction of ignorance in a classical universe.

Albert Einstein initially preferred an idea proposed in 1926 by Louis de Broglie. In this model the particles of radiation and matter are real and pointlike (or at any rate, very small) and their wavelike behavior is explained by their association with a kind of phantom field, which is detectable only through its effect on particles. This is, of course, the concept of guide waves. As we have seen, you can explain a great deal of what goes on in quantum by postulating some kind of invisible fine structuring to the world that can guide and jostle particles in a wavelike manner, describable by mathematicians in terms of hidden local variables.

However, Einstein soon came to realize the huge difficulties that nonlocality posed for this picture. In one of the most famous scientific papers of all time, written with Boris Podolsky and Nathan Rosen, he described the nub of the problem: After two particles have in some way interacted and traveled far apart, measuring one of them appears to have an instant effect on the other. The problem has been known ever since as the EPR paradox, or simply EPR.

Einstein hoped for a simple solution: Such long-range effects would turn out not to exist. Either quantum theory was incomplete and required modification or, more likely, there was some kind of error in the reasoning that implied that such “spooky forces” were operating. Einstein clearly thought that special relativity implied that no kind of influence could travel faster than light, irrespective of any quibbles about whether it could transmit information.

Nowadays, it is easy to borrow a glib psychologist’s phrase and say that he was in denial but at the time it was a perfectly reasonable position to prefer the implications of special relativity, which had been thoroughly tested, to those of quantum theory. At the time he was pondering these matters in the 1930s (and even when he died in 1954), there was no practical way to investigate the matter experimentally. It was not until the 1960s that John Bell formulated the theoretical basis for an experiment that would be both definitive and practicable, and not until the 1980s that Alain Aspect and others were able to turn the experiment into foolproof hardware. But now the test has been done many times, and there is no question about it.¹ Nonlocality is real. Einstein was wrong.

Niels Bohr is generally remembered as the father of the Copenhagen interpretation. Many textbooks describe the Copenhagen interpretation, formulated in dialogues between Bohr and others in and around that picturesque Danish city, as being the orthodox or mainstream interpretation of quantum mechanics. Yet there is no general agreement on what the Copenhagen interpretation actually is. At the lowest common denominator, it can be summed up in the following pair of statements:

1. The only real things are the results of experiments as measured by conscious, macroscopic observers; there is no deeper underlying reality.

2. Experiments yield results consistent with either wavelike behavior or particle-like behavior, depending on the design of the experiment, but never both at the same time.

But until the Copenhagen interpretation came along, the whole point of doing experiments was to formulate a picture of an underlying reality. Why, exactly, are we being forbidden to speculate further in this instance? Surely the idea that there are questions that must not be asked is contrary to the whole spirit of scientific endeavor.

Of course, there is nothing unreasonable about saying that a question is unanswerable because the result you get depends on the way that the question is asked. Consider, for example, a punchbag filled with a thixotropic fluid—one that acts like a liquid under gentle forces but like a solid if struck hard. The question “Are the contents of this bag liquid or solid?” can be answered only in the context of whether it is going to be squeezed or struck. But of course we can and do ask questions like: What is the threshold at which the behavior changes? Why does this happen? What, exactly, is going on at the molecular level? Bohr, by contrast, seemed to dismiss many questions about quantum as altogether meaningless, analogous to asking: “What color is up?”

Evidently Bohr felt confident that quantum theory as then formulated could answer all the questions that he felt it needful to ask of it. But he resisted further probing with wordy statements that have led

many to retreat in confusion ever since, convinced that the tougher questions they had dared to pose had indeed been foolish. To me, Bohr’s attitude seems uncomfortably reminiscent of those Buddhist sages who feel free to reply to certain questions with the response “Mu!” which means “The question is unsaid!” But many people have faith in such gurus.

My views on Bohr have recently undergone a partial change as a result of an intriguing paper by Don Howard, first delivered at a conference in Oxford.² Howard argues plausibly that the so-called unified Copenhagen interpretation was a myth invented retrospectively by Bohr’s enemies (or at any rate, enemies of his school of thought), Heisenberg and Popper. In Howard’s view, Bohr, far from being intentionally mystical in his replies, was merely being careful. If Howard is right, the nature of Bohr’s caution is perfectly described by an anecdote many people will have heard in different forms. In my version, a child, a physicist, and a philosopher are traveling in a train passing through a country none of them has previously visited. The train passes a field in which they see a black sheep.

“Wow,” says the child, “look at that. All of the sheep in this country are black!”

The physicist smiles. “We don’t know that,” he says. “All we can really tell is that some of the sheep in this country are black.”

The philosopher smiles. “We don’t know that,” he says. “All we really know is that at least one sheep in this country appears black on at least one side!”

In an everyday context, we might consider that the physicist was the most sensible of the three. But if we are visiting a truly unfamiliar place—such as the world of quantum—then the philosopher’s point that you should make statements only about the things you directly perceive, avoiding even the most reasonable-seeming inferences, is quite logical. Only by sticking to what you know for sure will you gain a reliable understanding.

When Bohr insisted that all it is legitimate to say about a quantum experiment is: “The experimenter observes such-and-such result,” as opposed to “The quantum system was in such-and-such state,” according to Howard he was merely being as careful as the philosopher

in the train story. He certainly was not assigning any mystically powerful role to conscious observers. Interpreting any of his statements as “Conscious observers are the agents who physically trigger quantum collapse” would then be as much of a blunder as the famous mistranslation of the canali (channels) that the astronomer Schiaparelli thought he had seen on Mars as “canals,” implying that Schiaparelli was postulating intelligent (and presumably conscious) Martians as the agents that created them.

I accept Howard’s claim that Bohr was, at worst, a cautious agnostic, rather than a mystic. Possibly he hoped that if other investigators followed his example of making statements about only what they observed, rather than what they presumed, then a fully objective picture of quantum would ultimately emerge. But to me there seems a touch of cowardice about his stance. It was certainly frustrating to talk to Bohr; famously, he once reduced Heisenberg to tears. Here is an example of a genuine Bohr statement quoted by Howard:

If you find this less than transparent, you have my sympathy. It sounds rather deep. But try rereading the passage, changing the words “quantum” to “Olympian” and “atomic phenomena” to “gods,” and you will see just how unsatisfactory the above statement is.

Bohr’s answer to the specific problem of wave-particle duality is particularly inadequate. He said, essentially, no more than that we should expect a particle-like result from a particle-oriented experiment, and a wavelike result from a wave-oriented experiment. To me, this is uncomfortably suggestive of an engineer’s rule-of-thumb. Imagine that you meet a hydraulics engineer who tells you the following story:

“I have two formulas that tell me exactly how fast water will flow through a channel of given size, under a given pressure difference,” he says. “One formula works well for flow through narrow pipes, as used in domestic plumbing. The other formula works well for large constructions, like canals and aqueducts.”

You take a look at his formulas. “But these are two completely different equations!” you exclaim. “They are supposedly describing the same thing, but would predict completely different results if they were applied to the same channel. What happens in a pipe of intermediate size, say one that is 10 centimeters in diameter?”

The engineer shrugs his shoulders. “I do only domestic plumbing and canals,” he says cheerfully. “I don’t need to know the answers for intermediate sizes.”

Apart from his lack of theoretical curiosity, this hypothetical engineer would be missing out on his appreciation of a most important phenomenon: turbulence. The different equations arise because the flow through a narrow tube tends to be smooth or laminar, whereas larger flows naturally break up into the swirls and eddies of turbulence. Understanding turbulence is not only of great theoretical interest; manipulating the conditions that trigger its onset is the key to harnessing the properties of fluid flow in all sorts of contexts.

Nowadays, we can do experiments involving behavior that is intermediate between particle-like and wavelike. We are beginning to understand a process called decoherence, which is arguably the real mechanism of quantum collapse and is in some ways quite analogous to turbulence. Bohr has absolutely nothing to say about these kinds of situations. Agnosticism is perhaps an intellectually respectable position, but it does not lead to progress. Bohr had not so much an interpretation of quantum mechanics as an absence of one.

The worst part of the Copenhagen legacy, though, is that it continues to give aid and comfort to those who, in the debate between physicists and philosophers over the meaning of quantum theory, could be described as at the extreme philosophical end of the spectrum—those who maintain that questions about reality beyond the scope of immediate personal observation are meaningless. This solipsist viewpoint is impossible to refute, just like such claims as, “You have actually been lying on a couch all your life, wired up to a virtual reality machine,” or “The world, complete with your memories and those of everybody else, has just been created in the last second.” But it is utterly barren and unhelpful to the scientist’s quest to build a meaningful picture of the universe. As Howard has pointed out, this idea has remained in the running largely because various claimants have muddied Bohr’s name by falsely associating him with this viewpoint in a Copenhagen synthesis that never was.

The mathematician John von Neumann’s major contribution to the world was to lay the foundations for the computer revolution that followed later in the 20th century. But he also worked on the quantum theory, and his book, Mathematical Foundations of Quantum Mechanics, published in 1932, was fundamental to the field.

Von Neumann was the first person to think really deeply about the problem of quantum collapse. He was troubled by the potential for infinite regress, which we have already come across. If system A is measured by being put in contact with a larger system B, the result is measured by being put in contact with a still larger system C, and so forth, where does the process stop? When does the universe decide, OK, that’s it, and settle down to a particular version of reality rather than tracing out yet more families of wavy variants? Von Neumann identified a physical need for collapse that goes beyond the philosophical problem of why we observe a single fixed version of reality. He realized that the equations of quantum are time symmetric. This, of course, contrasts with our macroscopic experience that there is a clearly defined arrow of time; eggs do not unscramble themselves, for example.

In the classical world, the arrow of time is associated with a steady increase of entropy, which can also be understood as a decrease of ordering. The universe started in an extremely ordered state, crammed into a tiny space, and even today most of its visible mass remains packed into stars occupying a very small fraction of its overall volume. The temperature difference between those stars and the cold emptiness of interstellar and intergalactic space provides the flow of energy that drives such processes as life on Earth.

But how does this tie in with the timeless quantum world, whose mathematical waves flow symmetrically without anything corresponding to an arrow of time? Von Neumann worked out that there is an entropy increase associated with quantum collapse, when multiple possibilities reduce to a single outcome. This is an interesting finding, but of course it requires physical collapse to occur at some point. Von Neumann reasoned that in the absence of any evidence for its happening earlier, the collapse should be assumed to take place at the point where a conscious observer inspects a quantum system.

To be fair to von Neumann, we must remember that he was writing before such basic thought experiments as Schrödinger’s cat and EPR had even been formulated. I strongly suspect that he would have revised his views if he had lived until a later era. The contrast between his granting quantum collapse an important physical role on the one hand, and attributing it to an almost mystical cause on the other, is bizarre. But the idea of a conscious observer with a mysterious power to collapse systems by looking at them has appealed so strongly to a certain breed of thinker that it has survived for many decades. For example, von Neumann’s ideas were still being extended in the 1960s by Eugene Wigner.

Wigner suggested that von Neumann’s hypothesis from four decades earlier should be taken literally. Thus in Schrödinger’s cat experiment, the point at which the cat’s fate is determined comes not even when the box is opened, but when a conscious observer becomes aware of the result. For example, if the cat box is in space out beyond Pluto, aboard an unmanned probe with an automated opening mechanism that reveals the box’s interior to a television camera, the cat’s fate is not decided until the TV signal reaches the inner solar system. How-

ever, if an astronaut observer has been sent out to watch from aboard a nearby spacecraft, the cat’s fate is decided as soon as he can see it, microseconds after the box opens. This is known as the paradox of Wigner’s friend. (One wonders what he proposed doing to his enemies.)

Wigner’s ideas have been rightly lampooned, by John Bell among others. Among the reductio ad absurdum questions one can ask are:

“What happens if there is a conscious observer in the box with the cat. Does the cat then die immediately, before the box is opened?”

“What exactly counts as a conscious observer? Is the cat a conscious observer? If so, what about a mouse, a frog, a slug? If not, what about a chimpanzee, or a Neanderthal? Where does the dividing line come? Does the observer need a PhD?”

The beautiful point has been made that in the context of cosmology, there were no conscious observers at all until a certain point (probably quite recent) in the universe’s history. Was the entire universe waiting to collapse into a definite state until the first ape-man came along?

Conscious observers with spooky powers to collapse systems up to the size of a universe seem rather implausible. In any case the conscious-observer-collapse hypothesis does nothing to resolve the real problem of quantum, nonlocality. Remember the lottery cards example where my partner went to Mars. What happened when we simultaneously scratched our cards? Did my observation collapse the universe, or did hers, or was it both? In whichever case, the effect must presumably have rippled out faster than light to ensure that the far-off lottery card got to be the correct color.

Other attempts to extend the early interpretations of quantum were more respectable. Perhaps the most heroic attempt to cling to a classical picture is found in the rather tragic story of David Bohm, an American physicist who came to England’s Birkbeck College in London when his services were no longer required on the Manhattan project.

(I cannot resist pointing out here a curious fact about the scientists who have contributed the most to our understanding of quantum theory: A remarkably high proportion have four-letter surnames beginning with “B.” Those we shall encounter include Born, Bohr, Bohm, and Bell; there was also Bose of Bose-Einstein condensate fame. And some claim there is nothing weird about the statistics of quantum!)

In the 1950s, Bohm effectively rediscovered and revitalized the pilot-wave theory which had been invented by de Broglie a quarter of a century earlier, but fallen out of favor because of its problems with nonlocality. Bohm was determined to make the pilot-wave theory work somehow, despite the apparent faster-than-light influences of EPR. Mathematically, his work was to an extent successful and his findings interesting. He discovered that pilot-wave theory could work after a fashion if you assumed that the guide waves continued indefinitely even when they were no longer associated with any particular particle. He found, however, that individual guide waves did not, as you might expect, die away with time, viewed from a macroscopic scale, like water waves decaying to ripples after the storm that caused them has died down. The guide waves can remain large in amplitude even at times and places remote from their last occupation by a surfing particle. One way to see intuitively why this must be is to reflect that warships carry charts of equal scale and detail covering every portion of the world’s oceans—because although there are places they are most unlikely to be ordered to, if they ever are, then the scale of maps required to navigate properly is just the same as for those regions they visit frequently. Pilot waves, if they do exist, must guide particles with accuracy through low-probability as well as high-probability regions.

(Ironically, as David Deutsch and others have pointed out, Bohm’s work is excellently supportive of many-worlds. If you forget the rather artificial notion that the waves are occupied by surfers whose positions define a single reality, then the waves are tracing out all possible world-lines with equal fidelity. But we shall come to this later.)

Bohmian mechanics, as it is now called, is more sophisticated than the simple surfing-particle story we constructed in the previous chapters. Guided by an extra field he called the quantum potential, his particles did not “tumble off” their guide waves on undergoing

measurement-like interactions; rather, the wave that they were riding underwent a subtle and pseudo-instantaneous change.

But of course this quantum potential has to operate in a nonlocal manner, and Bohm’s attempts to explain how this could happen became rather desperate. In a book written with Basil Hiley shortly before Bohm’s death (it was published posthumously), he introduces the notion of implicate order.³ The attempt to understand this concept has baffled many physicists, but I think the idea can be taken in two separate ways. We can understand it as saying either that there is a kind of limited but instantaneous linkage between all parts of the universe, which is not directly accessible from the macroscopic world where time’s arrow operates, or that the universe contains embedded in every part of itself encoded information about what is going on in the other parts.

The problems with the first view we have already examined. They include contradictions with the spirit of special relativity, implying backward-in-time causation at the micro scale, as well as the truly enormous amount of behind-the-scenes calculation that the universe must be assumed to do if every part of it can instantly affect every other part. The problem with the second view is that it implies predestination. If a billion projectors are playing exactly the same movie, without needing to communicate with one another, then even if each projector is showing only one particular area of each frame at maximum magnification, they must surely all have been loaded with the same film at the start of the performance.

There is nothing intrinsically impossible about predestination. Richard Feynman was struck by the symmetry between the processes by which radiation is absorbed and emitted. We normally think of radiation as going out from its source in all directions, but approaching a target from only one direction; yet it is just as reasonable to think of it converging on its target from all directions, an invisible noose drawn tight with perfect precision. Feynman and others have toyed with the idea that although, to our usual perception, the order of the universe

is decreasing, perhaps in subtle ways it is increasing. The universe originated in one rather special highly ordered state, and is progressing toward another, rather than toward pure disorder. From our point of view, this progress toward a future constraint looks exactly like predestination. As Professor Cope explained in Chapter 1, predestination can easily account for EPR correlations. Indeed, predestination can in principle explain any apparently nonlocal phenomenon. Think of two people a light-year apart, holding a conversation; if each knows exactly what the other is about to say and when, each can “react” to the other without any delay for a signal to go from one to the other. The idea that a specific, subtle kind of predestination can explain EPR and other puzzles of modern physics has been developed by Huw Price, and is described in his popular book Time’s Arrow and Archimedes’ Point.⁴ Unfortunately, Price has encountered considerable technical difficulties in trying to develop a predestination theory that takes account of the way subatomic particles are known to behave.⁵

But the real trouble with the postulates of predestination and instant everywhere-to-everywhere links is that they are much too powerful merely to explain EPR correlations. If such phenomena exist, the problem becomes: How does the universe implement such remarkably efficient prevention of apparent faster-than-light causal effects and faster-than-light communication of information? Where does the censorship come from? Italian physicist Antony Valentini has attempted to develop a kind of hidden-variable theory in which the early universe did have general faster-than-light causal links, which died away naturally, except at the microscopic level, due to thermodynamic considerations, but his views have not won wide acceptance. Valentini has been brave enough to suggest an experiment to test his ideas. Essentially, the idea is to use a powerful telescope to capture photons that were emitted very early in the history of the universe, and subject them to a two-slit experiment. He predicts that the usual interference pattern will not be found. I applaud his courage, but I (and many others) would be prepared to bet a substantial sum that no such anomalies will be found.

Another variation on the predestination theme is John Cramer’s transactional interpretation. Cramer invites us to imagine a “retarded

wave” spreading backward in time from the point at which a system is finally measured, for example, by the absorption of a photon in a specific spot, and interfering with the forward wave which we normally think of as constituting or guiding the photon. You could think in terms of a sort of “negotiation” or transactional discussion between the past and the future that decides whether, for example, Schrödinger’s cat lives or dies. But this way of thinking has proved too cumbersome to gain widespread acceptance.

The work of Price, Valentini, and Cramer actually represents the respectable end of an endeavor that has become increasingly unrewarding, trying to cling to quantum interpretations invented a lifetime ago. Other attempts have led even powerful intellects into dubious pathways. I recently heard a rather distressing talk by a former colleague of Bohm’s detailing how his immediate circle at Birkbeck developed some aspects of a cabal, complete with initiation rites, as an ever more isolated group attempted to explain away the contradictions of quantum with ideas borrowed from literary theory and even psychoanalysis. Bohm died, in the judgment of many who knew him, “badly bewitched by philosophy.” Philosophical discourse into quantum has taken some unhelpful turns, most especially with respect to the claim that asking certain questions about quantum systems is meaningless or forbidden. Because everything in the universe is in fact a quantum system, an extension of this attitude could pretty much spell the end of scientific endeavor.

We will eschew philosophical excuses and outdated notions. We are looking for a physical, visualizable solution that our common sense can accept. Because none of the ideas above properly address the PPQs we have formulated, we must look for newer ones.