Schrödinger’s Cat
Despite his success as a quantum pioneer, Erwin Schrödinger was among those rooting for Einstein to succeed in deflating the Copenhagen consensus. Their alliance had been forged at the Solvay Conferences, where Einstein played God’s advocate and Schrödinger looked on with a mix of curiosity and sympathy. It was a lonely struggle, Einstein lamented in a letter to Schrödinger in 1928: “The Heisenberg-Bohr tranquilizing philosophy—or religion?—is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused.”14
So it was not surprising that Schrödinger sent Einstein a congratulatory note as soon as he read the EPR paper. “You have publicly caught dogmatic quantum mechanics by its throat,” he wrote. A few weeks later, he added happily, “Like a pike in a goldfish pond it has stirred everyone up.”15
Schrödinger had just visited Princeton, and Einstein was still hoping, in vain, that Flexner might be convinced to hire him for the Institute. In his subsequent flurry of exchanges with Schrödinger, Einstein began conspiring with him on ways to poke holes in quantum mechanics.
“I do not believe in it,” Einstein declared flatly. He ridiculed as “spiritualistic” the notion that there could be “spooky action at a distance,” and he attacked the idea that there was no reality beyond our ability to observe things. “This epistemology-soaked orgy ought to burn itself out,” he said. “No doubt, however, you smile at me and think that, after all, many a young whore turns into an old praying sister, and many a young revolutionary becomes an old reactionary.”16 Schrödinger did smile, he told Einstein in his reply, because he had likewise edged from revolutionary to old reactionary.
On one issue Einstein and Schrödinger diverged. Schrödinger did not feel that the concept of locality was sacred. He even coined the term that we now use, entanglement, to describe the correlations that exist between two particles that have interacted but are now distant from each other. The quantum states of two particles that have interacted must subsequently be described together, with any changes to one particle instantly being reflected in the other, no matter how far apart they now are. “Entanglement of predictions arises from the fact that the two bodies at some earlier time formed in a true sense one system, that is were interacting, and have left behind traces on each other,” Schrödinger wrote. “If two separated bodies enter a situation in which they influence each other, and separate again, then there occurs what I have just called entanglement of our knowledge of the two bodies.”17
Einstein and Schrödinger together began exploring another way—one that did not hinge on issues of locality or separation—to raise questions about quantum mechanics. Their new approach was to look at what would occur when an event in the quantum realm, which includes subatomic particles, interacted with objects in the macro world, which includes those things we normally see in our daily lives.
In the quantum realm, there is no definite location of a particle, such as an electron, at any given moment. Instead, a mathematical function, known as a wave function, describes the probability of finding the particle in some particular place. These wave functions also describe quantum states, such as the probability that an atom will, when observed, be decayed or not. In 1925, Schrödinger had come up with his famous equation that described these waves, which spread and smear throughout space. His equation defined the probability that a particle, when observed, will be found in a particular place or state.18
According to the Copenhagen interpretation developed by Niels Bohr and his fellow pioneers of quantum mechanics, until such an observation is made, the reality of the particle’s position or state consists only of these probabilities. By measuring or observing the system, the observer causes the wave function to collapse and one distinct position or state to snap into place.
In a letter to Schrödinger, Einstein gave a vivid thought experiment showing why all this discussion of wave functions and probabilities, and of particles that have no definite positions until observed, failed his test of completeness. He imagined two boxes, one of which we know contains a ball. As we prepare to look in one of the boxes, there is a 50 percent chance of the ball being there. After we look, there is either a 100 percent or a 0 percent chance it is in there. But all along, in reality, the ball was in one of the boxes. Einstein wrote:
I describe a state of affairs as follows: the probability is ½ that the ball is in the first box. Is that a complete description? no: A complete statement is: the ball is (or is not) in the first box. That is how the characterization of the state of affairs must appear in a complete description. yes: Before I open them, the ball is by no means in one of the two boxes. Being in a definite box comes about only when I lift the covers.19
Einstein clearly preferred the former explanation, a statement of his realism. He felt that there was something incomplete about the second answer, which was the way quantum mechanics explained things.
Einstein’s argument is based on what appears to be common sense. However, sometimes what seems to make sense turns out not to be a good description of nature. Einstein realized this when he developed his relativity theory; he defied the accepted common sense of the time and forced us to change the way we think about nature. Quantum mechanics does something similar. It asserts that particles do not have a definite state except when observed, and two particles can be in an entangled state so that the observation of one determines a property of the other instantly. As soon as any observation is made, the system goes into a fixed state.20
Einstein never accepted this as a complete description of reality, and along these lines he proposed another thought experiment to Schrödinger a few weeks later, in early August 1935. It involved a situation in which quantum mechanics would assign only probabilities, even though common sense tells us that there is obviously an underlying reality that exists with certainty. Imagine a pile of gunpowder that, due to the instability of some particle, will combust at some point, Einstein said. The quantum mechanical equation for this situation “describes a sort of blend of not-yet and already-exploded systems.” But this is not “a real state of affairs,” Einstein said, “for in reality there is just no intermediary between exploded and not-exploded.”21
Schrödinger came up with a similar thought experiment—involving a soon-to-be-famous fictional feline rather than a pile of gunpowder—to show the weirdness inherent when the indeterminacy of the quantum realm interacts with our normal world of larger objects. “In a lengthy essay that I have just written, I give an example that is very similar to your exploding powder keg,” he told Einstein.22
In this essay, published that November, Schrödinger gave generous credit to Einstein and the EPR paper for “providing the impetus” for his argument. It poked at a core concept in quantum mechanics, namely that the timing of the emission of a particle from a decaying nucleus is indeterminate until it is actually observed. In the quantum world, a nucleus is in a “superposition,” meaning it exists simultaneously as being decayed and undecayed until it is observed, at which point its wave function collapses and it becomes either one or the other.
This may be conceivable for the microscopic quantum realm, but it is baffling when one imagines the intersection between the quantum realm and our observable everyday world. So, Schrödinger asked in his thought experiment, when does the system stop being in a superposition incorporating both states and snap into being one reality?
This question led to the precarious fate of an imaginary creature, which was destined to become immortal whether it was dead or alive, known as Schrödinger’s cat:
One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and th
rough a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out.23
Einstein was thrilled. “Your cat shows that we are in complete agreement concerning our assessment of the character of the current theory,” he wrote back. “A psi-function that contains the living as well as the dead cat just cannot be taken as a description of a real state of affairs.”24
The case of Schrödinger’s cat has spawned reams of responses that continue to pour forth with varying degrees of comprehensibility. Suffice it to say that in the Copenhagen interpretation of quantum mechanics, a system stops being a superposition of states and snaps into a single reality when it is observed, but there is no clear rule for what constitutes such an observation. Can the cat be an observer? A flea? A computer? A mechanical recording device? There’s no set answer. However, we do know that quantum effects generally are not observed in our everyday visible world, which includes cats and even fleas. So most adherents of quantum mechanics would not argue that Schrödinger’s cat is sitting in that box somehow being both dead and alive until the lid is opened.25
Einstein never lost faith in the ability of Schrödinger’s cat and his own gunpowder thought experiments of 1935 to expose the incompleteness of quantum mechanics. Nor has he received proper historical credit for helping give birth to that poor cat. In fact, he would later mistakenly give Schrödinger credit for both of the thought experiments in a letter that exposed the animal to being blown up rather than poisoned. “Contemporary physicists somehow believe that the quantum theory provides a description of reality, and even a complete description,” Einstein wrote Schrödinger in 1950.“This interpretation is, however, refuted most elegantly by your system of radioactive atom + Geiger counter + amplifier + charge of gunpowder + cat in a box, in which the psi-function of the system contains the cat both alive and blown to bits.”26
Einstein’s so-called mistakes, such as the cosmological constant he added to his gravitational field equations, often turned out to be more intriguing than other people’s successes. The same was true of his parries against Bohr and Heisenberg. The EPR paper would not succeed in showing that quantum mechanics was wrong. But it did eventually become clear that quantum mechanics was, as Einstein argued, incompatible with our commonsense understanding of locality—our aversion to spooky action at a distance. The odd thing is that Einstein, apparently, was far more right than he hoped to be.
In the years since he came up with the EPR thought experiment, the idea of entanglement and spooky action at a distance—the quantum weirdness in which an observation of one particle can instantly affect another one far away—has increasingly become part of what experimental physicists study. In 1951, David Bohm, a brilliant assistant professor at Princeton, recast the EPR thought experiment so that it involved the opposite “spins” of two particles flying apart from an interaction.27 In 1964, John Stewart Bell, who worked at the CERN nuclear research facility near Geneva, wrote a paper that proposed a way to conduct experiments based on this approach.28
Bell was less than comfortable with quantum mechanics. “I hesitated to think it was wrong,” he once said, “but I knew that it was rotten.”29 That, plus his admiration of Einstein, caused him to express some hope that Einstein rather than Bohr might be proven right. But when the experiments were undertaken in the 1980s by the French physicist Alain Aspect and others, they provided evidence that locality was not a feature of the quantum world. “Spooky action at a distance,” or, more precisely, the potential entanglement of distant particles, was.30
Even so, Bell ended up appreciating Einstein’s efforts. “I felt that Einstein’s intellectual superiority over Bohr, in this instance, was enormous, a vast gulf between the man who saw clearly what was needed, and the obscurantist,” he said. “So for me, it is a pity that Einstein’s idea doesn’t work. The reasonable thing just doesn’t work.”31
Quantum entanglement—an idea discussed by Einstein in 1935 as a way of undermining quantum mechanics—is now one of the weirder elements of physics, because it is so counterintuitive. Every year the evidence for it mounts, and public fascination with it grows. At the end of 2005, for example, the New York Times published a survey article called “Quantum Trickery: Testing Einstein’s Strangest Theory,” by Dennis Overbye, in which Cornell physicist N. David Mermin called it “the closest thing we have to magic.”32 And in 2006, the New Scientist ran a story titled “Einstein’s ‘Spooky Action’ Seen on a Chip,” which began:
A simple semiconductor chip has been used to generate pairs of entangled photons, a vital step towards making quantum computers a reality. Famously dubbed “spooky action at a distance” by Einstein, entanglement is the mysterious phenomenon of quantum particles whereby two particles such as photons behave as one regardless of how far apart they are.33
Might this spooky action at a distance—where something that happens to a particle in one place can be instantly reflected by one that is billions of miles away—violate the speed limit of light? No, the theory of relativity still seems safe. The two particles, though distant, remain part of the same physical entity. By observing one of them, we may affect its attributes, and that is correlated to what would be observed of the second particle. But no information is transmitted, no signal sent, and there is no traditional cause-and-effect relationship. One can show by thought experiments that quantum entanglement cannot be used to send information instantaneously. “In short,” says physicist Brian Greene, “special relativity survives by the skin of its teeth.”34
During the past few decades, a number of theorists, including Murray Gell-Mann and James Hartle, have adopted a view of quantum mechanics that differs in some ways from the Copenhagen interpretation and provides an easier explanation of the EPR thought experiment. Their interpretation is based on alternative histories of the universe, coarse-grained in the sense that they follow only certain variables and ignore (or average over) the rest. These “decoherent” histories form a tree-like structure, with each of the alternatives at one time branching out into alternatives at the next time and so forth.
In the case of the EPR thought experiment, the position of one of the two particles is measured on one branch of history. Because of the common origin of the particles, the position of the other one is determined as well. On a different branch of history, the momentum of one of the particles may be measured, and the momentum of the other one is also determined. On each branch nothing occurs that violates the laws of classical physics. The information about one particle implies the corresponding information about the other one, but nothing happens to the second particle as a result of the measurement of the first one. So there is no threat to special relativity and its prohibition of instantaneous transmission of information. What is special about quantum mechanics is that the simultaneous determination of the position and the momentum of a particle is impossible, so if these two determinations occur, it must be on different branches of history.35
“Physics and Reality”
Einstein’s fundamental dispute with the Bohr-Heisenberg crowd over quantum mechanics was not merely about whether God rolled dice or left cats half dead. Nor was it just about causality, locality, or even completeness. It was about reality.36 Does it exist? More specifically, is it meaningful to speak about a physical reality that exists independently of whatever observations we can make? “At the heart of the problem,” Einstein said of quantum mechanics, “is not so much the question of causality but the question of realism.”37
Bohr and his adherents scoffed at the idea that it made sense to talk about what might be beneath the veil of what we can observe. All we can know are the results of our experiments and observations, not some ultimate reality that lies beyond our perceptions.
Einstein had displayed so
me elements of this attitude in 1905, back when he was reading Hume and Mach while rejecting such unobservable concepts as absolute space and time. “At that time my mode of thinking was much nearer positivism than it was later on,” he recalled. “My departure from positivism came only when I worked out the general theory of relativity.”38
From then on, Einstein increasingly adhered to the belief that there is an objective classical reality. And though there are some consistencies between his early and late thinking, he admitted freely that, at least in his own mind, his realism represented a move away from his earlier Machian empiricism. “This credo,” he said, “does not correspond with the point of view I held in younger years.”39 As the historian Gerald Holton notes, “For a scientist to change his philosophical beliefs so fundamentally is rare.”40
Einstein’s concept of realism had three main components:
1. His belief that a reality exists independent of our ability to observe it. As he put it in his autobiographical notes: “Physics is an attempt conceptually to grasp reality as it is thought independently of its being observed. In this sense one speaks of ‘physical reality.’ ”41
2. His belief in separability and locality. In other words, objects are located at certain points in spacetime, and this separability is part of what defines them. “If one abandons the assumption that what exists in different parts of space has its own independent, real existence, then I simply cannot see what it is that physics is supposed to describe,” he declared to Max Born.42
3. His belief in strict causality, which implies certainty and classical determinism. The idea that probabilities play a role in reality was as disconcerting to him as the idea that our observations might play a role in collapsing those probabilities. “Some physicists, among them myself, cannot believe,” he said, “that we must accept the view that events in nature are analogous to a game of chance.”43
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