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,
small, that
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 through 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
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.
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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 Schrodinger’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 Schrodinger’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 Schrodinger’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 Schrodinger 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
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
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.
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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.
