einstein

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 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 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