Einstein: His Life and Universe

“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

Schrodinger 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 Schrodinger, 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 Schrodinger did smile, he told Einstein in his reply, because he had likewise edged from revolutionary to old reactionary.

On one issue Einstein and Schrodinger diverged. Schrodinger 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,” Schrodinger 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 Schrodinger 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, Schrodinger 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 Schrodinger, 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 ? 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 Schrodinger 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

Schrodinger 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, Schrodinger 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, Schrodinger 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 Schrodinger’s cat: