Walter Isaacson - Einstein - His Life and Universe

However, using his concept of complementarity, Bohr added a significant caveat. He pointed out that the two particles were part of one whole phenomenon. Because they have interacted, the two particles are therefore “entangled.” They are part of one whole phenomenon or one whole system that has one quantum function.

In addition, the EPR paper did not, as Bohr noted, truly dispel the uncertainty principle, which says that it is not possible to know both the precise position and momentum of a particle at the same moment. Einstein is correct, that if we measure the position of particle A, we can indeed know the position of its distant twin B. Likewise, if we measure the momentum of A, we can know the momentum of B. However, even if we can imagine measuring the position and then the momentum of particle A, and thus ascribe a “reality” to those attributes in particle B, we cannot in fact measure both these attributes precisely at any one time for particle A, and thus we cannot know them both precisely for particle B. Brian Greene, discussing Bohr’s response, has put it simply: “If you don’t have both of these attributes of the right-moving particle in hand, you don’t have them for the left-moving particle either. Thus there is no conflict with the uncertainty principle.” 12

Einstein continued to insist, however, that he had pinpointed an important example of the incompleteness of quantum mechanics by showing how it violated the principle of separability, which holds that two systems that are spatially separated have an independent existence. It likewise violated the related principle of locality, which says that an action on one of these systems cannot immediately affect the other. As an adherent of field theory, which defines reality using a spacetime continuum, Einstein believed that separability was a fundamental feature of nature. And as a defender of his own theory of relativity, which rid Newton’s cosmos of spooky action at a distance and decreed instead that such actions obey the speed limit of light, he believed in locality as well. 13

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.” 16Schrödinger 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.

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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