Walter Isaacson - Einstein - His Life and Universe

Because the two particles were far apart, Einstein was able to assert, or at least to assume, that “all physical interaction has ceased between them.” So his challenge to the Copenhagen interpreters of quantum mechanics, posed as a question to Rosenfeld, was simple: “How can the final state of the second particle be influenced by a measurement performed on the first?” 1

Over the years, Einstein had increasingly come to embrace the concept of realism, the belief that there is, as he put it, “a real factual situation” that exists “independent of our observations.” 2This belief was one aspect of his discomfort with Heisenberg’s uncertainty principle and other tenets of quantum mechanics that assert that observations determine realities. With his question to Rosenfeld, Einstein was deploying another concept: locality.* In other words, if two particles are spatially distant from each other, anything that happens to one is independent from what happens to the other, and no signal or force or influence can move between them faster than the speed of light.

Observing or poking one particle, Einstein posited, could not instantaneously jostle or jangle another one far away. The only way an action on one system can affect a distant one is if some wave or signal or information traveled between them—a process that would have to obey the speed limit of light. That was even true of gravity. If the sun suddenly disappeared, it would not affect the earth’s orbit for about eight minutes, the amount of time it would take the change in the gravitational field to ripple to the earth at the speed of light.

As Einstein said, “There is one supposition we should, in my opinion, absolutely hold fast: the real factual situation of the system S 2is independent of what is done with the system S 1, which is spatially separated from the former.” 3It was so intuitive that it seemed obvious. But as Einstein noted, it was a “supposition.” It had never been proven.

To Einstein, realism and localism were related underpinnings of physics. As he declared to his friend Max Born, coining a memorable phrase, “Physics should represent a reality in time and space, free from spooky action at a distance.” 4

Once he had settled in Princeton, Einstein began to refine this thought experiment. His sidekick, Walther Mayer, less loyal to Einstein than Einstein was to him, had drifted away from the front lines of fighting quantum mechanics, so Einstein enlisted the help of Nathan Rosen, a 26-year-old new fellow at the Institute, and Boris Podolsky, a 49-year-old physicist Einstein had met at Caltech who had since moved to the Institute.

The resulting four-page paper, published in May 1935 and known by the initials of its authors as the EPR paper, was the most important paper Einstein would write after moving to America. “Can the Quantum-Mechanical Description of Physical Reality Be Regarded as Complete?” they asked in their title.

Rosen did a lot of the math, and Podolsky wrote the published English version. Even though they had discussed the content at length, Einstein was displeased that Podolsky had buried the clear conceptual issue under a lot of mathematical formalism. “It did not come out as well as I had originally wanted,” Einstein complained to Schrödinger right after it was published. “Rather, the essential thing was, so to speak, smothered by the formalism.” 5

Einstein was also annoyed at Podolsky for leaking the contents to the New York Times before it was published. The headline read: “Einstein Attacks Quantum Theory / Scientist and Two Colleagues Find It Not ‘Complete’ Even though ‘Correct.’ ” Einstein, of course, had occasionally succumbed to giving interviews about upcoming articles, but this time he declared himself dismayed by the practice. “It is my invariable practice to discuss scientific matters only in the appropriate forum,” he wrote in a statement to the Times, “and I deprecate advance publication of any announcement in regard to such matters in the secular press.” 6

Einstein and his two coauthors began by defining their realist premise: “If without in any way disturbing a system we can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.” 7In other words, if by some process we could learn with absolute certainty the position of a particle, and we have not disturbed the particle by observing it, then we can say the particle’s position is real, that it exists in reality totally independent of our observations.

The paper went on to expand Einstein’s thought experiment about two particles that have collided (or have flown off in opposite directions from the disintegration of an atom) and therefore have properties that are correlated. We can take measurements of the first particle, the authors asserted, and from that gain knowledge about the second particle “without in any way disturbing the second particle.” By measuring the position of the first particle, we can determine precisely the position of the second particle. And we can do the same for the momentum. “In accordance with our criterion for reality, in the first case we must consider the quantity P as being an element of reality, in the second case the quantity Q is an element of reality.”

In simpler words: at any moment the second particle, which we have not observed, has a position that is real and a momentum that is real. These two properties are features of reality that quantum mechanics does not account for; thus the answer to the title’s question should be no, quantum mechanics’ description of reality is not complete. 8

The only alternative, the authors argued, would be to claim that the process of measuring the first particle affects the reality of the position and momentum of the second particle. “No reasonable definition of reality could be expected to permit this,” they concluded.

Wolfgang Pauli wrote Heisenberg a long and angry letter.“Einstein has once again expressed himself publicly on quantum mechanics (together with Podolsky and Rosen—no good company, by the way),” he fumed. “As is well known, every time that happens it is a catastrophe.” 9

When the EPR paper reached Niels Bohr in Copenhagen, he realized that he had once again been cast in the role, which he played so well at the Solvay Conferences, of defending quantum mechanics from yet another Einstein assault. “This onslaught came down on us as a bolt from the blue,” a colleague of Bohr’s reported. “Its effect on Bohr was remarkable.” He had often reacted to such situations by wandering around and muttering, “Einstein . . . Einstein . . . Einstein!” This time he added some collaborative doggerel as well: “Podolsky, Opodolsky, Iopodolsky, Siopodolsky . . .” 10

“Everything else was abandoned,” Bohr’s colleague recalled. “We had to clear up such a misunderstanding at once.”Even with such intensity, it took Bohr more than six weeks of fretting, writing, revising, dictating, and talking aloud before he finally sent off his response to EPR.

It was longer than the original paper. In it Bohr backed away somewhat from what had been an aspect of the uncertainty principle: that the mechanical disturbance caused by the act of observation was a cause of the uncertainty. He admitted that in Einstein’s thought experiment, “there is no question of a mechanical disturbance of the system under investigation.” 11

This was an important admission. Until then, the disturbance caused by a measurement had been part of Bohr’s physical explanation of quantum uncertainty. At the Solvay Conferences, he had rebutted Einstein’s ingenious thought experiments by showing that the simultaneous knowledge of, say, position and momentum was impossible at least in part because determining one attribute caused a disturbance that made it impossible to then measure the other attribute precisely.