Einstein: His Life and Universe

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 Schrodinger 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.”⁷ In 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.

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

Schrodinger’s Cat

Despite his success as a quantum pioneer, Erwin Schrodinger 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 Schrodinger looked on with a mix of curiosity and sympathy. It was a lonely struggle, Einstein lamented in a letter to Schrodinger 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 Schrodinger sent Einstein a congratulatory note as soon as he read the EPR paper.