einstein
einstein
einstein
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“Spooky Action at a Distance”<br />
CHAPTER TWENTY<br />
QUANTUM ENTANGLEMENT<br />
1935<br />
The thought experiments that Einstein had lobbed like grenades into the temple of quantum mechanics had done little damage to the edifice. In<br />
fact, they helped test it and permit a better understanding of its implications. But Einstein remained a resister, and he continued to conjure up new<br />
ways to show that the uncertainties inherent in the interpretations formulated by Niels Bohr, Werner Heisenberg, Max Born, and others meant that<br />
something was missing in their explanation of “reality.”<br />
Just before he left Europe in 1933, Einstein attended a lecture by Léon Rosenfeld, a Belgian physicist with a philosophical bent. When it was<br />
over, Einstein rose from the audience to ask a question. “Suppose two particles are set in motion towards each other with the same, very large,<br />
momentum, and they interact with each other for a very short time when they pass at known positions,” he posited. When the particles have<br />
bounced far apart, an observer measures the momentum of one of them. “Then, from the conditions of the experiment, he will obviously be able to<br />
deduce the momentum of the other particle,” Einstein said. “If, however, he chooses to measure the position of the first particle, he will be able to<br />
tell where the other particle is.”<br />
Because the two particles were far apart, Einstein was able to assert, or at least to assume, that “all physical interaction has ceased between<br />
them.” So his challenge to the Copenhagen interpreters of quantum mechanics, posed as a question to Rosenfeld, was simple: “How can the final<br />
state of the second particle be influenced by a measurement performed on the first?” 1<br />
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”<br />
that exists “independent of our observations.” 2 This belief was one aspect of his discomfort with Heisenberg’s uncertainty principle and other tenets<br />
of quantum mechanics that assert that observations determine realities. With his question to Rosenfeld, Einstein was deploying another concept:<br />
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<br />
other, and no signal or force or influence can move between them faster than the speed of light.<br />
Observing or poking one particle, Einstein posited, could not instantaneously jostle or jangle another one far away. The only way an action on<br />
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<br />
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<br />
time it would take the change in the gravitational field to ripple to the earth at the speed of light.<br />
As Einstein said, “There is one supposition we should, in my opinion, absolutely hold fast: the real factual situation of the system S 2 is<br />
independent of what is done with the system S 1, which is spatially separated from the former.” 3 It was so intuitive that it seemed obvious. But as<br />
Einstein noted, it was a “supposition.” It had never been proven.<br />
To Einstein, realism and localism were related underpinnings of physics. As he declared to his friend Max Born, coining a memorable phrase,<br />
“Physics should represent a reality in time and space, free from spooky action at a distance.” 4<br />
Once he had settled in Princeton, Einstein began to refine this thought experiment. His sidekick, Walther Mayer, less loyal to Einstein than<br />
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-yearold<br />
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.<br />
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<br />
Einstein would write after moving to America. “Can the Quantum-Mechanical Description of Physical Reality Be Regarded as Complete?” they<br />
asked in their title.<br />
Rosen did a lot of the math, and Podolsky wrote the published English version. Even though they had discussed the content at length, Einstein<br />
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<br />
originally wanted,” Einstein complained to Schrödinger right after it was published. “Rather, the essential thing was, so to speak, smothered by the<br />
formalism.” 5<br />
Einstein was also annoyed at Podolsky for leaking the contents to the New York Times before it was published. The headline read: “Einstein<br />
Attacks Quantum Theory / Scientist and Two Colleagues Find It Not ‘Complete’ Even though ‘Correct.’ ” Einstein, of course, had occasionally<br />
succumbed to giving interviews about upcoming articles, but this time he declared himself dismayed by the practice. “It is my invariable practice to<br />
discuss scientific matters only in the appropriate forum,” he wrote in a statement to the Times, “and I deprecate advance publication of any<br />
announcement in regard to such matters in the secular press.” 6<br />
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<br />
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<br />
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<br />
particle’s position is real, that it exists in reality totally independent of our observations.<br />
The paper went on to expand Einstein’s thought experiment about two particles that have collided (or have flown off in opposite directions from<br />
the disintegration of an atom) and therefore have properties that are correlated. We can take measurements of the first particle, the authors<br />
asserted, and from that gain knowledge about the second particle “without in any way disturbing the second particle.” By measuring the position of<br />
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<br />
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<br />
element of reality.”<br />
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<br />
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<br />
mechanics’ description of reality is not complete. 8<br />
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<br />
momentum of the second particle. “No reasonable definition of reality could be expected to permit this,” they concluded.