einstein

“Spooky Action at a Distance”
CHAPTER TWENTY
QUANTUM ENTANGLEMENT
1935
The thought experiments that Einstein had lobbed like grenades into the temple of quantum mechanics had done little damage to the edifice. In
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
ways to show that the uncertainties inherent in the interpretations formulated by Niels Bohr, Werner Heisenberg, Max Born, and others meant that
something was missing in their explanation of “reality.”
Just before he left Europe in 1933, Einstein attended a lecture by Léon Rosenfeld, a Belgian physicist with a philosophical bent. When it was
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,
momentum, and they interact with each other for a very short time when they pass at known positions,” he posited. When the particles have
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
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
tell where the other particle is.”
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.” 2 This 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 2 is
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
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-yearold
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.” 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.