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The New York Review of Books Volume 54, Number 13 · August 16, 2007 

Exchange Einstein: An Exchange By Jeremy Bernstein, Reply by Lee Smolin 

In response to The Other Einstein (June 14, 2007) 

To the Editors: 

I very much enjoyed Lee Smolin's recent article on Einstein ["The Other
Einstein," NYR, June 14]. Let me however comment on something at the end
concerning Einstein and the quantum theory. Rivers of ink have flowed on
this and it still arouses great interest—even passion. Smolin writes,
"As for Einstein's dissent from quantum mechanics, there remains the
stubborn fact that a significant fraction of those who have thought the
matter through find themselves in agreement with Einstein that quantum
mechanics must be understood as an incomplete approximation to a very
different theory." I have no idea which of my colleagues would be
included in the "significant fraction" but I would need to know to what
they are agreeing. If they are agreeing that quantum mechanics may
eventually be replaced by another theory that includes it, then I would
join them. If they are agreeing to Einstein's stated reasons then I
would say not only have they not "thought the matter through," but they
have not read enough Einstein. 

Einstein in the course of his life had several different views about the
quantum theory but his final view—you can read about it most clearly in
his communications with Max Born—was that the theory does not give a
complete description of properties of objects that have what Einstein
referred to as a "real existence." The position and momentum of a
particle being an example. In the Einstein, Podolsky, and Rosen thought
experiment cited in Smolin's article it is alleged that measurements can
be performed that confer a simultaneous value to these quantities, which
is a violation of the Heisenberg uncertainty principle. Hence you cannot
both believe in quantum mechanics and that a determination of these
quantities, which Einstein would say have a "real existence," can be
made. In fact, a quantum mechanician like Bohr would say that, in the
absence of an experiment to determine them, these quantities have no
existence at all. This is what Einstein objected to. He once walked back
from the Institute for Advanced Study in Princeton with the late Abraham
Pais. The moon was out and Einstein asked Pais, "Do you really believe
the moon is not there when you are not looking at it?" 

The EPR paper came out in 1935 and for at least two decades no one paid
much attention to it. However, in 1964, the late John Bell published a
paper that changed everything. He showed that Einstein's idea that the
results of quantum mechanics could be reproduced by a theory in which
Einstein's notions of realism were included could be tested in the
laboratory. Having spent a good deal of time talking to Bell I can tell
you that his heart was with Einstein. He often referred to Bohr as an
"obscurantist." But the experiments were carried out by Alain Aspect and
others and showed that Einstein was wrong and Bohr was right. I cannot
believe that anyone familiar with this would still agree with Einstein. 

Jeremy Bernstein Aspen, Colorado 

Lee Smolin replies: 

I have been all my life a great fan of Jeremy Bernstein's writings but
he incorrectly interprets the assumptions of Bell's theorem and the
related experiments. Bell's result depended on two assumptions:
Einstein's notion of the "real existence" of the properties of objects,
plus the assumption that physical effects are local. According to this
second assumption, what is real at one place cannot be influenced by the
setting of a switch on an experiment at another place. Quantum mechanics
violates this notion of locality because a single system can split into
two systems whose properties remain entangled even when they travel
apart from each other. 

The experimental disproof of Bell's result requires that at least one of
these assumptions is wrong. Some of us are happy to hypothesize that it
is the assumption that physical effects are local that is wrong. This
assumption can be denied while holding fully to Einstein's notion of
realism. We know this because there are theories that make the same
predictions as quantum mechanics and are fully consistent with
Einstein's notion of realism, but give up the assumption of locality.
Two examples are the pilot wave theory of Louis de Broglie and David
Bohm and the theory of spontaneous wave-function collapse of Ghirardi,
Rimini, and Weber. 

Theories such as these, which give a realistic description of quantum
phenomena, are called hidden variables theories. According to such
theories, quantum mechanics is statistical because it gives an
incomplete description of reality. Alain Aspect's experiment only ruled
out hidden variables theories that are local; but there are non-local
theories of hidden variables such as the ones I mentioned by de Broglie
and Bohm, and by Ghirardi, Rimini, and Weber. 

Indeed, some recent work in quantum gravity suggests that locality—the
assumption that physical effects are local—is only an approximate
concept that emerges during the early universe from a more fundamental
level of reality without space or locality. If this is true, then there
is no barrier to the possibility that quantum mechanics will be
supplanted by a more fundamental theory which satisfies Einstein's
notion of realism.

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