Friday, 23 October 2015

Was Einstein Wrong?

Some headlines from the past day or so:
This sort of thing is nothing new, of course.  Many  discussions of Einstein's attitude towards quantum mechanics suggest that Einstein simply refused to accept a straightforward consequence of quantum mechanics: non-locality.

Now, if you have no reason to believe something, and you don’t believe it, but it happens to be true, then, in one sense, you’re wrong in your belief But you can’t be accused of being unreasonable or irrational.  So, in an uninteresting sense, Einstein was wrong, in not having beliefs that scientists would later provide good evidence for.  But the same could be said of everybody, ever.

Einstein didn’t believe that nonlocality was a feature of the world. But should he have? Was he wrong not to believe in nonlocality?  What reason did anyone have, in 1935, or in the late ‘40s, when he wrote his fullest discussions of his attitude towards quantum mechanics, for thinking that nonlocality was a feature of physical reality?

We can’t fault Einstein for not having read Bell’s 1964 paper, or for not having read the December 20, 1982 issue of Physical Review Letters—after all, he was no longer alive by the time Doc Brown and Marty McFly arrived in 1955, and, besides, as far as we know, they didn’t bring literature of that sort with them. We can't fault him, either, for not having read this week's issue of Nature.  Hindsight is 20-20, and we now know two things that, as far as I know, nobody really knew during Einstein’s lifetime.
  1. Some quantum correlations are not locally explicable.
  2. Quantum correlations persist when the systems are separated by a large distance.
The first, of course, is Bell's theorem, and the second we know through a series of increasingly impressive experimental demonstrations of violations of Bell inequalities.  Re 1:  As Bell himself emphasized, the mere fact that outcomes of spatially separated experiments are correlated is no indication of any sort of nonlocality.  As you're reading this, someone else on the other side of the world might be reading the same words.  No spooky action-at-a-distance is needed to explain this; nor is there any need for direct influence between the two of you, as there's a common source for what's displayed on your computer screen, and what is being displayed on the other side of the world.
As I’ve emphasized in earlier blog posts (here and here),  Einstein was not nearly as dogmatic about locality as he is sometimes painted to be.  He did not regard the assumptions of locality and separability as non-negotiable; his attitude was that one could, indeed, drop them as requirements on a physical theory, given good reason.  But he saw no reason to do so.  He wrote, in 1948,

if I consider the physical phenomena with which I am acquainted, and especially those which are so successfully comprehended by means of quantum-mechanics, then, nevertheless, I nowhere find a fact which makes it appear to me probable that one has to give up requirement II [of locality and separability]. For that reason I am inclined to believe that the description afforded by quantum-mechanics is to be viewed … as an incomplete and indirect description of reality, that will again be replaced later by a complete and direct description (Einstein 1948; translation in Howard 1985).

Systems that are interacting or have interacted, and whose quantum-mechanical state is entangled, have probabilistically correlated behaviour.  That by itself isn’t enough to answer Einstein’s challenge to point to a phenomenon that would count as evidence for a breakdown of locality. Correlations are already familiar in classical probability theory: for example, if two classical  systems interact, a probability distribution that represents thermal equilibrium will involve correlations between the two.  An answer to Einstein’s challenge would have to give us some reason to think that quantum correlations aren’t like that.

My question for historians: did anyone even attempt to provide an argument of that kind, during Einstein’s lifetime?


Aspect, A., J. Dalibard, and G. Roger (1982). Experimental test of Bell’s inequalities using time-varying analyzers. Physical Review Letters 49, 1804-1807.

Bell, John S.,  (1964). On the Einstein-Podolsky-Rosen Paradox. Physics 1, 195-200.

Einstein, Albert (1948). Quanten-mechanik und wirklichkeit. Dialectica 2, 320–324.

Hensen, B. et al. (2015).  Loophole-free Bell inequality violation using electron spinsseparated by 1.3 kilometres. Nature, online 21 October 2015.

Howard, Don (1985). “Einstein on Locality and Separability.” Studies in History and Philosophy of Science 16, 171-201.

No comments:

Post a Comment