The impressive experiments described in a preprint by Ronald Hanson at Delft and colleagues have been widely reported (for example, here and here) as if to imply that they confirm quantum “spooky action at a distance” (in other words, entanglement). With all due respect to my excellent colleagues (who of course don’t write their own headlines), this is not true.
Einstein’s phrase is of course too nice to resist. But there’s a clue here. Einstein? You, know, the guy who wasn’t convinced by the Copenhagen Interpretation of quantum mechanics that reality is just what we can measure, and that nothing deeper lies “beneath”? Einstein, who suspected that there might be “hidden variables” that restore local properties to the quantum world?
Einstein’s “spooky action at a distance” was predicated on that view. It was action at a distance if, via this thing we call (that is, which Schrödinger called) entanglement, an influence at one location (via a measurement) is transmitted instantaneously to another. Only in some kind of local or hidden-variables view do you need to invoke that picture.
Quantum nonlocality – which is what is supported by a violation of Bell’s inequality, and what the new experiments now confirm by closing another of the loopholes that could have permitted a violation in other circumstances – is not spooky action at a distance, but the alternative to it. It says that we can’t always characterise the properties of a particle in ways local to that particle: its state is a smeared-out thing (to put it crudely) that may be correlated with the state of another distant particle. And so it appears to be. In this view, there is no action at a distance when we make a measurement on one particle – rather, there are nonlocal quantum correlations with the state of another. It is hard to find words for this. But they are not “spooky action at a distance.”
I don’t expect these words to make a blind bit of difference, but here they are anyway.
The word you are looking for is causality. There is non-locality indeed, but no causal connection.
ReplyDeleteHi George,
ReplyDeleteThanks for your comments. I know you’ve thought about this carefully, so I certainly want to think about your remarks carefully too. But I can say a few things right away.
First, I’ve given you the wrong impression about what I’m saying about Einstein. When I say “Einstein’s spooky action at a distance”, I mean “the thing that Einstein spoke about”, not “the thing that Einstein believed in”. Of course he didn’t believe in it. That was the whole point: the fact that he regarded the EPR experiment as requiring it, in the Copenhagen view, was why he thought the Copenhagen view was wrong.
Second, I have somewhat too casually conflated hidden-variables theories and local realism. That’s because Einstein’s HV view was, I believe it is fair to say, a local realist one. Today folks are talking about nonlocal HVs, but I don’t think Einstein ever contemplated such a thing. And so my point is that Einstein was implicitly assuming local realism all along – which is why he figured that the Copenhagen view could only preserve the correlations by some kind of action from one particle to the other.
I don’t see why action at a distance is forced on Copenhagen (with or without collapse) – and neither did Bohr, of course. As I say, the standard Copenhagen view today (to the extent that one can speak about such a thing at all) invokes nonlocality – so you don’t need anything to “get” from one particle to the other. David Mermin said it very clearly: the mystery of the EPR experiment is that “it presents us with a set of correlations for which there is simply no explanation”.
You’re clearly unhappy with “no explanation”, which is why you’re not ruling out some sort of “hidden causation”. I guess this shows you’re a local realist at heart. But why shouldn’t the world be nonlocal at this level? After all, there are folks looking for simple physical axioms out of which quantum nonlocality will fall quite naturally. It seems entirely possible to me that they’ll find them. And don’t the Bell tests anyway make it impossible to sustain both locality and realism?
Yes, we must talk about this some day George. It's certainly a puzzle how to talk about these correlations, but personally I don't see all versions of Copenhagen as mysticism opposed to materialism. One could of course argue, as Bohr did, that the purpose of physics/science is to provide predictions of what we'll see, and quantum mechanics does that. And even if there's action at a distance (though I see no need to invoke it), it's not clear how it can possibly be causal - I don't even know what superluminal causality could mean...
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