Dear Travis,
The standard simple example is found in Bell’s SPEAKABLE … on p. 111 (I think the same in both editions) and given in a figure on the bottom of that page. Bell seems to ascribe it to Wheeler, and maybe that is where it originated, but Bell’s version is very clearly worded. The basic issue is that two wave packets cross in empty space and the Bohmian particle ‘hops’ from one to the other, or you can say it “bounces”. You will see an example of this phenomenon in Lev Vaidman’s contribution to this workshop (In “Bohm’s Theory). Bell acknowledges that this behavior is counterintuitive: “It is vital here to put away the classical prejudice that a particle moves on a straight path in ‘field-free’ space …”
What Englert et al pointed out was that if you insert a ‘which way’ detector ahead of the location at which the “bounce” takes place there can be occasions when the detector is triggered despite the fact that the Bohmian particle takes the other path and never comes close to the detector, and this was their basis for declaring the Bohmian trajectory to be ‘surrealistic’. As I noted in my earlier post, the Bohmian defence was that SQM doesn’t tell one where the particle happens to be except if it is measured, and therefore their story was as good as any. My reply was (and is) that consistent histories (CH), which is a fully consistent extension of SQM minus various obscurities, can very well say something about where the particle is when it is not being measured, and in the situation under consideration it says there is no bounce where Bohmian mechanics calls for one. And I remark that the experimental program at CERN, or at least the interpretation of the experiments, would be put in serious doubt if physicists did not have the “classical prejudice” (justified by CH) that contradicts BM.
I agree that what we have access to at the macroscopic level is “pointer positions” and “preparation procedures”. But quantum mechanics was developed in order to have a physical description of the microscopic world, and when we say that it has been confirmed by experiments we mean that it provides a microscopic story that allows us to understand experiments which probe (or so most physicists believe) the microscopic world, even though their inputs and outputs are necessarily understood in macroscopic terms. I add that CH takes full account of experimental apparatus to the extent that it can be modeled at all in quantum terms; two chapters, 17 and 18, of my book along with a large number of toy models are devoted to that task.
So I think your criticisms miss the mark. On the other hand I don’t claim to be infallible, and if you, or Aurélien, can find some mistake in my analysis of the situation discussed above–see [1]–or of my general proof of quantum locality–see [2]–you will certainly be doing me and the community a favor.
Bob Griffiths
[1] R. B. Griffiths, “Bohmian mechanics and consistent histories”, Phys. Lett. A 261 (1999) 227. arXiv:quant-ph/9902059.
[2] R. B. Griffiths, “Quantum Locality,” Found. Phys. 41 (2011) 705;
arXiv:0908.2914.
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