Reply To: God knows where all the particles are!


Dear Dustin, Aurelien and Travis,
I guess this workshop is coming to a close, so let me try to wrap up some.

Dustin: My “hostility” to BM is only that I am totally underwhelmed by it. Given that, I did put way too much energy into discussions like this in the last year or so (on this particular occasion on the explicit invitation by Travis), during which I have clearly not succeeded to help any Bohmian think a new thought. So I have made the resolution to not divert that much energy from my scientific work anymore, so whatever “hostility” you sensed is over with this workshop.

Concerning your points:
1) It may be that you can adopt BM without being a believing realist. But there would be no point in that really. Why adopt a theory that in every situation asks you to solve additional (but irrelevant) equations?

2) Indeed, the micro to macro transition, the emergence of a classical world, and all that are complex issues. I agree that some of the Bohmians have done reasonable work in mathematical statistical mechanics. But they go all hazy when it comes to BM, and write pamphlets with fairly low mathematical content, discussing complex situations on the _assumption_ that things work out as they would like to have it. The “clarity” of BM is in just sticking with the equations and never ever solving them. The “simplicity” comes from not getting your hands dirty. So I am not at all impressed by the status of BM as a mathematical theory, although, as a mathematical physicist I would have enjoyed these discussions much more if Bohmians had done a better job and there was at least some mathematical substance to discuss (even though the physics would still be shaky).

For your final sentence in that post: I agree that we are all prejudiced (and in this case in opposite directions). I thought this exchange was about sorting some of that out, but maybe it wasn’t.

Hiding under a quantum carpet? Indeed I don’t have a definition of Reality, which I think is better than having a “definition” that sucks. This is because I am a realist in the broad sense of aiming at a kind of empirical science that wants learn from Nature, thereby getting closer and closer to whatever reality is. Just inventing a reality by identifying it with some objects in a mathematical framework concocted for that purpose is not a helpful step in that direction. So I am criticizing BM for not being realistic in the broad sense, and instead jumping to premature procalmations of “reality”.

Combine your reference to Popper with the Einstein quote. BM actually does decide what can be measured, and there is a Bohmian Theory of measurement, which is a rehash of the formal theory of measurement by von Neumann, with the twist that the pointer is assumed to be identified by its Bohmian positions. According to that theory, what is measurable is exactly what is measurable in QM, definitely excluding all trajectory features beyond equal time position configuration. Since nothing can be learned about the trajectories, Popper would indeed tell you to scrap the stuff that can never be falsified empirically. But Popper was not just known for formalized methodology, brilliantly ridiculed by Feyerabend. He was also very much concerned with enlightenment, the “enemies of open society”, and their immunized truths. Even within just science the falsification idea still has some value as a good practice rule (not as a rigid principle). BM is pretty well immunized, and that does make it less convincing for me, although I am not a strict Popperian.

Just a brief remark on weak measurement as supporting BM trajectories (as in that recent experiment by the Steinberg group). That is complete bullshit. Of course, you can make measurements of the probability current. It is just another QM operator, and I don’t want to discuss here to what extent it is actually determined by the weak measurement, so let us assume it does. But this is a point by point statistical measurement, and only when those data are all in you can play connect-the-dots, and draw those trajectories. Nothing whatsoever in that experiment suggests that anything moves on those lines. Embarrassingly, both Detlef Dürr and Shelley Goldstein have cited these experiments as providing empirical support for BM (I just checked the reference of Detlef’s paper, and realized it is actually a joint one with Dustin). That is just dishonest.

On the final sentence of your post: Whether BM will inspire any new ideas is anybody’s guess. Past experience shows that while the QM community has produced quantum information theory and many new results on statistical mechanics of large quantum systems (to mention just the corners I know best), the BM community has utterly missed these developments and has not come up with a single idea pointing to a hitherto unexplored phenomenon.

So Frankenstein it is? This stitching together of different theories is not just very common in physics, there is no way without, if you actually want to come to conclusions that have some bearing on the real world. You may not like that, and completely opt out of the enterprise of connecting to real experiments. That is why called the cozy Bohmian world of just two equations Platonic. Often the stitching is quite loose, and I would see my job in many cases in improving it, i.e., strengthening the logical connections between different branches (preferably by a theorem). But at the micro-macro divide the distinction is quite clear, including the movability of the cut. In an image I got from Berge Englert: I your are standing at the beach with your feet in the water the exact border between sea and land may be impossible to define. Nevertheless the distinction between sea and land is perfectly clear in the bigger picture. Nothing is “gruesomely stitched together” here.

Supposedly it is a virtue of BM to treat micro and macro on the same basis. I don’t think that is quite true when it comes to the theory of measurement, because all of that depends on assumptions of classicality (also made on the quantum/stat mech treatment of the theory) plus some extra ones on disjoint wave function supports for macroscopically distinct pointer positions, which are strictly speaking false (but Bohmians seem happy anytime to replace error estimates by hand waving, so no problem). Still the supposed virtue is that everything in the universe is treated by the same set of equations. But at the same time it is a problem, because you lose touch with reality.

I get your point about the electromagnetic invisibility: If there are no charged particles around, only Bohmian ones, you don’t have to worry about fields anyway. The physics of electrodynamics is supposedly all in the traced out quantum electrodynamical field. But actually this is just what I meant: Bohmian particles are not so important agents in the game (indicated partly by them not even producing a field) so they can easily be eliminated. Let me just dwell on this a bit and on the question of Bohmian furniture.

The standard picture of BM is that the positions of massive particles are real, spin isn’t and the quantum electrodynamical field isn’t either, since photons are not included with their trajectories. So the subalgebra of Real Things that you select is NOT maximally abelian, and you are happy to trace out some bits (like spin,…). So in order to see how the theory works, let us just play with that parameter a bit. Now I happen to be of the opinion (for the sake of this discussion) that Leptons are of secondary importance, since it is only their way of making Baryons move about which justifies even talking of them. Clearly, in good lowest order Born-Oppenheimer spirit, what defines the position of a pointer is its distribution of nuclei, which the electrons just help keeping together. So I think Leptons should be traced out along with the photons and the spins, i.e., should be De-Bohmified and stripped of their unearned Reality status. I think it is clear that all the things you usually say remains true in this simplified BM, in which only Baryons have trajectories and are Real. The joint distribution of Baryons will still be in BM equilibrium, but that distribution is now computed from the wave function (like formerly for spin), by tracing out the non-Real degrees of freedom. Good theory then, philosophically equivalent to BM.

Now let us carry this process a bit further. I started after my last post with the furniture of my house, which is now no longer real. It still looks the same, because seeing involves photons, and they were not real in the first place (only their wave functions needed to tickle some molecules in my Retina). But even to touch it is still the same, because my finger’s Baryons still have trajectories (i.e., are Real). They move much the same way, only now it is the average position of the table’s individual particles rather than the individual positions entering the equations of motion. Not much can go wrong here, because the wave functions are anyhow the same, and at the macroscopic level, according to the Bohmian handwaving principle, they will behave as expected.

You can see where this goes. I would next De-Bohmify Outer Space, then the rest of the world, including myself, but not you, Travis, because it would be impolite to take Reality away from a Bohmian. Imagine yourself to live in such a world (as you often asked me for BM). How would you like that? I think it should make you equally happy. Much happier than a solipsist, actually, who thinks he is alone in the world but isn’t, whereas your case would be the other way round.

All the best, Reinhard

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