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Hi Reinhard, Re: “gravitationally and electrodynamically invisible”, I think I understand what you meant, but I still think you are misunderstanding (or maybe it would be better to say misapplying) Bohm’s theory here. It really is, I remain convinced, just what I said in my comments in response to Richard: you insist on thinking of Bohm’s theory as a replacement for just the quantum part of the Frankenstein picture of the world you’re accustomed to from ordinary QM (I mean with the quantum part and the classical part gruesomely stitched together); I agree that if you think of it that way, it will seem pointless and stupid; but you should really try to appreciate that that is not the right way to think of it (not, at any rate, if you want to give it a fair hearing and try to understand what people who like it like about it).
Let me try to elaborate this just to make it as clear as I can (and so you can point out any mistakes I’m making in understanding your point of view). You wrote that, according to Bohm’s theory, “you should not take these particles as a source for the respective fields”. By which I think you mean the following: if I have some particle (or, say, a planet) here in the classical world, and I want to know how it will move, I figure out what the gravitational/electric fields are in its vicinity and then solve F=ma; to figure out what the fields are like, I add up the contributions from all of the sources; but if I treat the Bohmian particles (over on the quantum side of the shifty split) as sources, there will be cases where I get completely wrong predictions for the motion of the particle/planet; so obviously I shouldn’t treat the hypothetical Bohmian particles as sources in this way; which then means that the hypothetical Bohmian particles have no effects at all on me and the particles and planets over here on this side of the shifty split; which means I might as well simplify my quantum theory by just eliminating them. Is that a fair summary of your thinking?
The reason I think it’s wrong is that Bohm’s theory should be understood not just as a theory about “the quantum part” of some hybrid Frankenstein world, but instead as a theory about the whole world, full stop. And so if you want to know how some one particle affects some other particle or planet (or instrument pointer or your brain or whatever) you need to at least start by treating everything involved in a uniformly quantum (here, Bohmian) way. I think at this stage you just give up and dismiss the whole thing on the grounds that any such treatment (of, say, two planets colliding) in a “uniformly quantum way” will necessarily involve lots of idealizations and approximations and will therefore be, in your opinion, so “fuzzy” as to be completely meaningless. The Bohmian people see it differently, though, maybe because they are more comfortable with fuzzy things, or maybe because they are more able to appreciate that the details that are fuzzed over will not change the crucial points of principle. But in any case, to me at least, and I think to a lot of other people (but not to you, or at least not yet!), it is completely clear and obvious that a uniformly Bohmian-quantum treatment of two planets colliding, or whatever, will make perfect sense and will perfectly correspond to what we see in the real world: if there is some long-range gravitational attraction, but, say, something like a short range electrostatic repulsion, and the initial state (of the relevant particle positions and the wave function) is the sort that would give rise, for each planet in the absence of the other, to some more or less classical motion (nice wide gaussian wave packets or whatever), then what Bohm’s theory says is that the planets will speed up as they get closer together but then bounce off each other… just exactly the kind of thing we in fact observe happening. As I said before, I think it would be crazy to say that, according to the theory, the planets don’t affect each other (even if their interaction is “via” the wave function). And they are predicted to behave in just the way we observe such things to behave in such situations. So it is simply false to say either that the particles are pointless (in the sense that they don’t affect other things, other particles) or that if you do what you need to do to have the particles not be pointless, then you get wrong predictions. The wrong predictions you have in mind are simply *not* what Bohmian mechanics actually predicts. They are instead what some crazy Frankenstein theory that you have in mind, and that you mistake for Bohmian mechanics, predicts.