ttn said:
However, I do *not* think it's the case that God whispers this in our ear. That spacetime has a certain structure is an *inference* from more concrete experimental data, including such things as the measured invariance of the speed of light (e.g., the M-M experiment) and the failure of all attempts to send signals faster-than-light.
Of course. Let's say that taking on this structure for spacetime gives us then IMMEDIATELY the reason for:
1) the constancy of the speed of light,
2) the failure to send signals faster than light
3) the transformation properties in special relativity
4) the EXPLANATION of why ALL LAWS OF NATURE seem to be able to be expressed as Lorentz invariant quantities.
...
In other words, this is ONE SINGLE principle, from which a WHOLE LOT of consequences can be derived, and which ALL have been verified experimentally.
So, if some reasoning, which also makes OTHER hypotheses, leads us to conclude, that after all, this structure of spacetime cannot be true, it takes a really convincing argument that it is THIS point, and not all the other hypotheses, that need to fail.
The point is, if we should someday encounter some empirical evidence that Bell Locality is violated (i.e., that it is impossible to formulate dynamical laws in a Lorentz invariant way) then we will simply have to accept that, despite the tons of evidence for this conclusion that was found up to this point, the conclusion turns out to have been premature and wrong -- Lorentz invariance is *not* the fundamental/final word in spacetime structure.
This would be true if Bell locality were a direct consequence of the spacetime structure of Minkowski space. But there are extra hypotheses needed to do so, and the most evident OTHER hypothesis is the denial of that other great principle: the superposition principle.
This is basically nothing but our same old disagreement about whether or not locality is proved by experiment, whether or not MWI is a counterexample to my claims that nonlocality is proved by experiment. I take the outcomes of experiments ("naively" interpreted) as the rock-bottom givens. Theory, in my opinion, must always stand or fall with experimental data. So if (as I think) some experimental data is in conflict with the idea of Bell Locality (i.e., the fundamental principles of relativity theory) then it is so much the worse for locality/relativity. We must reject these ideas as wrong -- or at any rate non-universal. On the other hand, you seem to give this "untouchable rock-bottom" status to something very high up and abstract, namely the Lorentz invariance of dynamical laws. As I understand it your view is that this is sacred and untouchable, so if the experimental data "naively" appears to conflict with it, we need to find some way to reinterpret the experimental data so as to render it consistent with the sacred principle -- and hence is born this idea that, really, when Bob thinks his needle points left, he's deluded, and what's actually happening is that it's pointing both left and right at the same time for two different Bobs (or in two different universes or branches of the wf or whatever).
Yes. For TWO reasons, not only one. We already found one great principle, which is the Minkowski structure of spacetime, as being able to explain naturally points 1,2,3 and 4 in my little list above. These points include A LOT of empirical evidence, and none EXPLICITLY against it (that is, a DIRECT derivation of a result based upon the spacetime structure being a Minkowski space, in contradiction with experiment ; say, the wrong life times of muons or so, or clocks not behaving as computed).
But we discovered also ANOTHER great principle, which is the superposition principle, which ALSO explained a lot of empirical results. It gave rise to all of quantum mechanics, in its "bare bones" applications, from atomic spectral lines, solid state stuff, ...
Applying "naively" the superposition principle to "remote Bob and his needle" would imply that indeed, Bob exists in two states. But somehow we don't want to see that, so we state that this shouldn't be so.
So now we MAKE THE ASSUMPTION that 1) the superposition principle DOES NOT APPLY TO remote Bob and his needle, and 2) the Minkowski geometry of spacetime and we arrive at a contradiction with a prediction of a theory based upon the superposition principle - and some experimental support for it: namely the violation of Bell locality.
So it seems that if, at a certain point, you DO NOT ALLOW FOR THE SUPERPOSITION PRINCIPLE anymore, and you assume the superposition principle for microscopic systems, that you run into problems with Minkowski spacetime, both theoretically and experimentally.
Conclusion: Minkowski space is dead. Is it ? Or is it in the *assumption of the non-application of the superposition principle* that the error resides ?
It sounds a bit weird that we conclude about the non-validity of two principles, namely the superposition principle, and the Minkowski structure of spacetime, which were otherwise empirically very successful, simply because at a certain point we REFUSE to apply the superposition principle, no ?
As a philosophical summary of all this, I'd say that I'm much more of an empiricist and you're much more of a rationalist. What's sacred to me is basic perceptual facts like Bob sees his needle go left; all the abstract stuff about Lorentz invariance and such is, if need be (i.e., if empirical data requires it), negotiable. What's sacred for you is the abstract principle, while all the nitty-gritty perceptual facts (Bob seeing the needle go left) are, if need be (i.e., if the principle requires it), negotiable.
That's a bit carricatural. Every great principle is negociable, ON THE CONDITION that we have a better, more encompassing principle to replace it. One that has MORE explanatory power.
And just to hint at what I think is wrong with your approach: as an empiricist (who doesn't believe we have *any* a priori knowledge, no revelations from God, etc.) I think that we only get to abstract principles by organization/interpretation of the closer-to-perception type data. So your whole approach strikes me as circular: you're willing to radically reinterpret something like Bob's perception of a hunk of aluminum in front of him, in order to "save" some abstract principle which (I would argue) we only believe in in the first place because we accepted as given such things as Bob's ability to correctly perceive bits of aluminum in front of him.
This is often the case, that, when things are better understood, we realize that our perception of things were not what we thought it was, but nevertheless at a certain point we had to go by there to arrive where we are now.
You could say the same of, say, the "atomic hypothesis", which was regarded as a very hypothetical idea in the 19th century, but of which one had to recon, one could deduce quite some observed facts. One could argue a bit like you do here: in order to make the atomic hypothesis (which includes that objects are made of tiny little things, with lots of empty space in between them), we'd need to consider that we are "deluded" in thinking that we have massive objects of continuous matter around us, while these are essentially "empty" pieces of space, with some tiny tiny matter points in them. And the funny thing is, that to even MAKE the atomic hypothesis, we have to use instruments that are EXACTLY MADE OF MASSIVE, CONTINUOUS MATTER.
Now, it might be that using the atomic hypothesis, you might have an explanation of why atomic matter *looks and feels* like massive, continuous matter, but isn't it strange that in order to "save this abstract principle of atomic matter" one has to deny the existence of the continuous matter which we used in the first place to arrive at this "atomic matter". We used "continuous" electrical wires, "continuous" pieces of metal, ...
I don't agree. Signal Locality is a summary of empirical facts. We know from experience that it isn't possible (by any mechanism studied so far) to send signals faster than light. It is not unreasonable to formulate this summary as a principle and hypothesize that it is general, i.e., to expect future theories to also respect it. But you're right that, without some kind of prior certainty about the underlying cause of signal locality (minkowski spacetime structure) we can't be *certain* of this extrapolation. And that is perhaps frustrating... but if one is an empiricist at least, that's just the way things are, the way science works.
But the great advancements of science are exactly when one realizes that a general principle encompasses a lot of empirical facts.
I don't see how you could possibly know the latter. Minkowski geometry is one possible underlying cause for our in-practice inability to send signals FTL. But something like what you call a conspiracy is also a possible cause of this. The fact is, we just don't know for sure a priori.
Yes, but when you get a lot of "conspiracies" that can find their explanation in a principle otherwise, one should really consider that principle, no ?
What the cause is of our inability to send signals FTL will have to be ultimately settled by experiment and (future) theories based on experiment. Your attitude seems to be that all the experiments prior to (say) 1950 constitute logically sufficient proof that spacetime has this minkowski geometry, and that this principle is therefore now untouchable, and that we therefore have to go to these ridiculous crazy MWI ideas in order to respect that principle.
First of all, if one takes the attitude that one can never formulate a principle because one day it might be falsified, one isn't going to make much progress!
I'm not stating that the Minkowski structure is there once and for all (hey, we already know it is not the case thanks to general relativity!) I'm saying that NOTHING has ever been found that contradicted it. The only "contradiction" that is found, is when you are using another principle and violate it at the same time: the superposition principle.
Entanglement is a consequence of the superposition principle ; and saying that Bob CANNOT be in two states is denying this. Now from this double standard, you derive that Minkowski spacetime must come into troubles. My answer is: apply RIGOROUSLY the superposition principle, and apply RIGOROUSLY the Minkowski spacetime structure, and you don't have the problems you're talking about.
I completely disagree with that initial claim to certainty, though. It was known (or should have been known) all along that an "ether" type view was logically possible and consistent with all the empirical data (if inelegant). I'm not saying people should have believed in the ether, just that they shouldn't have foreclosed on it and claimed certainty about this so early. And, I think, that chicken is now coming home to roost, because the Bell/Aspect stuff (if you just accept all the experimental data at face value) proves that minkowski geometry is not the final word in spacetime structure.
So, spacetime is not Minkowski, but behaves in almost all respects AS IF it were. The superposition principle is not valid, but things behave AS IF it were valid on microscales. We know that if spacetime IS Minkowski, and the superposition principle IS valid, that there is no problem and that all empirical data can be explained too, but this goes against our intuition. Hmmm...
Bohmian Mechanics provides a nice counterexample of this claim. It violates Bell Locality (and the easiest way to formulate it is to have a preferred reference frame, contra Minkowski) and yet respects signal locality.
Bohmian mechanics also violates a priori signal locality. It is only when we equip it (with much difficulty!) with Lorentz-invariant dynamics apart from the quantum potential that it doesn't. In other words, Bohmian mechanics has NO EXPLANATION for signal locality ; it has to be put in there by hand.
So does unitary quantum theory BTW. But the difference is that unitary QM can be made COMPLETELY Lorentz invariant. As such, Minkowski spacetime can be considered, and serves then as an explanation for it. In other words, quantum theory can be considered over Minkowski spacetime. In Bohmian mechanics, as its formulation does not allow for Minkowski spacetime to exist, and there are explicit violations of this formulation to be Lorentz invariant. But SOME expressions must be Lorentz invariant while others aren't. And if you do the mix in the right way - with no good reason as of WHY - you can get out signal locality.
So this framework has much less explanatory power than the combination of Minkowski spacetime and the superposition principle.
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) you're then denying that the measurements really have outcomes prior to Alice and Bob meeting up later! This is like my saying: there's no fact of the matter about whether some distant star has supernova'ed or not (in say the rest frame of the earth) because facts about that distant star only become "real" when the "information" about them gets to my eyes. 