What Is an Element of Reality?

[vanesch]

But you miss an incredibly important point by putting it this way. In BM, that is what is consciously experienced *because* that is (part of) the actual physical state of the objects being observed.

Ok, let us put it differently: "part of" is the intuitive notion of an unfaithful mathematical representation. By that, I mean: if (A,B,C) is the state, then "part of" the state is a function of (A,B,C), f(A,B,C). If it is unfaithful, it simply means that it is not 1-1, so that it is not because I know f(A,B,C) that I know (A,B,C).
Now, in BM, we have the couple {pos,wf}. And I can define "part of" as f(pos,wf) = pos.

In a similar way, in (my version of) MWI, I have the state {wf, mytoken}. Now of course my "part of" is not as simple, but I can unambiguously define f(wf, mytoken) = mybranch, namely the term in wf that contains mytoken.

And this is what's crucial, what you try to hide below: "part" here means literally *part*. The particle positions are *real*. Yes, there's also the wave function and when we wake up in the morning and open our eyes, we see only particles, not wf.

I see what you mean, but it is very easily mathematically turned around. After all, my state {wf, mytoken} can just as well be transformed in a 1-1 relation into {wf, mybranch}. And then "mybranch" is literally "part of" my state. A state is always defined up to a 1-1 function of course ; after all, our mathematical representation is only that: a representation.

The particle positions are real, as real as the wf, and it's those particles (not a branch of the wf) that we observe. End of story.

Well, as I showed above, I can very easily find a mathematical transformation of my state that obeys exactly the same thing. You mean that it is allowed to take "the first element of a pair" as real, but not the "term that is indicated by the other part". Well, I then transform my state as not just {wf, token} but into {termwithtoken,wf} and we're in exactly the same situation. The first element of the pair is what is "real".

What about fields and other such things? What about other people's consciousnesses?

I think you should re-read my previous post. I only need one token for myself, but I can introduce as many tokens as I wish. I can even introduce a token for each individual system (say, each individual particle) and call it the "particle position", exactly as in BM. It indicates the branch the particle would observe consciously if ever it were conscious. If it isn't conscious, then the token of the particle doesn't play any role: it doesn't enter in any dynamics.

There are all sorts of things which aren't directly observable in classical physics, yet this doesn't raise any problems. That I can't experience your consciousness directly doesn't mean my perception of this coffee cup is invalid.

Yes, but now we're not dealing with classical physics.

OK, that's clarifying. I'm still worried that in order to have a real dynamics for all of this, you need some very clean rule for when the wf branches. Clearly this happens somewhere between your first and last expressions for psi. But when *exactly*? Wouldn't the moon crashing into the sun be such a massive macroscopic change that, after 8 minutes or whatever, your brain would get tangled in the superposition, whether you had looked through a telescope yet or not?

Of course. Every little interaction that "carries the message" is sufficient for my brain state to split, of course. In fact, it is sufficient that the event is in the past light cone of my brain and for sure SOMETHING (a cosmic proton, whatever) will probably interact and make my brain state split.
When exactly ?
Doesn't really matter: during the unitary transformation, you would have 3 terms:
a |brain1> |stuff> + b |brain1A> |stufff> + c |brain1B> |stufff>
You can already apply the Born rule here: if I remain in brain1, I haven't yet noticed anything, if I am put to brain1A I already have state brain1A. If after that, there is still interference, and brain1A splits partly back into |brain1> and |brain1A> then you simply apply the Born rule again...

Yes I see how you want this to work. A different question (that I think you answered before, but just to be clear): is there just *my* consciousness token, or does everybody have one? If everybody has one, there's near 100% probability that the other people's tokens are no longer in the same branch as mine, right? So all the people I see around me and talk to and (say) argue about physics with on the internet, are actually mindless hulks? (Not that I think that applies here... )

Yes, you got it. It is indeed the fundamental difference with BM (and in fact the only way to save relativity). But "mindless hulks" are behaviourally not distinguishable from conscious bodies. *this* is the "solipsist" part.
You could think up of many solutions to this apparent riddle (which, I repeat, is not observable). For instance, you could "start" new consciousnesses. Or you could say (I'm in fact philosophically really favorable for that one) that there is in fact only one true consciousness, which is my own. Or you can give a consciousness to every particle in the universe.
But all this doesn't make any observational difference, so you fill in whatever suits you. I only need one consciousness in the whole business to explain my observations and that's good enough.

I still don't understand clearly in what sense the dynamics is local in 4-d spacetime.

Well, the unitarily evolving wf has local dynamics (can be made a geometrical object). The token "worldline" is a mapping from an "eigentime" tau into a pair of objects: one is a point in 4d (an event) and the other is a state in the Hilbert space of "mybrain". As long as no split occurs, the unitary evolution of "mybrain" determines the following point in 4d and so on ; decoherence makes that this unitary evolution remains "lumped in space", so this 4d world line segment makes sense.
When an interaction occurs with another system, then (because of the locality of the Hamiltonian), this can only happen at the same event in 4d. This interaction is then responsible for the split (entanglement) of the state in the brain Hilbert space with the state of this other system we're interacting with, but which must be localized in 4-d at the same event. As a result of this interaction we apply the Born rule (which can of course be completely determined at the same 4-d event), to jump to another brain state (one of the two entangled states, say), also associated with the same 4-d event. So all of this happens on a fibre associated with one single spacetime event, where the local interaction of my brain with another system took place.
In this, I tacitly assumed that decoherence will make us work in the position basis in the relevant Hilbert spaces, but that's probably the case.
As I said, I never worked this out in rigorous detail, but this intuitive sketch should be sufficient to at least make it plausible that this dynamics is local in spacetime.

BTW, where do consciousness tokens go when people die? (That's not a joke, I really want to know how that's supposed to work in this theory.)

Be my guest. Some even say that this is the secret of eternal life. Look after "quantum suicide" in the quant-phys archive, there's a very funny article about it. In that there is a modification to the Born rule in that your consciousness will always pick only one of the "live" states. Or just terminate the token worldline at the event where the "bad" choice is made into a dead brain state :-)

Admit that such a view is much more spicy than this boring particle position stuff in BM

No, seriously, to me, the point is that we CAN save relativity.

 

[ttn]

Ok, let us put it differently: ...

I had no doubts that you could suitably redefine "part" to make Bohm and MWI parallel. And given the way you did that, you're absolutely right. But I still stand by my earlier claim, that there is a real and prima facie difference between the two on this point. So let's just leave it at that, yes?

Yes, but now we're not dealing with classical physics.

I realize that. My point was just that there is no essential difference b/w Bohm and Classical physics in regard to consciousness.

Of course. Every little interaction that "carries the message" is sufficient for my brain state to split, of course. In fact, it is sufficient that the event is in the past light cone of my brain and for sure SOMETHING (a cosmic proton, whatever) will probably interact and make my brain state split.
When exactly ?
Doesn't really matter: during the unitary transformation, you would have 3 terms:
a |brain1> |stuff> + b |brain1A> |stufff> + c |brain1B> |stufff>
You can already apply the Born rule here: if I remain in brain1, I haven't yet noticed anything, if I am put to brain1A I already have state brain1A. If after that, there is still interference, and brain1A splits partly back into |brain1> and |brain1A> then you simply apply the Born rule again...

It's the "doesn't really matter" that bothers me. It's true, FAPP, it doesn't really matter. But in terms of being able to write down a clean theory. As you know, Bell criticized OQM as being "unprofessionally vague and ambiguous." This wasn't because it violated Bell Locality (though it does) and it wasn't because Bohr was a dumbass (though he was) -- it was because the dynamics of OQM hinges on the meaning of a certain human concept ("measurement") which has no precise meaning *in the theory*. So it's vague. Yes, yes, it makes no difference FAPP. But it's still ugly. And it seems like your version of MWI shares that bit of ugliness... which maybe isn't too surprising, given that it comes in in precisely the same way: when do you apply the Born rule? Well, when {vaguely waving arms}... and it doesn't make any difference in practice when *exactly* you do it.

Yes, you got it. It is indeed the fundamental difference with BM (and in fact the only way to save relativity). But "mindless hulks" are behaviourally not distinguishable from conscious bodies. *this* is the "solipsist" part.

Yes, I see. By the way, both for you and anyone else still reading this zombie of a thread, David Albert has some really nice articles on MWI. The classic one is I believe called "Interpreting the Many Worlds Interpretation", where he (and maybe a co-author??) introduce all the stuff about consciousness, talk about the "many minds" version of the theory, worry about mindless hulks and such. It's a good read for anyone that takes MWI seriously. I think there is also some good material along these lines in his book, "Quantum Mechanics and Experience", which I would also highly recommend. But I can't remember for sure what's in there since I haven't read it in a while. I also find it interesting that, for Prof. Albert, all of this stuff is essentially a reductio of MWI. If you have to go to these lengths, worrying about mindless hulks and all that silliness, you've clearly gone off the track somewhere. I agree with that sentiment completely, though I recognize the reasons that others (like vanesch here) don't.

You could think up of many solutions to this apparent riddle (which, I repeat, is not observable). For instance, you could "start" new consciousnesses. Or you could say (I'm in fact philosophically really favorable for that one) that there is in fact only one true consciousness, which is my own. Or you can give a consciousness to every particle in the universe.
But all this doesn't make any observational difference, so you fill in whatever suits you. I only need one consciousness in the whole business to explain my observations and that's good enough.

Good enough for you anyway.

Well, the unitarily evolving wf has local dynamics (can be made a geometrical object). The token "worldline" is a mapping from an "eigentime" tau into a pair of objects: one is a point in 4d (an event) and the other is a state in the Hilbert space of "mybrain".

That's the piece I'm missing. What is this "event" in 4-D? A moment of awareness of the consciousness? But where does that happen? In the brain? But where in the brain? etc... It's just very funny for consciousness (and nothing else!) to be having a worldline through spacetime. Or maybe I'm totally misunderstanding something.

Be my guest. Some even say that this is the secret of eternal life. Look after "quantum suicide" in the quant-phys archive, there's a very funny article about it. In that there is a modification to the Born rule in that your consciousness will always pick only one of the "live" states. Or just terminate the token worldline at the event where the "bad" choice is made into a dead brain state :-)

I'll check it out. But not until I've drained the beer molecules out of my next glass-shaped configuration of silicon particles.

Admit that such a view is much more spicy than this boring particle position stuff in BM

It is, it's great fun. A nice way to spend a semi-intoxicated weekend, before getting back to serious clear-headed Bohmian physics Monday morning.

No, seriously, to me, the point is that we CAN save relativity.

I know. For what it's worth, I think we can save it too -- in the same way we can save the laws of thermo or "p=mv" when we discover microphysics or relativity -- as a limiting case or approximation that applies just fine in some domain. Believing that relativity isn't the last word in spacetime structure isn't the same as tossing it and everything it's influenced in the garbage-can-shaped configuration of...

 

[vanesch]

Believing that relativity isn't the last word in spacetime structure isn't the same as tossing it and everything it's influenced in the garbage-can-shaped configuration of...

But I'm with you, here, in keeping open the possiblitiy that relativity is not the last word. Not for the same reasons, though. I think that "stuff will happen" on the GR / QM unification front and this might change the entire vision on it. At that moment, we can then tell another story. But for the moment all this is wishful thinking and relativity is still strictly part of our formal toolset and it is THAT formal toolset that has to be made sense out of. As long as we don't have another principle that replaces it (if ever), I think it is not a good idea to toss it. My point is that you cannot "toss it a bit". Relativity stands or falls, completely. ALL objects are 4-dim geometrical objects, or the 4-dim geometry doesn't make sense.
See, my main complaint I have about BM is that it is a theory that is not relativistic at all, but "plays one on TV". All the elements in the theory absolutely don't have to be 4-dim geometrical objects (because that 4-dim geometry clearly has no meaning, given that *part* of the theory is not geometrical), but for some unexplained reason, everything that pertains to the wavefunction has to look like it is a 4-dim geometrical object. It is as if you'd say that most of the laws of physics have to obey a form which is translation invariant, but the dielectric constant of the vacuum is depending on the distance from the left big too of my grandmother ; nevertheless it is somehow in principle forbidden to find out, from measurements, where exactly that big too is, so "translation invariance" gives somehow the impression to be respected. Clearly, as is, translation invariance is NOT a principle of nature. Ok, the big too of my grandma has maybe some profound influence on nature. But then, why did the laws of nature have to mimick such a non-existing translation invariance ??
The answer "well, it will probably turn out later that translation invariance is some approximate symmetry of nature induced by another principle" doesn't satisfy me as long as we don't have the slightest hint as what that other principle is ; in any case that other principle will then totally alter the entire formalism and hence the entire interpretation, so this renders useless the current exercise anyway.

 

[ttn]

See, my main complaint I have about BM is that it is a theory that is not relativistic at all, but "plays one on TV". All the elements in the theory absolutely don't have to be 4-dim geometrical objects (because that 4-dim geometry clearly has no meaning, given that *part* of the theory is not geometrical), but for some unexplained reason, everything that pertains to the wavefunction has to look like it is a 4-dim geometrical object. It is as if you'd say that most of the laws of physics have to obey a form which is translation invariant, but the dielectric constant of the vacuum is depending on the distance from the left big too of my grandmother ; nevertheless it is somehow in principle forbidden to find out, from measurements, where exactly that big too is, so "translation invariance" gives somehow the impression to be respected. Clearly, as is, translation invariance is NOT a principle of nature. Ok, the big too of my grandma has maybe some profound influence on nature. But then, why did the laws of nature have to mimick such a non-existing translation invariance ??
The answer "well, it will probably turn out later that translation invariance is some approximate symmetry of nature induced by another principle" doesn't satisfy me as long as we don't have the slightest hint as what that other principle is ; in any case that other principle will then totally alter the entire formalism and hence the entire interpretation, so this renders useless the current exercise anyway.

I understand your point of view, but, not surprisingly, I don't agree with it.

In BM we do have a fair sense of at least one candidate for a new principle to replace Lorentz invariance: quantum equilibrium. Valentini had some very cool papers about this in the 90's. If you assume BM but *don't* assume that the initial particle positions are |psi|^2 distributed, you can do all sorts of crazy things: you can beat the Heisenberg uncertainty principle, you can signal faster than light, etc. In other words, a Bohmian universe in quantum equilibrium will forbid faster-than-light signalling *because of* quantum equilibrium -- it is quantum equilibrium that gives rise to the apparent Lorentz symmetry (and some other things like HUP). This is just a brief sketch, but it's the kind of point that can be elaborated more if you think following a Bohmian path is somehow "sterile" theoretically because you are left with no guiding principles (having junked fundamental relativity).

 

[vanesch]

I understand your point of view, but, not surprisingly, I don't agree with it.

In BM we do have a fair sense of at least one candidate for a new principle to replace Lorentz invariance: quantum equilibrium.

Ok, I don't know much about this (read some stuff when we had a previous discussion, but only sketchy). However, I think you don't see exactly what my criticism is. The point is not that you can add something (like quantum equilibrium) to restore Lorentz invariance. It is, by itself, not sufficient. The interactions, described by the unitary quantum evolution (the lagrangian, or the hamiltonian) need also to be Lorentz in variant, otherwise quantum equilibrium will not save it.
What I mean, for instance, is that in the Lagrangian of a quantum field theory, the lagrangian density needs to be a Lorentz scalar (a geometrical object over 4-dim spacetime). This is an enormously powerful principle, which, from gazillions of possible theories, only leaves out very few, and those that happen to work in the lab are exactly picked out of those few. It is this underlying 4-dim geometry, which is first destroyed in BM, and then restored with something like quantum equilibrium. But because of the destruction, it means that this 4-dim geometry is meaningless, so there's no reason, in the first place, to have this Lorentz-invariant Lagrangian density. And if you do not have that (you do not have a lorentz-invariant unitary evolution), then quantum equilibrium will not "restore" it for you. It can only restore it because it was already built into the unitary evolution. And THIS is incomprehensible to me: why would nature obey this 4-dim geometry, if the geometry is in fact meaningless ; in such a way that all *observable* stuff, again, seems to obey that 4-dim geometry. This seems to me to indicate that the intermediate step, which makes the 4-dim geometry meaningless, is misguided. And this intermediate step is exactly the non-geometric evolution equation for the particle positions.
Before you reiterate your (justified) critic of OQM, I agree that it suffers from exactly the same problem of course if you introduce an "ontological collapse" (and it suffers from a SECOND problem, which is unrelated to it, and which is the vagueness in what mechanism is responsible for the collapse - in a certain way, it is with this vagueness that I'm struggeling too ; everybody has his own problems :-).

It might be, of course, that a totally different underlying principle can tell us why nature DOES seem to obey this 4-dim geometry while it isn't there. It is something like that that I'm secretly hoping for, but it's not in sight, is it. And that's why I find it "dangerous" to toss out this 4-dim geometry before knowing what will force us to introduce it again.

 

[ttn]

Ok, I don't know much about this (read some stuff when we had a previous discussion, but only sketchy). However, I think you don't see exactly what my criticism is. The point is not that you can add something (like quantum equilibrium) to restore Lorentz invariance. It is, by itself, not sufficient. The interactions, described by the unitary quantum evolution (the lagrangian, or the hamiltonian) need also to be Lorentz in variant, otherwise quantum equilibrium will not save it.

Oh, I think you just misunderstood my point (perhaps because I misunderstood yours earlier?). I wasn't claiming that the concept of quantum equilibrium (in Bohm's theory) saves Lorentz invariance. It doesn't. If Bohm is right, then Lorentz invariance *as a fundamental principle* is dead, at least so far as I can tell. So my point wasn't to save it. I thought part of what you were saying before was that, without that basic theory-constraining principle, we'll be "lost at sea". Well, my point was that something like quantum equilibrium could provide a deeper, replacement principle that could perhaps guide future theory construction, guide people in making bold but not crazy predictions for worthwhile things to study empirically (eg, looking for plausible effects of quantum non-equilibrium), etc.

I wouldn't want to push this point too hard. Who knows if "quantum equilibrium" is the new "Lorentz invariance" for the 21st century. My point was more just the principle of the thing: just because you accept Bohm (and hence deny Lorentz invariance as a fundamental principle) doesn't mean you won't have any good ideas for how to move knowledge forward in the immediate future.

And THIS is incomprehensible to me: why would nature obey this 4-dim geometry, if the geometry is in fact meaningless ; in such a way that all *observable* stuff, again, seems to obey that 4-dim geometry.

Well the nice thing about the concept of quantum equilibrium is that it answers precisely this question. Hence, while on the one hand Bohm requires you to reject Lorentz invariance as a fundamental principle, it also provides a framework for actually making sense of the curious fact that Lorentz invariance seemed to serve so well as a guiding principle.

It might be, of course, that a totally different underlying principle can tell us why nature DOES seem to obey this 4-dim geometry while it isn't there. It is something like that that I'm secretly hoping for, but it's not in sight, is it. And that's why I find it "dangerous" to toss out this 4-dim geometry before knowing what will force us to introduce it again.

Your position is not unreasonable here, but it's also reminiscent of the view taken by, e.g., the "energeticists" in the late 19th century. They liked to remind each other that the continuous differential equations of classical thermodynamics had served us so well, that there seemed to be no good reason to reject that fundamental principle of continuity for some crazy idea like the atomic theory of matter. I'm sure they all thought that physics would be lost at sea without this fundamental principle of "smoothness" (as opposed to granularity, atom-ness) that had had so much success (all the differential equations of classical physics, etc). It's a valid point, but hardly strong evidence against the existence of atoms. In fact, I'd even admit that people were right to be somewhat suspicious until at least the atomists were able to show that their theories reduced to the continuous, macro, classical view in a certain limit.

All of this is rather parallel to the current debates over the interp of QM. The only difference is: now we know for sure there are atoms, but we don't yet know for sure that Bohm is right about QM.

 

[vanesch]

Hence, while on the one hand Bohm requires you to reject Lorentz invariance as a fundamental principle, it also provides a framework for actually making sense of the curious fact that Lorentz invariance seemed to serve so well as a guiding principle.

This is what I fail to see. If lorentz invariance is "dead", what stops me from writing terms in the Lagrangian which are not lorentz invariant, in which case I will see deviations from Lorentz invariance in the lab, no matter whether there is quantum equilibrium or not ? So how can quantum equilibrium "make sense" of the fact that Lorentz invariance IS actually observed (ie, that we need to write Lorentz invariant terms in the Lagrangian ?)
It seems that in doing so, we've LOST a lot of understanding, no ? You will argue that we've lost an illusion and are now ready to move on, but nothing *replaced* our previous illusion of an understanding.

Your position is not unreasonable here, but it's also reminiscent of the view taken by, e.g., the "energeticists" in the late 19th century. They liked to remind each other that the continuous differential equations of classical thermodynamics had served us so well, that there seemed to be no good reason to reject that fundamental principle of continuity for some crazy idea like the atomic theory of matter. I'm sure they all thought that physics would be lost at sea without this fundamental principle of "smoothness" (as opposed to granularity, atom-ness) that had had so much success (all the differential equations of classical physics, etc).

The analogy doesn't work completely, and the difference is my point: The proponents of an atomic theory REPLACED the continuity principle by another one FROM WHICH THEY COULD DERIVE the apparent continuity on macroscopic scales. I wanted to indicate that quantum equilibrium in BM DOES NOT allow you to derive apparent lorentz invariance if you do not put it in by hand in the unitary dynamics.

I can buy your argument in the following sense: the status of BM is that somehow they're convinced that smoothness is NOT, finally, a fundamental principle of nature, but they've not yet thought up the atomic theory.
My point is that such an argument is moot: I can then clearly state that whatever we know today are indeed NOT fundamental principles of nature, only, I haven't yet thought up any alternative :-) It is not going to get me very far unless I have a brilliant flash of inspiration in the coming days. In the mean time, I stick with what works today...

 

[ttn]

This is what I fail to see. If lorentz invariance is "dead", what stops me from writing terms in the Lagrangian which are not lorentz invariant, in which case I will see deviations from Lorentz invariance in the lab, no matter whether there is quantum equilibrium or not ? So how can quantum equilibrium "make sense" of the fact that Lorentz invariance IS actually observed (ie, that we need to write Lorentz invariant terms in the Lagrangian ?) It seems that in doing so, we've LOST a lot of understanding, no ? You will argue that we've lost an illusion and are now ready to move on, but nothing *replaced* our previous illusion of an understanding.

Ah, you're getting to know me well. If *fundamental Lorentz invariance* is an illusion, then it represents a step in the right direction to recognize this clearly. And by the way this is true whether anything replaces the illusion or not. It's better to stop believing one lie and have nothing to put in its place (right away) than to pretend one knows a lot by believing the lie.

The analogy doesn't work completely, and the difference is my point: The proponents of an atomic theory REPLACED the continuity principle by another one FROM WHICH THEY COULD DERIVE the apparent continuity on macroscopic scales. I wanted to indicate that quantum equilibrium in BM DOES NOT allow you to derive apparent lorentz invariance if you do not put it in by hand in the unitary dynamics.

Oh, I see your point. So you're saying if we take some N-particle dirac theory (just to give an example of a theory for which a Bohmian version is actually known to exist), we can prove that "superluminal signalling" is impossible only in quantum equilibrium, and thus (in a certain sense) explain the origin of observed Lorentz invariance. But you're saying: the Dirac equation is still Lorentz invariant, and we have no explanation for that.

That's true. I'm not sure what to say that I haven't said before, though. It's better to know that a certain feature is unexplained than to hold onto a false explanation of it. The first makes it possible that you'll eventually discover the true explanation; the second prevents this possible future progress.

I can buy your argument in the following sense: the status of BM is that somehow they're convinced that smoothness is NOT, finally, a fundamental principle of nature, but they've not yet thought up the atomic theory.
My point is that such an argument is moot: I can then clearly state that whatever we know today are indeed NOT fundamental principles of nature, only, I haven't yet thought up any alternative :-) It is not going to get me very far unless I have a brilliant flash of inspiration in the coming days. In the mean time, I stick with what works today...

This is a very strange attitude. Does anyone actually think that rapid progress in the "coming days" is going to be made by some first year grad student writing down only Lorentz scalar terms in the Lagrangian of some field theory? Nobody has made any serious progress on anything significant in that way for decades, as far as I know. In fact nowdays it's just the opposite: people postulate non-Lorentz-invariant terms to explain the observed violations of GZK cutoff for high energy cosmic rays, etc.

Real progress of the kind that interests me, at least, comes from understanding the correct ontology for microphysics. If Bohm is that correct ontology, then it is huge major step in the right direction even to recognize that for non-rel QM. I know it wasn't the mapping I intended originally, but maybe that's the fundamental parallel to the atomic theory. If theorists are more concerned these days with figuring out what terms they should permit students to write in Lagrangians than in figuring out what the real world is like at small scales, then something is seriously wrong.

 

[vanesch]

It's better to stop believing one lie and have nothing to put in its place (right away) than to pretend one knows a lot by believing the lie.

Then this is where we agree to disagree. I think at ANY moment in history (past or future) our understanding of nature will ALWAYS be "superficial", in that we will ALWAYS only know some "illusion". We can only hope for the fact that the illusion will be better and better, going from some paint on a cave wall over black and white photography to THX sound systems and 3-D goggles in a movie theatre.
I prefer watching the movie than somebody switching off the projector and yelling that it is just a movie :-) However, contrary to the epistemologists, I nevertheless think that in order for the illusion to be an acceptable illusion, it has to do the big game, and present an illusion of an ontology: a story (not correct of course) needs to come with the illusion of how things "really" are. But the story is only that: a story to complete the illusion. In more serious terms, what I call "illusion" is usually called "paradigm". And in THIS context, I think it should be clear (also to you, even if you don't agree), that it is not reasonable to do away with part of the illusion and not replace it with something else (which will, in any case, again be another illusion).
So, today, at the beginning of the 21st century, we have an "illusion" which explains quite a lot, and consists of 2 principles: relativity and (unitary) quantum theory. It is for this illusion that I want a story (and as you rightly point out several times, the USUAL story of OQM is not consistent of course).
Now, saying that the story is silly and that a much more "realistic" story, with particles, is the way to go, but doing away with part of the paradigm is, IMHO, not the thing to do, because you have now LESS explanatory power (relativity being gone and not replaced).
You are free of course to set up another illusion (paradigm), have a story that goes with it and so on, but in order for it to be an improvement, it need AT LEAST to explain everything which the previous illusion could explain. Saying that my illusion is fundamentally wrong is no news! I know that! And yours is just as wrong and the theories we'll have 500 years from now will be just as wrong.
I think the only REAL illusion, at any point, is to think we NOW know fundamentally what is ultimately right. It is the only statement which has been proven wrong systematically throughout all of history. THIS statement is probably fundamentally right :-) Eh...

Oh, I see your point. So you're saying if we take some N-particle dirac theory (just to give an example of a theory for which a Bohmian version is actually known to exist), we can prove that "superluminal signalling" is impossible only in quantum equilibrium, and thus (in a certain sense) explain the origin of observed Lorentz invariance. But you're saying: the Dirac equation is still Lorentz invariant, and we have no explanation for that.

Yes, exactly.

That's true. I'm not sure what to say that I haven't said before, though. It's better to know that a certain feature is unexplained than to hold onto a false explanation of it. The first makes it possible that you'll eventually discover the true explanation; the second prevents this possible future progress.

In a way I could return the argument and saying that you want to cling onto a configuration space of particles as in good old Newtonian physics prevents this possible future progress. As long as the future progress is not on the table, there's no way to tell !

This is a very strange attitude. Does anyone actually think that rapid progress in the "coming days" is going to be made by some first year grad student writing down only Lorentz scalar terms in the Lagrangian of some field theory? Nobody has made any serious progress on anything significant in that way for decades, as far as I know.

I'd say that theoretical physics hasn't done much ELSE the last 50 years ! From QED up to string theory.

In fact nowdays it's just the opposite: people postulate non-Lorentz-invariant terms to explain the observed violations of GZK cutoff for high energy cosmic rays, etc.

I don't know much about this but the point is of course that from the moment that you relax, for your specific application, some general principle, your "explanatory possibilities" become much larger. If it weren't already the case in this thread, as an example I'd cite Bohmian mechanics :-) The difficulty resides then in re-explaining, without the doomed general principle, everything which was already explained by it, the exercise I wanted you to do.

But I can understand your point too: if relativity is a wrong principle, then tossing it out is the first step in finding a better one ; nevertheless, the intermediate situation, where the "wrong" principle is tossed out, but the better one is not found yet, should remain confined to the studyroom of the genius that will find the better principle. This intermediate step is NOT a theory of course :-)
Otherwise I'll find an even better theory: relativity AND quantum theory are wrong. There. :-)

If theorists are more concerned these days with figuring out what terms they should permit students to write in Lagrangians than in figuring out what the real world is like at small scales, then something is seriously wrong.

I had the impression that was what people did in most of the 20th century. Something is rotten in this country :-)

 

[ttn]

...contrary to the epistemologists, I nevertheless think that in order for the illusion to be an acceptable illusion, it has to do the big game, and present an illusion of an ontology: a story (not correct of course) needs to come with the illusion of how things "really" are. But the story is only that: a story to complete the illusion. In more serious terms, what I call "illusion" is usually called "paradigm". And in THIS context, I think it should be clear (also to you, even if you don't agree), that it is not reasonable to do away with part of the illusion and not replace it with something else (which will, in any case, again be another illusion).

Well, we have a significant philosophical difference then. I think we actually know what we know (though of course we don't know everything). You think we know nothing, that everything we could believe is an illusion, and the only serious question is whether the illusion you posit is in some superficial sense consistent with your personal mental experience. I guess this explains why you advocate this strange solipsist version of MWI. Try as I might, I can't even bring myself to take that idea seriously. It just isn't a serious theory in the way meant by such scientific realists as Einstein and Bell.

So, today, at the beginning of the 21st century, we have an "illusion" which explains quite a lot, and consists of 2 principles: relativity and (unitary) quantum theory. It is for this illusion that I want a story (and as you rightly point out several times, the USUAL story of OQM is not consistent of course).
Now, saying that the story is silly and that a much more "realistic" story, with particles, is the way to go, but doing away with part of the paradigm is, IMHO, not the thing to do, because you have now LESS explanatory power (relativity being gone and not replaced).

"Explanatory power" is not a subjective game. It's about what is true, what corresponds to the way the real world actually is. Lies have zero explanatory power.

You are free of course to set up another illusion (paradigm), have a story that goes with it and so on, but in order for it to be an improvement, it need AT LEAST to explain everything which the previous illusion could explain. Saying that my illusion is fundamentally wrong is no news! I know that! And yours is just as wrong and the theories we'll have 500 years from now will be just as wrong.

And to think, all this time I wasted trying to convince you that your beliefs were a mere illusion. Now you say you knew it all the time. Sigh...

I absolutely do not believe that in 500 years we will all believe that the world is flat, that matter is not atomic, that DNA plays no role in inheritance, etc. We actually *know* some things, things about how the world acutally *is*, and science has as its goal finding out *more* things. All I can say is, if you disagree with that it's no wonder you don't see why Bohm is a serious and promising theory.

 

[vanesch]

You think we know nothing, that everything we could believe is an illusion, and the only serious question is whether the illusion you posit is in some superficial sense consistent with your personal mental experience.

Yes, although I would object to the "you think we know nothing". Knowing what is a good illusion is real knowledge that is useful.

I guess this explains why you advocate this strange solipsist version of MWI.

Exactly. I give myself more liberty with the "story" and I am more severe on the principles of the formalism, while you do the opposite.

Try as I might, I can't even bring myself to take that idea seriously. It just isn't a serious theory in the way meant by such scientific realists as Einstein and Bell.

Those whimps don't run in the same category as I do

"Explanatory power" is not a subjective game. It's about what is true, what corresponds to the way the real world actually is. Lies have zero explanatory power.

Hehe, you must be a bad poker player I'd say that there is a whole greyscale between "lies" and "what is true". Given the fact that "what is true" is an unattainable ideal (in my view), there's still stories that work well, stories that work better and stories that are obviously flawed.

And to think, all this time I wasted trying to convince you that your beliefs were a mere illusion. Now you say you knew it all the time. Sigh...

Yes, but YOU don't know that your beliefs are just as well an illusion

I absolutely do not believe that in 500 years we will all believe that the world is flat, that matter is not atomic, that DNA plays no role in inheritance, etc. We actually *know* some things, things about how the world acutally *is*, and science has as its goal finding out *more* things.

When I go hiking, I "believe" that the world is flat and I don't mind taking a flat map of the environment with me. Of course I know that it is an approximation (illusion?) and that this can be explained by the size of the earth. So it is a very useful "illusion" (which is, in your terminology, basically a lie). When I calculate the mechanical vibration modes in a steel structure, I "believe" that steel is a continuum allowing me to use simple equations in continuum mechanics ; even though it is a "lie".
In the same way, I "believe" that matter is made out of individual atoms, although I also think that this is not correct and that they are apparent structures that result from the interactions of quantum fields (yes, we differ here).
What I want to say is that even explanations for which we now KNOW that they are not correct, are still very useful explanations (now understood as approximations of a more "fundamental" explanation). In that sense, indeed, the good old explanations which work up to a point will remain with us for ever, and we will tell them to the kids. "Explanations", "paradigms", "illusions" and so on are like that Iceland cosmology: it is turtles all the way down, and probably at a certain point we will simply stop looking at underlying turtles, because we can't think anymore of any falsifiable statement that could reasonably be tested (and because maybe humankind will face more pressing problems, like total extinction).

All I can say is, if you disagree with that it's no wonder you don't see why Bohm is a serious and promising theory.

Ha, our mutual psycho-analysis has led us to understand each other's viewpoint and mindset, and leads us to fully disagree in peace

Nevertheless, I DO think of Bohm's theory as a useful illusion . First of all because it is an example of a class of theories that certain dumbasses thought couldn't exist (namely a deterministic hidden variable theory with identical predictions as QM), and second, you guys MIGHT be on something. The day that you can come up with a reason why lorentz invariance appears, without putting it in by hand, hence giving us back SR and GR, I will listen very carefully :-)
What I wanted to make you see is that MWI is ALSO a useful illusion, first of all because it is ALSO an example of a class of theories that certain dumbasses thought couldn't exist (namely a theory which fully respects relativity and has identical predictions as QM).

 

[ttn]

I'm not going to bother responding on all the philosophical stuff. I think it's clear to both of us and to anyone else reading what we each think, and how that influences the physics debates.

But one small but important point:

The day that you can come up with a reason why lorentz invariance appears, without putting it in by hand, hence giving us back SR and GR, I will listen very carefully

There is a funny kind of demand here if you think about it. My only claim in all of this has been that Lorentz invariance appears not to be fundamental. That is, Lorentz invariance seems to be a kind of emergent phenomenon that appears in certain situations and/or under certain conditions, rather than a fundamental fact about the causal structure of the world. What this means in practice is that I'm willing to accept some violations of Lorentz invariance in theories so long as they remain consistent with observed facts; you on the other hand want Lorentz invariance to be universal (so that no non-Lorentz-invariant theories or aspects of theories would be acceptable).

My question is: what in the world does any of this have to do with *explaining* Lorentz invariance? Neither one of us has an explanation for it. Taking that formal principle as universal, as a kind of axiom, isn't the same as providing an *explanation* for it. You just accept it and then demand it of theories. I also just accept it (but not as a fundamental) and then demand that theories be consistent with what is known empirically relating to it.

Maybe I'm not being too clear. My point is that I, as an advocate of a Bohm interpretation of QM, have no *special* responsibility to provide some underlying explanation of Lorentz invariance. It's not like you *already have* such an explanation, but that I've pulled the rug out from under it and now I have to provide a new rug. There was never any rug. Lorentz invariance is just something people accepted based on empirical evidence -- namely, the fact that observable effects all seemed to obey a certain kind of transmormation law. It turned out over the last 100 years to be a quite general rule, yes. So people started to believe that it might be or must be universal. But in the absense of some underlying causal argument that it must be universal, that's just plain hasty generalization. That it was hasty is, I think, proved by the prima facie evidence for a non-Lorentz-invariant structure provided by empirical violation of Bell's inequalities.

I fear I've again drifted from the point I actually want to make here. So let me start over. You said: "The day that you can come up with a reason why lorentz invariance appears, without putting it in by hand, hence giving us back SR and GR, I will listen very carefully." My point here is this: The day you can come up with a reason why Lorentz invariance should be accepted as a *universal* and *fundamental* principle, I will listen very carefully. You talk as if there is some special responsibility on my to reject your extrapolation of Lorentz invariance from "works as a description of a number of things so far observed" to "universal fundamental principle of nature". Well I reject that. The burden is on you if you want to extrapolate something into a fundamental law. *You* prove to me (by giving some "reason why lorentz invariance appears, without putting it in by hand") why I shouldn't entertain empirically adequate theories that aren't relativistic.

And on that I'll have to wish you luck given that Bell's inequalities are violated, so we know that no *serious* locally causal theory will be adequate.

 

[vanesch]

*You* prove to me (by giving some "reason why lorentz invariance appears, without putting it in by hand") why I shouldn't entertain empirically adequate theories that aren't relativistic.

Of course you can entertain that view ! But now you should explain to me the APPARENT lorentz invariance of things such as the Maxwell equations, the Dirac equation and so on. HOW COME that all of unitary quantum theory can be formulated in such a way that they can be written in a Lorentz invariant way ?
You can just shrug and say: "well, heh, that's simply how they are ; they could have been different, but they simply happen to be that way." I find that slightly unsatisfying, in the same way as it would have been unsatisfying to say: "equations in Newtonian physics can be written in 3-vector form, it just happens to be that way, it doesn't mean that there is something like isotropy of space."

I could perfectly well accept that Lorentz invariance is not a fundamental principle, but is an "emerging property". Only, you cannot derive that "emerging property" without putting it into ALL the field equations.

I do of course not have to explain WHY Lorentz invariance is a fundamental principle if I TAKE it as basic postulate that all objects in the theory are 4-dim geometrical objects. THIS then implies that all kinds of equations I will write down in a specific coordinate system will be lorentz invariant. So now I have an explanation, for which you can only say "well, it just happens to be that way".

This is where I say that explanatory power is lost in your case.

 

[ttn]

Of course you can entertain that view ! But now you should explain to me the APPARENT lorentz invariance of things such as the Maxwell equations, the Dirac equation and so on. HOW COME that all of unitary quantum theory can be formulated in such a way that they can be written in a Lorentz invariant way ?

You're right that I have no answer to "HOW COME?". My point was that you don't either, so you can't point to my inability to answer this question as a reason to reject my side in favor of your side. Neither side knows *why* Maxwell's equations (etc) are Lorentz invariant.

You can just shrug and say: "well, heh, that's simply how they are ; they could have been different, but they simply happen to be that way."

and I'd add: and hopefully someday we'll have a non-delusional explanation for this.

But my main point was that you shouldn't criticize me for saying "well, that's simply how things are, they happen to be that way." You say the same thing.

I do of course not have to explain WHY Lorentz invariance is a fundamental principle if I TAKE it as basic postulate that all objects in the theory are 4-dim geometrical objects. THIS then implies that all kinds of equations I will write down in a specific coordinate system will be lorentz invariant. So now I have an explanation, for which you can only say "well, it just happens to be that way".

That's precisely what I'm denying. Taking something as a postulate and showing that it is a consequence of something deeper (explaining or proving it) aren't the same thing. You can't *explain* why Maxwell's equations are lorentz invariant by saying "I accepted it as a postulate that all equations should be lorentz invariant." That's just dumb on the face of it. Plus, the only reason you accepted that postulate is because Maxwell's equations (and some other stuff) were lorentz invariant. So not only is the only relevant sense of "explanation" involved here pertaining to the wrong thing (namely, our beliefs rather than the physical facts), the explanation goes the wrong way: it's Maxwell's equations that (partially) explain your belief in some general postulate, not the other way around.

Well, whatever. I think maybe we've talked this one to death. But we've covered a lot of important ground and I've enjoyed it. You can have the last word if you want it.

 

[vanesch]

Taking something as a postulate and showing that it is a consequence of something deeper (explaining or proving it) aren't the same thing. You can't *explain* why Maxwell's equations are lorentz invariant by saying "I accepted it as a postulate that all equations should be lorentz invariant."

I agree with you there, if it were "and the equations are Lorentz invariant" (a certain way of writing formulas). If it were a statement like "and the equations only contain at most 7 terms" that would be an equally silly "principle".
You could write down the Maxwell equations, and show that there are less than 7 terms in it, and just say, "well, I don't know why there are only 7 terms but that's the way it seems to be", and I would be nervously jumping up and down and require you to EXPLAIN my superb postulate that all fundamental field equations in nature have less than 7 terms in them. You'd call me a <fill in derogatory statement about my intelligence> and you would be right. And I have the impression you take the same attitude about the statement of "lorentz invariance". If it were just a "property of the formulas we write down" you would be entirely justified to do so. But what I'm affraid you fail to see is that Lorentz invariance is A CONSEQUENCE of a physical idea, namely that all objects in the theory are 4-dim geometrical objects. This is as fundamental a statement as saying that there are only particles out there. Ok, I cannot tell you WHY the objects should be 4-dim geometrical objects, just as well as you cannot tell me WHY there are only particles out there ; that's always the case with the basic postulates: we cannot explain them. However, once I accept that statement, THERE IS NO OTHER WAY for the theory as to be written down in lorentz invariant equations, so as such, this postulate DOES EXPLAIN the lorentz invariance of the Dirac equation, the maxwell equation, the equations of gravity etc...: they are coordinate expressions of the 4-dim geometrical objects.

 

[metacristi]

"...when you have eliminated the impossible, whatever remains, however improbable, must be the truth." - Sherlock Holmes (Sir Arthur Conan Doyle)

Heh, again that phrase about 'eliminating the impossible' (often seen on forums 'packed' with 'skeptics' :-) )...It might work for detective work or more generally at the macro level (underdetermination is not a serious problem) but I don't think it works at the micro level. Besides even at the macro level there are problems, for example we have to rely on the auxiliary assumption (hidden) but not at all proved (in the sense of having sufficient reasons once and for all) that common sense really gives us a representation of an external reality, for example solipsism is still a perfectly valid solution in spite of the fact that science has a form of realism at its basis (it's laden with the assumption that there exist an external reality which we can perceive/model/understand at least partially).

Returning at the problem of 'nonlocality' the truth is that there is no good reason now to suppose that local realism (in its classical definition) has been thoroughly rejected, indeed nothing impedes the possibility of strong determinism at the most basic level of reality (in which case it is not at all surprising that classical locality still hold, of course to the expense of renouncing at free will). Or even that 'modus tollens' cease to work when applied at quantum level (at least in certain cases, the assumption of uniformity is inductive and no one has produced so far an argument justifying a general inductive method).

This does not mean that there are good reasons to prefer now such a research program (holding classical locality) as the first choice, no, the idea is that we/scientists should be much more humble, there is no crushing evidence that science really lead us to the Truth; Aspect's experiment is considered 'crucial' by many scientists but our actual interpretation might be in reality (not known by us) wrong.

 

[prabhuramkumar]

model question papers and guidance

sir
i have received ur mail on quantum mechanics.it is really nice.
but i want to prefer for iit and iisc.so am in need of model question papers.
pls mail to me at once.it will be helpful for my further studies.
yours
B.prabhu ramkumar