EPR experiments imply STR not correct?

[zenith8]

Quote:-
---Length contraction/time dilation real physical effects (not observational perspective) but unclear why objects enduring through time suffer these (reciprocal!) effects just because in relative motion.----

Why is the proposed Lorentz contraction any different.

Matheinste

Actual length contractions (physical squishing) occurs in both Lorentz and Einstein 3+1 relativity. In the former, this is postulated to occur because of motion with respect to the fixed ether (and actually calculated through the modified moving fields of Maxwell's equations), in the latter case it occurs due to relative motion with respect to anything else moving in a different frame..

Silly analogy for Lorentz : drive your car underwater - you feel a drag force because you are driving through water (and this drag force makes the car shorter(!) - there is a causal explanation for it).

Einstein 3+1 implies if you drive your car in space then you are physically contracted just because of relative motion to other moving things in space - i.e. there is no causal explanation for it - it is just stated to occur. Because this is clearly a bit silly (and because of the pluralistic fragmentation of reality into distinct spaces and times) most no-preferred-frameists eventually went for the Minkowskian interpretation, where the length contraction is a perspective effect in 4d spacetime.

Zenith

 

[zenith8]

The existence of EPR phenomena does not require Lorentzian relativity. They can occur equally in GR and SR. So your basic point is wrong. So is most of the other stuff in your last post.

OK - I'm interested. This is what I actually want to know - it is my original question. Tell me how EPR phenomena occur in SR.

Zenith

 

[matheinste]

Something odd here

From another thread

Quote:-


Length contraction in Lorentzian relativity

--------------------------------------------------------------------------------

Hi,

I'm trying to understand Lorentzian relativity (Lorentz ether theory, whatever) which is empirically equivalent to the Einsteinian STR. I have, however, a problem in comprehending length contraction.

In the Lorentz theory we have a preferred frame and length contraction is a real physical effect. It has a causal explanation in terms of motion of the body with respect to this absolute space which causes distortions in the electromagnetic field and hence in the intermolecular forces holding rods and clocks together.

Of course no experiment has ever been performed which checks length contraction directly, as there is no known way to accelerate a macroscopic object to relativistic speeds.

However, why doesn't the following imply a difference between Einsteinian STR and Lorentz?

Imagine we can build a spaceship which will travel at 0.995c. In the frame of a stationary observer, everyone agrees that the spaceship looks squished as it flies past (because of a perspective effect in Minkowski spacetime for STR, or because it actually is squished for Lorentz).

However, if I am actually on the ship then other things inside either should look squished (because they are - Lorentz) or they do not look squished (because all inertial frames are equivalent - Einstein). Now whenever I have seen this discussed one just reads that in Lorentz theory measuring rods are distorted too so I can't measure the effect. But surely if I'm going at 0.995c then things will just look distorted (spheres not being spherical etc) and I can tell the damned measuring rod is a lot shorter than it used to be (because it's now square, rather than a long rectangular metre rule).

So maybe it's because my eyes are distorted, or whatever - but isn't this dangerous? Being compressed to the thickness of a piece of cardboard can't be good for the human body surely..

What's the flaw here? All opinions gratefully received.

Cheers,
Zenith

Quick learner?

Matheinste

 

[zenith8]

Something odd here

Quick learner?

Matheinste

Sure. I knew f*ck all about relativity this time last week.

However, given the interpretations have different ontologies regarding whether one is physically squashed or not, it is not completely impossible that one might feel/see an observational difference at relativistic speeds. I mean, it's not like anyone has ever tried it..

The interpretation of `Bell's spaceship paradox' - see Wikipedia - I believe is still somewhat controversial - that involves physical length contractions leading to stress breaking a rope strung between two spaceships.

I just like to get these things clear in my mind.

Zenith

PS: Any smartarse saying 'and you still don't' in response to my first sentence can expect a quick unpleasant death.

 

[matheinste]

Hello zenith8

I just found it strange that you suddenly became a confident, apparently or truly, knowledgeable espouser of Lorentzian relativity.

Please don't call me smart.

Matheinste

 

[Fredrik]

The nonlocality experiments define a universe-wide absolute simultaneity.

You keep saying it, but it's still wrong. Your argument is flawed. The wavefunction isn't a measurable quantity, so its "collapse" can't be used to synchronize two clocks at spacelike separated events. The wavefunction also isn't an objective representation of the properties of a physical system. The experimenter who measures the spin of one member of the EPR pair "first" (in the frame we're using) would describe the current state of the two-particle system in a different way than the experimenter at the other end who hasn't measured anything yet (in the same frame).

 

[zenith8]

Hello zenith8

I just found it strange that you suddenly became a confident, apparently or truly, knowledgeable espouser of Lorentzian relativity.

Please don't call me smart.

Matheinste

Well, I do try to learn fast (I have a wife and baby and they don't give me much free time to study, so I kind of have to!). Sometimes I succeed, sometimes I don't, but truth be told I *still* don't quite get length contraction, which was the point of the other thread.

See, one does sometimes read that there *is* a difference between physical length contraction and apparent length contraction in the following sense. Bell assumed that only the physical objects in his thought experiment (the spaceships and the rope) contracted. But e.g. Petkov (see p.136 - Relativity and the Nature of Spacetime - Google Books) states that Bell was an idiot (!) because he doesn't realize that the space between the spaceships contracts as well, therefore the rope doesn't break, in contrast to what Bell said. And one must also consider the *reciprocity* of the effect i.e. if somebody in a 3rd spaceship accelerates past the two apparently stationary spaceships which are connected by a rope then Bell (according to Petkov) also implies that the rope would break even though the two connected spaceships never turned on their engines. Therefore - says Petkov - length contraction is not a real physical process. There seem to be a lot of misunderstanding between two clever fellows here - which I may need some time and a few cups of coffee to sort out.

I wasn't calling you smart, was I? The smartarse remark was aimed at generic wits who might try to come in and take advantage.

Zenith

 

[Fredrik]

The wave function of the universe has got nothing to do with MWI in particular.

Actually it does. The axioms of non-relativistic quantum mechanics can be expressed like this:

1. States are represented by the rays of a Hilbert space.
2. The time evolution of an isolated system is |\psi\rangle\rightarrow e^{-iHt}|\psi\rangle
3. A measurement of an observable changes the state into an eigenstate of the observable being measured, and the probability that that state is |a\rangle is |\langle a|\psi\rangle|^2.

The feature that distinguishes the MWI is that #2 is taken literally, and therefore assumed to apply to all isolated systems. Without that assumption, the notion of a wave function of the universe doesn't even make sense. The Copenhagen interpretation (CI) on the other hand, assumes that there's a boundary between the quantum world and the classical world. In particular, any device that displays the result of a measurement in a way that a human can understand, is a classical object. So in the CI, it makes no sense to use quantum mechanics to try to describe a physical system that includes, say, the screen in a double slit experiment.

 

[matheinste]

Hello zenith8.

No hard feelings.

I'm off to bed.

Matheinste.

 

[zenith8]

You keep saying it, but it's still wrong. Your argument is flawed. The wavefunction isn't a measurable quantity, so its "collapse" can't be used to synchronize two clocks at spacelike separated events. The wavefunction also isn't an objective representation of the properties of a physical system. The experimenter who measures the spin of one member of the EPR pair "first" (in the frame we're using) would describe the current state of the two-particle system in a different way than the experimenter at the other end who hasn't measured anything yet (in the same frame).

What you're implying - I think - is that the argument is QM interpretation dependent. And if so, you'd be right.

For interpretations without a realistic ontology (i.e. where the wave function represents 'knowledge' or 'information') then all bets are off. But these interpretations can - to my satisfaction at least - be put aside here.

In many-worlds, there is no nonlocality, but that's one of my allowed get-out clauses.

With a collapse interpretation like GRW - where, note, the wave function *is* an objective representation of the properties of a physical system - then the intermediate *history* of the EPR experiment depends on observational perspective but you end up at a unique *final* outcome. But given that the wave function is supposed to be real, you are not allowed to have a different history (who measured what first and caused the wave function to collapse, and what the intermediate state was).

With a hidden variables interpretation like pilot-wave theory where the wave function and the particles are ontologically real then not only the intermediate history but even the final outcome depends on observational perspective. And again, given the realistic ontology, this is not allowed.

The only thing to say is that all non-local interactions are instantaneous. Therefore they define a preferred frame.

Zenith

 

[Fredrik]

See, one does sometimes read that there *is* a difference between physical length contraction and apparent length contraction in the following sense. Bell assumed that only the physical objects in his thought experiment (the spaceships and the rope) contracted. But e.g. Petkov (see p.136 - Relativity and the Nature of Spacetime - Google Books) states that Bell was an idiot (!) because he doesn't realize that the space between the spaceships contracts as well, therefore the rope doesn't break, in contrast to what Bell said.

This is very very wrong, but at least it's not your blunder. It's Petkov's. (The page wasn't displayed at Google Books, but I found it at Amazon.com). The rope definitely breaks. I'm not going to go into the details here, because there are plenty of threads about this already.

 

[Mentz114]

The only thing to say is that all non-local interactions are instantaneous. Therefore they define a preferred frame.

You keep saying this without backing it up so I'm giving up too. There's no point in arguing with a Ph D (Wiki) who thinks he's had a great idea.

 

[zenith8]

Actually it does. The axioms of non-relativistic quantum mechanics can be expressed like this:

1. States are represented by the rays of a Hilbert space.
2. The time evolution of an isolated system is |\psi\rangle\rightarrow e^{-iHt}|\psi\rangle
3. A measurement of an observable changes the state into an eigenstate of the observable being measured, and the probability that that state is |a\rangle is |\langle a|\psi\rangle|^2.

The feature that distinguishes the MWI is that #2 is taken literally, and therefore assumed to apply to all isolated systems. Without that assumption, the notion of a wave function of the universe doesn't even make sense. The Copenhagen interpretation (CI) on the other hand, assumes that there's a boundary between the quantum world and the classical world. In particular, any device that displays the result of a measurement in a way that a human can understand, is a classical object. So in the CI, it makes no sense to use quantum mechanics to try to describe a physical system that includes, say, the screen in a double slit experiment.

Grin. In the light of what we know now the Copenhagen interpretation is the only interpretation we know of that is logically inconsistent and wrong - as anyone who has ever studied it in detail knows. Circular arguments of consistency disguised as arguments of inevitability - see Mara Beller's book.

The time evolution in many-worlds is just the time-dependent solution to Schrödinger's equation - a dynamical object used also in say, the Bohm pilot-wave theory (where it doesn't collapse either). So all I'm saying is that that wave function is not unique to MWI and calling it the "Everettian wave function" like some people do is a misnomer.

Zenith

 

[zenith8]

This is very very wrong, but at least it's not your blunder. It's Petkov's. (The page wasn't displayed at Google Books, but I found it at Amazon.com). The rope definitely breaks. I'm not going to go into the details here, because there are plenty of threads about this already.

Indeed - I only found the Petkov thing earlier this evening and his tone was so dismisive of Bell that I though I must have missed something. You've saved me the job of sitting down and reading it carefully..

Zenith

 

[Fredrik]

What you're implying - I think - is that the argument is QM interpretation dependent. And if so, you'd be right.

That's not what I'm saying. (I'm not even sure what that would mean). There's no need to consider interpretations of QM here (or ever really). We can simply view QM as an algorithm that tells us how to calculate probabilities of possibilities. I'm just trying to point out the flaws in your argument, but I think the best way for you to see why it's flawed is to try to make it stronger.

 

[atyy]

And one must also consider the *reciprocity* of the effect i.e. if somebody in a 3rd spaceship accelerates past the two apparently stationary spaceships which are connected by a rope then Bell (according to Petkov) also implies that the rope would break even though the two connected spaceships never turned on their engines.

My quick guess. In the "reciprocal" (don't think it is, but just to use the same word), the distance between the two ships will contract, as will the rope, so the rope will not break. In the "original", the distance between the two ships stays the same, the rope contracts, so the rope breaks.

 

[zenith8]

You keep saying this without backing it up so I'm giving up too. There's no point in arguing with a Ph D (Wiki) who thinks he's had a great idea.

Look, if I'm guilty of not backing things up (which I'm surprised at) then so are you. Our last exchange was as follows:

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> The existence of EPR phenomena does not require Lorentzian relativity. They can occur equally in GR and
> SR. So your basic point is wrong. So is most of the other stuff in your last post.

OK - I'm interested. This is what I actually want to know - it is my original question. Tell me how EPR phenomena occur in SR.
-----------------------------------------------------------------------------------------------------------------------------------------------

So I'm being polite. I'm interested in your point of view. You haven't replied.

So tell me how EPR phenomena occur in SR.

I've explained (several times) why I don't think they can occur in SR. I'm really interested to know how they can. I am *very* happy to be disabused of my view - it's what I'm here for.

Dr. Zenith Ph.D.

 

[atyy]

Try Asher Peres, Quantum Theory, discussion starting about halfway down p170:
http://books.google.com.sg/books?id=rMGqMyFBcL8C&printsec=frontcover#PPA170,M1

 

[Fredrik]

I've explained (several times) why I don't think they can occur in SR. I'm really interested to know how they can. I am *very* happy to be disabused of my view - it's what I'm here for.

You're asking "how" it can happen in SR, but I don't know what sort of answer you're looking for if the ones I've given you aren't enough. There's no way to use an EPR experiment to synchronize two clocks at spacelike separation. There's no transfer of information, since the wave function isn't measureable. Alice, who does the first measurement, doesn't even use the same wavefunction to represent the state of the two-particle system after her measurement as Bob at the other end at the same time in a frame where Alice's measurement happens first. So it's up to you to explain why you think EPR implies absolute simultaneity. Why would it, when there's no instantaneous transfer of information (or any transfer of information for that matter)? And even if information had been transferred at infinite speed in one inertial frame, why do you assume that the speed is infinite in all inertial frames?

 

[zenith8]

Try Asher Peres, Quantum Theory, discussion starting about halfway down p170:
http://books.google.com.sg/books?id=rMGqMyFBcL8C&printsec=frontcover#PPA170,M1

Hi Atyy,

Sorry I fail to see what Peres is saying that we haven't already said or is relevant to the argument. Can you be more specific?

Zenith

 

[zenith8]

You're asking "how" it can happen in SR, but I don't know what sort of answer you're looking for if the ones I've given you aren't enough. There's no way to use an EPR experiment to synchronize two clocks at spacelike separation. There's no transfer of information, since the wave function isn't measureable. Alice, who does the first measurement, doesn't even use the same wavefunction to represent the state of the two-particle system after her measurement as Bob at the other end at the same time in a frame where Alice's measurement happens first. So it's up to you to explain why you think EPR implies absolute simultaneity. Why would it, when there's no instantaneous transfer of information (or any transfer of information for that matter)? And even if information had been transferred at infinite speed in one inertial frame, why do you assume that the speed is infinite in all inertial frames?

OK. Consider a deterministic hidden-variables theory - de Broglie-Bohm pilot-wave theory, say. There are particles as well as the waves, and the particles follow the stream lines of the ordinary QM probability current. No extra equations. It's just QM.

Under ordinary circumstances (particles distributed as psi^2) you can't send instantaneous signals - even though when you measure spin-up then an instantaneous signal passes to the distant arm saying "You're spin down" - it is the case that there is no effect on the expectation values or on the probabilities. The statistical distribution of properties at one end are just the normal ones. and this masks any signalling. So quantum nonlocality cannot in fact be used for practical signalling at a distance. This means that if there were a preferred rest frame, it would be undetectable in practice.

But this isn't a fundamental constraint. From the hidden-variables perspective, it's a peculiarity of a special 'quantum equilibrium' distribution of the particles. The fact we can't detect the rest frame is not an uncomfortable conspiracy seemingly built into the laws of physics - it's just an accident of our living in a state of quantum equilibrium, whose statistical noise masks the underlying nonlocality.

If you do the analysis, hypothetical non-equilibrium distributions (particle distribution not equal to psi^2) do make it possible to use non-locality for instantaneous signalling (just like in stat mech, differences of temperature make it possible to convert heat into work). Proof slightly boring but obvious - trust me for the moment

So to synchronize clocks:

If experimenters at space time events A and B had access to non-equilibrium systems entangled between A and B, then they would be able to signal back and forth to each other instantaneously. In an arbitrarily short time (as measured at each wing) a long conversation could in principle take place, during which (for example) the experimenters agree to set their clocks to read time t=0. They could signal to each other to confirm that they did do this. In such conditions A and B have to be considered as simultaneous events, and the agreed-upon time variable would define an absolute simultaneity. Thus, using non-equilibrium matter, experimenters at remote locations could set their clocks to read the same instantaneous time.

This is true, even though practical difficulties might prevent us from *actually* doing the experiment. It raises the question of how these signals could mesh with the surrounding approximately classical spacetime. This question must have an answer , irrespective of the underlying microscopic theory of spacetime.

Now you might say, well - what if the two wings are in motion relative to each other. So synchronize the clocks at t=0, and then let one of them accelerate and go on a tour then come back to where it was. The clock readings will no longer be the same since the accelerated clock will have slowed down. But this doesn't matter - remember we know have an absolute time - the final clock readings will still be simultaneous events (as could be verified by non-local communication) yet, the readings will not be synchronous.

Note - and this is the fun bit - that if you synchronize your distant clocks by non-local signalling, then the speed of light will be measured to be isotropic *only* in the preferred rest frame. Recall that moving experimenters who assume that the speed of light is still c in all directions would adjust their clocks at different points in space with settings that differ by the term -vx/c^2 (to lowest order in v/c) - see Poincare. In quantum equilibrium, of course, such non-local signalling is impossible and the true rest frame can't be detected.

Which I think explains what you chaps were grumbling about earlier this evening.

Did I get anywhere?

Zenith.

 

[Mentz114]

EPR in SR - instantaneous signalling

The diagram has distance x on the horizontal axis and time t on the vertical. We see the worldlines of 3 observers who synchronise clocks and then part company. We are on the green ship so our worldline is vertical. At time Tau-green we send an instantaneous signal to red and blue. At the same time we send a light pulse ( yellow ). If red and blue recorded the time of arrival of the instant signal, we can then compare clock times, velocities and positions and calculate that indeed, the signal had taken no time on anyone's clock. Also notice that the white line, if extended will cut all observers worldlines. Furthermore, because all the taus are proper intervals, all inertial observers will agree on the clock times.
An observer not in on our deliberations wouldn't be able to tell who had originated the signal.
As far as I can see, there are no contradictions in this. No doubt you will tell me if there is one.

However, a more interesting thing occurs to me. Length contraction may be attributed to the impossibility of detecting both ends of a moving rod simultaneously. So if we could do this, there would be no length contraction and both SR and Lorentz relativity would be wrong, or at least irrelevant.

 

[atyy]

Quantum Information and Relativity Theory
Asher Peres, Daniel R. Terno
http://arxiv.org/abs/quant-ph/0212023

 

[Fredrik]

Under ordinary circumstances (particles distributed as psi^2) you can't send instantaneous signals
...
If you do the analysis, hypothetical non-equilibrium distributions (particle distribution not equal to psi^2) do make it possible to use non-locality for instantaneous signalling

I have no idea what this is supposed to mean. |\psi(x,t)|^2 is the probability density of finding the particle in a region of space near x at time t. If it isn't, it isn't QM.

 

[zenith8]

I have no idea what this is supposed to mean. |\psi(x,t)|^2 is the probability density of finding the particle in a region of space near x at time t. If it isn't, it isn't QM.

Yes, it is. You said in your earlier post that interpretations of QM don't matter - now you're saying the opposite. What I'm saying here is just the standard mathematics with a perfectly standard hidden variables interpretation.

The Schrödinger equation of 1926 is set up as if particles existed all the time (wave function depends on position of all the particles, the Hamiltonian has Coulomb interactions between point particles etc.). De Broglie set up his pilot-wave hidden variables theory of 1924-1927 along those lines - there were particles, and the Schrödinger wave 'guided' the particles as they move along the streamlines of the probability flow which explains the double slit experiment. Note the whole concept of a wave function was de Broglie's in the first place - using it to predict electron diffraction through a slit was what he got the Nobel prize for.

(Note that this is mathematically identical to what is normally called Bohmian mechanics or the Bohm interpretation - all Bohm did was add decoherence to de Broglie's original pilot wave theory in order to explain measurement).

So anyway, in a spirit of positivism fashionable at the time, Heisenberg and the others then claimed that particles didn't exist unless you measure them. So they then changed the meaning of the word probability to mean 'probability of finding the particle at point x in a suitable position measurement' instead of its original meaning of 'probability of the particle being at x'. This leads to essentially all the features of QM that are normally considered paradoxical, but that's by the by. This is the only difference between the standard QM viewpoint and the hidden variables viewpoint. It's the same mathematics, different interpretation.

In presentations of de Broglie-Bohm theory, it's normally said that there is an extra equation - the guidance equation - where the velocity is given by v = grad S, where S is the phase of the Schrödinger wave. In fact this is just the ordinary probability current over the density from the standard theory - again, the particles are following the streamlines of probability flow.. So there isn't an extra equation at all.

If you believe that particles and waves are logically separate entities then their distribution in space does not have to have any relation to each other. However, because the wave guides the particles then there is a natural 'equilibrium distribution' analagous to the ones in classical stat mech. That equilibrium distribution is found (by numerical simulations, or by maths) to be the square of the wave function. Now in standard QM the relation 'probability = psi^2' is simply given, with no reason or explanation - in hidden variables theories it has a causal explanation.

In extreme conditions (the early universe for example) it is postulated - see papers by Antony Valentini ,for example - that one can find non-equilibrium matter where p is not equal to psi^2, in which case non-local signalling becomes possible, as I've already stated.

This may sound like nutter stuff to you, but it's perfectly standard. You've just been conditioned to think from a positivist viewpoint just like Bohr told you to (note the view isn't even correctly applied here, as it is the wave function which is not measurable; the particle positions are!).

Anyway, given that such signalling is possible (i.e. in theory, ignoring practical problems) one then must consider the implications of instantaneous signalling for the ontology of space and time, which I believe is the point of this discussion.

Remember again, it is standard QM. If you don't believe that, you need to do some reading.

Zenith

 

[zenith8]

The diagram has distance x on the horizontal axis and time t on the vertical. We see the worldlines of 3 observers who synchronise clocks and then part company. We are on the green ship so our worldline is vertical. At time Tau-green we send an instantaneous signal to red and blue. At the same time we send a light pulse ( yellow ). If red and blue recorded the time of arrival of the instant signal, we can then compare clock times, velocities and positions and calculate that indeed, the signal had taken no time on anyone's clock. Also notice that the white line, if extended will cut all observers worldlines. Furthermore, because all the taus are proper intervals, all inertial observers will agree on the clock times.
An observer not in on our deliberations wouldn't be able to tell who had originated the signal.
As far as I can see, there are no contradictions in this. No doubt you will tell me if there is one.

As you wish. You are claiming this shows you can have EPR in SR where all inertial frames are equivalent. Then your arrangement is not relevant as you need to have two different lines of simultaneity (supposedly equivalent inertial frames) to show up the paradox.

So just have your red and green ships (forget the blue one) and have them travel at the same speed with respect to each other. They have a non-equilibrium EPR device to communicate with each other instantaneously, and each ship is rigged to flash its own colour in ordinary light when it either sends or receives such a signal.

Then let *my* ships, which are pink and purple should you ask, be traveling at the same speed as each other, but with a different speed to the red and green ones. They also have instantaneous EPR transmitters to communicate with each other.

Because these two pairs of spaceships are traveling at different speeds, their lines of simultaneity are tilted with respect to each other. Let the green ship transmit instantaneously to the red one, and arrange that pink is somewhere near red in space. If you arrange it so that pink transmits instantaneously to purple as soon as he sees the red ship flash red, then you could arrange the tilt such that the original green ship instantaneous transmission is still in the future of pink and purple on the occasion of the pink instantaneous transmission. So if the purple ship is anywhere near you and your green ship in space when he receives the transmission from pink, then he can arrange to blow you and your green ship out of the sky with a photon torpedo before you even sent out the original signal. In which case.. you fill in the rest..

Only way out of this is to have all EPR correlations occur in a single unique reference frame - the absolute space, ether, whatever.

Zenith

 

[Fredrik]

Yes, it is. You said in your earlier post that interpretations of QM don't matter - now you're saying the opposite.

You obviously need postulates that tell you how to interpret the mathematics as predictions about the results of experiments. What I just said about |\psi|^2 is such a postulate. The "interpretations" of QM deal with a different set of issues, like e.g. the question of whether the terms in a superposition represent "different worlds" or not. What I was trying to say earlier is that there's no need to consider those questions here.

(I have a lot of opinions about "interpretations" of QM, but I won't post them here. I might however start another thread in the philosophy subforum soon).

I don't know the Bohm theory very well, so I can't comment on every detail of what you said, but there's definitely nothing in the standard formulation of QM that let's you use the EPR effect to synchronize clocks at events with spacelike separation. If there's something in Bohm's theory that let's you do that, then it can't be described as an "interpretation" of QM, or even as a "physically equivalent theory". It would be a different theory altogether.

If it isn't a completely different theory, then you should be able to use the standard formulation of QM to make your case for absolute simultaneity.

 

[zenith8]

You obviously need postulates that tell you how to interpret the mathematics as predictions about the results of experiments. What I just said about |\psi|^2 is such a postulate. The "interpretations" of QM deal with a different set of issues, like e.g. the question of whether the terms in a superposition represent "different worlds" or not. What I was trying to say earlier is that there's no need to consider those questions here.

(I have a lot of opinions about "interpretations" of QM, but I won't post them here. I might however start another thread in the philosophy subforum soon).

I don't know the Bohm theory very well, so I can't comment on every detail of what you said, but there's definitely nothing in the standard formulation of QM that let's you use the EPR effect to synchronize clocks at events with spacelike separation. If there's something in Bohm's theory that let's you do that, then it can't be described as an "interpretation" of QM, or even as a "physically equivalent theory". It would be a different theory altogether.

If it isn't a completely different theory, then you should be able to use the standard formulation of QM to make your case for absolute simultaneity.

Well - as I said, the Bohm theory in fact follows from the usual theory by a single change in the meaning of one word (though I must say this is not widely appreciated, due to silly ways of presenting it common in the past). If you choose to say that this gives a different theory, instead of a different interpretation, well then it's your choice.

I say it's the same mathematics therefore it's the same theory. The change in the meaning of the word just gives the theory different implications, that's all. One of which is superluminal signalling.

Anyway, the question of whether one can get superluminal correlations is not in doubt (discarding my original four unlikely get-out-clauses, or whatever people have subsequently made of them). It seems to me that elevating this to the question of whether one can exploit the phenomenon to send messages is kind of missing the point. OK - in some ways of looking at it - then sending instantaneous messages is perfectly possible - in some other ways it's not possible. But the very existence of superluminal correlations has implications for spacetime structure, irrespective of who's right about the messages.

Zenith

PS: and what you said was a 'postulate' - p=psi^2. Well with my change in the meaning of a word, I can derive that postulate. So it's fundamental status as a postulate is not God-given.

 

[Fredrik]

It seems to me that elevating this to the question of whether one can exploit the phenomenon to send messages is kind of missing the point. OK - in some ways of looking at it - then sending instantaneous messages is perfectly possible - in some other ways it's not possible. But the very existence of superluminal correlations has implications for spacetime structure, irrespective of who's right about the messages.

If you can't use these correlations to synchronize clocks, then there are no implications for spacetime structure.

 

[zenith8]

If you can't use these correlations to synchronize clocks, then there are no implications for spacetime structure.

Whether or not that is true, I don't care. I'm interested in the following question. Given that in at least some interpretations of QM (all 'hidden variables' ones now I think of it) one can in principle exploit nonlocality to send instantaneous messages, what implications does this have for spacetime structure?

Dear me, it seems a perfectly reasonable question to ask.