Questions on QFT & QM: Is QM or QFT Absolute Time?

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In summary, the conversation discusses the issue of locality in Quantum Theory (QT) and Quantum Field Theory (QFT). It is debated whether QFT is fundamentally non-local, as Schroedinger's equation evolves wave-functions in a non-local way, causing a conflict with the local realism of Relativity. Some argue that QFT is provably local and there is no issue with superluminal propagation or causality. However, others suggest that there is a conflict between quantum physics and special relativity, and a deeper theory is needed to fully describe individual experiments. This could mean embracing a preferred global notion of time and abandoning the relativity of simultaneity. Ultimately, the conversation highlights the ongoing debate and potential implications for the nature of
  • #36
DarMM said:
If the ##t## in the Schrodinger equation can be transformed into the ##t## in a different coordinate system with a different notion of simultaneity then time isn't absolute in the Newtonian sense.
I'll try to clarify my own misunderstanding.

How I'm interpreting it is that you have the quantum system under consideration, with time 't' being provided by a clock outside the system. You're saying this can be transformed into the 't' of another frame.

I'm imagining a system of two relatively moving observers where the time is given by a clock outside the system i.e. common time for both observers and this 't' can be transformed to the 't' of another frame.
 
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  • #37
Lynch101 said:
The quotes taken from the perimeter roundtable are perhaps not meant for a lay audience and may presuppose a level of background knowledge, but the statement about "time which is absolute" is so explicit and it's juxtaposition with the "relative and dynamical" time of GR so intentional that I struggle to see how any level of background knowledge could alter the meaning
And yet you have read it wrong. As @weirdoguy has said they are discussing background dependence, not time being absolute in the Newtonian sense. The contrast is between the background dependence of QM and the background independence (i.e. dynamic background) of GR.

Again it's not about absolute time in the Newtonian sense.
 
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  • #38
Lynch101 said:
I'm imagining a system of two relatively moving observers where the time is given by a clock outside the system i.e. common time for both observers and this 't' can be transformed to the 't' of another frame.
Yes and can be done so in a way that is Lorentz invariant.
 
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  • #39
weirdoguy said:
And I see no justification of such claims in what you've quoted. Mathematics of Minkowski spacetime is internally inconsistent with the notion of absolute time, period.
I'm basing it on Poincaré's derivation of the mathematics. Did he not derive the mathematics of Minkowski independently of him? That is why I ask if they are "baked" into the mathematics.
 
  • #40
weirdoguy said:
Which, per PF rules, is not a suitable source for a discussion. The main issue is not absolute time, because there is none in QFT, it's the 'background dependence' of the theory.
apologies, we can limit it to the statement from the perimeter roundtable
 
  • #41
DarMM said:
And yet you have read it wrong. As @weirdoguy has said they are discussing background dependence, not time being absolute in the Newtonian sense. The contrast is between the background dependence of QM and the background independence (i.e. dynamic background) of GR.

Again it's not about absolute time in the Newtonian sense.
ah, OK. I've read a bit about the background dependence vs independence problem. I'll go back and have a look at that. Thanks
 
  • #42
Lynch101 said:
Did he not derive the mathematics of Minkowski independently of him? That is why I ask if they are "baked" into the mathematics.

Apparently he did, but if he only focused on mathematics he could not base it on the notion of absolute time since time is not a mathematical concept. It sais in your quote that he 'formulated the notion of Lorentz group' and the thing is that the mathematical structure of Lorentz group is inconsistent with absolute time. Actually the structure of Lorentz group leavs barely no room for different physical interpretations.
 
  • #43
Lynch101 said:
Apologies, I've come across the idea of NRQM before but don't fully understand why the likes of Smolin and the others I quoed would talk NRQM when QFT supercedes it.
I fail completely to understand why Smolin talks at all...
 
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  • #44
weirdoguy said:
Apparently he did, but if he only focused on mathematics he could not base it on the notion of absolute time since time is not a mathematical concept. It sais in your quote that he 'formulated the notion of Lorentz group' and the thing is that the mathematical structure of Lorentz group is inconsistent with absolute time. Actually the structure of Lorentz group leavs barely no room for different physical interpretations.
I see. Would it be compatible with timelessness?

If time is not a mathematical concept, would that not mean that relativity of simultaneity is not "baked" into the mathematics of Minkowski?
 
  • #45
DarMM said:
Yes and can be done so in a way that is Lorentz invariant.
Why isn't that at odds with relativity? I thought that there is a time 't' or each particle in relativity, such that if you were to have a clock outside the system, then that would be a separate reference frame
 
  • #46
vanhees71 said:
I fail completely to understand why Smolin talks at all...
:biggrin:
 
  • #47
Lynch101 said:
Why isn't that at odds with relativity? I thought that there is a time 't' or each particle in relativity, such that if you were to have a clock outside the system, then that would be a separate reference frame
Lorentz transforming between different ##t## coordinates is directly a part of Relativity, I can't see how it would contradict it. You're free to use any reference frame to anaylse the situation and able to transform among them. That's all in accord with relativity. I don't see the contradiction.
 
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  • #48
DarMM said:
Lorentz transforming between different ##t## coordinates is directly a part of Relativity, I can't see how it would contradict it. You're free to use any reference frame to anaylse the situation and able to transform among them. That's all in accord with relativity. I don't see the contradiction.
I don't see one myself, it's just that I don't have a clear idea of it.

I'm not sure if the following makes sense, but I'm imagining a system of let's say two particles. How I'm understanding it is that QM uses a common time for both particles, while relativity would have relative, dynamical time for each particle.

EDIT: or maybe it's more a case of there is a common time for the experimenter in the lab and the reatively moving particle? As I say, I'm not entirely clear on it myself
 
  • #49
It's just as it is in Relativity the various systems can be analysed in any other frame. ##t## is just the time of the frame in which one performs the analysis.

Do you know the mathematics of Relativity.
 
  • #50
DarMM said:
It's just as it is in Relativity the various systems can be analysed in any other frame. ##t## is just the time of the frame in which one performs the analysis.

Do you know the mathematics of Relativity.
Unfortunately not. I wish I knew in high school what I know now. I would have made different choices.

Am I wide of the mark when I think that QM can have a common time for a system of relatively moving particles, while relativity would have relative time for each particle?
 
  • #51
Lynch101 said:
Unfortunately not. I wish I knew in high school what I know now. I would have made different choices.

Am I wide of the mark when I think that QM can have a common time for a system of relatively moving particles, while relativity would have relative time for each particle?
QFT would be no different from Relativity in this regard. There's a time for each of the particles and then the time for the frame you are using to analyse these particles.
 
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  • #52
Lynch101 said:
Pablo Acuña L. On the Empirical Equivalence between Special Relativity and Lorentz’s Ether Theory.

As @weirdoguy has already noted, this does not say what you claimed. It says that Poincare derived in detail the mathematical properties of the Lorentz group, which was indeed a very useful mathematical background for later work. But it has nothing whatever to do with "absolute time".

Lynch101 said:
PhysicsForums own policy on LET and Block Universe suggests the predictive equivalence between the two interpretations.

Sure, but that still has nothing to do with what Poincare did.

Lynch101 said:
I would question the idea that relativity of simultaneity is baked into the mathemaical formalism.

This is not only wrong, but egregiously wrong. How much time have you spent working actual problems using the mathematical formalism?

Lynch101 said:
it's confusing as to why these scientists would even consider raising such an issue if QFT completely negated it.

They raise it because they are considering the hypothesis that QFT as we now have it is not a fundamental theory. In other words, they are looking at possible theories that could have QFT as we now have it as an emergent approximation. Those theories might have some kind of "absolute time" in them even if QFT as we now have it does not. They are doing this because that is their current approach to trying to find a theory of quantum gravity.

In other words, when these people talk about "absolute time", they are not talking about anything that has been established by actual measurements or evidence or anything that is claimed by our best current theories at the most fundamental level those theories have. They are talking about a speculative hypothesis that they personally favor. That speculative hypothesis happens to be somewhat similar to how non-relativistic QM works, so they use non-relativistic QM as an analogy to illustrate what their speculative hypothesis is like.
 
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  • #53
PeterDonis said:
As @weirdoguy has already noted, this does not say what you claimed. It says that Poincare derived in detail the mathematical properties of the Lorentz group, which was indeed a very useful mathematical background for later work. But it has nothing whatever to do with "absolute time".

Sure, but that still has nothing to do with what Poincare did.
This is not only wrong, but egregiously wrong. How much time have you spent working actual problems using the mathematical formalism?
But he derived it from relatively moving frames, in a manner similar to how Einstein did. But his interpretation of the mathematics included absolute time and simultaneity. How would this be possible if the mathematics necessitates RoS?
PeterDonis said:
They raise it because they are considering the hypothesis that QFT as we now have it is not a fundamental theory. In other words, they are looking at possible theories that could have QFT as we now have it as an emergent approximation. Those theories might have some kind of "absolute time" in them even if QFT as we now have it does not. They are doing this because that is their current approach to trying to find a theory of quantum gravity.

In other words, when these people talk about "absolute time", they are not talking about anything that has been established by actual measurements or evidence or anything that is claimed by our best current theories at the most fundamental level those theories have. They are talking about a speculative hypothesis that they personally favor. That speculative hypothesis happens to be somewhat similar to how non-relativistic QM works, so they use non-relativistic QM as an analogy to illustrate what their speculative hypothesis is like.
The question I had though, was how such an issue could even be raised if QFT already took care of this.

From the thread I started on entanglement it would appear that it comes down to a question of realism/representationalism vs anti-realism.
 
  • #54
Lynch101 said:
he derived it from relatively moving frames

Basically, yes.

Lynch101 said:
his interpretation of the mathematics included absolute time and simultaneity

His interpretation of the math is not the math. If there is nothing in the math about absolute time and absolute simultaneity, which there isn't, then the math doesn't have those things.

Also, the reference you gave says nothing whatever about his interpretation of the math. Where are you getting his interpretation of the math from?

Lynch101 said:
The question I had though, was how such an issue could even be raised if QFT already took care of this.

Did you read what I said? I'll quote it again:

PeterDonis said:
they are considering the hypothesis that QFT as we now have it is not a fundamental theory

If QFT as we now have it is not a fundamental theory, then it can't "take care of" anything, because it's not fundamental, and a more fundamental theory could have different properties.
 
  • #55
PeterDonis said:
Basically, yes.

His interpretation of the math is not the math. If there is nothing in the math about absolute time and absolute simultaneity, which there isn't, then the math doesn't have those things.
Does that not imply that relativity of simultaneity is also not in the math, but rather an interpretation of it?

PeterDonis said:
Also, the reference you gave says nothing whatever about his interpretation of the math. Where are you getting his interpretation of the math from?
The paper I referenced talks about the Lorentz-Poincaré interpretation and its basis of absolute time and simultaneity.
PeterDonis said:
If QFT as we now have it is not a fundamental theory, then it can't "take care of" anything, because it's not fundamental, and a more fundamental theory could have different properties.
Does it boil down to the realism vs anti-realism debate?
 
  • #56
Lynch101 said:
Does that not imply that relativity of simultaneity is also not in the math, but rather an interpretation of it?

No. Relativity of simultaneity--the fact that simultaneity is different in different Lorentz frames--is directly in the math. The "interpretation" under which "absolute simultaneity" is claimed simply consists of saying that one particular Lorentz frame (but we can never know which one) is the "absolute" one, and its (relative) notion of simultaneity is called "absolute simultaneity" (and similarly its relative notion of time is called "absolute time"). But there's nothing like that in the math, because there's nothing in the math that picks out anyone particular Lorentz frame as being different from the others; they're all the same in the math.

Lynch101 said:
Does it boil down to the realism vs anti-realism debate?

I don't think so; I think that question is a different question from the question of whether QFT as we currently have it is truly fundamental, or whether it's emergent from some other more fundamental theory.
 
  • #57
PeterDonis said:
No. Relativity of simultaneity--the fact that simultaneity is different in different Lorentz frames--is directly in the math. The "interpretation" under which "absolute simultaneity" is claimed simply consists of saying that one particular Lorentz frame (but we can never know which one) is the "absolute" one, and its (relative) notion of simultaneity is called "absolute simultaneity" (and similarly its relative notion of time is called "absolute time"). But there's nothing like that in the math, because there's nothing in the math that picks out anyone particular Lorentz frame as being different from the others; they're all the same in the math.
But the Lorentz group was derived from the relative motion between the two [absolutely] moving inertial frames. I was thinking that the absolute reference frame could be considered as "scaffolding" and later removed. This would leave just the two relatively movinng frames but with no relativity of simultaneity.

My understanding is that the relativity of simultaneity is derived from the metaphysical assumption of the one-way SoL, as opposed to being a necessary consequence of the mathematics?
PeterDonis said:
I don't think so; I think that question is a different question from the question of whether QFT as we currently have it is truly fundamental, or whether it's emergent from some other more fundamental theory.
Can QFT be considered fundamental if it doesn't explain what happens in individual experiments?
 
  • #58
Lynch101 said:
the Lorentz group was derived from the relative motion between the two [absolutely] moving inertial frames

This would be a valid description of one way of deriving the Lorentz group if the word "absolutely" were not there. But with the word "absolutely" included, it's wrong. There is nothing whatever in the math that picks out any inertial frame as being different from any other, or that picks out particular frames as being "moving" or "at rest".

How familiar are you with the math of SR? I think you said before that you had never studied it. If you haven't, you should really, really, really take the time to do so before making claims about it.

Lynch101 said:
My understanding is that the relativity of simultaneity is derived from the metaphysical assumption of the one-way SoL, as opposed to being a necessary consequence of the mathematics?

Your understanding is wrong. Go study the math.

Lynch101 said:
Can QFT be considered fundamental if it doesn't explain what happens in individual experiments?

That's a matter of opinion. Some physicists think it can, others think it can't. Smolin and his colleagues appear to be among the latter group.
 
  • #59
PeterDonis said:
This would be a valid description of one way of deriving the Lorentz group if the word "absolutely" were not there. But with the word "absolutely" included, it's wrong. There is nothing whatever in the math that picks out any inertial frame as being different from any other, or that picks out particular frames as being "moving" or "at rest".
I understand that point. Apologies if this is over speculative but I'm just thinking out loud when I say that [I imagine] we could take Poincaré's derivation where he starts with the absolute reference frame and we could treat that absolute reference frame as "scafffolding" which can later be removed. This would leave us with the mathematics of Minkowski but without RoS.

I'm not certain about this, again something that occurred to me as I've been learning more about this; an alternative would be to amend Einstein's derivation with a roundttrip light principle, given that the one-way SoL hasn't been (or possibly can't be) measured. I imagine this should also return the mathematics of Minkowski without RoS "baked" in.

PeterDonis said:
How familiar are you with the math of SR? I think you said before that you had never studied it. If you haven't, you should really, really, really take the time to do so before making claims about it.

Your understanding is wrong. Go study the math.
I'm familiar with the math and others have applied it in discussions and explained how it works, so I haven't formally studied it but I have an understanding of it. The point that RoS comes from the one-way SoL is based on statements made by others who are familiar withh the math. They have stated that the one-way speed of light is encoded in the math and that RoS follows from Einstein's 2 postulates - the 2nd being the one-way SoL.
PeterDonis said:
That's a matter of opinion. Some physicists think it can, others think it can't. Smolin and his colleagues appear to be among the latter group.
ah, I see.
 
  • #60
Lynch101 said:
we could take Poincaré's derivation where he starts with the absolute reference frame

You're missing the point: Poincare's derivation does not start with an absolute reference frame. There is no absolute reference frame anywhere in the math. The only "absolute reference frame" was in Poincare's personal interpretation.

Lynch101 said:
The point that RoS comes from the one-way SoL is based on statements made by others who are familiar withh the math.

Please give specific references.
 
  • #61
PeterDonis said:
You're missing the point: Poincare's derivation does not start with an absolute reference frame. There is no absolute reference frame anywhere in the math. The only "absolute reference frame" was in Poincare's personal interpretation.
I'm basing it on this:
the transformations involved three reference frames. Frame S0 is at rest in the ether, S is a Galilean frame moving with velocity v with respect to S0 , and S' is an auxiliary frame that also moves with velocity v with respect to S0 . S0 and S are connected by the Galilean transformations, whereas S and S' are connected by the [Lorenntz transformations]. Combining these two transformations we obtain the transformations connecting So and S'
Pablo Acuña L. - On the Empirical Equivalence between Special Relativity and Lorentz’s Ether Theory

The thrust of the paper is to give further reasoning for favoring Einstein's interpretation over the Lorentz-Poincare interpretation, but in the paper he tries to create a steelman of the LP interpretation where he says that the Ether can be removed from the theory to leave just the absolute reference frame. My thinking is that this can be removed as well.

PeterDonis said:
Please give specific references.
I'll take a look for these.
 
  • #62
Lynch101 said:
I'm basing it on this

What does this have to do with Poincare's derivation? The footnote it appears in in the paper you referenced talks about a work by Lorentz. Also, the quote you give talks about a Galilean transformation as well as a Lorentz transformation, so whatever it is, it isn't a derivation of the Lorentz transformation.
 
  • #63
PeterDonis said:
What does this have to do with Poincare's derivation? The footnote it appears in in the paper you referenced talks about a work by Lorentz.
Does it not form the basis for Poincaré's derivation?

PeterDonis said:
Also, the quote you give talks about a Galilean transformation as well as a Lorentz transformation, so whatever it is, it isn't a derivation of the Lorentz transformation.
Is that not how the Lorentz transformation was derived by Lorentz?
 
  • #64
Lynch101 said:
Can QFT be considered fundamental if it doesn't explain what happens in individual experiments?
Yes, since this is indeed what's observed in the experiments: Depending on the preparation of the measured system the outcome of measurements of all but maybe a few observables is indetermined.

Of course, whether or not this is inherent randomness of nature or just our ignorance of some hidden variables (i.e., observables not yet known and thus not taken into account), one can of course not decide. However, as the very well established violation of Bell's inequalities prove, if there's a deterministic HV theory describing all the facts QT is describing, it must be a non-local theory, and to find such a theory is at least very difficult in connection with relativity. Nevertheless as far as I know, there's also no mathematical proof of a corresponding "no-go theorem", i.e., it could be that indeed such a non-local deterministic theory exists. If so, it's not found yet.
 
  • #65
Lynch101 said:
Does it not form the basis for Poincaré's derivation?

Not what is referred to in that footnote, no.

Lynch101 said:
Is that not how the Lorentz transformation was derived by Lorentz?

Not what is referred to in that footnote, no.
 
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  • #66
I've never seen Poincare's derivation of the LT. Can you point me to the paper (if possible in English or German translation; my French is close to inexistent :-(().
 
  • #67
vanhees71 said:
I've never seen Poincare's derivation of the LT.

As far as I can tell from looking at sources available online, neither Lorentz nor Poincare actually derived the LT at all, in the sense of starting from some more basic axioms. They simply observed that Maxwell's Equations were invariant under the LT, not Galilean transformations, and tried to draw inferences from that fact. So the first actual derivation of the LT might well be Einstein's in his 1905 paper.
 
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  • #68
Sure, the great breakthrough by Einstein was to realize that the invariance of Maxwell's equations under Lorentz transformations (as we'd formulate it today) is not simply a mathematical curiosity but a fundamental discovery for all of physics, not only for electrodynamics. It lead to the unique idea that the description of space and time for all of physics has to be changed.

The mathematics was known much earlier. The earliest reference I know is a paper by Woldemar Voigt:

https://eudml.org/doc/180122

It's not the final version of the LT though.
 
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  • #69
PeterDonis said:
Not what is referred to in that footnote, no.
Not what is referred to in that footnote, no.
My apologies, I was under the impression that Einstein derived the LT from his two postulates, with the one-way SoL being the 2nd postulate and that the relativity of simultaneity was a consequence of the one-way SoL.
 
  • #70
Lynch101 said:
I was under the impression that Einstein derived the LT from his two postulates

You didn't mention Einstein in the post of yours that I responded to in the response you quoted in post #69. You mentioned Poincare and Lorentz. Their derivations of the LT (to the extent they are "derivations" at all--see my post #67) are not the same as Einstein's. You can't keep moving the goalposts whenever you are told you have an incorrect understanding.

Lynch101 said:
with the one-way SoL being the 2nd postulate and that the relativity of simultaneity was a consequence of the one-way SoL

Have you read Einstein's 1905 paper? It's here:

http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdf
Read it and what you think. It's always best to go to the primary source instead of relying on second-hand accounts.
 

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