QFT & QM Questions (about time, locality, gravity, etc.)

In summary, the conversation revolves around the concept of locality in quantum theory. While some sources suggest that quantum field theory (QFT) is provably local and there is no issue with superluminal propagation, others argue that there is a conflict with relativity due to the non-locality and/or absolute time in QFT. There is also a discussion about the choice between QM being the final theory or a hidden variables theory, and how both lead to the abandoning of relativity of simultaneity. The questions raised include whether Schroedinger’s equation evolves wave-functions in a non-local way, if this implies a problem with superluminal propagation, whether the idea of non-locality is still relevant in QFT, and
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Lynch101
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Apologies if there are already an abundance of threads on related questions. I have tried searching for threads on here and have read quite a few, as well as reading other sources. I've kind of reached a point where I need help to parse some of the information that I have been reading and to get a clearer understanding. Some of the information I will be posting in this thread is information I posted in response to a different thread, but I want to ask some more specific questions wrt to the info. I am approaching this from a lay backgroud, so that is the source of my information - which I know is not necessarily ideal.Locality in Quantum Theory

I was always under the impression that QT was fundamentally non-local. But the more I read about QFT the more it seems as though non-locality isn't an issue. But this seems to conflict with statements like this:
Problem of localization in a quantum field theory. Schroedinger’s equation evolves wave-functions in a non-local way, so there seems to be a problem with superluminal propagation.
https://www.perimeterinstitute.ca/research/conferences/convergence/roundtable-discussion-questions/what-are-lessons-quantum

This seems to imply that there is a "problem of localisation" in QFT because Schroedinger’s equation evolves wave-functions in a non-local way. They seem to be saying that there is some issue with superluminal propagation. This would appear to be a conflict with the local realism of Relativity.

I have, however, read that QFT is provably local and that there is no issue of superluminal propagation with regard to signaling - and so there is no such conflict with Relativity.

I take some context from what Lee Smolin says in Time Reborn p.142
despite the successes of quantum field theory, many physicists, beginning with Einstein, have wanted to go beyond it to a deeper theory that gives a complete description of each individual experiment--which, as we have seen, no quantum theory does. Their searches have consistently found an irreconcilable conflict between quantum physics and special relativity.

As long as we’re just checking the predictions of quantum mechanics at the level of statistics, we don’t have to ask how the correlations were actually established. It is only when we seek to describe how information is transmitted within each entangled pair that we need a notion of instantaneous communication. It’s only when we seek to go beyond the statistical predictions of quantum theory to a hidden-variables theory that we come into conflict with the relativity of simultaneity.

This makes it sound like there is actually a conflict but if we don't probe too far [or if we are unable to], then it isn't a problem. I'm reminded of a quote from Wolfgang Rindlers Relativity: Special, General, and Cosmological where he's talking about length contraction: "We cannot and need not know the details of all this, but we know a priori that there must be a detailed mechanical explanation".

Smolin goes on to say
To describe how the correlations are established, a hidden-variables theory must embrace one observer’s definition of simultaneity. This means, in turn, that there is a preferred notion of rest. And that, in turn, implies that motion is absolute. Motion is absolutely meaningful, because you can talk absolutely about who is moving with respect to that one observer--call him Aristotle. Aristotle is at rest. Anything he sees as moving is really moving.
End of story.

In other words, Einstein was wrong. Newton was wrong. Galileo was wrong. There is no relativity of motion.

This is our choice. Either quantum mechanics is the final theory and there is no penetrating its statistical veil to reach a deeper level of description, or Aristotle was right and there is a preferred version of motion and rest.
Smolin appears to be saying that the choice is between QM being the final theory or a hidden variables theory (which would necessitate a preferred reference frame and therefore absolute motion/simultaneity).

He goes on to say
This means giving up the relativity of simultaneity and embracing its opposite: that there is a preferred global notion of time. Remarkably, this does not require overthrowing relativity theory; it turns out that a reformulation of it is enough. The heart of the resolution is a new and deeper way of understanding general relativity theory which reveals a new conception of real time.
He seems to be suggesting that both choices above - QM as a final theory or a hidden variables theory - lead to the abandoning of relativity of simultaneity.

This would seem to imply that the non-locality and/or absolute time of QM as a final theory conflicts with relativity, or that the requirement, in a hidden variables theory, for a preferred reference frame conflicts with relativity.

Questions
  • Does Schroedinger’s equation evolve wave-functions in a non-local way?
  • If so does this imply a problem with superluminal propagation?
  • Is the idea of non-locality gone from QM with local QFT replacing it?
  • How does QFT resolve these issues of simultaneity and non-locality?
  • Is non-relativistic QM finished and replaced by QFT or are there areas where ordinary QM applies that QFT doesn't?
  • Are there other areas, besides particle physics, where QFT applies but QM doesn't?
  • Can QFT be formulated without relativity of simultaneity?
Time in Quantum Mechanics

The idea that Quantum Mechanics employs an absolute notion of time is one I had started to take for granted, because I had encountered it so often.
In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical
Quanta Magazine article (source of citation in wikipedia: the Problem of Time)

In quantum mechanics the situation is rather similar. There is a t in the quantum state and the Schroedinger equation, but it is time as measured by an external clock, which is not part of the system being modeled
A Possible Solution For The Problem Of Time In Quantum Cosmology. (Kaufmann & Smolin).

Quantum mechanics has one thing, time, which is absolute. But general relativity tells us that space and time are both dynamical so there is a big contradiction there. So the question is, can quantum gravity be formulated in a context where quantum mechanics still has absolute time? Or does time have to give. The answer, yes or no, is interesting. If the answer is no, then perhaps some experiment can probe whether or not time is absolute?
https://www.perimeterinstitute.ca/research/conferences/convergence/roundtable-discussion-questions/what-are-lessons-quantum

But, again, QFT would seem to contradict this because it employs the notion of time used in SR which is relative and dynamical, and incorporates the relativity of simultaneity as opposed to absolute time.

Questions
  • Is the Perimeter Roundtable quote correct, does QM have "time which is absolute"?
  • If QM has "time which is absolute", how does QFT resolve this?
QFT and Quantum Gravity

I know the search for a theory of Quantum Gravity is ongoing and not without its issues. One such issue is that of "the Problem of Time". The above quote from the Perimeter Roundtable, with regard to QM having "time which is absolute", is made in the context of QG.

There are other problems referenced in lay literature, such as:
[/quote]After working with the Standard Model for several decades, we are now simultaneously more confident that it’s correct within the limited domain in which it has been tested and less confident of its extendability outside that domain.[/quote]
Lee Smolin Time Reborn

Relativity gives nonsensical answers when you try to scale it down to quantum size, eventually descending to infinite values in its description of gravity. Likewise, quantum mechanics runs into serious trouble when you blow it up to cosmic dimensions. Quantum fields carry a certain amount of energy, even in seemingly empty space, and the amount of energy gets bigger as the fields get bigger. According to Einstein, energy and mass are equivalent (that’s the message of E=mc2), so piling up energy is exactly like piling up mass. Go big enough, and the amount of energy in the quantum fields becomes so great that it creates a black hole that causes the universe to fold in on itself. Oops.
Relativity versus quantum mechanics: the battle for the universe (Guardian website)

Put this in the context of this quote from João Magueijo Faster than the Speed of Light (p.250)
The root of all the evil was clearly special relativity. All these paradoxes resulted from well-known effects such as length contraction, time dilation, or E=mc2, all basic predictions of special relativity. And all denied the possibility of establishing a well-defined border, common to all observers, capable of containing new quantum gravitational effects. Quantum gravity seemed to lack a dam—its effects wanted to spill out all over the place; and the underlying reason was none other than special relativity.

Questions
  • In the Smolin quote about the Standard Model above, do the limitations apply to QFT also?
  • Are there approaches to QG that seek to unify QFT? Do String Theory, LCQ, and MWI attempt to do this?
  • Are there resolutions to the issues that Magueijo mentions above?
Wave Function Collapse Thought Experiment

I may have this all wrong, but I will outline my question and hopefully someone can point out where I've gone astray.

The standard thought experiment with Alice and Bob each being sent one of a pair of entangled particles. I think I've seen it formulated before whereby [in a given referecence frame], Alice and Bob measure the particles simultaneously. Then, in relatively moving reference frames the order of them measuring the particles changes depending on the motion of the frame (as a result of relativity of simultaneity).

From my understanding, the wave function for each of them collapses the instant it is measured by one of them, but because they cannot communicate/signal each other faster than the speed of light, there is no violation of causality. Am I even in the right ball park with this?Questions

What about in the case where only one of them makes a measurement? Let's say Bob just doesn't bother to measure his particle. Does the wave function for his particle still collapse? If so, and in accordance with the relativity of simultaneity, will there be a reference frame in which the wave function of Bob's particle spontaneously collapses and causes the collapse of Alice's particle? Is the wave function in QM completely non-physical?
Again, am I even in the right city with this one (not to mention the right ball park)?
 
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  • #2
That's far too many questions in one thread.

I propose that this thread is closed by a moderator, and you create a few new independent threads with one or two of your questions at a time, in the order of perceived importance, referring to your opening post here as background, so that you do not need to repeat yourself. And then after some time a few more, etc. ...
 
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  • #3
A. Neumaier said:
That's far too many questions in one thread.

I propose that this thread is closed by a moderator, and you create a few new independent threads with one or two of your questions at a time, in the order of perceived importance, referring to your opening post here as background, so that you do not need to repeat yourself. And then after some time a few more, etc. ...
Each section could be separated into separate threads. I'll copy the body of the post and save it and start the separate threads. Thanks.

I just started typing and wanted to get everyting down.
 
  • #4
A. Neumaier said:
That's far too many questions in one thread.

what he said (very small).jpg


Lynch101 said:
I just started typing and wanted to get everyting down.
Suggestion. Type your posts in a text editor before posting then break it into threads as appropriate. You'll find that particularly helpful if something happens and you lose what you've typed so far (it happens).
 
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  • #5
Lynch101 said:
Each section could be separated into separate threads. I'll copy the body of the post and save it and start the separate threads. Thanks.
Some of the sections still contain too many questions. For an orderly discussion you need to focus!
And please don't start 10 threads at the same time, or each one will get much less attention.
 
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  • #6
A. Neumaier said:
Some of the sections still contain too many questions. For an orderly discussion you need to focus!
And please don't start 10 threads at the same time, or each one will get much less attention.
In addition to which, getting some of the questions answered may obviate some of the other questions.
 
  • #7
A. Neumaier said:
That's far too many questions in one thread.

I propose that this thread is closed by a moderator, and you create a few new independent threads with one or two of your questions at a time, in the order of perceived importance, referring to your opening post here as background, so that you do not need to repeat yourself. And then after some time a few more, etc. ...
Agreed.

@Lynch101, please take the above suggestion by @A. Neumaier to heart. Accordingly, I am closing this thread.
 
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Related to QFT & QM Questions (about time, locality, gravity, etc.)

1. What is the difference between quantum field theory (QFT) and quantum mechanics (QM)?

Quantum mechanics is a theory that describes the behavior of particles at the microscopic level, while quantum field theory is a more comprehensive theory that combines quantum mechanics with special relativity to describe the behavior of particles and fields at the subatomic level.

2. How does time work in quantum field theory?

In quantum field theory, time is treated as a parameter rather than a fundamental concept. This means that time is not considered to be a measurable quantity, but rather a variable that is used to describe the evolution of a system.

3. Is locality violated in quantum field theory?

No, locality is not violated in quantum field theory. While quantum mechanics allows for non-local effects, quantum field theory is a locally causal theory, meaning that interactions between particles are limited to their immediate vicinity.

4. How does gravity fit into quantum field theory?

Gravity is not currently included in the framework of quantum field theory. While there have been attempts to incorporate gravity into the theory, such as in string theory, a complete and consistent theory of quantum gravity has not yet been achieved.

5. Can quantum field theory explain the concept of time travel?

No, quantum field theory does not provide a framework for time travel. While some theories, such as the Wheeler-DeWitt equation, suggest the possibility of time travel, it is still a highly debated and speculative topic in the field of quantum physics.

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