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- Summary
- I've heard that QFT is the unification of SR and QM but much of the stuff I have read in lay literature seems to suggest certain conflicts that [should] theoretically remain. I'm hoping to resolve my confusion.

It was recommended that I start separate threads, as I have quite a number of questions on QM & QFT. I'm including all the relevant information/quotes in this thread just for the sake of reference, but there are fewer questions. It might seem like an excessive amount of information but it's all relevant to the questions I have - as the questions arise out of the info. The questions are posted spearately and clearly at the end, if anyone gets fed up with reading through the post.

To give my background:

I am approaching this from a lay backgroud, so that is the source of my information - which I know is not necessarily ideal. 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.

My main area of interest is the nature of time and this has naturally brought me into contact with Relativity and Quantum Theory, so my questions tend to be focused more in that context.

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:

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 hence no issue with regard to causality - and so there is no such conflict with Relativity.

I take some context from what Lee Smolin says in Time Reborn p.142

Smolin goes on to say

He goes on to say

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.

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.

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.

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 is often how the problem of time is expressed.

There are other problems referenced in lay literature, such as:

Put this in the context of this quote from João Magueijo Faster than the Speed of Light (p.250)

I have more, related questions and even these ones could be expanded, but I will see where these ones go.

To give my background:

I am approaching this from a lay backgroud, so that is the source of my information - which I know is not necessarily ideal. 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.

My main area of interest is the nature of time and this has naturally brought me into contact with Relativity and Quantum Theory, so my questions tend to be focused more in that context.

__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:

Perimeter Institute RoundtableProblem 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.

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 hence no issue with regard to causality - and so there is no such conflict with Relativity.

I take some context from what Lee Smolin says in Time Reborn p.142

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 Rindlersdespite 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.

*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

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).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.

He goes on to say

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 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.

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.

__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.

Quanta Magazine article (source of citation in wikipedia: the Problem of Time)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

A Possible Solution For The Problem Of Time In Quantum Cosmology. (Kaufmann & Smolin).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

Perimeter Institute RoundtableQuantum 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?

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.

__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 is often how the problem of time is expressed.

There are other problems referenced in lay literature, such as:

Lee Smolin Time RebornAfter 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.

Relativity versus quantum mechanics: the battle for the universe (Guardian website)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.

Put this in the context of this quote from João Magueijo Faster than the Speed of Light (p.250)

And this: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.

Andrei Khrennikov (2016)it is meaningless to try to unify QFT so heavily suffering of infinities with GR. We also highlight difficulties of the QFT-treatment of entanglement.

*The Present Situation in Quantum Theory and its Merging with General Relativity*__Questions__- Has QFT completely superceded Quantum Mechanics or is there a domain where QM applies but QFT doesn't and vice versa - not necessariy only with regard to Quantum Gravity?
- Is the "problem of superluminal propagation" referred to in the Perimeter Roundtable quote above an issue in any way, in the way that Smolin suggests (i.e. going beyond "the statistica"), or is it competely resolved?
- Is there any issue whatsoever with regard to the conceptualisation of "time" between QM and GR; does QM have "time which is absolute" while GR has relative time; is the relativity of simultaneity indispensible in QFT or future theories of Quantum Gravity.

I have more, related questions and even these ones could be expanded, but I will see where these ones go.

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