I General Relativistic Quantum Theory?

[PeterDonis]

The physicists reasonings might be that they are no phenomena at low energy not explainable by our GR and QFT

Exactly.

kindly give models where the ultimate framework occurs at low energy

There aren't any, for the very reason you gave and I said "exactly" to above.

 

[jake jot]

It's right there in what you quoted from me: "He was never convinced that QM was a necessary part of a fundamental theory." He thought a classical unified field theory was the way to go, so that's what he worked on.

But there was already an electron. What is Einstein thought of electron classically? Even if he was not convinced of QM. Still the electron exists. How would he embed electrons in the unified field theory? Maybe electron as some kind of geometry? What is it?

 

[PeterDonis]

But there was already an electron.

That was true even in 1905 when Einstein published his original paper on special relativity. (The electron was discovered in 1897.) That doesn't mean Einstein was required to produce a theory of it.

What is Einstein thought of electron classically? Even if he was not convinced of QM. Still the electron exists. How would he embed electrons in the unified field theory?

I don't know if Einstein considered the electron or any charged particles specifically in his unified field theory work. I think he had a very general idea that a charged "particle" would be described as a solution of the classical field equations that looked like a dense bundle of energy in a very small volume. I don't think he ever actually found such solutions mathematically, however.

 

[bhobba]

What is Einstein thought of electron classically?

I do not think he had any specific ideas, except maybe as part of some unified theory he was working on at the time (he worked on a few). Actually Dirac discovered classically the electron had its problems:
https://cds.cern.ch/record/419756/files/9912045.pdf

He fully believed in QM, except at the beginning where his famous debates with Bohr were part of the reason he, and every other physicist I am aware of, came to believe in QM. I have posted it before, but here is his interpretation of QM:
https://www.informationphilosopher.com/solutions/scientists/ballentine/AJP72.pdf

It is still around today, being called the Ensemble Interpretation. It is the interpretation in one of the most respected modern QM books:
https://www.amazon.com/dp/9814578584/?tag=pfamazon01-20

The only real issue Einstein had was till his dying day he believed it incomplete, and part of a more classical unified field theory he spent the latter part of his life working on. In fact, for what it's worth, so do I, except I do not think it is likely 'classical'.

Thanks
Bill

 

[atyy]

He fully believed in QM, except at the beginning where his famous debates with Bohr were part of the reason he, and every other physicist I am aware of, came to believe in QM. I have posted it before, but here is his interpretation of QM:
https://www.informationphilosopher.com/solutions/scientists/ballentine/AJP72.pdf

It is still around today, being called the Ensemble Interpretation. It is the interpretation in one of the most respected modern QM books:
https://www.amazon.com/dp/9814578584/?tag=pfamazon01-20

Ballentine's ensemble interpretation is not Einstein's interpretation. Ballentine's textbook has many fundamental errors and cannot be recommended except to experts.

 

[atyy]

It's right there in what you quoted from me: "He was never convinced that QM was a necessary part of a fundamental theory." He thought a classical unified field theory was the way to go, so that's what he worked on.

But there was already an electron. What is Einstein thought of electron classically? Even if he was not convinced of QM. Still the electron exists. How would he embed electrons in the unified field theory? Maybe electron as some kind of geometry? What is it?

Couldn't he have used a classical Dirac field? Or would that not count that as having an "electron" since there is no quantization?

This has nothing to do with Einstein's classical unification efforts, but here's an interesting reference for the classical Einstein-Maxwell-Dirac equations:

The Einstein-Dirac-Maxwell Equations - Black Hole Solutions
Felix Finster, Joel Smoller, Shing-Tung Yau
https://arxiv.org/abs/gr-qc/9910030

 

[PeterDonis]

Ballentine's textbook has many fundamental errors

Can you give some specific examples?

 

[PeterDonis]

Couldn't he have used a classical Dirac field?

In principle he could have considered that, but I don't know of any evidence that he did. AFAIK the only fields he was considering were the metric tensor $g_{\mu \nu}$ and the EM field tensor $F_{\mu \nu}$.

 

[atyy]

Can you give some specific examples?

Ballentine lacks a clear statement of collapse or state reduction. He misrepresents the Copenhagen interpretation, and suggests that the Copenhagen interpretation is in conflict with experiment (Chapter 9). Ballentine's lack of collapse makes him give the wrong result in conflict with experimental outcomes on the "watched pot" experiment.

 

[jake jot]

It depends on what you call "spacetime". If you call all 10 (or 11) dimensions postulated by string theory "spacetime", then 6 of those dimensions are the ones in the Calabi-Yau spaces, yes. But if you only call the 4 dimensions we actually observe "spacetime", then no, the 6 dimensions in the Calabi-Yau spaces are different from those.

My understanding (which might possibly be mistaken; perhaps other experts on this forum can weigh in here) of how "spacetime emerges" in string theory is that the string mode that looks like a massless spin-2 field at low energy only affects the 4 dimensions we actually observe, not the others. If that is correct, then the dimensions contained in the Calabi-Yau spaces do not emerge that way; and that was what you were asking about.
No. You have our normal spacetime of 4 dimensions, and then 6 (or 7) other dimensions that are compactified.
Yes.
Not the EFE of GR, no. That EFE is specifically for 4 dimensions.

About the "That EFE is specifically for 4 dimensions.". Does it mean whenever there are 4 dimensions, there is automatically GR? Or are there 4D worlds that are not described by GR? If so, what would change if the world is 4D but doesn't have Equivalence principle and if you put feather and iron core, they don't fall at same time?

In LQG, there is always the comment it can't recreate GR. So I wonder if LQG can somehow create 4D world but not described by GR.

Also Hossenfelder wrote in Sabine Hossenfelder: Backreaction

"Personally, I find it misleading to say that in this case, time is not real. It’s like claiming that because our theories for the constituents of matter don’t contain chairs, chairs are not real. That doesn’t make any sense. But leaving aside that it’s bad terminology, is it right that time might fundamentally not exist?

I have to admit it’s not entirely implausible. That’s because one of the major reasons why it’s difficult to combine quantum theory with general relativity is that… time is a dimension in general relativity. In Quantum Mechanics, on the other hand, time is not something you can measure. It is not “an observable,” as the physicists say. In fact, in quantum mechanics it is entirely unclear how to answer a seemingly simple question like “what is the probability for the arrival time of a laser signal”. Time is treated very differently in these two theories. "Is it possible Quantum Mechanics lives not in the 4D spacetime of GR but it has its own space where time is different?

All this is to prepare for reading Julian Barbour new book "The Janus Point" (which she mentioned at the end).

The Janus Point: A New Theory of Time: Barbour, Julian: 9780465095469: Amazon.com: Books

In a review

"In his radical new book, Julian Barbour argues that...time flows in not one, but two ways... Such an argument might seem overly technical, but it's explained simply and accessibly for all to understand."
―BBC Science Focus

But a reader has this to say:

"Left me baffled
Reviewed in the United Kingdom on December 29, 2020

I found this book incomprehensible - even with a strong science background. The explanations given for the Janus point and the diagrams illustrating it are not well explained and the text left me totally baffled in too many places.".

This thread can prepare or make me see even slightly or catch a glimpse through the cloudy dense veil of Julian Barbour uncanny ideas. Thank you.

The paper isn't investigating when "GR is no longer applicable". It is investigating how large the compactified dimensions could be without conflicting with observations. That has nothing to do with GR not being applicable. See below.
Not as geometry, no. As above, GR describes only the geometry of the 4 ordinary spacetime dimensions we observe. As far as GR is concerned, these "extra dimensions", or more precisely their effects as manifested in things like new particles or fields beyond the ones we already know of (in the Standard Model of particle physics) would appear as part of the stress-energy tensor not the spacetime geometry. That doesn't mean GR is "not applicable"; it just means GR doesn't describe the "extra dimensions" as spacetime geometry.
There isn't one. Why do you think there is? I don't see anything saying this in the paper you linked to.

 

[PeterDonis]

About the "That EFE is specifically for 4 dimensions.". Does it mean whenever there are 4 dimensions, there is automatically GR?

No. Mathematically, you can have 4-dimensional manifolds that don't even have a metric.

what would change if the world is 4D but doesn't have Equivalence principle and if you put feather and iron core, they don't fall at same time?

Such questions are meaningless unless you give us a specific alternative theoretical model to GR to use when answering them.

In LQG, there is always the comment it can't recreate GR.

This tells us nothing without a specific reference.

Is it possible Quantum Mechanics lives not in the 4D spacetime of GR but it has its own space where time is different?

This is personal speculation and is off topic for this forum.

All this is to prepare for reading Julian Barbour new book "The Janus Point"

If you want to discuss one of Barbour's models, you need to look at his peer-reviewed papers and start a separate thread on one of them.

 

[PeterDonis]

This thread has run its course and is now closed.