Why Do We Need a Quantum Theory of Gravity?

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In summary: Standard Model?In summary, the physicists at Not Even Wrong think that we need to quantize gravity because it will lead to a more fundamental version of the Standard Model. They also believe that this is something that can be done rigorously and that it is important to do because it will help us understand how the universe works.
  • #36
No need to panic. :smile:

As to the "not friendly towards MWI", MWI indeed seems to provoque, as only critique, emotional statements like "rubbish", or "meaningless words" or something of the kind. I've never seen a constructive, argumented discussion of which the conclusion is that MWI suffers from logical problems. Most arguments go along the "naah, can't be true" line.

So I'm going to find Leggett's article and read it, to see if he does anything more than the others. IF there's some substance to it, I'll discuss it in the QM forum, or in the philosophy forum.
 
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  • #37
vanesch said:
So I'm going to find Leggett's article and read it, to see if he does anything more than the others. IF there's some substance to it, I'll discuss it in the QM forum, or in the philosophy forum.

Given that there is no substance to it, I can just as well continue here :-)

Ok, I read the paper (I now recall having read it before). The first part is OK, he refers to the 3 major lines of interpretation:
1) statistical ensemble (no objective reality is existing, or is described by QM, which is seen as a problem)
2) Copenhagen (there's somehow a change from micro to macro) and an argument is presented why this is inconsistent.
3) MWI (no reasoned argument presented, just stated that "must be a meaningless collage of words", as I predicted).
So logically he says:
QM <=> (1) or (2) or (3).
Argument against (1) = "no reality" (ok)
Argument against (2) = "inconsistency" (ok)
Argument against (3) = "must be a meaningless collage of words" (?)
hence NOT(QM). Hmmm.
Given the weakness of the "argument" against (3), I'd say:
QM <==> (3)
That's all I'm saying too!

It could of course be that we have NOT(QM) and NOT(3). But you cannot use the non-argument NOT(3) to conclude NOT(QM), because it is not a well-build up argument.

That said, apart from this weakness about NOT(3), the article is a good read.
 
  • #38
vanesch said:
Given that there is no substance to it, I can just as well continue here :-)
Ok, I read the paper (I now recall having read it before). The first part is OK, he refers to the 3 major lines of interpretation:
1) statistical ensemble (no objective reality is existing, or is described by QM, which is seen as a problem)
2) Copenhagen (there's somehow a change from micro to macro) and an argument is presented why this is inconsistent.
3) MWI (no reasoned argument presented, just stated that "must be a meaningless collage of words", as I predicted).
So logically he says:
QM <=> (1) or (2) or (3).
Argument against (1) = "no reality" (ok)
Argument against (2) = "inconsistency" (ok)
Argument against (3) = "must be a meaningless collage of words" (?)
hence NOT(QM). Hmmm.
Given the weakness of the "argument" against (3), I'd say:
QM <==> (3)
That's all I'm saying too!
It could of course be that we have NOT(QM) and NOT(3). But you cannot use the non-argument NOT(3) to conclude NOT(QM), because it is not a well-build up argument.
That said, apart from this weakness about NOT(3), the article is a good read.
:smile: :smile: :smile: :smile: Anyway, I am going to quit discussing this, since athough I acknowledge it is logically consistent: MWI has many other problems too (probability you are in one universe = zero) and my common sense is just smacked to the ground by this fairy tale.

I think it is good anyway that by discussing this we point out that the cat is still alive unless you partially digest it by zombies. :biggrin:

Cheers,

Careful
 
  • #39
Careful said:
MWI has many other problems too (probability you are in one universe = zero)

Not in the right versions of MWI. You're right with the "state counting" versions of it. But those accepting an a priori Born rule, have not this difficulty.

and my common sense is just smacked to the ground by this fairy tale.

This is my point exactly. It's the only "argument" I've ever seen. :-p
 
  • #40
vanesch said:
Not in the right versions of MWI. You're right with the "state counting" versions of it. But those accepting an a priori Born rule, have not this difficulty.
This is my point exactly. It's the only "argument" I've ever seen. :-p
Hi Patrick,

I think there exist serious arguments against your interpretation by the following: as everyone knows, local realism cannot reproduce all quantum predictions (it can reproduce in principle all experiments till now though). Local realism consists of three components : (a) realism (b) arrow of time (c) locality. By giving up (b) to a reasonable extend, that is by allowing for (limited) backwards causation the EPR QM correlations can be recovered (the electrons figure out what the detector state will be in the future, hence opening fully the locality loophole). Therefore, QM has a spacetime realist and local realisation which is much more ECONOMIC than the standard QM formalism and certainly not more crazy than what your interpretation of MWI is concerned. Consequently, by Occam's razor, backwards causation is a superior alternative to your MWI.

Cheers,

Careful
 
  • #41
Careful said:
Therefore, QM has a spacetime realist and local realisation which is much more ECONOMIC than the standard QM formalism and certainly not more crazy than what your interpretation of MWI is concerned. Consequently, by Occam's razor, backwards causation is a superior alternative to your MWI.

I would be all takers, you know. But I don't know of any formalism where this backward causation is implemented and which shows equivalence to QM. Any readable references on it ?
 
  • #42
vanesch said:
I would be all takers, you know. But I don't know of any formalism where this backward causation is implemented and which shows equivalence to QM. Any readable references on it ?
:smile: :smile: I will look them up, this is old stuff from the seventies and eighties as far as I remember (so from my childhood time). Mind: the papers I have scanned only concern EPR type correlations but I think we agree that this is sufficient. It must be clear, moreover, that I feel this option is close to the level of zombies :biggrin: :biggrin:

Always beat something crazy by something which is only slightly less unbelievable I say... :smile: :smile:
 
  • #43
Careful said:
Mind: the papers I have scanned only concern EPR type correlations but I think we agree that this is sufficient. It must be clear, moreover, that I feel this option is close to the level of zombies

After a short search, I fell onto some papers by Wharton or so, which are not really helpful (just words about how a backwards causation can (of course!) solve an EPR situation in a local way), and then of course the transactional interpretation:
http://mist.npl.washington.edu/ti/TI_toc.html
I should probably look deeper into this one...

BTW, too bad that the author has a total misunderstanding of MWI...
 
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  • #44
vanesch said:
After a short search, I fell onto some papers by Wharton or so, which are not really helpful (just words about how a backwards causation can (of course!) solve an EPR situation in a local way), and then of course the transactional interpretation:
http://mist.npl.washington.edu/ti/TI_toc.html
I should probably look deeper into this one...

BTW, too bad that the author has a total misunderstanding of MWI...
Hi Patrick,

The author which I remember is Olivier Costa de Beauregard who has written in the seventies technical papers on this subject (however I am unaware of an internet source for them). A paper with some references is quant-ph/9804069 and another semi technical paper can be found on http://atkinson.fmns.rug.nl/public_html/narlikar.pdf

It appears to be a funny/tragic story, de Beauregard seems to have gotten lots and lots of critique by many people, amongst which Alain Aspect (it was easy to attack him since he believed in paranormal phenomena- but so do you :wink: ).

Cheers,

Careful
 
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  • #45
Careful said:
Hi Patrick,

The author which I remember is Olivier Costa de Beauregard who has written in the seventies technical papers on this subject (however I am unaware of an internet source for them). A paper with some references is quant-ph/9804069 and another semi technical paper can be found on http://atkinson.fmns.rug.nl/public_html/narlikar.pdf

An interesting read. I'll start a thread on the transactional interpretation in the QT forum, because I would like to learn more about the differences in the formalism and it supposed isomorphism with the formalism of unitary QM. In fact, it is my gut feeling that the superposition principle will STILL hold, and that my body will STILL appear in several states if it is included in the theory. But in order for that, I need to learn more about the formalism behind the transactional interpretation, and what exact mathematical structure is supposed to be the description of nature. Apparently, it is NOT the wave function as we know it in unitary QM.
 
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  • #46
vanesch said:
In fact, it is my gut feeling that the superposition principle will STILL hold, and that my body will STILL appear in several states if it is included in the theory. But in order for that, I need to learn more about the formalism behind the transactional interpretation, and what exact mathematical structure is supposed to be the description of nature. Apparently, it is NOT the wave function as we know it in unitary QM.

Indeed, it is a different ``classical´´ approach AFAIK; that is why I said it is superior (no second quantization). To tell you more, I should have a good look at the papers (books) of de Beauregard. Now you perhaps better see why starting out from pure local realism (for quantum gravity) is not so dangerous at all (if you are prepared to reconsider the arrow of time if necessary).

Cheers,

Careful
 
  • #47
Careful said:
Indeed, it is a different ``classical´´ approach AFAIK; that is why I said it is superior (no second quantization).

But then it is not an "interpretation" of the quantum formalism but radically a different theory (which might, or might not, be empirically equivalent to quantum theory). IF it turns out to be empirically equivalent, then it must also be COMPUTATIONALLY vastly superior (we've had this discussion before). But I'd like to see a proof of this equivalence first. Well, I'd like to see the *theory* first! Because in the things I found, none is really presented. Just some very limited aspects.

(if you are prepared to reconsider the arrow of time if necessary).

I don't see any conceptual problem with that. In fact, relativity already did that, no ? Cfr our discussion about the relationship between the "time coordinate" on the static blob of spacetime, and our subjective experience of it :wink:

EDIT: but as this has not much to do with quantizing gravity anymore, I would like to invite you to the thread I started on the topic in the quantum physics forum, in order not to get this thread completely derailed.
 
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  • #48
** But then it is not an "interpretation" of the quantum formalism but radically a different theory (which might, or might not, be empirically equivalent to quantum theory). IF it turns out to be empirically equivalent, then it must also be COMPUTATIONALLY vastly superior (we've had this discussion before). But I'd like to see a proof of this equivalence first. Well, I'd like to see the *theory* first! Because in the things I found, none is really presented. Just some very limited aspects. **

Well, I am going to check out the papers of de Beauregard and others.


** I don't see any conceptual problem with that. In fact, relativity already did that, no ? Cfr our discussion about the relationship between the "time coordinate" on the static blob of spacetime, and our subjective experience of it :wink: **

Well yes, but then you have the problem why for big macroscopic objects we have an arrow of time (future not influencing the past) - this is what I wrote in PM to you. My first idea would be that there is some timescale involved which is inverse proportional to the mass.

Moreover, it is pretty clear that we are dealing here with a different theory, since the idea behind it is to be able to *understand* the correlations predicted by QM in terms of physical spacetime mechanisms (and/or suitable boundary conditions); while in QM these correlations are merely kinematical. Let me add that this zigzagging picture in time was also suggested by Feynman, Dirac and others...

** EDIT: but as this has not much to do with quantizing gravity anymore, I would like to invite you to the thread I started on the topic in the quantum physics forum, in order not to get this thread completely derailed. **

IMO it has everything to do with QG ! A spacetime realist theory of QM would basically solve QG.
 
  • #49
Does the need to find a quantum theory of gravity imply that the
gravitational field must be quantized? Physicists working in quantum
gravity routinely assume an affirmative answer, often without being
aware of the metaphysical commitments that tend to underlie this
assumption. The ambition of this article is to probe these commitments
and to analyze some recently adduced arguments pertinent to the issue
of quantization. While there exist good reasons to quantize gravity, as
this analysis will show, alternative approaches to gravity challenge the
received wisdom. These renegade approaches do not regard gravity as
a fundamental force, but rather as effective, i.e. as merely supervening
on fundamental physics. I will urge that these alternative accounts at
least prove the tenability of an opposition to quantization.
 

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