Why quantize gravity?

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Jee, you are chatty today :rofl: I will be brief since it is bed time over here and I shall only react upon the points where I disagree :biggrin: just because I am a grumpy person.

(a) A semantic issue about the incompatibility of QM and GR: I guess we both agree upon what it means but we disagree upon the formulation. So, lets quit that one, it is getting boring.

(b) How to reach QG? My point of view is very clear: we have NO experimental guideline whatsoever (at least Einstein did have some indications for GR) so we are entirely lost. The good way to proceed in this case then is to radically sacrifice one of both theories and build upon the other one, i.e. see how far one theory ``deviates´´ from the other.
(1) LQG, STRT and CDT builds upon QM and it is far from sure that they get out GR in the classical limit.
(2) Local realism starts from GR and has to get out microscopic physics (note: this is NOT necessarily equal to QM).

Approach (1) has a long very active history and according to me, a key problem on the road there is the measurement (or micro - macro) problem in QM with the bourlesque name: Schroedinger's cat. This problem is especially important since I want to know how the macroworld, with its local realism can be understood to emerge from a microscopic world with an operational ontology. IMO, this problem is unsolvable within contemporary QM and little or no progress has been made in this field.
Penrose OR proposal is one of the more intelligent proposals I have heard concerning this problem. So (1) has a really bad record if you remember my list of four laws.

Now (2) is not experimentally falsified and it is really a mystery for me why people have not put more effort into this:
(a) there are no ontological problems whatsoever (so the approach is self consistent)
(b) spacetime locality and covariance are the MOST simplifying principles to construct a theory
(c) There EXIST already results which reasonably indicate that local realism (LR) can reproduce quantum mechanical results (think about stochastic electrodynamics and the Barut self field approach)
...


The reason why (2) is unpopular I guess is because it sacrifies QM (which is outcasting GR). Moreover, the deliberate misinformation about Bell test results is not really contributing to an honest evaluation (I have still some unfinished business on the QM forum). So, the best you can reasonably do is propose concrete test models and TEST them :smile: That is actually Einsteins methodology which has proven to be highly succesful.

So, I am not contradicting myself here: the first *logical* step is to choose radically for one theory and option (2) is leading much faster to experimental falsification. Actually, this is the big *advantage* of (2) : we have pleanty of experimental results to test against: check out any book on quantum chemistry. Then, when it would turn out that (2) is reasonably not possible (which would require testing lots of continuum models for elementary particles and so on), *then* we might think about CAREFULLY abandonning spacetime locality (and not that wild as happens in QM). Next, if we would find such deviation which is in concordance with all experimental results, THEN we could try to derive this theory from basic new physical principles. This seems to me the road to succes.


** Haven't you already made an assumption that you can not be sure of?
I surely would not want to INcrease them. **

Sure, but now you should think of other models available on the market and do the (rough) counting there (you shall see there is no substantial deviation). You must admit that it is a fair way to get a rough estimate (and that is all I wanted to provide).

Your Nestle and wheel analogies can be easily answered but that will have to wait. It is nice talking to you too :rolleyes:

Cheers,

Careful
 
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  • #27
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Hey Careful,

Careful said:
(c) There EXIST already results which reasonably indicate that local realism (LR) can reproduce quantum mechanical results (think about stochastic electrodynamics and the Barut self field approach)
...
The reason why (2) is unpopular I guess is because it sacrifies QM (which is outcasting GR). Moreover, the deliberate misinformation about Bell test results is not really contributing to an honest evaluation (I have still some unfinished business on the QM forum).
I have no proplem with sacrifying QM - I don't particularly like it.

Could you give me some references on the above (preferably on the arxiv)?

Thanx, sleep well,

S.
 
  • #28
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hossi said:
Hey Careful,
I have no proplem with sacrifying QM - I don't particularly like it.
Could you give me some references on the above (preferably on the arxiv)?
Thanx, sleep well,
S.
Tomorrow: most of them are Phys Rev though (but I assume you have easy acces to that too). Let me add the obvious which is that altough these approaches make correct predictions in some cases, they are also known to deviate *slightly* (which is actually hopeful) from experiment in other circumstances (this despite the fact that entanglement is *entirely* given up).

:zzz: :zzz: :zzz:

Here are some SEMICLASSICAL approaches (it is not what I have ultimatly in mind, but it is a good start)

Phys rev A, 55, 3879 (1997) Fourth-order interference in the Wigner representation for parametric down - conversion experiments

Phys rev A, 39 (6), 2796, Quantum electrodynamics based on self-fields, without second quantization
Phys rev A, 38 (9), 4405, idem


European Physics Journal D, 1, 317-327 Semiclassical theory of Compton and photoelectric effects
 
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  • #29
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To quantize or not?

In fact, one does not need to quantize the spacetime manifold to derive the Einstein-Hilbert action. One need first assume a flat Lorentzian manifold, and perform one-loop quantum field effects to generate the effective dynamics of standard GR. See the following works of Visser, Liberati, and Barcelo, who extend the ideas of Sakharov that "Einstein gravity is an emergent low-energy long-distance phenomenon that is insensitive to the details of the high-energy short-distance physics."


http://arxiv.org/abs/gr-qc/0204062

http://arxiv.org/abs/gr-qc/0111111

http://arxiv.org/abs/gr-qc/0106002

Cheers,
Maaneli
 
  • #30
vanesch
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Careful said:
Approach (1) has a long very active history and according to me, a key problem on the road there is the measurement (or micro - macro) problem in QM with the bourlesque name: Schroedinger's cat. This problem is especially important since I want to know how the macroworld, with its local realism can be understood to emerge from a microscopic world with an operational ontology. IMO, this problem is unsolvable within contemporary QM and little or no progress has been made in this field.
Now, *that* I don't agree with! An MWI view (with a few additional postulates about the link between the ontology of the world and the subjective perception of it) CAN do it. You may not like it (you call it "believing in ghosts"), you may be convinced that it is not the right approach, but you cannot say that there is no solution to the problem of Schroedinger's cat without (non-local) collapse.

You should acknowledge this, as much as it should indeed be acknowledged that LR has not been outruled by experimental results.
 
  • #31
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vanesch said:
Now, *that* I don't agree with! An MWI view (with a few additional postulates about the link between the ontology of the world and the subjective perception of it) CAN do it. You may not like it (you call it "believing in ghosts"), you may be convinced that it is not the right approach, but you cannot say that there is no solution to the problem of Schroedinger's cat without (non-local) collapse.
You should acknowledge this, as much as it should indeed be acknowledged that LR has not been outruled by experimental results.
Hi vanesch,

Well, the MWI approach is (a) incomplete (give me a theory of consciousness and state projection; when does it take place etc...) (b) highly uneconomic (parallel universes, conscious beings, zombies and all that). Moreover, macrorealism means a bit more than ``why is the cat percieved alive or dead´´: for example one must also show that the conscious observations (in different universes) must satisfy more or less locally causal laws of motion (no entanglement). Anyway, more information is to be found in the paper of A. Leggett which I referred you to by PM a while ago.

Cheers,

Careful
 
  • #32
vanesch
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Careful said:
Hi vanesch,
Well, the MWI approach is (a) incomplete (give me a theory of consciousness and state projection; when does it take place etc...) (b) highly uneconomic (parallel universes, conscious beings, zombies and all that). Moreover, macrorealism means a bit more than ``why is the cat percieved alive or dead´´: for example one must also show that the conscious observations (in different universes) must satisfy more or less locally causal laws of motion (no entanglement).
I think you still didn't get exactly the meaning of MWI! But we're not going to have this discussion here again, right...

Anyway, more information is to be found in the paper of A. Leggett which I referred you to by PM a while ago.
Yes, I tried to find that with the info you PMed me, but couldn't. Any more precise reference of it ?
 
  • #33
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**I think you still didn't get exactly the meaning of MWI! **

Ah, you might have been confused by my use of ``state reduction´´ (which is my personal terminology for the assignment of consciousness to a particular physical state).

** But we're not going to have this discussion here again, right... **

No need to, it is impossible to forget such story which had the same impact on me as Haensel und Gretchen of the Grimm brothers :biggrin:

**
Yes, I tried to find that with the info you PMed me, but couldn't. Any more precise reference of it ?**

Testing the limits of quantum mechanics : motivation, state of play, prospects. J. Phys, condens matter 14, R415-R451

You can get it for free at the homepage of Leggett. You will notice that he is less friendly to the relative state interpretation than I am :smile: : ``a meaningless collage of words´´ to quote the good man.

Cheers,

Careful
 
  • #34
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If this is going to turn into yet another discussion of MWI, doesn't it belong on the quantum physics forum?
 
  • #35
Chronos
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So it would appear, Vanesch. Would a lamp appear to be too Dionysian?
 
  • #36
vanesch
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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.
 
  • #37
vanesch
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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
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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.
:rofl: :rofl: :rofl: :rofl: 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
vanesch
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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. :tongue2:
 
  • #40
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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. :tongue2:
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
vanesch
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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
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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 ?
:rofl: :rofl: 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... :rofl: :rofl:
 
  • #43
vanesch
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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 [Broken]
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
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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 [Broken]
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 [Broken]

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
vanesch
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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 [Broken]
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
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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
vanesch
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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
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** 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
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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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