Motivation behind quantum gravity

[jostpuur]

So far all theories of physics have been motivated by the need to explain physical phenomena, but is there any phenomena that requires quantum gravity for explanation? Is this a situation, where the motivation stems from a need to have a model of some specific form, instead of having explanations for the behaviour of nature?

The Podkletnov effect could be one application of quantum gravity, but only if it is real.

 

[marcus]

Bojowald has suggested an explanation of the dark energy effect which uses LQG
(it does not require any exotic dark energy field, so it is in some sense SIMPLER than other explanations. It doesn't have to put in any extra junk---only what you need already to quantize GR)

Reuter has gotten inflation in the early universe to happen in his model without needing any kind of "inflaton" field. He gets inflation by the correct amount (exp(60) or 60 e-folds) and a graceful exit and reheating WITHOUT ANY NEW PHYSICS. He does not need any energy-condition-violating exotic scalar field, or any "slow roll" potential. All he needs is what he already must have to quantize GR.

So you are asking WHAT EFFECTS CAN QUANTUM GRAVITY EXPLAIN? and I guess you just have to look at what are the most interesting effects which people have noticed about the universe and which currently puzzle people. I will try to spell this out in more detail:

========================

*Horizon problem----observed uniform CMB temperature

Bojowald explains by bounce
Reuter explains by natural inflation
Neither need to put in extra junk to get explanation

*Structure problem---observed scaleinvariant spectrum of CMB

Reuter explains by natural inflation. Does not need "inflaton" scalar field junk.

*Small positive Lambda problem---observed slight acceleration in late universe

Bojowald has LQG explanation, details still need work
Reuter has Lambda run so same thing explains both early inflation and late acceleration.

*CMB entropy problem---observed huge entropy (10⁸⁸ natural units) has been there since last scatter (13 Gy ago) where did this entropy come from?

Reuter's model predicts it, again in an economical way without having to add extra junk.

*Before big bang problem---what was before bang and how much can we know about it?

LQC removes singularity. Bojowald recent paper gets a handle how much we can know of conditions before bounce.

So I would say that the MOTIVATION is that GR is incomplete, it has singularities and it leaves certain puzzles---so the natural step is to quantize GR and to get a Quantum Cosmology. this is what people are doing, and the PAYOFF is that they are coming up with BETTER ANSWERS to the problems that everybody is interested in: currently what are the most exciting puzzles.

===================

There is already so much immediately visible motivation, based on things which have already been observed.
So I do not think one needs to worry about things like "anti-gravity" effects which have not been observed.

 

[jostpuur]

I have difficulty imagining quantum effects in the cosmology. So QG would be a theory where the classical limit is different from the usual classical gravity?

 

[marcus]

I have difficulty imagining quantum effects in the cosmology. So QG would be a theory where the classical limit is different from the usual classical gravity?

Look at this Reuter paper
http://arxiv.org/abs/0706.0174

Typically a QC model will recover the classical (up to current limits of observation) AWAY FROM SINGULARITIES.

The classical model does not work very well near the singularities, and various puzzles have arisen in classical cosmology because of this.
It is exactly there that the quantum model can give different results (near the singularities) and can possibly resolve some problems.

Then there is the question of TESTING. Because the QC model normally replicates GR in largescale lowenergy limit at least approximately up to present limits of observation, one must find SMALL DIFFERENCES to look for----e.g. in the CMB---so that the QC models can eventually be tested and either falsified or shown to be superior to classical.

this is the general picture, there is an exception which you can see in this paper
http://arxiv.org/abs/0705.4398
this paper is preliminary and needs more work. In this paper it is proposed that LQG can actually predict a measurably different history of acceleration WITHOUT any dark energy----by the cumulative effect of many quantum correction terms distributed throughout space.
This idea was just proposed this year and it needs to be followed up and confirmed. If Bojowald follows up on it, then it exposes LQG to falsification if the acceleration history at higher redshift z is measured and if the history FAILS to agree with the prediction derived from LQG.

that is the only late-universe effect that I know. the other stuff is early universe that one can expect to check for in the CMB and structure formation datafor survey and introduction to Reuter's work, here are slides and audio of his June talk at Loops 07
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3

 

[jostpuur]

I was afraid that difficult questions would give me difficult answers :/

 

[genneth]

A really simple motivation is internal consistency. We like our physics to be consistent, because we're weird, or something. General relativity says that the time coordinate is meaningless, and can be arbitrarily re-assigned. Quantum mechanics intrinsically involves time. This is already a big enough issue with special relativity that quantum field theory has to get involved before the maths gets so hard that it's difficult to see if there's still a consistency issue. Quantum field theory also has some pretty serious issues even if you just attempt it on a curved, but classical, spacetime. The biggest motivation for theorists is this internal inconsistency. You can learn more by looking at any quantum gravity review paper:
[1] http://xxx.lanl.gov/abs/gr-qc/9310031
[2] http://xxx.lanl.gov/abs/gr-qc/0108040
[3] http://xxx.lanl.gov/abs/hep-th/0303185