# LQG makes predictions?

1. Feb 2, 2006

### ZapperZ

Staff Emeritus
At least LQG tries to make measurable predictions! When is String theory going to even try?

http://physicsweb.org/articles/news/10/2/2/1

Zz.

2. Feb 2, 2006

Staff Emeritus
It's good to see the beginning of some empirical tests in quantum gravity, but I have to say that string theorists have been trying to come up with checkable predictions - trying very hard, too - for nigh on to thutty years now. It's just that they have always failed.

3. Feb 2, 2006

### marcus

I remember that Parampreet Singh gave a seminar talk about this very thing last Fall, at Penn State. the slides and/or the audio recording were online. I will see if I can find some links.

I dont know if this can be put to observational use NOW. But it is at least a serious try at getting an observable signature of some QG.

Here is a free version of their article:

http://arxiv.org/abs/gr-qc/0506129
Quantum evaporation of a naked singularity

Authors: Rituparno Goswami, Pankaj S. Joshi, Parampreet Singh
Comments: 4 pages, 2 figures. Minor changes to match published version in Physical Review Letters
Report-no: IGPG-05/6-8
We investigate here quantum effects in gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that non-perturbative semi-classical modifications near the singularity, based on loop quantum gravity, give rise to a strong outward flux of energy. This leads to the dissolution of the collapsing cloud before the singularity can form. Quantum gravitational effects thus censor naked singularities by avoiding their formation. Further, quantum gravity induced mass flux has a distinct feature which may lead to a novel observable signature in astrophysical bursts.

Last edited: Feb 3, 2006
4. Feb 2, 2006

### marcus

Here is Parampreet Singh's October seminar talk
http://www.phys.psu.edu/events/index.html?event_id=1302;event_type_ids=0;span=2005-08-20.2005-12-25

Both the audio and the lecture-note/slides are available online at that link.

the talk was given 21 October several months after the article was submitted to Phys. Rev. Letters. so it must be reasonably up-to-date.

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

I think this signature "flicker" in the Gammaray burst of a certain type of stellar collapse is only ONE OF SEVERAL ways that Loop researchers are getting into position to test. It may not even be the best or farthest along. I don't mean to criticize their efforts---I think it's great. but these things all seem tentative to me and nothing has reached the stage of a cut-and-dried ironclad test that you can actually do next year. it's "getting there" but not there yet. my personal opinion. Parampreet Singh may say different.

5. Feb 3, 2006

### Nucleonics

Zapper, I was under the impression that some models of string theory HAD made measurable predictions -- namely the deviation from inverse-square-law gravity at the sub-millimeter scale. Is this not generally accepted as a measurable prediction of ST?

physicsweb.org/articles/news/7/2/14/1

Also, "split symmetry" models of ST predict gauginos and higgsinos at the TeV scale. There are also other models of ST at the TeV scale. They make predictions that would be testable at the LHC. So, am I missing something when I think that ST does actually make testable predictions?

Best wishes,
Nucleonics

6. Feb 3, 2006

### josh1

Hi guys,

My understanding of LQG is that they dont know whether it can describe the universe at the low energies we see it today. So Im wondering what you guys mean when you say that LQG makes testable predictions. For example, even if somebody did find evidence of area quantization or some effect depending on it, they cant claim that theyve verified LQG since they cant say that LQG describes the universe in which they did their experiment. In fact, from what I understand, detecting this kind of discreteness in the spectrum of whatever you are measuring doesnt necessarily imply quantization. It could simply be revealing a fundamental length in the theory rather than a genuine quantization of the observables spectrum. So can someone give me an example of an experiment that can be performed that could verify LQG? Maybe someone knows some way that LQG is falsifiable. What you guys are claiming just doesnt make sense to me since the only universe experimentally accessible to us is one which LQG hasnt been able to describe. By the way, please dont post some quotes from or links to a paper. Just explain it to me yourself. If I wanted to go read papers, I wouldn`t have posted this.

7. Feb 4, 2006

### marcus

This is a good point. At least it tries, and I think it's fair to say that SOME VARIANTS of LQG do make testable predictions or are coming close to that goal.

One thing to mention is that there is no one unique LQG. People use LQG as a catch-all term for a number of leading QG alternatives to string.
These include canonical LQG, spinfoam, LQC (loop quantum cosmology, a symmetry-reduced version of canonical LQG), Thiemann's masterconstraint, Gambini's discretized LQG.

One can't say which of these are equivalent to which others. In some the dynamics have been worked out and the semiclassical or largescale limit has been checked. In others not.

LQC is an example of where the dynamics have been worked out and low energy limit checked. It gives the same answers as ordinary Gen Rel except around the big bang ('big bounce'). LQC can make predictions IN ITS OWN RIGHT and needs to be tested in its own right. Since the logical relation with canonical LQG is not clear, you have people working on LQC phenomenology. Parampreet Singh has given some seminar talks about this at Penn State---audio and visuals are online.

There is also an attempt to make GENERIC predictions that would be valid consequences of several different LQG models regardless of whether the details of dynamics and classical limit have been worked out in each separate case. An example of this is DSR. Some people argue that a variety of LQG models will not work without some modification of SR symmetry. Unfortunately there is disagreement about this. There is time pressure to get this issue resolved by 2007, or whenever the GLAST satellite is launched. It is argued that a number of LQG models require a slight energy-dependence of the speed of light. This gives reason to hope that one might RULE OUT SUCH MODELS regardless of whether details have been established and checked in each case---by finding gammaray dispersion in the opposite sense to that predicted. But this issue is still unresolved.

Zapper's main point is a sound one. Researchers pursuing various LQG approaches are indeed making an effort to arrive at tests which could falsify some or all of the different LQG models.

Last edited: Feb 4, 2006