# Tests of loop quantum gravity?

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1. Oct 16, 2014

### 21joanna12

I am doing a project of theories of gravity, and am having some trouble understanding whether loop quantum gravity is testable or not. I had some questions which I cannot seem to find the answers to:

1. Is it theoretically possible to probe distances of $10^{-35}$ or would the concentration of energy required result in the formation of a black hole?

2. Is loop quantum gravity testable? If so what are the tests? There seems to be some discrepancy about this online because I have read some sources saying that it does not make any testable predictions, and other saying that the radiation released by black holes, or the CMB, could be used to test for loop quantum gravity, although I cannot find details about this.

Last edited: Oct 16, 2014
2. Oct 16, 2014

### marcus

Some papers:
http://arxiv.org/find/grp_physics/1/au: barrau/0/1/0/all/0/1
The most recent appeared this month:

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain
(Submitted on 7 Oct 2014)
Quantum gravity has long been thought to be completely decoupled from experiments or observations. Although it is true that smoking guns are still missing, there are now serious hopes that quantum gravity phenomena might be tested. We review here some possible ways to observe loop quantum gravity effects either in the framework of cosmology or in astroparticle physics.
25 pages, 8 figures. Draft chapter for a volume edited by A. Ashtekar and J. Pullin, to be published in the World Scientific series "100 Years of General Relativity"

Here's a direct link to the PDF:
http://arxiv.org/pdf/1410.1714v1.pdf

There are a number of phenomenologists (theory testers, whose specialty is figuring out how to test physics theories by observation and experiment) working out how to test LQG. Aurelien Barrau seems to be the most prominent. Others are Julien Grain, Jakub Mielczarek, Thomas Cailleteau...

Last edited: Oct 16, 2014
3. Oct 16, 2014

### marcus

I'd say the key parameter scale is planck density. One expects to see evidence of quantum gravity effects when the density (say at the core of a gravitational collapse, or the beginning of cosmological expansion) reaches planck scale.

The Barrau Grain paper explains this---one is not expecting to observe reactions at a length scale of 10-35 meter or planck length. : ^)
or to accelerate particles to planck energy : ^))
The focus is on studying natural processes where planck energy density is presumably attained and checking the predictions of QG theory against observation.

EDIT: I went back and looked at the Barrau Grain October 2014 paper. It is too technical! I'll try to find another paper that is written more for wider audience.
However it does have this passage:
==quote Barrau Grain page 20==
In loop cosmology, the Friedmann equation is modified by quantum gravitational effects by a term determined by the ratio of ρ to a Planck scale density ρPl. The quantum gravity regime seems to be reached when the energy density of matter reaches the Planck scale, ρ ∼ ρPl. The point is that this may happen well before relevant lengths l become planckian. The bounce is due to a quantum- gravitational repulsion which originates from the Heisenberg uncertainty and does not happen when the universe is of planckian size but instead happens when the energy density reaches the Planck density. Quantum gravity could become relevant when the volume of the universe is some 75 orders of magnitude larger than the Planck volume. ..
==endquote==
The density is denoted by the lower-case Greek letter rho.

Last edited: Oct 16, 2014
4. Oct 16, 2014

### 21joanna12

I admit that I don't quite understand the paper, but the quote you picked out is really helpful! I know of the equation $H^2=\frac{8\pi G}{3} \rho \left(1-\frac{\rho}{\rho_c}\right)$, so I suppose it is referring to this. I think maybe I am just thinking about loop quantum gravity in the wrong way. I saw somewhere that the 'loops' have something to do with volumes, although I keep thinking about them as physical little loops, much like strings in string theory...
So I suppose using photons to probe the grainyness of spacetime wouldn't work anyway, even if high enough photon energies could be achieved?

5. Oct 16, 2014

### marcus

Hi Joanna,
I found an article that is written for wider audience:
http://arxiv.org/pdf/1206.1192v1.pdf
It is 7 pages and has only 3 equations. It is by the same authors, mostly communicates by words, but also has some charts that show the predictions of various theories as curves, that could be compared with observations
http://arxiv.org/abs/1206.1192
Quantum gravity in the sky
Aurelien Barrau, Julien Grain
(Submitted on 6 Jun 2012)
Quantum gravity is known to be mostly a kind of metaphysical speculation. In this brief essay, we try to argue that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". With the example of Loop Quantum Gravity, we detail some expected features.
7 pages. Brief essay written for the Gravity Research Foundation.

About how to think of Loop QG and how to imagine "graininess"... I think that any quantum theory is more concerned with interactions, measurements, events, observables, than with "what is really there". For example particles do not have continuous trajectories, they are observed at various points along the way, going thru a slit, bumping something, scattering, being absorbed by a detector etc. A quantum theory is less concerned with what Nature is and more concerned with how it responds to measurement.

So "grainy is as grainy does". Graininess would be manifested by evidence of a minimal length, or area. Curvature is reciprocal to these things...e.g. the radius of curvature. One might see a minimal area in the form of a maximal curvature. Maybe there are no actual GRAINS, there is only a graininess of measurement. No halfway detections. A photon a quantum of energy is either detected or it is not. An atom can only be in a such and such discrete energy levels. Never halfway between. And yet energy does not actually exist in grains---the discreteness occurs in the interactions, the measurements, the emission and absorption events.

So I would not expect the world to be made of grains, I would look for granularity in quantum processes. Admittedly that is something of a disappointment, and not so intuitive. Well, maybe it IS made of grains at some deep level.

But the Loop QG people would be happy to see evidence of a minimal area---thus a maximal curvature, and a maximal energy density (the two are related)---and therefore a bounce

Last edited: Oct 16, 2014
6. Oct 16, 2014

### marcus

I still need to say something about LQG, in general. Why is it called "Loop"? Back in early 1990s loops were the main mathematical tool those researchers were using as they imagined probing of geometry at very small scale. So it was was reasonable to apply the word Loop to what they were doing.
If you join several loops, so that they share some vertices and edges, you get a network. A network labeled with some geometric information (about what happens to you as you travel around in the network, or about the volumes you might measure at the nodes where the links come together) turns out to be a convenient way of summarizing geometric information. Roger Penrose had already been studying and writing about what he called "spin networks" as finite geometric descriptions. It was just a small step, the Loop researchers shifted focus from loops to spin networks. But the labels on the network were not necessarily spins. Nomenclature tends to get stuck, and doesn't always keep up with current practice.

The main thing is they use finite combinatorial objects to characterize quantum states of geometry. We still call the research Loop even though the combinatorial objects now might not be labeled loops, they might be finite labeled networks. "Combinatorial" means something you can describe in very simple finite terms like vertices, edges, or nodes and links, or triangles. You combine these simple things to approximate complicated ones.
It seems irresistible at this point to mention tinkertoy. Or lego. Do you recall playing with tinkertoy or lego as a kid? You made combinatorial objects, which might have been a kind of shorthand skeletal description of continuum objects.

A quantum state has a certain kind of finiteness because it is specified by the results of a finite number of measurements. I'm not sure I'm saying the right thing, or even making sense. This is not an explanation of what LQG is, it is more like making excuses for the fact that it is called "Loop" when they have moved on, since the early 1990s, to some more efficient combinatorial objects to stand for quantum states of geometry (and also for transitions between those states, I omitted mention of how they keep track of transitions).

7. Oct 17, 2014

### enorbet

Do we have any good estimates on the frequency that GRBs occur so that Integral or something like it can repeat it's astonishing experiment showing no graininess to orders of magnitude below Planck Scale?

8. Oct 17, 2014

### marcus

Hi Enorbet, I regret to say I don't understand the question. Could you indicate an equation, by number or page in the paper? or point me to a figure?
Or maybe someone will respond who understands the question better.

9. Oct 17, 2014

### enorbet

Please accept my apologies for being less than clear. I've read several QG books including 3 by Lee Smolin and a few papers but I think I'm lucky if I truly grasp 10-15% of their content.

I'm sure you're aware of ESA's Integral observations regarding GRB 041219A that seem to tell us that any graininess apparently must be on a scale orders of magnitude smaller than Planck Scale - 10^-43 in fact. While this particular GRB and it's study had all the best breaks, it is still only one observation, albeit an important one. I can't help but wonder how easy or difficult it may be to be able to repeat this to get additional similar data to begin to build a statistical base and rule out idiosyncrasies or flukes, let alone errors. It bothers me a little that ESA seems bold about their data here
http://www.esa.int/Our_Activities/Space_Science/Integral_challenges_physics_beyond_Einstein

but even papers specifically just about GRBs like
http://arxiv.org/pdf/1302.4847.pdf

barely mention this rather earth-shaking data.

So my question is essentially how long must we likely wait for repetition to help confirm if such a bold title is right and proper.

10. Oct 18, 2014

### marcus

Hi Enorbet,
I understand better now. There is a confusion. Lorentz Invariance Violation (LIV) is not a predictions of LQG. So these astrophysical observations are not relevant as a test of LQG, although they may be relevant to some other quantum gravity theories (I don't know which though.)
Between 2005 and 2008 some people thought they could prove LIV was a prediction of LQG and they tried, but failed. Later Rovelli and Speziale actually proved the opposite (around 2010 as I recall) namely that LQG actually satisfies Lorentz invariance. Yes! it was 2010! Here is the paper:
http://arxiv.org/abs/1012.1739

"Graininess" is a vague concept and can enter into a theory in different ways and have different effects. These ESA people were looking for LIV.
Here is a paper that I think your press release refers to:
http://arxiv.org/abs/1106.1068 (you see it is about LIV, and Philippe Laurent is an author, and it is about that GRB, and the paper came out June 2011)
If they wanted to test LQG they were looking for the wrong effect.

But it is good that somebody was checking for Lorentz invariance violation. By now it has been pretty much ruled out so it does not seem so "earth shaking". But if they actually had FOUND some LIV in the light that had traveled for a long time, this would have been a bit of a shake-up. A lot of physics theory would have been thrown into question! Including LQG (because of the 2010 result I mentioned) but it would have been just one of many.

Last edited: Oct 18, 2014
11. Oct 18, 2014

### craigi

As I understand it, there are 3 areas where we may find new physics to test QG theories

* The early universe
* Blackholes and collapsing stars
* Decoherence

We can't say that QG doesn't make any testable predictions, because we don't have a working theory of QG yet. I would expect QG to lead to testable predictions in at least one of these three areas, if not all. I think the misnomer arises from the fact we don't have a single theory that explains both QM and GR, but that isn't to say that we expect QG just to merge the two. We should expect that they are limiting cases of QG.

There's an overview paper that might be easier to understand here:
http://arxiv.org/abs/1010.3420

Bekenstein has proposed a tabletop test for Planck scale quantisation of space.
http://arxiv.org/abs/1211.3816

Last edited: Oct 18, 2014
12. Oct 18, 2014

### Chronos

The problem with probing the planck scale is the energies required are unimaginable, and probably beyond the reach of human technology. That reduces us to drawing inferences from cosmological evidence - which is rife with uncertainties. Math is a powerful tool, but, not every problem is a nail. At some point we should accept the proposition not every mathematical possibility is realized in nature, IMO.

13. Oct 18, 2014

### enorbet

Thank you, Marcus. While your response didn't answer my stated question it went one better and addressed the underlying concern with ESA's article as to just how earth-shaking it is. Kudos. Thanks to all this is a very interesting thread since almost all areas of Science must deal with the observation vs/ theoretical mathematics issue. A syllogism in any language can be constructed around nonsense. The thorn for pure "observationists" is that mathematics is the purest, clearest language in existence and has a very strong track record in prediction, but I suspect I'm now just "preaching to the choir". In any case I will continue to watch this thread and others like it. Good stuff!

14. Oct 18, 2014

### Doug Huffman

Thanks for the mention above of Roger Babson's Gravity Research Foundation, now run by George Rideout, Jr. Note that an apparently complete catalogue of the award winning essays full free access PDF's is at their web site.

My set of print copies is complete from the beginning to 1962.