Wednesday, February 17, 2016

Dear Dr Bee: Can LIGO’s gravitational wave detection tell us something about quantum gravity?

“I was hoping you could comment on the connection between gravitational waves and gravitational quanta. From what I gather, the observation of gravitational waves at LIGO do not really tell us anything about the existence or properties of gravitons. Why should this be the case?”

“Can LIGO provide any experimental signature of quantum gravity?”

“Is gravity wave observation likely to contribute to [quantum] gravity? Or is it unlikely to be sensitive enough?”


It’s a question that many of you asked, and I have an answers for you over at Forbes! Though it comes down to “don’t get your hopes up too high.” (Sorry for the extra click, it’s my monthly contribution to Starts With a Bang. You can leave comments here instead.)

31 comments:

Daniel de França MTd2 said...

There is probably a gamma ray burst associated with this black hole merger:

http://gammaray.nsstc.nasa.gov/gbm/publications/preprints/gbm_ligo_preprint.pdf

Daniel de França MTd2 said...

More here: http://arxiv.org/pdf/1602.04488v1.pdf

Andrew Thomas said...

Nice article, thanks.

CapitalistImperialistPig said...

Great article Bee! Thanks for writing it.

Topi Rinkinen said...

Sabine,

In the detected binary BH case, is there estimate what is the energy density (W/m2) of the gravitational waves passing us by?

If more traditional waves (e.g. EM) passes through us with the same energy density and same wavelength, is the perceived strain in the same range?

BR, -Topi

Sabine Hossenfelder said...

Topi,

Have a look at this paper, eq (2.29). It has the flux in convenient units, and for the event under question it seems to be about 1 erg/(s*cm^2). Now to convert this into W/m^2... Have some pity on me poor theorist, I think you can do it yourself :) Probably multiply it with 4 c's or so.

As to your second question, don't know what you mean by strain - an electromagnetic wave doesn't deform space-time. And as to what we 'perceive' of electromagnetic waves, that's an even more difficult question depending on the absorption rate of the human body. Best,

B.

Shantanu said...

Sabine: slightly offtopc to this. but just wanted to point to readers to free online proceedings of 1957 chapell hill conference proceedings where there were lots of discussions of reality of gravitational waves and many of the doubts were clarified by
Feynmann, Bondi, Dewitt etc
http://www.edition-open-access.de/media/sources/5/Sources5.pdf

Uncle Al said...

Your Forbes exposition of gravitons and photons is attractive and lucid. Rubber sheet spacetime deformation is defective geometrically (hyperbolic versus elliptic "surface" distortion) and dynamically (arXiv:1312.3893). The Earth as a strain gauge gravitational wave detector via temblors and geophone outputs is unreported. Has anybody looked?

"violations of the equivalence principle" EP violation requires an out-of-founding-postulate geometric test consistent with required baryogenesis symmetries, e.g., atom scale-emergent enantiomorphic test masses. Achiral theories requiring chiral curve fittings are diagnostic.

Daniel de França MTd2 said...

That's around 4miliWatt. 4 millionth of the sun. https://en.wikipedia.org/wiki/Magnitude_(astronomy)
Around the brightness of the full moon.

Charlie said...

Thanks Sabine! Your blogs and articles are really wonderful to read. If you will forgive me, I'd like to ask a very basic question about LIGO (and similar systems to measure gravitational waves). Apology due to the fact that this has been asked before by others, but the answer still eludes me:

How can LIGO measure changes in space-time when the "ruler" itself (light) is affected by those changes?

I feel like the answer must be #4 of the list below, simply because options #1-3 violate what little intuition I feel like I have about this topic:

1. The physical apparatus (a chunk of WA or LA) is not expanding/shrinking with the gravitational waves;
2. The light bouncing back and forth is not expanding/shrinking with the gravitational waves;
3. The two above are being expanded/shrunk differently by gravitational waves, allowing a difference to be measured;
4. My list of possible answers displays a fundamental lack of understanding.

I have some vague notion that the answer has to do with the fact that the waves are in space-time, not space. Not sure if this is on the right track.

Plato Hagel said...

Dear B,

Is it possible that gravitational waves carry a type of "information" away from it's source?

Best,

Sabine Hossenfelder said...

Charlie,

Please see these references

http://arxiv.org/abs/gr-qc/0702079
http://arxiv.org/abs/gr-qc/0511083

The brief answer is 3) the light (being light-like) is not affected the same as the arms of the interferometer

Plato Hagel said...

Second follow up question.

Is Gravity now part of the standard model?

Sabine Hossenfelder said...

Plato,

A most excellent question! Yes, they can, and they do. They must carry away all information that deforms the initial configuration away from the rotational symmetry that it must have at the end because of the no-hair theorem. At last that's what General Relativity predicts (leaving aside some loopholes in these theorems).

Plato Hagel said...


Bee Said: "A most excellent question! Yes, they can, and they do. They must carry away all information that deforms the initial configuration away from the rotational symmetry that it must have at the end because of the no-hair theorem. At last that's what General Relativity predicts (leaving aside some loopholes in these theorems)."

Okay.....then not like the photon or the electrons, so as to tune a radio to hear, what and how would we discern the nature of the information of the gravitational waves from the initial configuration? Can it ever be that specific?

Best

Voice Of Reason said...

Sabine,

Thanks for the article. I'm excited to find this site and I've always had a layman's interested in these topics. I do have a question regarding the LIGO's results which are interpreted as validating Einstein's theory. Without getting into the fine details, do the LIGO results differentiate between Einstein's theories wrt gravitational waves and what would be expected based on Newtonian physics? Even Newtonian physics would lead to an expected varying gravitational field under these circumstances, right?. How does the signal that was received valid gravitational waves and distinguish them from a varying gravitational field from a Newtonian perspective?

Thanks,
Gregg

Topi Rinkinen said...

Sabine,

" an electromagnetic wave doesn't deform space-time"

Why? Em wave is carrying energy, ie mass, away from source. During the course the mass (photons or em field energy) is deforming space-time similar way any other mass does? I don't see why this should be different from more tangible mass.

Or is the deformation due gravitational waves somehow different than deformation around e.g. sun?

Br, -Topi

MarkusM said...

"From what I gather, the observation of gravitational waves at LIGO do not really tell us anything about the existence or properties of gravitons".
That is not right, they put an upper limit on the mass of the graviton, which may be regarded as a small experimental contribution to a theory of quantum gravity.

MarkusM said...

@Plato:
"Is Gravity now part of the standard model?"
Why should it ? My simple answer would be "No". Einstein who predicted gravitational waves in 1918 (http://adsabs.harvard.edu/abs/1918SPAW.......154E) had no idea what the Standard Model is.

Sabine Hossenfelder said...

Topi,

I think this is a terminology issue. You mean that an electromagnetic wave would also be accompanied by a deformation of space-time. This is correct. This is qua definition not a gravitational wave - a gravitational wave is a ripple in what is otherwise vacuum.

Sabine Hossenfelder said...

Voice,

There are no gravitational waves in Newtonian gravity. (A wave-equation requires a local theory.)

Sabine Hossenfelder said...

Plato,

Well, we *do* infer information about the initial state, such as the masses and their orbits and so on. But the finer the detail, the higher the frequency and the weaker the signal. So most of the details get lost in noise. Regarding the question whether GR is part of the SM, that's a question of terminology. SM is normally used to refer to the particle sector. If you combine both, you get what is known as quantum field theory in curved space - which is a theory that works as an approximation but is fundamentally inconsistent. Best,

B.

Plato Hagel said...

Markus and Bee,

I too am very excited about the confirmation of Gravitational wave detection and I understand we have a long way to go. The finer the detail and the problems with the noise, so what would be the next step regarding early universe understanding of the gravitational waves in the CMB if not with directing attention to the B modes as they are seen and understood?

Would it not have to be consistent that early universe detection of gravitational waves need to become part of the SM in order for that progress in the early universe understanding?

Best,

Plato Hagel said...

MarkusM: Why should it ? My simple answer would be "No". Einstein who predicted gravitational waves in 1918 (http://adsabs.harvard.edu/abs/1918SPAW.......154E) had no idea what the Standard Model is.

No I think it is more about the direction of where we are going today. There needs to be a confirmation regarding quantum theory regarding early universe formation so having gravity become part of the standard model serves to initialize the further ability to measure at these finer levels. Having this attribution as becoming part of the standard model, would go toward understanding how quantum theory as Bee has explained, is a deeper understanding of quantum mechanics and gravity being joined.

Any help and understanding here would be most appreciated. What are the requirements of gravity becoming part of the standard model as a measure of quantum theory?

Best,

MarkusM said...

@Plato
"Any help and understanding here would be most appreciated. What are the requirements of gravity becoming part of the standard model as a measure of quantum theory?"

My point of view is, that it is too early to ask this question (and put the cart before the horse). The unification of gravity with the other forces is supposedly at extremely high energies and there is a lot that is to be understood at lower energies yet (which could be "game changing" once understood), like dark energy and dark matter. In particular, I don't think that the quest is to unify the Standard Model (as given at the moment) with gravity, rather we probably first need an appropriate extension, a Grand Unified Theory (GUT), and there are many candidates around at present.
It is a very interesting question what we would see if we detected gravitational waves from the very early universe and could look beyond the surface of last (optical) scattering. That probably depends on inflation being there or not. Therefore my bet would be that one of the next big things in physics will be the upcoming polarization measurements. Hope you can soon get excited again, like with the gravitational waves, once these results come around :-)

Noa Drake said...

In a context for a search for quantum gravity : Since waves need a medium to propagate, and since we have now proven that ripples can be produced, does this imply that space (spacetime) itself must be regarded as a medium, in need of quantisation ? I mean you can't produce ripples from/in 'nothingness'.

MarkusM said...

@Plato
P.S. The "Cosmology Large Angular Scale Surveyor" may be able to detect primordial gravitational waves soon:
http://news.yahoo.com/video/moving-johns-hopkins-telescope-001554308.html

Sabine Hossenfelder said...

Noa,

Gravitational waves are disturbances in space-time. They are "made of" the same thing, whatever you want to call it. Space-time is not nothing, I don't know why you think so.

Plato Hagel said...

Thank You MarkusM,

BICEP 3 would be an advancement toward gravitational process understanding in early universe, indeed.

Bee and Markus,

Regarding Dark Matter and Dark Energy, so indeed, a large part requiring some understanding of the universe in our determinant makeup of "all there is."

A couple of things then come to mind about "the matter," as an idea of the early universe and the detection of gravitational waves with regard to our perspective around our current understanding of the quark Gluon plasma.

Is there not a "relativistic interpretation," regarding the viscosity of heavy ion collision process at LHC that takes us back to the early universe for examination?

Best,

Phillip Helbig said...

" Rubber sheet spacetime deformation is defective geometrically (hyperbolic versus elliptic "surface" distortion) and dynamically (arXiv:1312.3893)"

True but irrelevant. The Bohr model is wrong, but atoms still exist. :-)

Shantanu said...

Hello GW afficionados,
check out fig 1 of http://arxiv.org/pdf/1602.06833v1.pdf
if this is right, maybe first evidence of deviation from GR?
(of course I don't myself believe and think its detector noise).