# Quantized Gravity

1. Mar 24, 2014

### L.Newton

Hey guys hope everybody is doing well. Here is my question. From what I understand, according to GR, gravity is present because of the curves in space-time. I also believe, please correct me where I am wrong, that those curves also create ripples in space in time. So that's what einsteins gravitational waves are. So if that's the case why would there be a graviton? After all that's what quantum gravity is...right? I'm very confused. Perhaps some good links or references to helpful literature could go a long way. Thanks...forgive me if I have made everybody dumber for having asked my question...haha

2. Mar 24, 2014

### Cosmobrain

That's a good question. To me, there is no need for a Graviton. A Graviton is just a frustrated attempt to include gravity in the Standard Model. Until a graviton is actually detected, I still say that gravity can be explained by GR. A graviton has never been observed, so if they claim that it exists, I want proof. Otherwise it is going to be just a hypothesis.

3. Mar 24, 2014

### L.Newton

So then doesn't that beg the question...Is gravity a fundamental force? If there is not, and there could be, a carrier particle than is it a fundamental force? That is what quantum gravity is right? As what the name implies. gravity quantized. Or is it a different concept?

4. Mar 24, 2014

### euquila

Ripples / waves in spacetime are not the same thing as gravitons. These ripples are predicted by general relativity and can stretch and squeeze a region of space as it passes by.

Gravitons are the hypothetical/unconfirmed mediator particles of the gravitational force (just as photons mediate the electromagnetic forces or gluons mediate the strong force).

Edit: what do you mean by "fundamental" ;)

5. Mar 24, 2014

### Staff: Mentor

"Classical" (non-quantum) GR works fine as far as current experimental observations are concerned. As far as I know, there are no experiments or observations on gravity that directly require something "beyond" classical GR. All observations so far are at very "macroscopic" scales where quantum effects in general are negligible.

Nevertheless, many physicists expect classical GR to break down when we get close enough to the Big Bang, or close enough to the center of a black hole. They expect that in those regimes, quantum effects become important, and that we will need a quantum theory of gravity in order to describe those situations better. And of course we will need better observations in order to test such theories adequately.

6. Mar 25, 2014

### L.Newton

euquila: Yes I'm know. I'm sorry if I misrepresented my understanding. As far as "fundamental force" I would have to employ some basic textbook definition. That's the limit of my knowledge on the subject. As a matter of fact I will be taking modern physics in the fall and I am a math junior about to be a senior after this semester. Is there a deeper or less known use of the term "fundamental" in physics?

7. Mar 25, 2014

### atyy

A graviton is a quantized gravitational wave, just as a photon is a quantized light wave.

If the BICEP2 observation of primordial B-modes holds up, that is potentially a sign of quantum gravity (a theory involving gravitons): http://www.nature.com/news/how-astronomers-saw-gravitational-waves-from-the-big-bang-1.14885. The caveat is that even if the observation holds up, there could be classical ways of producing the primordial B-modes, even though the quantum gravitational model is the dominant hypothesis at the moment: http://motls.blogspot.com/2014/03/bicep2-primordial-gravitational-waves.html (see the discussion in the section "Implication 3: quantization of the gravitational field").

Last edited: Mar 25, 2014
8. Mar 25, 2014

### Cosmobrain

Yes, but as far as I know, a force carrier particle (boson) isn't always necessary. That really depends on the definition of a force, which can be not as simple as it may seem.

By the way, there is something I can't quite understand. If gravitons hypothetically mediate the gravitational force, that means that photons of light (which are affected by gravity and have their own small gravitational field as well) must interact with it. I can't get my head around the idea that such particles could interact with one another and the photon doesn't change its momentum or direction. I hope someone can clarify this to me.

cb

9. Mar 25, 2014

### ChrisVer

Well in my opinion a theory is complete when it can be quantized. Since GR is not quantizable (or better put it's not renormalizable), it still remains a good classically "effective" theory... The problem is that it can really give answers for things happening in large distances, but itself it's inconsistent at small distances (where a Quantum theory comes in hand).
The graviton is not in the Standard Model. Of course most lecturers present it as such, but that's not the case. The Standard Model is a Yang Mills gauge theory which contains (roughly speaking) strong+weak+electromagnetic interactions and Spontaneous symmetry breaking. If I'm not mistaken it's renormalizable (otherwise it couldn't predict anything). Gravity is not.
So in Standard Model we accept there is no gravity, and when gravity comes in game we expect new physics to occur (like GUT, string theories etc). We just say that as the known forces have one mediator after quantization, so should a consistent gravity theory which we call graviton.

I don't understand what you mean Cosmo. The graviton coupling with matter is very weak in contrast to the other forces (this is near the hierarchy problem). So even if it happens you cannot see it. What you see is the effective large-distance general relativity.

10. Mar 25, 2014

### Staff: Mentor

Glad you put not renormalisable. Theories that are not renormalisable in fact can still give finite answers up to a cut-off beyond which it's not valid - such is called the Effective Theory approach:
http://arxiv.org/pdf/gr-qc/9512024v1.pdf

The real issue with gravity is not that its not able to be quantized - it can be. Its that you can only extract finite answers up to a certain cutoff. Actually without going into the details while renormalisable theories give finite answers without an explicit cut-off (they actually do have a cut-off, but have the very nice property that the answers don't actually depend on what that cut-off is - Wilson sorted this out and got a Nobel price for it), there are strong reasons to suspect they too cant be trusted beyond some cut-off.

The real issue with gravity is while in other theories like electodynamics the interesting physics occurs below that cutoff, the interesting physics for gravity is beyond the cutoff - damn.

Thanks
Bill

Last edited: Mar 25, 2014
11. Dec 31, 2014

### Ross D

All of the other fundamental of forces of nature, such as electromagnetism, are mediated by what's called gauge bosons. These things transfer energy between particles, thereby creating the forces we see. However, all attempts to view gravity in this fashion have failed, probably due to the fact that the Einstein Field Equations require a continuous fluid in which to work. The graviton is the theoretical particle that mediates gravity, while gravitational waves are ripples in space-time.

12. Dec 31, 2014

### Staff: Mentor

http://arxiv.org/abs/1209.3511

Gravitational waves are in fact one of the easiest things to handle using QFT. The QFT of spin 2 particles leads to linearised gravity from which gravitational waves emerge naturally.

For the approach of deriving GR from linearised gravity see Ohanians text:
https://www.amazon.com/Gravitation-Spacetime-Hans-C-Ohanian/dp/1107012945

Thanks
Bill

Last edited by a moderator: May 7, 2017
13. Dec 31, 2014

### LachyP

Yes, I think that I partially have to agree with you on that, but I do think that there is more to it than you think. It isn't the curves in space-time themselves that form gravitational waves, it is actually the interference of two curves in space-time of which form the gravitational waves. In gravitational waves, these gravitons are the quanta of which make up the waves, but as I understand it, are not necessarily important, but in the case of usual gravity such as here on Earth, it may apply usefully. Just think of it on the quantum level, two non-attracted atoms would not just pull each other inwards towards each other, but here is where the graviton comes in to play. The graviton, being massless, can travel at extreme velocities mediating the gravitational force upon "contact" and can create this "attraction". Hopefully this may help you to better understand the graviton.

14. Jan 1, 2015

### Staff: Mentor

It's the solution to the EFE's - where you got the above from has me beat - even what it means has me beat.

Thanks
Bill

Last edited: Jan 1, 2015
15. Jan 1, 2015

### ShayanJ

But the very fact that there is a cut-off the other side of which we don't understand, makes that other side to be "interesting physics"! So there is still trouble with electrodynamics. Oh...now that I remember...do we have non-perturbative QED? what about QCD? Or electroweak? I suspect all we have is lattice gauge theory. Right?

16. Jan 1, 2015

### Staff: Mentor

Of course - the issue is we have a lot of interesting physics in QED below its cut-off - beyond which we also have interesting physics in the electroweak theory. In gravity below the cut-off, the area that EFT works, it isnt telling us anything much new.

Non pertubative methods are still in their infancy - what they will tell us eventually - who knows. It has shed new light on virtual particles - which has been discussed in other threads.

Thanks
Bill

17. Jan 1, 2015

### LachyP

Yeah, that's what I gathered from different sources, so it may not be correct, but the main part of that whole answer, was the last half.. sorry about that.

18. Jan 3, 2015

### Ilja

That's wrong. In Wilson's approach, non-renormalizable theories are effective field theories, which are wrong below a critical distance but can predict nicely whatever you want for larger distances.

Roughly speaking, the way one can deal with this infinite number of terms which appear in a naive attempt to construct a renormalizable theory by a simple trick: One assumes they have all the same order at the critical length. This has a simple consequence: Most of them become irrelevant very fast for larger distances, and one can savely ignore them. What survives are only those terms which are the most unproblematic ones, the closest thing to a renormalizable term. They also decrease much faster than renormalizable terms, but much slower than all this infinity of other terms. This makes gravity extremely weak in comparison with the other forces, but the lowest level - the tree approximation - works nicely for a large range of distances.

19. Jan 3, 2015

### ChrisVer

That's in the language of Effective Field Theories. For me the cut-off you add is rather unphysical. Take for example the QCD cut-off, although it's connected to pions etc, it's rather unphysical and ugly for me [in terms of QCD]. I prefer the breaking of the symmetry and the dealing with discrete symmetry left-overs , lattice QCD.
And why would you ask for the theory to behave nicely at large distances and not at small? and vice versa. Except for if you are assuming new physics in the mean-way (like in QED at Landau's Pole).

20. Jan 3, 2015

### Ilja

For me, the cutoff is clearly physical.

I don't understand your preference for lattice QCD in comparison with QCD with cutoff, because a lattice theory is, IMHO, simply a variant of a theory with a cutoff, with the lattice length as the critical distance.

Because I think field theories are only meaningful as continuous approximations of some underlying discrete reality. Of course, one may hope that we have already found some really fundamental field - and those "spacetime-interpretations" of general relativity heavily suggest such an interpretation of the field $g_{\mu\nu}(x,t)$ as fundamental. But I think this is naive, the field $g_{\mu\nu}(x,t)$ simply influences the behaviour of clocks without having a fundamental meaning, and defines some continuous approximation of some properties of an underlying discrete reality.