# Gravity Wave Discovery and Gravitons

1. Feb 12, 2016

### ptalar

Does the discovery of gravity waves imply the existence of gravitons?

2. Feb 12, 2016

### Staff: Mentor

No.
If gravity can be quantized - and that is the usual expectation - then gravitons should exist, but that knowledge didn't need the direct observation of gravitational waves. Under this assumption, the direct detection allows to put upper limits on the graviton mass, but those are worse than indirect upper limits from the existence of galaxy clusters.

3. Feb 12, 2016

### ShayanJ

Its true that this is the first time people are directly observing GWs, but people were expecting to see them. There were much evidence in favor of GWs. So it seems to me that this observation doesn't change anything about theoretical approaches toward QG, except about those that were trying to prove that horizons don't form for some reason, because this observation is clear evidence in favor of the existence of horizons.
But the main part of my answer to you is from here. There were times when EM waves were just waves, ripples in EM fields, a classical field. So clearly the existence of some classical field doesn't mean it has to be quantized, e.g. no one quantizes the temperature field of a room!
But at some time, people said photons exist. What happened? Two things: 1-There were observations that couldn't be explained by classical EM fields(at least it seemed so to the people back then). 2- A self-consistent theory of photons was born that was also consistent with observations. These two points made people say photons exist.
But about gravitation. The first reason, as far as I know, is almost established. Almost all physicists in the field think that gravitation has to be quantized. But we still don't have a self-consistent theory of QG that is also consistent with observations and this direct observation of GWs doesn't change that.

4. Feb 12, 2016

### eXorikos

All observations are consistent with GR as we have know it for a century. Gravitons are massless, travel at the speed of light and have spin 2. I too am wondering if this would help in some way, but I guess it just eliminates some exotic theories. It would be nice to see an overview of exactly what limits are placed by these 3 conditions. I do not have any answer to that question.

5. Feb 12, 2016

### Staff: Mentor

Unless the room is filled by a superfluid. Second sound.

6. Feb 12, 2016

### bcrowell

Staff Emeritus
It's more than an expectation or an assumption. It's been known since ca. 1927 that you can't couple a quantum-mechanical field to a classical field without creating serious problems, such as nonconservation of energy and momentum. For example, in the Bohr-Kramers-Slater theory, the electromagnetic field was classical, but atoms were quantized. An example of a problem was that there was then no way to prevent a particular electromagnetic wave packet from having its energy absorbed by two different atoms, violating conservation of energy. So in such a theory, energy was conserved only on a statistical basis. Although the arguments are totally generic, here are a couple of papers discussing the case of gravity.

Carlip, "Is Quantum Gravity Necessary?," http://arxiv.org/abs/0803.3456

Adelman, "The Necessity of Quantizing Gravity," http://arxiv.org/abs/1510.07195

This is an apples-oranges comparison. By "field" we don't mean just any function that varies across spacetime. We mean something that carries energy and momentum. Quantization is not optional, for the reasons I described above.

But if we're going to have a lengthy discussion of this, we should start a separate thread in the BTSM section.

7. Feb 12, 2016

### ShayanJ

I can't back what I said with physical arguments because of my lack of knowledge. But there is surely some discussions going on about whether its really necessary to quantize gravity. People are even designing experiments!
Another thing I find worth mentioning, is the emergent spacetime/gravity approaches. We still can't say they're wrong. If these approaches turn out to be correct, then it seems reasonable to expect that gravity itself isn't quantum mechanical, but only the underlying theory is!

8. Feb 12, 2016

### gamow99

First, what exactly does 'quantized' mean? Does that just mean composed of particles? I would think that it would be logically impossible for a wave to not be composed of particles or at least sections. The only way a wave can go up and down is if it is composed of parts or particles and one part of the wave goes up while the other part goes down. If gravity waves were composed of one thing then by 'one' I mean it has all of the same non-spatiotemporal properties which to me is logically impossible for waves since in order for there to be a wave some parts of it must be up while other parts of it must be down.

Again, I'm not sure what you mean by quantized but temperature is certainly the result of quanta or particles. It's the speed of the particles that humans sense which gives room its temperature. So I'm not sure your inference: 'because it's possible that temperature is not quantized, it is also possible that gravity is not quantized' is correct.

9. Feb 12, 2016

### Staff: Mentor

Nothing goes up and down in quantized electromagnetism, and nothing would go up and down in quantized gravity either. Assuming the direction of propagation of the signal is not called "up" or "down".

10. Feb 12, 2016

### ShayanJ

True, but we don't get those particles by quantizing the temperature fields, i.e. atoms and molecules are not quanta of temperature fields!

11. Feb 12, 2016

### ChrisVer

I am not sure that you can define temperature everytime via the velocity of particles... Temperature can have several definitions [take for example the case when you can define negative temperatures, just by defining them from the occupation distributions alone]
Another example I can think of is the temperature of the photons, which can vary, but their velocity is always c.

12. Feb 12, 2016

Not having made it past high school physics, I almost don't feel worthy to even post a question here amongst all you eggheads :) Can someone explain to me in simple terms why gravity waves don't imply the existence of gravitons? Is it an entirely different kind of force? I always had thought there was an associated particle with every force. Maybe that's true only for electromagnetic forces? So is a gravity wave just a ripple in spacetime itself without the need for a quantum field? Is that what Einstein figured? And by the way, congratulations to the teams who created such a fantastically sensitive experiment.

13. Feb 12, 2016

### bcrowell

Staff Emeritus
[/PLAIN] [Broken]

I've asserted that statement X is true and noncontroversial, for a number of theoretical reasons. I explained an example of the theoretical reasons, which are quite simple, and gave references to papers on the topic. You've pointed out that people would also like to do experiments to verify X by direct experiments. Your statement and my statement do not contradict each other.

Last edited by a moderator: May 7, 2017
14. Feb 12, 2016

### A/4

Although the GW that was detected is a classical object (i.e. a prediction of a classical theory), there is some connection to the quantum nature of gravity. That is, if the graviton is massless, we expect GWs to travel at the speed of light. A massive graviton will slow the wave down, and also introduce other polarization modes. At yesterday's press conference, Kip Thorne commented the data allows a constraint on the potential mass of the graviton of $m_G \leq 10^{-55}~$ g.

15. Feb 12, 2016

### ChrisVer

Because they don't have to... Gravity waves were by themselves a prediction of classical GR (that was discovered now), and so it's not something that came to be added to searches beyond GR (into quantum gravity)...

Gravity and the rest of the known forces are still something different.

That's what people believe and they suggest the particle named graviton, which we don't know from where it comes from yet [neither have seen it].

16. Feb 12, 2016

### bcrowell

Staff Emeritus
Gravitational waves have to travel at c regardless of whether they're quantized.

17. Feb 12, 2016

### ChrisVer

That's extremely small...

18. Feb 12, 2016

### A/4

But not if the graviton has mass, as the rest of my post says.

19. Feb 12, 2016

### ShayanJ

Yeah, yeah...But he clearly didn't have those in mind(or probably didn't know them). I just wanted to make the point that he has a wrong picture of what is quantization.