# I Graviton spin?

1. Mar 8, 2016

### bbbl67

I'm somewhat familiar with the General Relativity description of gravity, at least conceptually. So I thought I'd ask about the graviton theory of gravity. Specifically, I've read elsewhere that a graviton must be a Spin-2 boson. Okay, given that as it may, how does a spin-2 boson differ from a spin-1 or spin-0 boson? What is the difference in behaviour between these three types of bosons? Also, all of the familiar force carrier bosons, like the photon, the gluon, the W & Z bosons are spin 1, while the Higgs boson is spin-0, I gather. So what is it that makes the photon, gluon, and W & Z similar to each other, while being different from the Higgs, and how are they all different from the graviton?

Also do any of the graviton theories come close to reproducing General Relativity? Somewhere I read that graviton theory so far only reproduces Newtonian Gravity, rather than GR?

2. Mar 8, 2016

### dextercioby

While this is the proper place to ask all these questions, I'm afraid clear answers don't really exist. Or if they do, they are not clear to me. There's no valid (=renormalizabile like QCD) graviton theory, unless there's no cross interaction between gravitons, hence no curvature or... gravity. In other words, the only theory which makes sense is either the quantized linear gravity, or an effective field description of gravity. The latter is reviewed here: http://arxiv.org/abs/gr-qc/9512024

Last edited: Mar 8, 2016
3. Mar 8, 2016

### bbbl67

Okay, I understand, no real theory of gravitons vs. gravity yet. So what exactly is linearized gravity? A linear approximation of General Relativity?

So what about the other part of the question, the differences between boson spins?

4. Mar 8, 2016

### DelcrossA

My understanding of it is that the graviton is said to have be spin-2 because it is described as a rank-2 tensor field. More specifically, the polarization tensor for a graviton is given by the direct product of two polarization vectors (the ones used to describe a photon field).
$$R^\mu_\nu \ R^\rho_\sigma \ \epsilon^\nu_{\pm} \ \epsilon^\sigma_{\pm}$$

Note: the +- denotes the helicity of the particle. So the total product with will be changed by a net phase of $\pm2\theta$ ($\theta$ is the rotation angle about the chosen axis that the R operators rotate about) so that it transforms as if it had a helicity of +-2. Meaning,
$$\epsilon^\nu_{\pm} \ \epsilon^\sigma_{\pm}=\epsilon^{\nu\sigma}_{2\pm}$$

5. Mar 10, 2016

### bbbl67

No idea what those equations mean, whatsoever.

Let's go about this in a different direction. Does the graviton hypothesis imply that gravity is an energy field again, like under Newton. Because under Einstein, it became a negative energy field.

6. Mar 10, 2016

### Staff: Mentor

Quantum field theories are even more alien to classical mechanics than general relativity - the Newtonian notion of position and thus of force as something that affects the second derivative of that position doesn't appear.

7. Mar 10, 2016

### bbbl67

Okay, but doesn't the idea of a quantum particle, the graviton in this case, imply that it's a particle of energy?

8. Mar 10, 2016

### Staff: Mentor

No. It is "a quantized excitation of a quantum field" - and if you think this is less than helpful when you want a math-free intuitive picture of what's going on, I agree with you.

9. Mar 10, 2016

### Staff: Mentor

Energy is not a substance, or "stuff"; it's a property of something.

10. Mar 12, 2016

### bbbl67

Leon Lederman in his book Beyond The God Particle, considers energy to be stuff, while mass is just a property of that stuff.

Also this video from PBS Spacetime also considers energy to be stuff, while mass is just a property of the energy. That conclusion starts at 7:48 of the video.

11. Mar 12, 2016

### Staff: Mentor

Those are both popularizations, and neither is an acceptable source under the PhysicsForums rules.

There's nothing wrong with popularizations as long as you understand their limitations... But one of those limitations is that they oversimplify in many ways. "Energy is stuff" and "mass is just a property of the energy" are statements so vague as to be lacking in any meaning unless backed up with some serious mathematical rigor.

12. Mar 12, 2016

### bbbl67

Well, both were done by well-respected physicists. If you consider energy to be a property of stuff, then what do you consider "stuff" to be?

13. Mar 12, 2016

### Feeble Wonk

Ahhh... the question of ontological reality. It seems like a reasonable question to me.

14. Mar 12, 2016

### Staff: Mentor

Many excellent popularizations (and some rather bad ones too) are done by well-respected physicists. This is a good thing, because too few people have the opportunity to learn enough math to take on the real thing - but when you read them you have to remember that you're not taking on the real thing.

That question is too imprecise to answer - there's just no way to attach a rigorous meaning to the phrase "energy is a property of 'stuff'" so no way to consider it true or false. However, we've drifted far from your original question about spins.... For that, I am aware of no answer that is both the real deal and less technical than http://arxiv.org/abs/hep-ph/9405255

15. Mar 12, 2016

### atyy

The paper that dextercioby refers to by Donoghue says that gravitons = classical GR + small quantum corrections. So there is a good theory of quantum gravity (including most of the nonlinear part of GR, using gravitons) in the same sense that we have a good theory of quantum electromagnetism (using photons and electrons, as worked out by Tomonaga, Feynman, Schwinger and others).

The quantum gravity theory does not include full GR, in the sense that it does not include spacetime very near black hole or big bang singularities. However, that is the part of GR that we do not trust - in part because the quantum theory fails, so the basic answer is that we do have a good theory of quantum gravity that reproduces the successful parts of classical GR.

Last edited: Mar 12, 2016
16. Mar 13, 2016

### bbbl67

But doesn't GR work fabulously up until the event horizon of a black hole? It's only when you get past the event horizon towards the center of the BH that we have idea about. Since you said that linear gravity doesn't "include spacetime very near a black hole"? Or do you mean very near a black hole singularity?

17. Mar 14, 2016

### haael

Physicists will rot in hell for making simple things sound hard.

We have different kinds of fields.
The simplest is the scalar field (aka spin 0), where each point is assigned a scalar. Example: pressure or temperature.
Then we can have vector fields (aka spin 1), where each point has an associated vector. Example: wind.
We can also have fields where each point is assigned a matrix. These are called tensor field. Sometimes with restrictions, i.e. it has to be symmetric. We call them also spin-2 fields.

What kind of field is gravity, according to GR? It's literally the field of deformation. What kind of field is it? It's a symmetric tensor field. (Try Wikipedia: deformation tensor.)

The statement "graviton has to have spin 2" means really: "Gravity is deformation of spacetime, deformation is a tensor field and graviton is the quantum of that field."

In quantum mechanics we don't really have scalars, vectors and tensors, but their quantized relatives: quantum scalars, quantum vectors and quantum tensors, but you don't have to care about that right now.

18. Mar 14, 2016

### bbbl67

Amen! ;-)

19. Mar 14, 2016

### atyy

No, I was not talking about linear gravity. The graviton theory includes nonlinearities. Your quote of my post is not correct.

20. Mar 14, 2016

### nikkkom

You ask for answers, but when they are given to you, you don't like them. That's unproductive.

The answer to "why graviton must be a spin-2 particle" is mathematical: only spin-2 particles will result, in classical limit, in a force which looks like GR. If you don't understand the math which shows this, it is your problem, not math's. BTW, this particular math is slightly above my head too, but I don't go down the road "I don't understand it, so I'll demand a different explanation". There is no non-mathematical explanation to it.