Gravitons and gravity vs curvature of space-time.

[Liger20]

Hey, I've been a little confused on the concept of gravitons. I know that they are the messenger particle of the gravitational force, but I thought that gravity was a result of the warping of the fabric of spacetime. If a large star warps spacetime, therefore attracting things around it, then why would a messenger particle be needed in order to "tell" other pieces of matter that there is a gravitational force there?

 

[Voltage]

Personally I think of them as accounting units rather than actual particles. For example, when you spend some money, there's no host of little pennies flying over the counter. But then again, I am something of a relativity fan rather than a QM buff, so don't take my opinion as gospel.

 

[Norman]

You can consider the Graviton as the language of Quantum Field Theory trying to describe gravity.

The curvature of the spacetime manifold description is the language of General Relativity.

A quantum description of gravity is currently lacking. GR on the other hand works beautifully at large scales. So for a "large star" this is no longer a quantum description and therefore GR is an excellent description of this process and there is no need for the graviton at all. BUT, if you were to try to develop a Quantum Theory of Gravity that is compatible with QFT, then you will find that the gauge boson of gravity (called the graviton) is a spin 2 particle.

 

[affinefield]

You can basically divide the practicioners of General Relativity into two main camps:First,we have the "geometrodynamic camp" whose standard-bearer would be(after Einstein himself) John Wheeler and many of his students,though not all.Then,we have the "particle-physics camp",whose attitude toward any "interaction" is strongly influenced by their experiences with quantization and its inevitability.R.P.Feynman,S.Coleman,Steven Weinberg,belong firmly in the "QFT" camp.I strongly recommend that you read the short article in "Studies In The History and Philosophy of Modern Physics" called "A psi is just a Psi,Pedagogy,practice and the reconstitution of General relativity"(I think that is the title,but I'm sure it is close enough).Your question can really be answered quite simply:It is a matter of personal predjudice whether you seriously believe that "geometry is all" or whether the QFT vacuum is really the answer,and that Einstein's GR is really only an approximation to the "truth"(quantum gravity).I belong to the latter group,...,I believe that it is simply more "physical" to side with the quantum-theorists than with the "geometers",Ciao,...

 

[affinefield]

Also,try to read articles(or chapters in texts,say) with titles like "Gravity as a Gauge Theory".Contrast Weinberg's "Gravitation and Cosmology" with MTW,and notice the difference.Think about scale,like "Norman" mentioned.Think about a type of "Bohr-Rosenfeld"analysis of the gravitational interaction,...think about the energy scales involved in making gravitons the dominant source of interaction,...

 

[Haelfix]

I don't see a contradiction with the graviton treatment of GR and the usual one alla MTW. They both are dual descriptions of the same thing as both end up with the same identical field equations. I find it personally rather satisfying actually, quantize a spin 2 particle and you end up with a unique theory of gravity in the classical limit, namely GR.

Of course the quantum treatment necessarily requires a weak field approximation to calculate with, but that's really just a matter of technology and not something particularly deep.

People get horribly confused (wrongly imo) with the tension between gauge transformations and diffeormorphisms.

 

[affinefield]

Thank you.Your comments prompted me to go back and read two papers by Weinberg from the Physical Review:"Photons and Gravitons in S-Matrix Theory",from 1964(V 135 #4b),and "Photons and Gravitons in Perturbation Theory",from 1965(V 138 #4b).I have always sided with Weinberg's observation that General Relativity is the UNIQUE low-energy description of the massless spin-2 "graviton".This has cured my momentary and glaring ignorance displayed in my previous correspondence,...,

 

[trampjuice]

I do think there are two distinct camps, its not fair to say one view (Relativity's bending of space time) is macro and the other is simply on the micro/quantum scale but they are all the same and logically consistant with each other, just the scale differs.

The standard model has the graviton as the yet unobserved force transmitter for the weak, but infinite acting force of gravity. In this model gravity is transmitted at the speed of light between mass particles. i.e. if the sun disappeared the Earth would still rotate around it's gravity for 8 minutes before the mysterious gravitons stop transmitting the gravity force at the speed of light and the Earth flys off into space.

However, my understanding of Relativity in its bending of space time doesn't require gravitons as the force transmitter. Instead, space and time are themselves warped due a property of the presence of mass, and gravity is the name of this 'fabric space-time dimension warping' acting on both massive or massless particles (other force carriers like photons). i.e. Space-time is in itself the transmission medium for the force, no gravitons needed.

Why has this graviton force boson has not been observed. The other force carrier which acts over infinite distances has been easily observed, blocked, deflected etc.. (i.e. the electro-magnetic/electro-weak force which uses photons as force). If gravitons exist, as the force boson, and act over infinite distances like the electro-magnetic force, then why then can we not interact with them in any way to detect them - for example to block them?

 

[Naty1]

Everybody is "confused" by that issue...not to worry. It's like asking is light a particle or a wave? There is far more that we don't understand that we do understand; Hence my signature below.

 

[blechman]

In QED, the electromagnetic field is quantized in wavepackets that we affectionately call "photons." We picture them as particles. Yet we all know that macroscopic electromagnetic fields (such as those that zap us when walking across a carpet or the refrigerator magnet) are described by classical fields that make no mention whatsoever about photons. Is this a contradiction?

No. The way we "glue" these two descriptions together is as follows: the physical (quantum) state that corresponds to the classical electromagnetic field is a COHERENT STATE of photons - these states are not eigenstates of the number operator, but are a special linear combination of wavefunctions of 1, 2, 3, ... photons! Remember, this is quantum mechanics and we can do this.

Those who have studied QM should know that when you have a simple harmonic oscillator, the coherent states form the "semiclassical" states; that is, states whose quantum numbers describe (average) position and momentum. So these states behave as classical states.

If you take QED, and study NOT the states of definite photon number, but these coherent states, you will find that CLASSICAL E&M emerges!

Now to the extent that the graviton approximation makes sense (low energy, low curvature, etc): this argument follows word-for-word in the case of GR! Instead of using the "graviton-number states" we use the "graviton-coherent-states", and we find that these states correspond to CLASSICAL gravitational fields described by the classical Einstein equation.

So there is no contradiction between the "graviton" description and the "manifold" description of gravity, just like there is no contradiction between the "photon" and "field" description of E&M.

Of course, gravity is much more complicated than E&M, since it's a nonlinear theory, etc. But to the extent that the "graviton approximation" makes sense, this is not a real concern. It just means that the "coherent states of quantum gravity" are more complicated than the corresponding QED states.

Hope that helps!

 

[pallidin]

Gravity. A phenomenon that attracts mass; yet no-one knows what mediates this attraction(if there IS a mediating carrier), or even the very definition of mass in this respect.
Enough to want one to have a cold beer...