why do we need the graviton?

da_willem

I'm under the impression that general relativity with it's curved space-time as a description of the gravitational force is commonly accepted in the physics-community.

However there is another interpretation of the gravitational force in terms of a particle, the graviton. This seems to me as a totally different mechanism. Is it true that only one of these descriptions can be right?

So my question is: why do we need the graviton, when we've got this highly accurate theory of general relativity?

Norman

If we want a quantum mechanical view of gravity, which I, atleast, would like to see in my lifetime, we would need to talk about the quanta of the gravitational field. Hence the graviton. General relativity is a classical theory, therefore gravity on small scales is not addressed. My understanding of GR is very crude, but I think that a theory will eventually come about which reconciles the geometric interpretation of curved backgrounds with the notion of a quantum field for gravity. Are we anywhere close to having quantum gravity... I guess it depends on how optimistic you are.
Cheers,
Norm

Monique

Could someone at the same time explain the difference between gravitons and Higgs bosons, and why/how they give rise to gravitational force?

Janitor

The Higgs is expected to be massive, electrically neutral, and spinless.

The graviton is expected to be massless, electrically neutral, and spin-2.

Monique

I was just going to ask you the 'why' of

"The Higgs is expected to be massive, electrically neutral, and spinless.
The graviton is expected to be massless, electrically neutral, and spin-2."

But I guess your title will explain more [;)]

Norman

There is actually a fundamental difference between the two. The graviton is the particle which mediates the gravitational force, much like the vector bosons of weak interactions and the photon of electromagnetism or the gluons of the strong force. The Higgs is thought to be the field by which particles acquire mass. The idea is that through interaction with the field all particle acquire their specific masses. This is one of the most fundamental questions in physics... why does the proton have a mass of 938.280 MeV or any other particle for that matter?
Cheers,
Ryan

da_willem

It seems quite strange to me to have two different explanations of the gravitational force. One for the large and one for the small scale where it should obey QM.

How can a theory of a force ever be taken seriously if there is such a seperation. Obviously gravity doesn't work by both curving space-time AND exchanging virtual messenger particles like the graviton, or does it...?

ZapperZ

But we still don't know if there is a "separation". That's why this is still an on-going research work. Maybe one is a generalization of the other, very much like SR and Newtonian laws.

Besides, it isn't that strange to have more than one different "explanations" for the same thing. QM, for instance has several different types of formulation (Matrix, Schrodinger, path integral, 2nd quantization, etc, etc...). All of them appear to look different from each other. Even classical mechanics have two separate dichotomy in approach - Newtonian "forces" and Hamiltonian/Lagrangian/Least Action Principle. They all "meet" somewhere and agree on the outcome. I wouldn't be surprised if GR and QFT meet the same way (but I wouldn't hold my breath).

Zz.

Nereid

No matter how one 'interprets' a physical theory, it must account for experimental results, and make predictions which, when tested, are verified. Both QFT and GR do this, to some extraordinary degrees of accuracy (in some cases).

However, QFT and GR, as they are currently formulated, cannot BOTH be correct - there are regimes where they produce conflicting predictions. Unfortunately (or maybe fortunately [6)] [;)] ), there are no nearby black holes for us to test those conflicting predictions (and we're not about to make a black hole in the lab anytime soon [:(] [g)] ).

Norman

Nereid,

Could you maybe comment on where they give conflicting predictions? Or direct to somewhere it has already been discussed?
Thanks,
Norm

Nereid

I'll see what I can dig up, in detail. For now, Greene's "Elegant Universe" has a chapter or two which talk about this, and I'm sure many PF members - especially those who post to Strings&LQG - can contribute too.

Haelfix

The graviton naively does produce general relativity.

If you take a course in field theory, you can heuristically show that a spin 2 particle will output something that looks like Einsteins field eqns (up to a troubling constant).

In essence, you will show that a graviton will output something known as the tetrad formulation of GR, a highly linearized version of GR.

The problem is, this theory is nonrenormalizable... Hence the need for String Theory or something else.

For a quick, simple and nontechnical (for field theory) look at this, I suggest reading A. Zee 'Quantum field theory in a nutshell'

Monique

I'm going to attend a lecture by Brian Greene next week [:D]
Maybe he'll go into this issue..

Janitor

Last week I noticed Greene's "Fabric" book at a general-purpose warehouse store for $16.95. Has anybody here read that one?

Monique

I've heard mixed reviews: most people complain it is too general, too many analogies, repetitive compared w/ his previous book and other authors in the field. But still a very good read, especially when you haven't read his previous book in a long time (as I have :P).

I've recently ordered it, so I'll find out soon what new things he had to say :D

Janitor

Thank you for the information.

Monique

Are you still going to buy the book? I think you should. People were disappointed because they wanted a physics/mathematic rigorous book, instead they got an interesting read w/ many inspiring and identifiable analogies..

What Brian himself says about his two books: