why do we need the graviton?

Javier

General relativity does describe the nature of spacetime and the resulting gravitational effects quite well, though you have to keep in mind that any theory (except by definition a fundamental theory from which all else follows) has its regime of applicability, beyond which it will give nonsense results.
Take as the prototype: Newtonian vs. Qauntum mechanics. QM is supposed to be the more fundamental theory, which it is, from which Newtonian physics should arise, which it does. No one would doubt (except cranks perhaps) that Newtonian physics works fine for everyday "human" scales...and even though QM is supposed to be more fundamental theory, this doesn't mean we go out and apply it to describe planes in flight. The reasons: (1) Describing the system in terms of the consituents is too complicated...also, talking about the protons, neutrons, and electrons making up the plane and surrounding air in motion is not very useful to describe the macroscopic object. (2) Newtonian mechanics is a very good approximation when talking about a collective object like a plane, so using QM and talking about all the particles making up the plane would be overkill in accuracy anyway.
The same discussion applies to general relativity, where this now plays the role of the effective theory describing macroscopic scales quite nicely. However, the theory does not apply below a certain distance scale: for example, it can't handle extreme spacetime curvatures well (singularities are the signal here). Therefore, it is commonly believed in the physics community that there is a more fundamental theory describing the nature of spacetime, and that general relativity should emerge from it at the distance scales where it is a good approximation. In addition, spacetime is dynamical, and we would expect quantum mechanics to apply to it as well...why should only electrons, light, and everything else *other than* spacetime behave fundamentally qauntum mechanically?
Gravitons arise as a description that tries to mirror what was done for *all the other known constituents of the universe*: by using quantum field theory. The idea here is that gravitons arise as quantum excitations of spacetime, but in a way that is still an approximation: we consider only fluctuations of spacetime about a given background spacetime that is smooth down to the smallest distance scales. There are arguments that the reason this description doesn't work as a truly fundamental theory of spacetime is that spacetime structure is *not smooth* down to all distance scales...that the geometry of spacetime is quantized...it, too, has fundamental constituents!
Some physicists work on such an attempt at quantizing spacetime starting with the classic general relativistic theory. Gravitons would arise as a more macroscopic (albeit a tiny macroscopic) approximation to what's really going on. This approach is called "Qauntum Geometry" to distinguish it from any other qauntum gravity theories.
A larger group of physicists work in string theory, which is any possible way to find a fundamental theory of spacetime (and with it, everything else tied together!). More ambitious, no? This is the camp that gets more attention...like Nova specials and magazine articles.
In the end, one must be careful to remember that neither of these paths (quantum geometry and string theory) have any testable predictions yet, and so they are hypotheses for the way things are...so talking about gravitons and strings as if they are known to exist is misleading. The Standard Model of particle physics, on the other hand, has been (and continues to be) tested and though it is still not the most fundamental description of things, it is a beautiful thing.
On that note, good night.

Janitor

I got home today and found several interesting and informative posts by you. You wrote, "neither of these paths... have any testable predictions yet..."

Do you think it is possible that some day some approach to gravity may actually allow us to derive Newton's gravitational constant G as the (macroscopic at least) coupling constant? So far, are theorists inserting G into their theories as a given parameter instead of trying to allow it to come to the surface naturally, so to speak?

Javier

So far, it is not clear why the value of Newton's constant (and the speed of light, etc) is what it is. In "fundamental theories", there are a very few (maybe one or hopefully someday zero) free parameters, and things like Newton's constant can be expressed in terms of them...but we leave them free to see if there is a mechanism that fixes these parameters such that G=what it should.
There have been similar issues explored in the successful standard model of the weak and electromagnetic unification: masses and couplings of things we detect are determined in terms of a fewer number of more fundamental couplings. However, in the Standard model of particle physics, there are still a relatively large number of parameters whose values remain unexplained. In models like GUTs, where a unification of strong with electroweak interactions is attempted, the values of couplings and masses are determined by yet a smaller set of parameters (the couplings and masses would be determined via group theory: when a gauge group breaks into a number of other groups, like SU(5)-->SU(3)xSU(2)xU(1), the new fields and interactions are given masses and couplings dictated by the group theory of how the decomposition occurs).
In my opinion, Newton's constant will be determined in terms of a more fundamental theory...but there is no guarantee!

Janitor

I was at that particular store yesterday, and I went down their book aisle, and there are no more of that particular book. I may go look in some other store on of these days.

SAZAR

Gravity = Nuclear force

Sariaht

Why we need gravitons? Because we don't, and we don't need football either!

shrumeo

I'll be hanged for this but someone told me that String Theory would one day reconcile QM and GR, especially when it comes to gravity. Supposedly, in String Theory, gravity is our universe's (or brane's) interaction with other branes through >3 dimensions. Could be total poo poo.

Lankyman

the theory of general relativity canot be acepted as acurate or corect because no one has been able to conduct adequete experiments to prove it right or wrong.

that is why it is only a theory and not a law thefor it cannot be used as the basis of any other theory without the consideration of it all being wrong.

humanino

GR is correct. It predict itself where it is gonna fail.
QFT is so succesful only because it is effective. We later succeded to extract first principle of QFT, but it was not originally formulated from them, contrary to GR.

GR is more like a mathematical theorem : there is no way out of it. I am sorry, but I must say that Lankyman seems to be a good joker.

sol2

The defintion of time in relation to the graviton is not the same, if you alter Sr results like LQG is doing. If the presence of the theoretial graviton can move freely between dimensions how shall we see this feature if not in relation to a continous nature of "time" so the geometry is very much different between the two.

Overall this perception can be comment on and corrected, but it helps orientate the thinking of our "times"

I left out the issue of background dependence and independance for a very good reason, since "space" can be a issue as well?

For more on the graviton

Nereid

Actually, GR has been tested in rather a large number of experiments and observations, and has passed every test with flying colours. This 2001 review gives a good snapshot of the status at that time. Since then there have been several other, more stringent tests, for example Cassini. And we're all waiting for the first results from Gravity Probe B and LIGO!

While there are some aspects of GR that aren't yet tested, the extent of the tests done to date isn't bad - ranging from ~1% to 1 part in 100,000.

C. Michael Turner

That question truely answered is the missing..................................... Space - time should be understood as the resulting and continious actions of a process. Space, time, and gravitational wave synchronization are all actions of a process. A process that has a beginning, middle and eventual end. Space and time are actions in the middle. The process is that discrete matter combines for an overall release of mass into the gravitational wave. THE REASON SPACE AND TIME SEEMED CURVED is a misunderstanding of a fundemental nature of the universe. Mass evaporates into the gravitational wave creating the actions of time, the unfolding of space and a sychronization of gravitational waves that through the path of least resistance brings matter together.Gravitational relativity: Point of origin, mass to energy transfer in wave form. Time and space, the graviton, will never come in quanta. They are continious actions of mass independent yet relative!