gravity - gravitons or warped space-time?

[tomc]

Hi,

 

A bit confused - is gravity a result of graviton interaction, warped spacetime, or a combination of the two?

 

Tom C

[swansont]

One is a quantum description, the other is classical. It depends on how you are describing things.

[Reaper]

Yeah, that's pretty much it. General Relativity says that gravity is due to the geometry of space-time itself (and that objects with mass cause this warping), while gravitons are still hypothetical particles in quantum mechanics.

 

So far, there is much evidence to support Relativity's description of gravity.

[thedarkshade]

Just to add another thing and create a parallel.

In classical physics an electron (for example) is thought to create an electric field around itself, and Feynman's electrodynamic theory describes the electron as being sorrounded by things he called 'vitual photons' (because they behaved like photons) that behaved like EM waves. So when you have two electrons, one electrons absorbs this wave created by the other electrons, and this wave sort of transmits some momentum which will eventually results with a push. So in this case, the carriers of the electris field are these virtual photons. For strong nuclear force you have gluons, and similalry (why would it be different?) for gravity physicists think there are these gravitons.

 

Hope I haven't been confusing:-)

[D H]

General Relativity says that gravity is due to the geometry of space-time itself (and that objects with mass cause this warping), while gravitons are still hypothetical particles in quantum mechanics.

General relativity is a classical (non-quantum) theory. Physicists have not yet been able to reconcile general relativity and quantum mechanics. The bolded statement (bolding mine) is critical. Other than some gross characterizations (gravitons must be massless and have spin 2), physicists do not have a good model of what gravitons are and how they mediate gravity.

 

That is not to say that such a reconciliation is not achievable, just that it hasn't been achieved yet.

[ajb]

You should not really think of warping of space-time and gravitons as "two competing ideas". Without any well formulated quantum theory of gravity the best we can really do is discuss gravitons in the context of general relativity.

Gravitons are to general relativity what photons are to electromagnetism.

 

You should think of them as the particles associated to fluctuations in the geometry.

 

The important difference is that naive (perturbative) quantum general relativity is not well founded. However, what you can do is treat it as an effective theory and you can work with gravitons. In that sense lots is known about gravitons, you can work to first loop order ok. So for example graviton scattering amplitudes to low order can be calculated. Also you can show that any quantum theory that has general relativity as a classical limit should have massless spin-2 particles as their quanta.

 

There is evidence that quantum general relativity may be well founded outside of perturbation theory. However, as gravitons are perturbative it is possible that quantum gravity is not a theory of gravitons. But I will leave that for now.

[Baby Astronaut]

Is a single graviton believed to itself exert gravity? Or does an object instead "radiate" a bunch of gravitons in all directions and whatever they land on is attracted in that direction?

 

If the latter, does the graviton keep going through whatever it landed on, and attract the next thing in line? For example, if it went through A, does it continue on to B, and then to C and beyond, attracting everything it made contact with towards the direction of its home source?

 

Also, is there any material that can hypothetically "block" the passage of gravitons?

[ajb]

Well, if you look at the Einstein-Hilbert action you see that the action is highly non-polynomial and indeed gravitons self-interact.

 

Your question classically is "does the gravitational field gravitate". The answer is yes.

[Baby Astronaut]

Well, if you look at the Einstein-Hilbert action you see that the action is highly non-polynomial and indeed gravitons self-interact.

 

Your question classically is "does the gravitational field gravitate". The answer is yes.

My apologies for not understanding, but does that "yes" answer part "B" of the first question, the second question, the third question, or does it answer "yes" for all three questions at once?

[ajb]

Just yes to the first part. Gravitons have self-interactions.

[Martin]

...

There is evidence that quantum general relativity may be well founded outside of perturbation theory. However, as gravitons are perturbative it is possible that quantum gravity is not a theory of gravitons...

 

Good point. Something to keep in mind.

[ajb]

Some thing that, coming from a "standard" QFT background is hard to accept. But then another "moto" to remember is that quantum field theory is exactly that, a quantum theory of fields. The association of a particle relies on perturbation theory and a distinguished vacuum. Just from semiclassical gravity it looks like we will have to think well beyond this when thinking of quantum gravity.

 

So Poincare group + quantum fields = particles.

 

But this is by no means a general statement when we pass to Diff. invariance.

[Baby Astronaut]

Your question classically is "does the gravitational field gravitate". The answer is yes.

Another question if you don't mind. This is a long-brewing set of questions for which I haven't found answers, so please bear with me.

 

If a single graviton does create its own gravity from where it stands (or if it's believed to), then does that gravity pull into all possible directions at the same time?

 

Or does the graviton pull in a linear direction only? For example, in an object the gravitons of the surface atoms would pull in the same direction as gravitons in underlying atoms, and those gravitons would pull in the same direction as gravitons in even deeper atoms within the object.

[galen]

Yeah, that's pretty much it. General Relativity says that gravity is due to the geometry of space-time itself (and that objects with mass cause this warping), while gravitons are still hypothetical particles in quantum mechanics.

 

So far, there is much evidence to support Relativity's description of gravity.

Space-time geometry was a postulate by Einstein to accomodate the instant non local effect of gravity, which he found unacceptable. In essence it allows all gravity effects to be local. This is not incompatible with gravitons as force carrying probability waves. Gravitons have not been detected yet, but would be expected to travel at c and show wave properties. Photon waves store energy at creation and release energy at interaction. Graviton waves at interaction convert + vector gravity force instantly at the current locations of BOTH source and interaction.

ps Gravitational lensing can be cited as wave property evidence for gravitons, but what puzzles me is why no reflection, diffraction, interference etc evidence?

Also gravitational shielding is expected with probability wave, but problematic in space-time.

Edited March 18, 2010 by galen

[toastywombel]

One is a quantum description, the other is classical. It depends on how you are describing things.

 

You know, I always wondered whether there were gravitons, or instead it was just space time being warped like, but I think its both, I think both is happening at the same time.