Graviton and general relativity

[Ratzinger]

Is the idea of a graviton reconcilable with general relativity? Is that already quantum gravity or something that works perfectly with existing gr?

 

[El Hombre Invisible]

Apparently not. Space-time in GR is smooth while the graviton idea of gravity is quantised. GR, as I understand it, cannot be renormalised because of this. Your second question, what did you mean by 'it'? The graviton? In which case, no. I don't think it's in the standard model, if that's what you mean. GR? No again. GR is not a quantum theory.

 

[Ratzinger]

With the word 'that' in my first sentence I’m referring to the graviton and consequently a quantised gravity theory.
So when all these books assuming the existence of gravitons, then they denying the GR as put
forward by Einstein?

 

[El Hombre Invisible]

It won't DENY GR, in the same way GR does not deny Newton's theory of gravitation. It is hoped that a quantum theory of gravity would be more complete than GR, particularly in explaining gravity on incredibly small scales (around the Planck scale).

 

[Ratzinger]

But isn't gravity according to GR something complete different from all the other forces? It's not a force but a curvature of spacetime induced by mass/ energy, as I read again and again in all my (mostly popular) books. But then I read often in the same books some chapters later that gravity is transmitted by gravitons just like the electromagnetic force by photons or the strong by glouns. So all of the sudden gravity is treated equally and just a force like the others.

 

[misogynisticfeminist]

Every force has its field, including the gravitational force. The graviton is simply the quanta of that field. GR talks about relations between spacetime and the G field. But quantizing that G field is something remote and strange to GR. We are simply quantizing this gravitational field, and everything which is quantized has its quanta, which in this case is the graviton.

The only thing different when someone talks about the graviton is that the G force is not smooth, but rather comes in discrete lumps. To put it very crudely, the approach of LQG, trying to quantize gravity, quantizes spacetime together with it.

but remember, that no one has seen the graviton, so we can't say with certainty that it exists.

 

[Art]

But isn't gravity according to GR something complete different from all the other forces? It's not a force but a curvature of spacetime induced by mass/ energy, as I read again and again in all my (mostly popular) books. But then I read often in the same books some chapters later that gravity is transmitted by gravitons just like the electromagnetic force by photons or the strong by glouns. So all of the sudden gravity is treated equally and just a force like the others.

I'm no doubt wrong but I thought gravity as portrayed under GR is a curvature of space/time caused by the presence of mass. If that mass were to suddenly disappear then the reshaping of spacetime around the hole left by the disappeared mass would create waves which would travel at the speed of light and it is these waves which are called gravitons?

 

[misogynisticfeminist]

no, simply, gravitons are the quanta of the G field. you have a G field, you'll find gravitons.

you can look at G fields in the same way as other fields, you have an electron field, you'll find electrons.

i find it a little uncomfortable talking about gravitons as if they are experimental facts actually.

 

[Art]

no, simply, gravitons are the quanta of the G field. you have a G field, you'll find gravitons.

you can look at G fields in the same way as other fields, you have an electron field, you'll find electrons.

i find it a little uncomfortable talking about gravitons as if they are experimental facts actually.

To expand a little further. GR didn't require gravitons as it doesn't claim any direct interaction between two masses (only indirect through the curvature of spacetime) whereas quantum mechanics assumes there is a direct interaction. As far as I know this asssumption by QM is more expectation than deduction; based on previous successes in explaining the other 3 forces through the exchange of particles - gauge bosons.
So maybe my understanding is correct afterall as nobody has yet found gravitons

 

[El Hombre Invisible]

Art, Mysogynisticfeminist (what kind of name is that?): I think you're both right. Both GR and quantum gravity theorists expect gravitational waves which, I believe, have not yet been observed with certainty. The gravity wave in the quantum gravity theory is a coherent state of gravitons. It is to gravitons what the electromagnetic wave is to photons.

 

[Ratzinger]

It is to gravitons what the electromagnetic wave is to photons.

But these gravitons quantize spacetime with it, as misogenistfeminist (strange name indeed) 'put it crudely'. So they (as far as they exist) are something different, because gravity is something different as Einstein, Art and I say. Or not?

 

[Mortimer]

Note that what follows is very speculative and certainly not proven. Don't take my word for it.
Photons have velocity $c$ in 3D, regardless of reference frame. For all of them $ds^2=0$ which means that their time "stands still". Essentially, the time dimension does not exist for the photon (at least if treated as a particle; this may not apply when treating photons as a wave).
Mass particles show similar behavior in 4D where they have velocity $c$ (the 4-velocity $U^{\mu}$ is invariant $c$), again regardless of reference frame. "Hyperspacelanders" would see mass particles move like we see photons move (even wave-particle duality may be similar, yet in +1 dimensions).
There are ways to show (by means of relativistic Lagrangians; see posts in thread "relativistic mass and energy") that energy-momentum shows similarities too, if the extra dimension for mass particles is taken into account.
So the role for the boson of gravity may perhaps better fit mass particles themselves, provided something else can play the role of gravity's fermion. An obvious candidate would be black holes that would then be 5D "particles". The pattern may repeat itself for higher dimensions.

 

[Art]

Note that what follows is very speculative and certainly not proven. Don't take my word for it.
Photons have velocity $c$ in 3D, regardless of reference frame. For all of them $ds^2=0$ which means that their time "stands still". Essentially, the time dimension does not exist for the photon (at least if treated as a particle; this may not apply when treating photons as a wave).
Mass particles show similar behavior in 4D where they have velocity $c$ (the 4-velocity $U^{\mu}$ is invariant $c$), again regardless of reference frame. "Hyperspacelanders" would see mass particles move like we see photons move (even wave-particle duality may be similar, yet in +1 dimensions).
There are ways to show (by means of relativistic Lagrangians; see posts in thread "relativistic mass and energy") that energy-momentum shows similarities too, if the extra dimension for mass particles is taken into account.
So the role for the boson of gravity may perhaps better fit mass particles themselves, provided something else can play the role of gravity's fermion. An obvious candidate would be black holes that would then be 5D "particles". The pattern may repeat itself for higher dimensions.

If gravity is a result of a higher dimensional influence outside the observable universe would this mean byebye to a theory of everything?

 

[ohwilleke]

no, simply, gravitons are the quanta of the G field. you have a G field, you'll find gravitons.

you can look at G fields in the same way as other fields, you have an electron field, you'll find electrons.

i find it a little uncomfortable talking about gravitons as if they are experimental facts actually.

A graviton would be analogous to photons, not to electrons.

There is nothing about a field theory the demands quanta. It has so happened that electromagnetic field theory can be seen as emerging from quantum behavior. One could have a gravitational theory that was classically smooth and did not have quanta. Indeed, one of the real issues in efforts to unify gravity and general relativity is whether you can do it with gravitons, whether space-time is itself discrete, and whether you need both gravitons and a discrete space-time to explain general relativity -- perhaps one or the other would suffice.

Quantum theories with carrier particles (gluons, W, Z, photons) are batting 3 for 3 in explaining the fundamental forces so far. But, nothing prevents the the fourth time from being the charm and working some other way.

 

[pervect]

Is the idea of a graviton reconcilable with general relativity? Is that already quantum gravity or something that works perfectly with existing gr?

The problem with gravitons is that they won't renormalize. This places the problem with the graviton on the quantum side, not on the GR side. Quantum Field Theory (QFT) has some unsightly infinities. Renormalization is the trick that allows us to sidestep these infinities. Unfortunately this trick won't work with GR, so we are stuck with nonsensical infinite results when we attempt to use standard methods to quantize gravity.

We can deal with non-quantized gravity just fine with classical GR. The problem with the idea of the gravition is with the quantum mechanical aspects of quantizing gravity.

 

[Chronos]

Actually, there is no conflict between gravity and quantum theory in the low energy realm. It quantizes nicely using the usual field theory techniques, is renormalizable and yields well behaved quantum predictions. The 'conflict' [a term I feel is a bit misleading] occurs in the high energy realm:

Introduction to the Effective Field Theory Description of Gravity
http://arxiv.org/abs/gr-qc/9512024
Author: John F. Donoghue (Univ. of Massachusetts, Amherst)

'This is a pedagogical introduction to the treatment of general relativity as a quantum effective field theory. Gravity fits nicely into the effective field theory description and forms a good quantum theory at ordinary energies.'

 

[pervect]

Thanks for the link. I'm gather I'm guilty of the "old way of thinking" mentioned in the introduction, which I'll quote

The conventional wisdom is that general relativity and quantum mechanics
are presently incompatible. Of the “four fundamental forces” gravity is said
to be different because a quantum version of the theory does not exist. We feel
less satisfied with the theory of gravity and exclude it from being recognized
as a full member of the Standard Model. Part of the trouble is that we
have tried to unnaturally force gravity into the mold of renormalizable field
theories. In the old way of thinking, only the class of renormalizable field
theories were considered workable quantum theories. For this reason, general
relativity was considered a failure as a quantum field theory. However we
now think differently about renormalizability. So-called non-renormalizable
theories can be renormalized if treated in a general enough framework, and
they are not inconsistent with quantum mechanics[1]. In the framework of
effective field theories[2], the effects of quantum physics can be analyzed
and reliable predictions can be made. We will see that in this regard the
conventional wisdom about gravity is not correct; quantum predictions can
be made.

 

[Mortimer]

If gravity is a result of a higher dimensional influence outside the observable universe would this mean byebye to a theory of everything?

I guess not. If such repeating patterns would exist in higher dimensions, this could be a unification by dimensional symmetries. Like a fractal.