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Is the idea of a graviton reconcilable with general relativity? Is that already quantum gravity or something that works perfectly with existing gr?
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???Ratzinger said: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.
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.misogynisticfeminist said: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.
If gravity is a result of a higher dimensional influence outside the observable universe would this mean byebye to a theory of everything?Mortimer said:Note that what follows is very speculative and certainly not proven. Don't take my word for it.
Photons have velocity [itex]c[/itex] in 3D, regardless of reference frame. For all of them [itex]ds^2=0[/itex] 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 [itex]c[/itex] (the 4-velocity [itex]U^{\mu}[/itex] is invariant [itex]c[/itex]), 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.
A graviton would be analogous to photons, not to electrons.misogynisticfeminist said: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.
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.Ratzinger said:Is the idea of a graviton reconcilable with general relativity? Is that already quantum gravity or something that works perfectly with existing gr?
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.
I guess not. If such repeating patterns would exist in higher dimensions, this could be a unification by dimensional symmetries. Like a fractal.Art said:If gravity is a result of a higher dimensional influence outside the observable universe would this mean byebye to a theory of everything?