Equivalence Principle Test Kits

[Edward Wij]

Nothing at all. GR would still be the classical limit of whatever quantum gravity theory was underneath. So at the level at which we talk about gravitational waves, the classical level of GR, nothing would be changed.
The spin-2 particles wouldn't radiate the wave; they would be the wave, just as spin-1 particles (photons) are electromagnetic waves.

Let's differentiate between photons and virtual photons, gravitons and virtual gravitons

Photons are the quanta of electromagnetic wave
virtual photons are quanta of the electromagnetic force (electric field)

So
Gravitons are the quanta of gravitational wave
virtual gravitons are the quanta of the gravitational force (gravity field)

In pure quantum gravity where spin-2 particles rule. You have virtual gravitons carrying the gravity force. It is not a curvature of spacetime, so how can virtual gravitons produce gravitational wave. It's like saying virtual photons produce electromagnetic wave (again recall virtual photons is the electric field).

In pure quantum gravity. There are only virtual gravitons. Unless you are saying these virtual gravitons are spacetime itself and hence can produce gravitational wave (produce gravitons)? So virtual gravitons produce curvature which produce gravitons which produce gravitational wave? I'm thinking how virtual photons can produce electromagnetic wave.. Is there an analogy where virtual photons produce "curvature" (what is the equivalent) which produce photons which produce electromagnetic wave?

 

[PeterDonis]

Photons are the quanta of electromagnetic wave
virtual photons are quanta of the electromagnetic force (electric field)

This is not really what "virtual" means in reference to virtual particles in quantum field theory. But it will do as a very rough approximation for this discussion.

So
Gravitons are the quanta of gravitational wave
virtual gravitons are the quanta of the gravitational force (gravity field)

"Interaction" is a better word than "force", because gravity is not a "force" in GR, and the reason for that (that objects moving solely under gravity feel zero acceleration, unlike objects moving under other interactions like EM) does not change if GR is an effective classical field theory on top of an underlying quantum field theory.

In pure quantum gravity where spin-2 particles rule. You have virtual gravitons carrying the gravity force.

Ok (with the caveats and qualifications above).

It is not a curvature of spacetime

Incorrect. Again, the classical limit of this theory is still GR, and in GR, gravity is still spacetime curvature. (More precisely, tidal gravity is spacetime curvature.) So at the classical level, the result of all those virtual gravitons is still spacetime curvature.

In pure quantum gravity. There are only virtual gravitons.

Incorrect. There's nothing preventing real gravitons from existing in a quantum field theory of gravity, any more than the quantum field theory of electromagnetism prevents real photons (and hence electromagnetic waves) from existing. Remember that, if there is a correct underlying theory of quantum gravity, it has to account for all the phenomena that our current classical theory (GR) accounts for, just as the correct quantum theory of EM has to account for all the phenomena that the classical theory of EM (Maxwell's Equations) accounts for. So classical EM fields and EM waves still exist in quantum electrodynamics, and spacetime curvature and gravitational waves still exist in quantum gravity.

Unless you are saying these virtual gravitons are spacetime itself

That's one way of looking at it, yes. Se above.

and hence can produce gravitational wave (produce gravitons)?

Just spacetime by itself doesn't "produce" gravitational waves. There has to be a source (matter or energy wiggling around) somewhere. But given a source, spacetime by itself can propagate gravitational waves (fluctuations in spacetime curvature), and at the quantum level, this is modeled as gravitons (real ones) propagating.

 

[Vanadium 50]

Edward, much of what you have written about gravitons is wrong. I recommend that in the future you reference what you write and try to avoid putting your own theoretical ideas in. It's confusing, and this is one reason why PF has rules against personal theories.

 

[Edward Wij]

This is not really what "virtual" means in reference to virtual particles in quantum field theory. But it will do as a very rough approximation for this discussion.
"Interaction" is a better word than "force", because gravity is not a "force" in GR, and the reason for that (that objects moving solely under gravity feel zero acceleration, unlike objects moving under other interactions like EM) does not change if GR is an effective classical field theory on top of an underlying quantum field theory.
Ok (with the caveats and qualifications above).
Incorrect. Again, the classical limit of this theory is still GR, and in GR, gravity is still spacetime curvature. (More precisely, tidal gravity is spacetime curvature.) So at the classical level, the result of all those virtual gravitons is still spacetime curvature.
Incorrect. There's nothing preventing real gravitons from existing in a quantum field theory of gravity, any more than the quantum field theory of electromagnetism prevents real photons (and hence electromagnetic waves) from existing. Remember that, if there is a correct underlying theory of quantum gravity, it has to account for all the phenomena that our current classical theory (GR) accounts for, just as the correct quantum theory of EM has to account for all the phenomena that the classical theory of EM (Maxwell's Equations) accounts for. So classical EM fields and EM waves still exist in quantum electrodynamics, and spacetime curvature and gravitational waves still exist in quantum gravity.
That's one way of looking at it, yes. Se above.
Just spacetime by itself doesn't "produce" gravitational waves. There has to be a source (matter or energy wiggling around) somewhere. But given a source, spacetime by itself can propagate gravitational waves (fluctuations in spacetime curvature), and at the quantum level, this is modeled as gravitons (real ones) propagating.

Ok. So quantum gravity has to be account for classical limit of general relativity as having spacetime curvature. What you didn't differentiate what is the difference between virtual gravitons and real gravitons. In quantum field theory.. virtual photons are the electric field. While real photons are the EM wave. In quantum gravity. We still have both virtual gravitons and real gravitons. What are their relationship? What is the difference between virtual gravitons and real gravitons? Please compare it to the difference between virtual photons and real photons. In the latter.. whenever we have virtual photons (electric field) do we automatically have real photons? I want to compare the difference to virtual gravitons and real gravitons.

This is not really what "virtual" means in reference to virtual particles in quantum field theory. But it will do as a very rough approximation for this discussion.
"Interaction" is a better word than "force", because gravity is not a "force" in GR, and the reason for that (that objects moving solely under gravity feel zero acceleration, unlike objects moving under other interactions like EM) does not change if GR is an effective classical field theory on top of an underlying quantum field theory.
Ok (with the caveats and qualifications above).
Incorrect. Again, the classical limit of this theory is still GR, and in GR, gravity is still spacetime curvature. (More precisely, tidal gravity is spacetime curvature.) So at the classical level, the result of all those virtual gravitons is still spacetime curvature.
Incorrect. There's nothing preventing real gravitons from existing in a quantum field theory of gravity, any more than the quantum field theory of electromagnetism prevents real photons (and hence electromagnetic waves) from existing. Remember that, if there is a correct underlying theory of quantum gravity, it has to account for all the phenomena that our current classical theory (GR) accounts for, just as the correct quantum theory of EM has to account for all the phenomena that the classical theory of EM (Maxwell's Equations) accounts for. So classical EM fields and EM waves still exist in quantum electrodynamics, and spacetime curvature and gravitational waves still exist in quantum gravity.
That's one way of looking at it, yes. Se above.
Just spacetime by itself doesn't "produce" gravitational waves. There has to be a source (matter or energy wiggling around) somewhere. But given a source, spacetime by itself can propagate gravitational waves (fluctuations in spacetime curvature), and at the quantum level, this is modeled as gravitons (real ones) propagating.

Ok. So quantum gravity has to be account for classical limit of general relativity as having spacetime curvature. I'll remember that from now on. What you didn't differentiate is what is the difference between virtual gravitons and real gravitons? In quantum field theory.. virtual photons are the electric field. While real photons are the EM wave. In quantum gravity. We still have both virtual gravitons and real gravitons. What are their relationship? What is the difference between virtual gravitons and real gravitons? Please compare it to the difference between virtual photons and real photons. In the latter.. whenever we have virtual photons (electric field) do we automatically have real photons? I want to compare them to the the difference between virtual gravitons and real gravitons. Thanks.

 

[PeterDonis]

In quantum field theory.. virtual photons are the electric field. While real photons are the EM wave.

As I said in my previous post, this is a very rough approximation; it's not really correct, and you should not be using it if you really want to understand quantum field theory.

In any QFT, there are many different possible field states. Some of those states correspond to what, at the classical level, we would call a "static force field" like the EM field (or a gravitational field, in the case of a QFT for gravity). Other states correspond to what, at the classical level, we would call a "traveling wave" like an EM wave (or a gravitational wave, in the case of a QFT for gravity). Still other states have no simple classical analogues (for example, the states in a superconductor).

The distinction between these different types of states, however, is not the same as the distinction between real and virtual particles. Real particles are particles that are "on the mass shell", i.e., they obey the relativistic energy-momentum relation $E^2 - p^2 = m^2$ (where as is usual in QFT, I am using units in which $c = 1$). Virtual particles are particles that do not obey this relation. The existence of such particles is possible because of the uncertainty principle (at least, that's one way of viewing it); the more a virtual particle's energy or momentum differs from the "on shell" value, the shorter the length of time it can exist and the shorter distance in space it can cover.

However, before we can even talk about virtual vs. real particles, we have to look at the concept of particles itself. In QFT, particles are not fundamental entities; fields are. What we think of as "particles" are just particular states of the quantum field--and the states with a useful interpretation as "particles" are not necessarily the states that I talked about above as corresponding to classical force fields or even classical waves.

The point of all this is that there are a lot of complexities involved in quantum field theory. (And we're not even sure that, if we do end up finding a correct quantum theory of gravity, it will be a QFT of the kind we've been talking about.) I would strongly advise you to take time to learn about those complexities (and if you want further discussion of them on PF, you should start a separate thread in the Quantum Physics forum, it's not really on topic in this forum) before trying to speculate about quantum gravity.

 

[Edward Wij]

Thanks for the distinctions and clarifications. I can now grasp books on relativistic quantum field theory and general relativity and understand papers of quantum gravity.

Now when I think of matter, energy and space time. I know I have to remove old ways of thinking Newtonianly (or I will never understand them) and have to think in terms of wave function and spacetime curvature. And the challenge of thinking how these two are coupled...