Shouldn't quantum gravity be an interaction between mass and spacetime?

In summary: Loop quantum gravity (LQG) [23, 24,..., 28]."I'm not sure how to make it any clearer that the answer to your question is "yes", we should be looking for a quantum version of spacetime.
  • #36
Demystifier said:
GR does not say that. Perhaps it's how GR is represented in popular literature, but it's not what GR really is. The primary interaction in GR is the interaction between energy-momentum tensor and spacetime metric tensor. The energy-momentum tensor may or may not be associated with mass. The reason why the mass seems essential is because the big massive bodies such as planets and stars usually have more energy-momentum than massless stuff like light. But at the fundamental level, massive electron and massless photon have comparable energies and momenta, so they are equally important in GR.
You are missing my point that gravity is SECONDARY. I think the issue is that people are confused what "gravity" means. It is not "a distortion of spacetime", it is not the primary interactions in GR, it is "an attractive force between masses (and equivalent energy)." The spacetime distortions that GR is about (which I consider primary interactions) are not "gravity." Gravity does not accelerate anything, it curves spacetime which changes how momentum is transfered (as well as how time flows locally). GR describes what is "actually" going on, which is a curvature in spacetime (which appear straight in a local frame). As you state, it is momentum that is changing, not some "force at a distance" that needs to be propagated with a graviton.

I'm just confused why anyone thinks that "gravity" would have a boson or a field to parameterize when GR expressly states that gravity is not a real force, and is not why masses (and energy) move through spacetime.
 
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  • #37
Vanadium 50 said:
This thread suffers from a couple of things, and maybe it can be put back on track.

I am not sure this can possibly be discussed at B-level. That's saying "I understand neither GR nor QM, except in a the most cursory manner, yet I am sure that people who do are all doing it wrong.": At a minimum, it's a tough sell.

It also suffers from the misconception that physics theories are all about words, and getting them in the right order. What does "an interaction between mass and spacetime" even mean in QM? The words all sound good, but how can you measure this? If this interaction were twice as strong or half as string, how would anybody know?

For this to make any sense at all, it needs to be quantitative. If the OP hasn't studied enough QM and GR to do that, that's a pity, but it's a necessary prerequisite to having any sort of useful discussion.
I don't know that it is "putting it back on track" to call the OP completely ignorant on GR and QM as you have no idea who I am.

I am absolutely not saying that people are "doing it all wrong" I'm saying that GR states that gravity is not a force between masses (can we agree on this part?) and yet it seems that many quantum gravity theories (specifically those involving gravitons) are focused on a mass<->mass interaction (please correct me if I am wrong on this front - but please note that a correction is not "You are an idiot").

I strongly disagree with a "this has to be quantative to make any sense." If you cannot describe the generalities of a theory without simply re-printing an equation then you really don't understand it. (Just because Ptolemy could predict the motions of the planets better than Copernicus doesn't mean he was a fool to put forward a heli-centric theory until Kepler could work out the rest of the details. --Einstein would be Copernicus in this analogy, btw.)

Gravity was first nailed down as a force by Newton. If gravity was a force in that way we would all feel similar inertial forces in freefall as we do when accelerated by a car or train. While gravity does direct our momentum, and increase our velocity, it does this by curving and distorting space and time (spacetime), NOT by putting a force on our mass (or energy-mass equivalent).

It seems (this is where my question lies) a mass-to-mass boson, like a graviton, would impart a force on us which should result in inertial forces through a massive body, which are not present in nature.

Can someone explain to me why a mass-to-mass (for example a Earth's mass to a free-falling human body) exchange would not experience inertial drag like we would when in a rocket?
 
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  • #38
TerranIV said:
You are missing my point that gravity is SECONDARY.
No, it isn't. See post #29.

TerranIV said:
I think the issue is that people are confused what "gravity" means. It is not "a distortion of spacetime"
Yes, it is. It's spacetime curvature. See post #29. Any GR textbook will tell you that.

TerranIV said:
The spacetime distortions that GR is about (which I consider primary interactions) are not "gravity."
Yes, they are. You need to learn GR.

TerranIV said:
Gravity does not accelerate anything, it curves spacetime
No, gravity is spacetime curvature. It is correct that spacetime curvature, by itself, does not accelerate anything--in the sense that it does not give anything proper acceleration. Objects moving solely under the influence of the geometry of spacetime are in free fall, with zero proper acceleration.

TerranIV said:
which changes how momentum is transfered (as well as how time flows locally)
Neither of these claims are true.

TerranIV said:
I'm just confused why anyone thinks that "gravity" would have a boson or a field to parameterize when GR expressly states that gravity is not a real force
"Not a real force" is vague ordinary language. In classical (non-quantum) GR, it just means what I said above, that objects moving solely under the influence of spacetime geometry are in free fall, with zero proper acceleration. That means they feel no force.

When you try to quantize GR, however, you come face to face with the fact that, first, the relationship between spacetime curvature and the presence of matter and energy, which in classical GR is given by the Einstein Field Equation, now has to be due to some kind of quantum interaction, whose classical limit is the Einstein Field Equation. (I say "interaction" rather than "force" because many quantum interactions do not result in the simplistic kind of Newtonian forces that many people imagine when they use the word "force".) The quantum field theory of the massless spin-2 field, which is what the term "graviton" refers to and which was developed by many physicists in the 1960s and early 1970s, includes, as I said in an earlier post, both graviton-mass vertices (interactions) and graviton-graviton vertices (interactions), the latter arising because the field is nonlinear. And the classical limit of this QFT is known to be the Einstein Field Equation. The main reason this QFT is not considered a final solution to the question of how to quantize gravity is that it is not renormalizable. But it is generally considered to be a valid (although, as I said before, not experimentally testable now or in the foreseeable future) effective field theory of the relationship bewteen spacetime and matter in the appropriate regime.

You would be well advised to become familiar with this body of work, as well as with classical GR. You can't criticize what you don't know.

TerranIV said:
I don't know that it is "putting it back on track" to call the OP completely ignorant on GR and QM as you have no idea who I am.
He's basing his criticisms on what you have posted in this thread. His criticisms look valid to me.

TerranIV said:
It seems (this is where my question lies) a mass-to-mass boson, like a graviton, would impart a force on us which should result in inertial forces through a massive body, which are not present in nature.
It might seem this way to you, but that is because you don't know what the theory you refer to, namely the QFT of a massless, spin-2 field that I described above, actually says. What it actually says is that all of the predictions of classical GR are perfectly valid in the classical regime, including the prediction that objects moving solely under the influence of spacetime geometry feel no force and are in free fall.

As for how that arises from the underlying quantum field interaction, the simplest way to view it is the same way classical GR views it: that the interaction in question (whether you call it spacetime geometry or a spin-2 field) acts on all matter the same way and imparts the same motion to all matter. So matter acted on solely by this means will feel no force since all of the matter is moving exactly the same. One can view this as a manifestation of the equivalence principle.
 
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  • #39
PeterDonis said:
Please do not encourage personal speculation, which is off limits here at PF.
OK if the speculation is clearly disconnected from reality.
 
  • #40
Maarten Havinga said:
OK if the speculation is clearly disconnected from reality.
There is no such provision in the PF rules. Personal speculations, personal theories, personal research are off limits period.
 
  • #41
The OP question has been answered. Thread closed.
 

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