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.
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Einstein showed (via general relativity) that spacetime is curved by mass, mass moves in relation to this curvature, and that gravitation arises as secondary effect. Why then are we looking for quantum gravity as some sort of mass<->mass interaction?

Aren't the fundamental interactions better thought of as between mass and spacetime (i.e. mass<->spacetime<->mass)? Shouldn't we be looking for a quantum form of spacetime? Or maybe at least two kinds of gravitons? (A mass->spacetime graviton and a spacetime->mass graviton.)

In other words, if gravitation is no more fundamental than the centripetal force, why are we looking for a quantum form of gravity?
 
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  • #2
GR is a theory of spacetime that has no theoretical support in terms of arising from the interactions of elementary particles. An explanation of GR and curved spacetime in terms of elementary particle interactions is what is needed in terms of quantum gravity.
 
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  • #3
TerranIV said:
Einstein showed (via general relativity) that spacetime is curved by mass
That's not exactly true. The Riemann curvature tensor can be non-zero even when the matter energy-momentum tensor is zero. In lay language, this means that spacetime can be curved even without mass. This indeed happens during the propagation of gravitational waves. In quantum gravity we need to quantize these gravitational waves. The quantization of gravitational waves implies the existence of gravitons.
 
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  • #4
Demystifier said
"In quantum gravity we need to quantize these gravitational waves. The quantization of gravitational waves implies the existence of gravitons."
Right, but my question is why are gravitions formulated as being exchanged between masses? Shouldn't gravitions be exchanging interaction between mass and spacetime (or propagating through spacetime just like photons)?
 
  • #5
TerranIV said:
Demystifier said

Right, but my question is why are gravitions formulated as being exchanged between masses? Shouldn't gravitions be exchanging interaction between mass and spacetime (or propagating through spacetime just like photons)?
Here is an overview of the various competing theories:

https://arxiv.org/pdf/1108.3269.pdf
 
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  • #6
My question is more about why would any theory think gravitons would be exchanged between masses? I thought Einstien is pretty explicit in general relativity in saying that gravity is not a force between masses. Why would any post-General Relativity theory go back to Newton's ideas of gravity?

Does anyone know any theories that formulate gravitions as exchanging an interaction between mass and spacetime?
 
  • #7
TerranIV said:
My question is more about why would any theory think gravitons would be exchanged between masses? I thought Einstien is pretty explicit in general relativity in saying that gravity is not a force between masses. Why would any post-General Relativity theory go back to Newton's ideas of gravity?
This highlights how little you understand about the attempts to find a theory of QG.
TerranIV said:
Does anyone know any theories that formulate gravitions as exchanging an interaction between mass and spacetime?
You could start here:

https://en.wikipedia.org/wiki/Graviton
 
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  • #8
TerranIV said:
Why would any post-General Relativity theory go back to Newton's ideas of gravity?
QM is a whole lot less Newtonian than GR! Newtonian physics is the local, low-speed, weak-field limit of GR. The proposed theories of QG are not Newtonian.
 
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  • #9
PeroK said:
This highlights how little you understand about the attempts to find a theory of QG.
I am asking a question on this forum to understand. Obviously if I knew the answer I wouldn't need to ask the question, right? If the answer is obvious, please answer my question, or allow someone else to answer if you don't know, instead of just telling me to read everything about QM and the graviton.

It is a simple question; why would a quantization of an interaction that GR indicates is between mass and spacetime not include spacetime in the interaction?
 
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  • #10
TerranIV said:
I am asking a question on this forum to understand. Obviously if I knew the answer I wouldn't need to ask the question, right? If the answer is obvious, please answer my question, or allow someone else to answer if you don't know, instead of just telling me to read everything about QM and the graviton.
That's a fair point.
TerranIV said:
It is a simple question; why would a quantization of an interaction that GR indicates is between mass and spacetime not include spacetime in the interaction?
It's not a simple question. Nothing to do with QG is simple. You already got one answer today:

https://physics.stackexchange.com/questions/745697/why-is-gravity-not-a-force

As far as I understand your question, you are asking about a fundamental non-quantum theory of gravity. Or, at least, that the interaction is not between the particles as described in the link above, but is a different type of interaction altogether. Where particles interact independently via the medium of spacetime.

I'm not sure how much research there is on that.
 
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  • #11
PeroK said:
GR is a theory of spacetime that has no theoretical support in terms of arising from the interactions of elementary particles. An explanation of GR and curved spacetime in terms of elementary particle interactions is what is needed in terms of quantum gravity.
After I set the thread level to "B", haven't you just answered the titled question with a "yes" since mass is caused by elementary particle interactions?
 
  • #12
fresh_42 said:
After I set the thread level to "B", haven't you just answered the titled question with a "yes" since mass is caused by elementary particle interactions?
It's the justification for curved spacetime that is the issue, rather than the justification that elementary particles have mass.
 
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  • #13
TerranIV said:
Right, but my question is why are gravitions formulated as being exchanged between masses? Shouldn't gravitions be exchanging interaction between mass and spacetime (or propagating through spacetime just like photons)?
For a start, let us use a correct language. In Feynman diagrams, virtual particles are exchanged between real particles. The particles (either real or virtual) can be matter particles (this corresponds to "mass" in your language) and gravitons (this corresponds to "spacetime" in your language). Now with the established correct language, the answer to your question is that gravitons are exchanged between both matter particles and gravitons.
 
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  • #14
PeroK said:
Here is an overview of the various competing theories:

https://arxiv.org/pdf/1108.3269.pdf
"So far, no less than 16 major approaches to quantum gravity have been proposed in the literature. Some of them make a direct or indirect use of the action functional to develop a Lagrangian or Hamiltonian framework. They are as follows.

1. Canonical quantum gravity [16, 17, 43, 44, 32, 99, 100, 6, 54, 144].

2. Manifestly covariant quantization [116, 33, 94, 74, 7, 152, 21, 103].

3. Euclidean quantum gravity [68, 90].

4. R-squared gravity [142].

5. Supergravity [64, 148].

6. String and brane theory [162, 98, 10].

7. Renormalization group and Weinberg’s asymptotic safety [129, 106].

8. Non-commutative geometry [26, 75].

Among these 8 approaches, string theory is peculiar because it is not field theoretic, spacetime points being replaced by extended structures such as strings.

A second set of approaches relies instead upon different mathematical structures with a more substantial (but not complete) departure from conventional pictures, i.e.

9. Twistor theory [122, 123].

10. Asymptotic quantization [67, 5].

11. Lattice formulation [114, 22].

12. Loop space representation [133, 134, 136, 145, 154].

13. Quantum topology [101], motivated by Wheeler’s quantum geometrodynamics [159].

14. Simplicial quantum gravity [72, 1, 109, 2] and null-strut calculus [102].

15. Condensed-matter view: the universe in a helium droplet [155].

16. Affine quantum gravity [105].

After such a concise list of a broad range of ideas, we hereafter focus on the presentation of some very basic properties which underlie whatever treatment of classical and quantum gravity, and are therefore of interest for the general reader rather than (just) the specialist. He or she should revert to the above list only after having gone through the material in sections 2–7."
 
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  • #15
Demystifier said:
For a start, let us use a correct language. In Feynman diagrams, virtual particles are exchanged between real particles. The particles (either real or virtual) can be matter particles (this corresponds to "mass" in your language) and gravitons (this corresponds to "spacetime" in your language). Now with the established correct language, the answer to your question is that gravitons are exchanged between both matter particles and gravitons.
Gravitons would be exchanged between matter particles AND energy particles, which is why I used mass instead. (Another reason why it seems that any real gravitons seem more likely to be eschanging energy and information with some form of a spacetime structure or field, not another mass.)
You didn't answer my question at all. I understand why gravitons are thought to be exchanged between matter particles and gravitons (as well as photons and any other mass or energy particle).
My question is, why does this seem to ignore General Relativity which states that gravity is a secondary result of the primary interaction between mass and spacetime? Why would particle physicists seemingly ignore Einstein for 100 years and think they would get any results? It seems like it would be smarter to try a different tactic (one supported by Einstein) rather than continue to bang our heads against the wall of trying to make gravity be a mass<->mass interaction.
 
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  • #16
Maarten Havinga said:
First, nice original idea! That's something good in itself, IMO. But there are problems as others already pointed out.

If gravitons would be attracting mass and spacetime to each other, this would mean an interaction between graviton and spacetime which absorbs the graviton, thus 'bending' spacetime - otherwise, the gravitational force carriers cannot transfer the force. That means we're far from an inverse square law for gravity, since the absorbed gravitons would add an exponentially decreasing factor to multiply with. This contradicts many confirmations of Newtonian dynamics and general relativity. Alas, this will not work.

But the relation between gravitons and spacetime is something I want to memorize. Since gravitational waves could leave an imprint on spacetime, and are measurable as ripples in spacetime, well... perhaps gravitons are the quantized wave packets of spacetime waves. Or maybe that's already known or defined as such in a theory; loop quantum gravity comes to mind. In any case, TerranIV had a refreshing though invalid idea.
I wouldn't think gravitons would "attract" mass and spacetime together. They would do what GR says mass does, i.e. bend spacetime or change the properties of spacetime so that momentum follows curved paths like GR says it does. While gravitation is a force, the "true" and "primary" interaction would not need to be a force per se, it would just need to be an interaction between mass and space that bends spacetime in the way that GR says it does.

I don't see how this would preclude an inverse square law, as gravitons would be traveling through space and spreading out as they travel, meaning the density distribution of gravitons would lead directly to an inverse square effect. Obviously gravitation obeys an inverse square law so any formulation of gravitons would have to include that to be valid.

I do appreciate you giving the idea some thought. I am disappointed that nobody seems to be able to point to a reason for the prevailing theories of gravity to seemingly ignore Einstein and General Relativity and try and make it into a force when the most successful theory of gravitation says it is no such thing.

My suspicion is that the nature of spacetime is so misunderstood (I mean we don't really even understand the true geometry of space! Eucledian? Hyperbolic? Ecliptic? Riemannian?) Maybe someday if we can narrow down what exactly space and time are, we will get closer to understanding how exactly mass distorts the behavior of inertia, momentum, impulse, etc to create gravity, time-dilation, non-accelerating curved motion, etc.
 
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  • #17
TerranIV said:
I am disappointed that nobody seems to be able to point to a reason for the prevailing theories of gravity to seemingly ignore Einstein and General Relativity and try and make it into a force
Huh?? I don't bother to point to such a "reason" because I don't see anyone (who actually understands GR and QFT) trying to do what you say (i.e., "make it into a force").

TerranIV said:
My suspicion is that the nature of spacetime is so misunderstood (I mean we don't really even understand the true geometry of space! Eucledian? Hyperbolic? Ecliptic? Riemannian?)
Or maybe it's just you who doesn't understand. Our best theories and their experimental evidence supports "Riemannian" as the best model.

TerranIV said:
Maybe someday if we can narrow down what exactly space and time are, [...]
Space and time are artifacts of our imagination (see the Einstein quote in my signature block below). They are useful concepts to construct mathematical models of physical (i.e., experimentally perceived) events. What's real are the correlations exhibited by (various configurations of) correlata, i.e., quantum fields, or, in a suitable limit, classical fields+particles.
 
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  • #18
It looks like your question has been answered several times, but I think there are some basic things you’re misunderstanding, so I’ll try to keep this as B level as possible.

You’re wondering why gravitons don’t interact with spacetime, when in fact they do. You have to first understand: what is spacetime, such that gravitons would interact with it? The answer is that spacetime is just a list of numbers. At any given point in spacetime, we can figure out a list of numbers (called a metric) that tells us where/when we are, and we can figure out the local curvature. When we have a list of numbers at every given point in a space, we call this a field. Einstein’s general relativity is a field theory (a classical one), but there are other field theories, for instance electromagnetism. When we go from a classical field theory to a quantum one, we find that the fields are excited in discrete quantities. For the electromagnetic field, this discrete excitation is called the photon. For the field representing spacetime, this excitation is called the graviton.

So the graviton is spacetime, or at least a quantum excitation of the tensor field representing spacetime. So gravitons interacting with gravitons is essentially what you’re asking for.
 
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  • #19
TerranIV said:
My question is, why does this seem to ignore General Relativity which states that gravity is a secondary result of the primary interaction between mass and spacetime?
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.
 
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  • #20
TerranIV said:
I am disappointed that nobody seems to be able to point to a reason for the prevailing theories of gravity to seemingly ignore Einstein and General Relativity and try and make it into a force when the most successful theory of gravitation says it is no such thing.
Maybe I can point to the reason. It's about general covariance. When GR is written in a general covariant form, then the force does not appear in the equations. But in this form, one cannot actually solve equations. To solve equations one must choose some particular coordinates, which breaks general covariance and usually the force appears in the equations. In perturbative quantum gravity, where Feynman diagrams appear, one does not know how to write the equations in a general covariant form, so forces appear naturally. In loop quantum gravity the general covariance is more explicit, forces typically do not appear in equations, but with this theory it's hard to make concrete calculations.

So it's not that GR does not have forces at all. Instead, it's that the theory can be written in two ways, general covariant and general non-covariant. In the covariant form one has mathematical elegance and no forces, but concrete calculations are hard. In the non-covariant form the mathematical elegance is lost and forces appear, but concrete calculations are easier.

The confusion may arise when people compute non-covariantly but think covariantly. I even have a metaphor for this; the non-covariant practice is like sex, the covariant ideology is like love, and confusion may arise when people don't see clearly the difference between love and sex. :oldbiggrin:
 
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  • #21
TerranIV said:
I wouldn't think gravitons would "attract" mass and spacetime together. They would do what GR says mass does
I'm afraid you lost me here. If you want to have effect on my opinions, you must write down how the interaction between mass and spacetime works mathematically. If I take your statement at face value, you're unifying GR and QM only in name, by dubbing GR an "interaction between mass and spacetime". It means nothing unless you offer background mathematics. For instance I need to know why you wouldn't have to quantize spacetime in order to get this "quantum interaction" via gravitons. I can't make any prediction or even qualitative statement about nature based on your text.
 
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  • #22
TerranIV said:
energy particles

There can be no such thing. Energy is a property of things, not a thing itself.
 
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  • #23
TerranIV said:
Einstein showed (via general relativity) that spacetime is curved by mass, mass moves in relation to this curvature, and that gravitation arises as secondary effect. Why then are we looking for quantum gravity as some sort of mass<->mass interaction?

Aren't the fundamental interactions better thought of as between mass and spacetime (i.e. mass<->spacetime<->mass)? Shouldn't we be looking for a quantum form of spacetime? Or maybe at least two kinds of gravitons? (A mass->spacetime graviton and a spacetime->mass graviton.)

In other words, if gravitation is no more fundamental than the centripetal force, why are we looking for a quantum form of gravity?
Many comments already but here is another one....

The graviton concept arises from treating spacetime metric as another field and then apply the "quantization procedure" that we have good confidence in from subatomic physics. Other reasearch programs change the variablea to something else than the metric. So picking the right "variable" and then quantize is one idea.

Another idea is to question wether gravity (or some variable related to it) should be treated with standars qm at all? The problem is that qft typically needs an background spacetime wich effectively is an external observer.
But there is no such thing in gr. So what todo? Does QM itself need to deform? Noone knows.

There are many problems here and until a theory is found and mature that unifies all forces the question is open.

/Fredrik
 
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  • #24
In contrast in particle physics there IS an effective external observer. The whole classical laboratory, where they can prepare and repeat the same experiment over and over again. The capacity of informationa of the surrounding lab is dominant relative to the atomic scale.

This situation is very different in a comsological perapective. So it is not i think obvious that the paradgim of atomic physics makes sense for cosmological models or cases where an effective background at distance is not practical.

/Fredrik
 
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  • #25
strangerep said:
Huh?? I don't bother to point to such a "reason" because I don't see anyone (who actually understands GR and QFT) trying to do what you say (i.e., "make it into a force").Or maybe it's just you who doesn't understand. Our best theories and their experimental evidence supports "Riemannian" as the best model.Space and time are artifacts of our imagination (see the Einstein quote in my signature block below). They are useful concepts to construct mathematical models of physical (i.e., experimentally perceived) events. What's real are the correlations exhibited by (various configurations of) correlata, i.e., quantum fields, or, in a suitable limit, classical fields+particles.
You must not have read the responses to the questions if you dont think that people are calling gravity a force here so far. If you had read my question you would also know that I'm not asking about it being a force or not, I'm asking about it being an interaction between masses rather than between mass and spacetime.

If you think that the debate over the nature and topology of space is settled you are ignorant to the issues involved. Eienstein used a pseudo-Riemannian manifold in GR because it was the best way to explain how GR works, but there are many other geometries which work just as well (which is why currently it is impossible to say which is correct). Overview of issues: https://en.wikipedia.org/wiki/Shape_of_the_universe

Space and time are absolutely not "artifacts of our imagination." This is a extremely silly thing to argue for in a physics forum. A random quote of Einstein does not trump his published peer-reviewed time-tested formalized theories. The way we experience space and time are obviously limited, but there is no doubt they relate to real underlying properties of the universe.

Again, the question for this thread is why is gravitation treated as an interacting between masses, and not simply as an interaction between mass and spacetime as General Relativity presents it.
 
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  • #26
TerranIV said:
If you think that the debate over the nature and topology of space is settled you are ignorant to the issues involved.

I would rather say that you are ignorant - all of the geometries that you linked to are pseudo-Riemannian.
 
  • #27
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.
 
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  • #28
TerranIV said:
why does this seem to ignore General Relativity which states that gravity is a secondary result of the primary interaction between mass and spacetime?
In GR, matter interacts with the metrical properties of space-time. In quantum gravity, matter interacts with gravitons; but a graviton is made out of the metric. A graviton is a quantum perturbation of the metric of space-time. It's the particle-field duality of quantum mechanics, applied to the metric field.
 
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  • #29
TerranIV said:
Einstein showed (via general relativity) that spacetime is curved by mass, mass moves in relation to this curvature, and that gravitation arises as secondary effect.
No. Gravitation is spacetime curvature in GR. It's not a "secondary effect" of spacetime curvature.

TerranIV said:
Why then are we looking for quantum gravity as some sort of mass<->mass interaction?
Who said we were? Do you have any references that show this?
 
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  • #30
TerranIV said:
why are gravitions formulated as being exchanged between masses?
The graviton theory (which is not considered a full quantum gravity theory but only an effective theory which might be valid in certain regimes--but we have no way of experimentally testing this now or in the foreseeable future) contains both graviton-mass vertices and graviton-graviton vertices. The latter arise because the QFT of a massless, spin-two particle (the graviton) is nonlinear.
 
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  • #31
TerranIV said:
why would any theory think gravitons would be exchanged between masses?
Because gravitons are exchanged between anything that has energy.
 
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  • #32
TerranIV said:
why would a quantization of an interaction that GR indicates is between mass and spacetime not include spacetime in the interaction?
It does. See post #32.
 
  • #33
Maarten Havinga said:
In any case, TerranIV had a refreshing though invalid idea.
Please do not encourage personal speculation, which is off limits here at PF.
 
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  • #34
TerranIV said:
why does this seem to ignore General Relativity which states that gravity is a secondary result of the primary interaction between mass and spacetime?
GR doesn't state that. See post #29.

TerranIV said:
Why would particle physicists seemingly ignore Einstein for 100 years and think they would get any results?
They aren't. See above.
 
  • #35
TerranIV said:
I am disappointed that nobody seems to be able to point to a reason for the prevailing theories of gravity to seemingly ignore Einstein and General Relativity and try and make it into a force when the most successful theory of gravitation says it is no such thing.
Well first off, some quantum gravity theories start with the Einstein-Hilbert action.

Secondly, what is meant by "force" in say particle physics is different from the concept of force in Newtonian physics.
 
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