Is Gravity Part Of The Standard Model?

• I
Mentor
I am watching a show about the Standard Model and The Theory of Everything (which of course we do not have right now) and the lecturer mentioned gravity is not part of the Standard Model. Yet other books include it. What is the general consensus?

Thanks
Bill

Gold Member
From what I understand, the Standard Model is background dependent, classical theories of gravity are background independent [1], making them incompatible. I can't speak to the success of current 'quantum gravity' or string theory solutions to the problem but I think string theory's approach is to claim everything is background independent [2].

[1] https://en.wikipedia.org/wiki/Background_independence
[2] https://arxiv.org/abs/hep-th/0507235

Delta2
Dr.AbeNikIanEdL
Yet other books include it.

Can you give an example?

I guess it somewhat depends on what you mean by included. I have not seen it included in general treatments of the SM, other than saying somewhere at the beginning that gravity is too weak to play a role for what we are going to do anyway.

Delta2
Mentor
2022 Award
I am watching a show about the Standard Model and The Theory of Everything (which of course we do not have right now) and the lecturer mentioned gravity is not part of the Standard Model. Yet other books include it. What is the general consensus?

Thanks
Bill
Interesting question. I guess it is pretty simple in the end:
Traditionally there are four forces: EM, weak, strong, gravity.
However, the Standard Model is ##U(1)\times SU(2) \times SU(3)## which does not include gravity.

As gravity as a force is questionable, the traditional view is not the view by QM. And how to include gravity into the model is the question they are all hunting for.

bhobba, *now* and ohwilleke
Staff Emeritus
I have usually heard it called "The Standard Model of Particle Physics" and thus far there is not a particle physics calculation that involves gravity.

But isn't this stamp collecting?

bhobba and ohwilleke
thus far there is not a particle physics calculation that involves gravity.
But isn't this stamp collecting?
$$\mbox{Gravitational Potential} = \mbox{Newton} \left( 1 – \mbox{GR} ( \mbox{correction} ) – \mbox{SM} ( \mbox{correction} ) \right) ,$$
$$V(r) = - \frac{G_{N}mM}{r} \left( 1 - \frac{G_{N}(M + m)}{c^{2}r} - \frac{127}{30 \pi^{2}} \frac{G_{N} \hbar}{c^{3}r^{2}}\right) .$$
That is a particle physics calculation. It is a serious correction not “stamp collection”.

bhobba, dextercioby, ohwilleke and 2 others
Gold Member
And how to include gravity into the model is the question they are all hunting for.
If one accepts the modern view of QFT http://de.arxiv.org/abs/hep-th/9702027 that Standard Model is just an effective theory anyway, then inclusion of gravity is straightforward, which leads to definite predictions such as the one mentioned by @samalkhaiat in the post above.

bhobba and ohwilleke
Staff Emeritus
I have usually heard it called "The Standard Model of Particle Physics" and thus far there is not a particle physics calculation that involves gravity.

Sure. Particles fall. Everything falls. But to understand how particles interact does not require gravity. For example, the decay (mu- e+) -> (mu+ e-) can proceed gravitationally. But it's never been observed, and I don't even know if its been calculated.

Hence the "stamp collecting" comment. We're not arguing about what happens, we're arguing how to categorize it. Would people prefer "botany"?

Fra and ohwilleke
Gold Member
I would agree that gravity is not part of the Standard Model of Particle Physics. A popular term to describe the Standard Model of Particle Physics plus General Relativity is "core theory."

Delta2 and *now*
Andelue
Gravity is not a force, but a consequence of the un-even distribution of mass which creates a curvature in space time itself as described by Einstein's theory of relativity

This is one of the fundamental reasons a black hole is able to do what it does : Suck everything, including light into itself. Space time behaves like a sort of "rubber sheet" in which everything including light travels along. A black hole is like a big sinkhole in that rubber sheet that distorts "space" into a cone.

Gravity by itself has no influence generally in governing the properties of how the "atom behaves" yet there are some situations whereby gravity can really do some messed up S*** to atoms.

Super nova: When a star dies, its hydrogen core gets squashed under the presence of its own gravitational mass so tightly that the nucleus components of the hydrogen atoms, mainly electrons and protons can either fuse together to create a densely packed ball of neutrons. (hence the name neutron star.)

Delta2
Elias1960
I am watching a show about the Standard Model and The Theory of Everything (which of course we do not have right now) and the lecturer mentioned gravity is not part of the Standard Model. Yet other books include it. What is the general consensus?
The Standard Model of particle physics does not include gravity. But I have also heard that $\Lambda$CDM was named the "Standard Model of Cosmology". $\Lambda$CDM is, of course, based on GR.

ohwilleke
Mentor
Can you give an example?

For example Feynman's Lectures on Gravity which GR is developed from spin 2 particles.

Thanks
Bill

dextercioby
Mentor
If one accepts the modern view of QFT http://de.arxiv.org/abs/hep-th/9702027 that Standard Model is just an effective theory anyway, then inclusion of gravity is straightforward, which leads to definite predictions such as the one mentioned by @samalkhaiat in the post above.

Of course, As a QFT it has exactly the same status as any other QFT's except that damnable non-renormalizability. But a lot of people these days think they all are just effective theories.

Thanks
Bill

Demystifier
Gold Member
I never understood one thing about GR as a spin-2 gauge QFT. This is very naive, but hopefully somebody knows.

Surely this QFT is defined on a Minkowski background, or at least it has always seemed to be so defined to me. The near vacuum particle states (i.e. a few gravitons) are fine and there are interaction terms out to arbitrary order that in total are equivalent to the Einstein-Hilbert action. Fine.

However this all takes place on a Minkowski background. What are we to take Schwarzschild spacetime as then? Is it just some sort of coherent state of the graviton field on Minkowski or something.

Gold Member
But a lot of people these days think they all are just effective theories.
I'm in that camp too.

ohwilleke and bhobba
Mentor
I never understood one thing about GR as a spin-2 gauge QFT. This is very naive, but hopefully somebody knows.

I used to post a lot on sci.physics.relativity when a lot of high quality people like John Baez and Steve Carlip posted. But gradually it became overrun by cranks. Gradually the good people left and it became ever more infested by nut cases so I too eventually left. But before I left I managed to sort out a number of issues with these people - one was this issue with Steve Carlip. Its obvious when someone points it out but may not have occurred to people otherwise - its not in textbooks I have read. Its simple really - there is no way to tell if gravity curves space-time or simply interacts with objects to make them act as is space-time was curved. So we can say space-time is flat and gravitons simply make it act as if it was curved or space-time is actually curved.

This raises all sorts of other issues such as a background free theory and why flat space-time is so special - all best confined to another thread.

Thanks
Bill

romsofia and DarMM
Gold Member
Thanks, as you said that is simple but I'd never seen anybody just state it. The issues you mentioned are the ones that automatically occurred to me and made think I was wrong, but if they are acknowledged discussion points that makes sense.

I remember speaking to somebody who was into de Sitter special relativity, so that expansion came from the background space time and local gravitational effects from gravitons. Apparently there are difficulties with expansion being a graviton effect, but I know very little.

For "nonperturbative" spaces like Schwarzschild they can't be seen in terms of finite graviton exchanges. I assume. So are they a coherent state of the field or something?

Mentor
For "nonperturbative" spaces like Schwarzschild they can't be seen in terms of finite graviton exchanges. I assume. So are they a coherent state of the field or something?

That I do not know,

Thanks
Bill

ohwilleke and DarMM
Gold Member
For example Feynman's Lectures on Gravity which GR is developed from spin 2 particles.

Thanks
Bill

In my humble opinion, as a non-professional who has read a lot, I think that the quantum field theory of a massless spin-2 boson that couples in proportion to the mass-energy of other particles, but does not have electromagnetic or weak force or strong force charge (i.e. as the graviton of Feynman's lectures), is a close approximation of GR, but is not identical to GR (at least as conventionally applied).

For example, in GR, gravitational energy cannot be localized, while in a spin-2 graviton theory, it is localized.

The differences are, I believe, not just theoretical and technical points for mathematicians, but have phenomenological implications.

If I had to put money on it, moreover, I would bet on the spin-2 graviton in Minkowski space approach, rather than the classical GR approach, being correct in observations where their implications can be distinguished (alas, at the moment, I'm not convinced that there are any).

Mentor
In my humble opinion, as a non-professional who has read a lot, I think that the quantum field theory of a massless spin-2 boson that couples in proportion to the mass-energy of other particles, but does not have electromagnetic or weak force or strong force charge (i.e. as the graviton of Feynman's lectures), is a close approximation of GR, but is not identical to GR (at least as conventionally applied).

Feynman shows it is exactly the same as GR. However I just know what I read in the book, others may know more details. A book that follows a similar approach classically is Ohanian - Gravitation:
https://www.amazon.com/dp/1107012945/?tag=pfamazon01-20

Its different - but equivalent - to the usual and perhaps more elegant geometric approach as is shown in the book.

Thanks
Bill

cosmik debris
I used to post a lot on sci.physics.relativity when a lot of high quality people like John Baez and Steve Carlip posted. But gradually it became overrun by cranks. Gradually the good people left and it became ever more infested by nut cases so I too eventually left. But before I left I managed to sort out a number of issues with these people - one was this issue with Steve Carlip. Its obvious when someone points it out but may not have occurred to people otherwise - its not in textbooks I have read. Its simple really - there is no way to tell if gravity curves space-time or simply interacts with objects to make them act as is space-time was curved. So we can say space-time is flat and gravitons simply make it act as if it was curved or space-time is actually curved.

This raises all sorts of other issues such as a background free theory and why flat space-time is so special - all best confined to another thread.

Thanks
Bill

I remember those days, there were some good people on there, Burns (don't remember his first name), Stephen Spiecer (or something). You were a novice in those days, you seem to have come a long way since then.

The difference between the two interpretation of GR are just the field versus geometrical interpretations mentioned in another thread aren't they?

Cheers

Mentor
The difference between the two interpretation of GR are just the field versus geometrical interpretations mentioned in another thread aren't they?

Basically - yes.

Thanks
Bill

Gold Member
I never understood one thing about GR as a spin-2 gauge QFT. This is very naive, but hopefully somebody knows.

Surely this QFT is defined on a Minkowski background, or at least it has always seemed to be so defined to me. The near vacuum particle states (i.e. a few gravitons) are fine and there are interaction terms out to arbitrary order that in total are equivalent to the Einstein-Hilbert action. Fine.

However this all takes place on a Minkowski background. What are we to take Schwarzschild spacetime as then? Is it just some sort of coherent state of the graviton field on Minkowski or something.

How is it necessarily defined on a Minkowski background? In the quantum theory, the Einstein equations become Schwinger-Dyson equations for the fields which are satisfied inside correlation functions. So for example, one can consider the Schwarzschild solution to be a particular saddle point one could expand the path integral around. One can also in principle write down a computation for any correlation function in (regulated) quantum gravity in appropriate limits which are not necessarily Minkowski.

But I agree that the "graviton" picture is maybe too Minkowski-specific. That is, if you perturb around the Minkowski solution, you get weakly-interacting spin-2 particles, but I am not sure you are guaranteed to have such a simple spectrum or picture if you consider other limits. There's also no guarantee that something like the Schwarzschild solution is perturbatively connected to the Minkowski limit, so trying to visualize it as coming from some infinite set of graviton Feynman diagrams (or a coherent state of gravitons) might not be conceptually correct.

ohwilleke
Gold Member
How is it necessarily defined on a Minkowski background?
In the path integral approach you mention what is being integrated over? Spin-2 fields ##h_{\mu\nu}## or the metric? In other words do we have a quantized causal structure?

coherent state of gravitons
Coherent states aren't necessarily perturbatively connected to the vacuum right? i.e. it being nonperturbative doesn't preclude that right?

Gold Member
In the path integral approach you mention what is being integrated over? Spin-2 fields $h_{\mu \nu}$ or the metric? In other words do we have a quantized causal structure?

I don't think I know what you mean by "quantized causal structure." But if you are defining $h_{\mu \nu} = g_{\mu \nu} - \eta_{\mu \nu}$, this is just a field redefinition and so the choice has no effect on the path integral provided your range of integration is changed appropriately.

Coherent states aren't necessarily perturbatively connected to the vacuum right? i.e. it being nonperturbative doesn't preclude that right?

I suppose this depends on your definition of coherent states. I think of them being defined in terms of a particular set of states in a Fock basis, but a generic quantum field theory is not necessarily spanned by a Fock basis. In particular, in referring to a "coherent state of gravitons" I was explicitly thinking of constructing coherent states out of the graviton particles present near the Minkowski limit. But perhaps you are using a different definition?

bhobba and DarMM
Gold Member
I'll read a bit more first and come back with better questions on another thread.

charters
Coherent states aren't necessarily perturbatively connected to the vacuum right? i.e. it being nonperturbative doesn't preclude that right

I suppose this depends on your definition of coherent states. I think of them being defined in terms of a particular set of states in a Fock basis, but a generic quantum field theory is not necessarily spanned by a Fock basis

The claim in string theory at least is all low energy gravitational backgrounds are indeed coherent states of graviton mode/spin 2 strings. This is discussed in Tong's note ch 7, https://arxiv.org/abs/0908.0333, and these papers from Nick Huggett, http://philsci-archive.pitt.edu/11116/, http://philsci-archive.pitt.edu/15434/

bhobba and DarMM
Gold Member
Feynman shows it is exactly the same as GR.

Do you know any place that has those in open access? I have a lot of his other writings (at least as dead trees) but not that one.

Mentor
ohwilleke
Do you know any place that has those in open access? I have a lot of his other writings (at least as dead trees) but not that one.

There are three people who showed essentially the same thing. Feynman in the aforementioned book, Deser did it by a bootstrap method as well as by deriving the full nonlinear GR as the unique theory from the classical limit of the quantum gravity of a spin 2 field, and Gupta as well independently.

Amusingly the quantum gravity derivation is actually (imo) the cleanest.
https://inspirehep.net/record/1461
Note that there are several technical details that make answering some of the previous questions a little challenging. For instance spin2 GR is strictly speaking *not* identical to the full Einstein theory b/c of the Vainshtein discontinuity, however there are fixes for this.

DarMM, dextercioby, ohwilleke and 2 others
Auto-Didact
Feynman shows it is exactly the same as GR. However I just know what I read in the book, others may know more details. A book that follows a similar approach classically is Ohanian - Gravitation:
https://www.amazon.com/dp/1107012945/?tag=pfamazon01-20

Its different - but equivalent - to the usual and perhaps more elegant geometric approach as is shown in the book.

Thanks
Bill
It has been a while, but from what I recall this equivalency certainly wasn't true at the level of rigour of mathematical physics instead of at the level of rigour of theoretical physics, where it is of course generally accepted as being true.

Myself, I'd wager on the geometric interpretation being more correct with the field theoretic interpretation being merely an approximation: this wager is based purely on mathematical intuition, biased by more sophisticated geometric and (complex-)analytic aesthetics.

ohwilleke