What is the rationale behind gravitons?

[Eelco]

Can someone explain the rationale behind gravitons to me?

My background is computational physics, and as such i may be biased towards physics that is actually computable, such as LQG and regge calc. I have some clue what this is all about, but i have some questions:


Is there any reason (beyond aestetics which i disagree with anyway) to favor a particle over a geometric explanation? Any sort of empirical matter gravitons may help explain?

How is something like gravitational lensing explained in a flat spacetime with gravitons? Are there force-carrier-to-force-carrier interactions in such a model? I have a hard time imagining how youd explain bending of light with gravitons. It seems likea pressing question to me, but no one else seems to care, as far as i can tell.


If your response is anything like the reasons listed on the wikipedia page on gravitons, then please try a different angle. Perhaps there is something to these arguments, but the way they are stated, they seem like garbage to me.

 

[Rebel]

Can someone explain the rationale behind gravitons to me?

My background is computational physics, and as such i may be biased towards physics that is actually computable, such as LQG and regge calc. I have some clue what this is all about, but i have some questions:


Is there any reason (beyond aestetics which i disagree with anyway) to favor a particle over a geometric explanation? Any sort of empirical matter gravitons may help explain?

How is something like gravitational lensing explained in a flat spacetime with gravitons? Are there force-carrier-to-force-carrier interactions in such a model? I have a hard time imagining how youd explain bending of light with gravitons. It seems likea pressing question to me, but no one else seems to care, as far as i can tell.

You'll find in any area of physics place for computations.

The only reason to favor a particle explanation is the success of QED and QCD for describing Electromagnetic and weak and strong interactions, and is not a bad reason at all, don't you think so? Also, the aesthetical guidings are lead by knowledge and predjuices, so also a geometrical point of view for all forces could be at least as aesthetical as a particle/field explanation for gravity and the other interactions as well (is nice to see for example some discussion of Rovelli about aesthetics in http://arxiv.org/abs/gr-qc/0604045).

The existence of gravitons were speculated very likely a little before than formalizing QED, as was growing QFT. That was just to associate a particle to the spin-2 wave that comes from general relativity in the linearized limit, so as can be associated the photon to the waves in Maxwell theory. The plane waves that results in linearized gravity are analogue of the plane waves in electromagnetism, and actually the same equations for linearized gravity can be obtained from the Fierz-Pauli action. As happens in electromagnetism that real phenomena is not described by plane waves but a superposition of infinite plane waves for instance, and the calculations can be made with perturbative expansions, one could take also more and more nonlinear terms in a expansion of the equations when dividing the metric in flat plus a perturbation. In this way the particle way was (and is) hopping that the bending of light, and in general the curvature of space-time observed will be an effect of the interactions of gravitons among them and with matter; continuing analogies it could be checked the scattering of light when passing throguh electric and magnetic fields, a point that reminds me also another recent post about a paper of Smolin.

 

[tom.stoer]

Personally I don't like gravitons very much:
- nobody has ever seen or measured a single graviton
- any attempt to formulate a physically realistic theory based on gravitons failed (due to infinities etc.)
- string theory contains something like gravitons - but the prize to pay is very high (extra dimensions and all that)

So if "gravitons" are real physical entities they could be something completely different than ordinary quantized plane waves. Supergravity may we a way out - quantized gravity with a finite perturbation expansion in terms of gravitons - let's wait and see ...

 

[humanino]

- nobody has ever seen or measured a single graviton

I saw a paper once where it was argued that they might not even be detectable in principle. Please note that gluons can not either.

- any attempt to formulate a physically realistic theory based on gravitons failed (due to infinities etc.)

That is simplistic. There are many self consistent non-relativist calculations which can be done with gravitons. Anybody remembers references for non-trivial metrics calculated by graviton exchanges ?

 

[tom.stoer]

I saw a paper once where it was argued that they might not even be detectable in principle. Please note that gluons can not either

So let's weaken the requirement to some sort of indirect effects like 3-jet events, a well-known signal for one hard gluon in deep inelastic scattering.

There are many self consistent non-relativist calculations which can be done with gravitons. Anybody remembers references for non-trivial metrics calculated by graviton exchanges ?[/QUOTE]
I haver no idea what a non-relativistic graviton exchange should be.

What I want to say is that even if gravitons are not detectable in the sense mentioned above, they should be at least a tool for doing certain calculations. Then we could treat them as some physically relevant entity (compare it to virtual particles, where I see two different interpretations: a) either you take QFT seriously and talk about asymptotic states plus the exchange of virtual particles; then you should agree that virtual particles and asymptotic states are physical entities; or b) you say that there are no asymptotic states at all because in the very end you must do a measurement, so the asymptotic states are absorbed by a detector which turnes them into virtual particles exchanged between the event and the detector)

So if you have a calculation in quantized gravity using plane wave gravitons to produce a finite and reasonable result, then I would again start to think about the existence of gravitons.

I would accept a concept from QED and QCD: If you quantize QED in the hamiltonian formulation and do an quantum mechanical gauge fixing via resolving the Gauss law, you have to invert a certain differential operator D, which results in a non-local operator 1/D in momentum space; this is just the (static) Coulomb-potential between current densities. If you do the same calculation in QCD (you will need more paper to write it down :-) the operator becomes 1/D[A] where A are certain momentum space modes of the gluon field. So again you find a Coulomb-potential, which now depends on the full dynamic gluon field.

I accept those kind of affects as something showing the relevance of photons and gluons as "physical entities", even if they are not measurable directly.

Perhaps you can convince me if with finite supergravity theories ...

 

[Eelco]

Thank you for your replies; glad to see I am not the only one why isn't too keen on gravitons.

When i say that i disagree with the aesthetics, i mean to say that i think the geometrical explanation of curved space is more aestetically pleasing to me. Insofar such matters should factor into it at all, but that's another debate.

You'll find in any area of physics place for computations.

There is computations and computations. Has someone ever computed something as simple as the bending of light around the sun by pertubative graviton calculations? Thats all I am asking for. Pictures, or it didnt happen.

The only reason to favor a particle explanation is the success of QED and QCD for describing Electromagnetic and weak and strong interactions, and is not a bad reason at all, don't you think so?

I think that's a horrible reason. Things should be made as simple/unified as they can be, but no more. Should we also have a centripedal particle? After all, that's really the same thing. Gravity is a 'fictional' force in the same sense that centripedal force is. It is conceptually a very different thing than the other forces.

As happens in electromagnetism that real phenomena is not described by plane waves but a superposition of infinite plane waves for instance, and the calculations can be made with perturbative expansions, one could take also more and more nonlinear terms in a expansion of the equations when dividing the metric in flat plus a perturbation. In this way the particle way was (and is) hopping that the bending of light, and in general the curvature of space-time observed will be an effect of the interactions of gravitons among them and with matter; continuing analogies it could be checked the scattering of light when passing throguh electric and magnetic fields, a point that reminds me also another recent post about a paper of Smolin.

You lost me there. Can you or someone else rephrase that?Id like to press my question: how does one explain refraction of light without either an interaction with other particles, or by an intrinsic curvature of space? Bonus points awarded for not getting lost in vague generalities.

That i do not see this question getting asked can mean only two things: either it has a very simple, trivial answer. Or it has no answer at all. If that question has no answer, then how are gravitons anything other than bogus?

 

[humanino]

Just remember there is a difference between calculating a scattering amplitude through (virtual) graviton exchange (which allows simple results) and a real graviton (which is the quantum of a gravitational wave). When I said "non-relativistic graviton" I was referring to the former, while it has been argued that the latter might never be detectable.

So if you have a calculation in quantized gravity using plane wave gravitons to produce a finite and reasonable result, then I would again start to think about the existence of gravitons.

I need to find the reference, but yes I have read at least one from arXiv.

 

[Eelco]

This thread seems relevant to my question:

https://www.physicsforums.com/showthread.php?t=77845

In the paper linked, it is claimed that Einsteins equations can not be recovered from any graviton theory. This claim is disputed, but it does confirm a suspicion of mine: I am not the only one to whom this does not make sense: nobody has any clue whatsoever what he is talking about when it comes to these matters.

The easiest person to fool are you yourself. As such, I don't care to hear your explanation of gravitons unless you can phrase it in physical terms: Regge calculus can give me a picture of spiraling black holes. Can pertubative graviton doohickies even account for the bending of light around the sun?

I repeat: Pictures, or it didnt happen.

Metaphorical pictures will do too, but arguments in terms of 'Hilbert actions' or somesuch need not apply, because unfortunately I can only interpret them as evidence that you are horribly confused.

 

[Eelco]

An interesting feature of gravitons in string theory is that, as closed strings without endpoints, they would not be bound to branes and could move freely between them. If we live on a brane (as hypothesized by some theorists) this "leakage" of gravitons from the brane into higher-dimensional space could explain why gravity is such a weak force, and gravitons from other branes adjacent to our own could provide a potential explanation for dark matter. See brane cosmology for more details.

By the way, does garbage like this make anyone elses toes curl?

I mean, of course it could be true, but id much prefer 'the flying spaghetti monster did it'. Its much more concise, way less pretentious, and it has exactly the same intellectual content!

 

[Haelfix]

Hawking radiation and the area-entropy law is a prediction of the graviton formalism. You can't have one without the other.

 

[humanino]

[...]

Is you strategy to challenge people by being arrogant and insulting so that they contribute to your thread ? We agree on one thing : it easy to fool oneself, and he only person we should care to convince are ourselves. Have a good day.

 

[Eelco]

Hawking radiation and the area-entropy law is a prediction of the graviton formalism. You can't have one without the other.

I am unable to make that connection myself, but it is something tangible, so thanks.

Tangible in a limited sense though. Has anyone ever observed Hawking radiation? Nope.

And where does the 'law' in area-entropy law come from? As far as i can tell, it is nothing but a rather daring cross-application of a concept far out of its verifiable domain.


Any single shortcoming of gravitons to explain any actual observed phenomena, would brush these theoretical constructions right into the trash can as far as I am concerned, and i wouldn't shed a tear.

 

[Eelco]

Is you strategy to challenge people by being arrogant and insulting so that they contribute to your thread ? We agree on one thing : it easy to fool oneself, and he only person we should care to convince are ourselves. Have a good day.

Thats an unorthodox interpretation of feynmans words; not at all what he intended, id say.

Either way, I am asking a question, feel free to ignore it, but I am trying to make clear what i do and do not consider to be answers. Make of that as you please, i just don't want to waste anyones time writing long epistels in a language in do not speak. If the conclusion should be that these matters are too complex to be understood by me, then fine.

In summary: i am hoping to shortcut what could be a very long discussion. If you can refer me to a simulation, a picture, i will instantly believe that my question has an answer. If no one is even trying to make such a picture, i consider that to be a very bad signal indeed.

Oh, and a good day to you as well. Mine is awesome, summer is finally arriving!

 

[humanino]

if you have a calculation in quantized gravity using plane wave gravitons to produce a finite and reasonable result, then I would again start to think about the existence of gravitons.

I'm not sure that will convince you but I found the paper I was thinking of
Quantum Gravitational Corrections to the Nonrelativistic Scattering Potential of Two Masses
Phys.Rev. D67 (2003) 084033

Also, I think it is worthwhile to glance at
Can Gravitons Be Detected?
Found.Phys. 36 (2006) 1801-1825

 

[Eelco]

From what i understand, most physicists are invested in graviton-ish theories. So maybe asking people to seriously inquire as to why their investment might be 'sub prime' is expecting too much of 'a special kind of honesty'. Its called special for a reason.

I was hoping somone whos biases swing my way might comment, Marcus for instance. Someone who has his money on background independent, discrete geometrical theories of space, shouldn't have any inhibitions from putting his mind towards dealing some serious blows to gravitons.

I havnt been able to find such a critique though, so that leads me to suspect i am misunderstanding gravitons. Am I? (or more accurately: to what degree am I?). How can gravitons, on a conceptual level, possibly account for the observations we already have, such a gravitational lensing?

Or does the answer depend on obscure mathematics, that does not translate back into a physical mechanism? If so, shouldn't you want to simulate that, to have at least one check that you are not fooling yourself with all your fancy mathematical jargon? (im not trying to attack anyone here, that happens to me all the time!)

 

[humanino]

I also dug out interesting references for calculating quantum corrections to well-known metrics.

$\hbar\alpha$ corrections : Quantum Corrections to the Reissner-Nordström and Kerr-Newman Metrics Phys.Lett. B529 (2002) 132-142

$\hbar G$ corrections : Quantum Corrections to the Schwarzschild and Kerr Metrics Phys.Rev. D68 (2003) 084005; Erratum-ibid. D71 (2005) 069904

Please note that classical leading terms ($\hbar=0$) are (re)derived

 

[atyy]

Is there any reason (beyond aestetics which i disagree with anyway) to favor a particle over a geometric explanation? Any sort of empirical matter gravitons may help explain?

In classical general relativity, is there any reason to favor a gravity wave over a geometric explanation?

We do have a correct quantum theory of gravitons at low energies:
http://relativity.livingreviews.org/Articles/lrr-2004-5/
http://arxiv.org/abs/gr-qc/9607039

So even LQG must have gravitons if it is to be correct:
http://arxiv.org/abs/0812.5018

 

[D H]

The only reason to favor a particle explanation is the success of QED and QCD for describing Electromagnetic and weak and strong interactions, and is not a bad reason at all, don't you think so?

An even better reason is that it would provide a deeper answer, which ultimately is what physics is all about. Stopping at a geometrical explanation of curved space is aesthetically displeasing. It is akin to those "because I said so" answers that physicists received from their parents when they were kids after having asked "why?" one too many times.

The classical Maxwell's equations provide a good enough answer for almost any problem that a power engineer will face. Invoking quantum mechanics to explain what happens in a high voltage transmission line is confounding rather than helpful. That doesn't mean that quantum mechanics is useless.

Classical Newtonian gravity provides a good enough answer for almost any problem except for precision timing that a low Earth orbit aerospace engineer will face. Invoking general relativity to explain the motion of a satellite in low Earth orbit is confounding rather than helpful. The error from ignoring general relativity are orders of magnitude smaller than the uncertainties induced by the relatively poor knowledge of the characteristics of the upper atmosphere (one, maybe two decimal places of accuracy is about as good as it gets). That doesn't mean that general relativity is useless.

Aside: How does one even model the non-spherical nature of the Earth in a general relativistic formulation? The typical approach in high precision satellite modeling or planetary ephemerides computations is to model general relativistic effects as a perturbative force in an otherwise Newtonian universe.

 

[Eelco]

In classical general relativity, is there any reason to favor a gravity wave over a geometric explanation?

The gravity wave is part of the geometric explanation. It seems we are wielding different terminology.

We do have a correct quantum theory of gravitons at low energies:
http://relativity.livingreviews.org/Articles/lrr-2004-5/

All of the explicit calculations of the previous sections are performed for weak gravitational fields, which are well described as perturbations about flat space. This has the great virtue of being sufficiently simple to make explicit calculations possible, including the widespread use of momentum-space techniques. Much less is known in detail about effective field theory in more general curved spaces, although its validity is implicitly assumed by the many extant calculation of quantum effects in curved space [19], including the famous calculation of Hawking radiation [87, 88] by black holes. This section provides a brief sketch of some effective-field theory issues which arise in curved-space applications.

'Its validity is implicitly assumed'

Uhm, yeah. Thanks for reiterating my point.

Reading things like that, this is really all I have left to say to any graviton-theorists:

1) Give me a picture of a simulation of light deflected by gravitons, corresponding to an observed case: case closed, gravitons are a possibility
2) Make an honest attempt at doing such a thing: ill suspend judgement
3) Dont even try: I have lost all confidence this is going anywhere whatsoever. You are shamelessly shaping your physical theory to the constraints of real analysis, with complete disregard for experiment: the least you could do is admit it. Go learn to program a computer.

Im leaning towards 3 right now.

So even LQG must have gravitons:
http://arxiv.org/abs/0812.5018

LQG is a dynamic field with many competing theories, so perhaps this is again a matter of terminology. LQG is not my point: my point is that there are competing theories without gravitons.

 

[Eelco]

An even better reason is that it would provide a deeper answer, which ultimately is what physics is all about. Stopping at a geometrical explanation of curved space is aesthetically displeasing. It is akin to those "because I said so" answers that physicists received from their parents when they were kids after having asked "why?" one too many times.

This doesn't make any sense.

Why does matter emit gravitons?

Its not about a race to the deepest answer. Its about finding a model that fits the data. If you have a model that fits all the data (and I am not saying we do), then asking why once more, is simply a stupid question. Because, that's why. Maybe you can find an even simpler model. Thats cute, but no more of an answer to your 'why'. To say gravitons qualify as the simpler model seems bizarre to me, but whatever floats your boat.

The important question is: how can gravitons ever possibly fit the data of gravitational lensing?

 

[Rebel]

...From what i understand, most physicists are invested in graviton-ish theories. So maybe asking people to seriously inquire as to why their investment might be 'sub prime' is expecting too much of 'a special kind of honesty'. Its called special for a reason.

I was hoping somone whos biases swing my way might comment, Marcus for instance. Someone who has his money on background independent, discrete geometrical theories of space, shouldn't have any inhibitions from putting his mind towards dealing some serious blows to gravitons.

I havnt been able to find such a critique though, so that leads me to suspect i am misunderstanding gravitons. Am I? (or more accurately: to what degree am I?). How can gravitons, on a conceptual level, possibly account for the observations we already have, such a gravitational lensing?

Or does the answer depend on obscure mathematics, that does not translate back into a physical mechanism? If so, shouldn't you want to simulate that, to have at least one check that you are not fooling yourself with all your fancy mathematical jargon? (im not trying to attack anyone here, that happens to me all the time!)

I don't get how one can diminish so easy the results and success of Yang-Mills theories as to put it as a horrible reason to follow its procedures...
as bending of light is an effect of atraction as any other gravitational effect, then you should make the same redundant questions you did for graviton also for photons and W and Z's in electoweak theory, and gluons for strong force, that is, how they make possible attraction and repulsion possible? then you'll get the virtual particles concept. Remarking (again) that gravitational lensing would be as good as usual gravitational atraction of matter the way things are seen now with GR, so your question could be as well, how gravitons can describe gravitational atraction? Then you'll see that gravitational waves would lead to suspect that there can be an associated particle as in QM and there could be virtual gravitons.

I saw a mention to graviton propagator and deflection of light by mean of interchange of gravitons in Zill's "QFT in a Nutshell". (If you stop putting things in that arrogant mood then won't appear that you are attacking anyone.)

 

[Rebel]

An even better reason is that it would provide a deeper answer, which ultimately is what physics is all about. Stopping at a geometrical explanation of curved space is aesthetically displeasing. It is akin to those "because I said so" answers that physicists received from their parents when they were kids after having asked "why?" one too many times.

I totally agree with all next of course. But... there could be well a theory for QG with final roots in geometrical description, don't you think so? A particle view is what past theories has thaught as a fruitful model, but gravity hasn't fit yet... Many of us cannot be sure of which scheme would be better despite our preferences with this kind of guides. This reminded me a writing of 't Hooft "The mathematical basis for deterministic QM" http://arxiv.org/abs/quant-ph/0604008

 

[D H]

Eelco: Your problem with gravitons (or whatever the answer is) arises in the original post:

How is something like gravitational lensing explained in a flat spacetime with gravitons?

You are unintentionally creating a straw man of gravitons here.

Any deeper answer to the question "what is gravity" than general relativity had better explain why spacetime is curved. Spacetime is not flat, and AFAIK, a graviton solution will not say that it is.

But... there could be well a theory for QG with final roots in geometrical description, don't you think so?

Sure. I should have been more explicit. I will be so here. Saying that physicists should stop with a general relativistic explanation of gravity is aesthetically displeasing. For one thing, there are those nasty singular solutions. For another, it is in essence saying that there is no reason to continue with theoretical physics. We have the standard model and general relativity. Done! All you theoretical physicists can go back to school and learn a new trade.

 

[atyy]

The gravity wave is part of the geometric explanation. It seems we are wielding different terminology.

Exactly. So gravitons are part of a quantum theory of gravity as geometry.

 

[humanino]

Give me a picture of a simulation of light deflected by gravitons, corresponding to an observed case

Do you realize that I have posted a link where you can find an elementary calculation of the metric around a BH using graviton exchange ? You want a picture, but a picture based on GR will give the same result as a picture obtained from the above kindergarden calculation, and as a bonus you even get quantum corrections. You decided to reject the idea of graviton because you think you understood GR so you do not even want to try to understand QFT. Why should we care that you ban from your culture such an important and beautiful construction ?

 

[wolram]

I am only a tyro, so what potential energy does a graviton have, consider a body in a void
coming into contact with another gravitating body, would not that gravitating body have to ramp up its graviton count to maintain the status quo?

 

[DaveC426913]

Any deeper answer to the question "what is gravity" than general relativity had better explain why spacetime is curved. Spacetime is not flat, and AFAIK, a graviton solution will not say that it is.

I would argue that "curvature" is part of the GR model of gravity, not independent of it. A graviton model will have to explain what we see* out there, such as gravitational lensing, but it is not obliged to do so using a curved space analogy.

*we do not see space-time curvature. Rather, space-time curvature is a model that tries to explain what we see.

 

[Eelco]

I would argue that "curvature" is part of the GR model of gravity, not independent of it. A graviton model will have to explain what we see* out there, such as gravitational lensing, but it is not obliged to do so using a curved space analogy.

*we do not see space-time curvature. Rather, space-time curvature is a model that tries to explain what we see.

Absolutely. Finally someone who seems to be talking about the same thing.

Photons are affected by the presence of the sun in some way. I do not see anyone suggesting that gravitons interact with photons. So how are these photons affected? Is there some indirect influence I am missing?

 

[Eelco]

Do you realize that I have posted a link where you can find an elementary calculation of the metric around a BH using graviton exchange ? You want a picture, but a picture based on GR will give the same result as a picture obtained from the above kindergarden calculation, and as a bonus you even get quantum corrections. You decided to reject the idea of graviton because you think you understood GR so you do not even want to try to understand QFT. Why should we care that you ban from your culture such an important and beautiful construction ?

I understand QFT to some degree, and i have no objections to it. I am just not sure why it should apply to gravity.

I noticed they calculate metrics. That had me confused, perhaps you can elaborate. All gravity can be explained in terms of curvature encoded in such a metric. If these gravitons in effect carry the metric, ie, if there presence deforms spacetime, then there is not any reason for them to be absorbed in the same way a photon is, to transfer its momentum, right? Because then youd be double-counting gravity. Are they absorbed without any effect, or not absorbed at all?

 

[Eelco]

Exactly. So gravitons are part of a quantum theory of gravity as geometry.

Ehm, no. When i talk of geometry, i talk of metric. Gravity can be explained purely in terms of a hyperbolic evolution of metric. No gravitons needed.

 

[Finbar]

Ok so as i see it the facts are that you can use QFT as an effective field and recover classical GR. This means you can use QFT to account for the bending of light. Here i refer to the papers of Donoghue et al already cited by humanino.

The question then remains. Can we interpret these results as graviton exchange? I think yes we can but we probably have to widen our ideas about what a "particle" really is. In particular we can't really stick with the idea that a graviton is a particle that "moves in space" because space and time are only defined in relation to the graviton. On the other hand its clear that the gravitational field once quantized can a)only transfer momentum in discrete packets and b) that information cannot travel faster than light. This must be true if you combine GR with QM. Given a) and b) its natural to want to interpret any given theory of QG based just on the principles of GR and QM as a "particle theory". But because of the nature of gravity the particle interpretation breaks down in the case of QG.

I think though that if we accept that quanta of the gravitational field are probably a reality but that these do not conform to our normal QM idea of a particle then it would perhaps be better to say that a graviton is a "quanta of the gravitational field" rather than a "particle".


Further more i would like to add that gauge fields e.g. photons also have geometrical interpretation. When we quantize them the geometrical interpretation doesn't go away but the interpretation of particles is then valid

 

[Eelco]

I don't get how one can diminish so easy the results and success of Yang-Mills theories as to put it as a horrible reason to follow its procedures...
as bending of light is an effect of atraction as any other gravitational effect, then you should make the same redundant questions you did for graviton also for photons and W and Z's in electoweak theory, and gluons for strong force, that is, how they make possible attraction and repulsion possible? then you'll get the virtual particles concept.

Do the other force carriers interact with oneanother? Not that I've ever heard of, but I could be wrong there. Is there such a thing as a gluon-photon interaction? If not, how does a graviton-photon interaction fit into the picture?

Remarking (again) that gravitational lensing would be as good as usual gravitational atraction of matter the way things are seen now with GR, so your question could be as well, how gravitons can describe gravitational atraction? Then you'll see that gravitational waves would lead to suspect that there can be an associated particle as in QM and there could be virtual gravitons.

I saw a mention to graviton propagator and deflection of light by mean of interchange of gravitons in Zill's "QFT in a Nutshell". (If you stop putting things in that arrogant mood then won't appear that you are attacking anyone.)

It seems i have to put more effort into deciphering your writing, than you put into writing it down. I am trying to be precise as to what I am asking, and if you can not give a precise and directed response, please do not respond at all.

I am aware of my 'arrogance'; I think being blunt is funny, but feel free to disagree. If you do not like my writing style, please do yourself the favor of not responding.

 

[Eelco]

Eelco: Your problem with gravitons (or whatever the answer is) arises in the original post:

You are unintentionally creating a straw man of gravitons here.

Any deeper answer to the question "what is gravity" than general relativity had better explain why spacetime is curved. Spacetime is not flat, and AFAIK, a graviton solution will not say that it is.

Are you implying that general relativity does not predict curved spacetime? I am confused.

Indeed, spacetime does not appear to be flat.

Curvature of spacetime directly leads to 'gravity' for all waves propagating through said space. If a graviton solution will not say that it is flat, ie, is just another way to formulate the evolution of metric, then would you say momentum is transferred by the absorption of the particle, or that it is a curvature effect?

Sure. I should have been more explicit. I will be so here. Saying that physicists should stop with a general relativistic explanation of gravity is aesthetically displeasing. For one thing, there are those nasty singular solutions. For another, it is in essence saying that there is no reason to continue with theoretical physics. We have the standard model and general relativity. Done! All you theoretical physicists can go back to school and learn a new trade.

Singular solutions can be dealt with in other ways, such as CDT.

Im not saying there are no open questions, but it seems a lot as if you are saying QG is a make-work project for physicists. I was hoping for a honest intellectual error, but maybe I am too much of an optimist :).

 

[atyy]

Ehm, no. When i talk of geometry, i talk of metric. Gravity can be explained purely in terms of a hyperbolic evolution of metric. No gravitons needed.

Exactly. No gravity waves needed.

 

[humanino]

I do not see anyone suggesting that gravitons interact with photons.

Photon do interact with gravitons. Not that gravitons carry electric charge, but photons carry energy-momentum, which is what the graviton couple to.

Please try to keep in perspective the difference between a real and a virtual graviton. It is unclear whether we can ever detect a single (real) graviton. Real graviton would be quantum of a gravitational wave, which essentially we can picture as propagating over a given metric. Virtual graviton on the other hand allow us to compute the amplitude for scattering in a gravitational potential order by order, that works fine in a non-relativistic limit, and we can compute the metric from that. Virtual "particles" are not constrained to stay inside a light-cone, a single virtual graviton exchange has a non-zero amplitude to violate all sorts of things including causality, but everything is restored when including the other terms, especially the interference between the first order and the lowest one (without any exchange) restores things together when neglecting higher order contributions.

Anyway, if you are confident you trust your QFT, I strongly suggest reading Feynman's lecture on gravitation. You may find there a wonderful discussion for how you can derive Einstein's GR from massless spin-2 exchange.