Are Gravitons the Key to Understanding Gravity?

[math_04]

According to particle physics, there is a certain particle called a graviton which is the messenger particle of gravity but it so happens that Einstein's theory of general relativity says that mass affects the space time fabric which is responsible for gravitational force. So it is like the apple on tissue paper analogy. Place an apple on a tissue paper and the tissue paper will bend downwards.

The thing that I do not understand is that if general relativity says that if mass affects space time (gravity), then why do scientists keep searching for the graviton particle? Surely a graviton particle would somehow violate general relativity?

Secondly, since gravity affects space and time, it seems intriguing that graviton particles can affect time? I know gravitons have something to do with string theory and trying to unify general relativity and quantum mechanics but still it seems strange.

 

[math_04]

oh wait, its alrite, don't answer this i got the answer somewhere else haha.

 

[GTrax]

The thing that I do not understand is that if general relativity says that if mass affects space time (gravity), then why do scientists keep searching for the graviton particle? Surely a graviton particle would somehow violate general relativity?

Gravitons? The Higg's Boson?
We now have a quite workable (quantum) model to describe the way things are. It is a conceptual fairy-story, a mixture of mathematical rules around little characters, yet put together to provide predictive results of astonishing accuracy - except for gravity! We could build an alternative model with new characters 1/3 the size of the present ones, but it would only be saying the same thing in a different way.

Despite its feebleness compared to the other forces, gravity does something the others do not. It reaches across galaxies! So if there must be little gravitons, there needs to be enough that each particle in the universe can interact with every other. This does strain credulity somewhat.

Thus for me, noting the astronaut in orbit who "feels" weightless, it is not too hard to regard his situation as Newton's "in a state of uniform motion". I just left out the bit that says"in a straight line". Simply because we cannot tell any difference between the effects "an acceleration from being shoved along" from "acceleration due to gravity", we should maybe suspect that the space that astronaut was in really was "curved". After all, if the Earth were to suddenly disappear, the astronaut would shortly head off in a (nearly) straight line. Within a curved space, lines that curve exactly to match will be perceived "straight", just like your local dragstrip.

Very qualitative I admit, but I am not so learned as others in this forum.

 

[peter0302]

Despite its feebleness compared to the other forces, gravity does something the others do not. It reaches across galaxies! So if there must be little gravitons, there needs to be enough that each particle in the universe can interact with every other. This does strain credulity somewhat.

You lost me there. Light reaches across galaxies too! And light is possible due to the EM force, another one of the four fundamentals. Likewise, gravity and EM follow the same inverse square law.

Also, why does the idea of multiple gravitons for every massive particle bother you? Just think how many GLUONS there are out there - gigantic numbers for every quark in every nucleus in every atom.

Plus, who's to say every particle gets one graviton? Maybe it takes more than one particle to make a graviton? Who knows?

Anyway, he idea of gravitons is not conceptually difficult at all IMO. The problem is not how they work at _large_ distances, but how they work at _small_ distances. When you try to quantize gravity using QFT, and thereby come up with a workable model for quantum gravity, you get absurd results that don't jive with experiments, to say the least.

 

[GTrax]

Hi Peter

OK - a piece at a time..

Why does the idea of multiple gravitons for every massive particle bother you? Just think how many GLUONS there are out there - gigantic numbers for every quark in every nucleus in every atom.

Its in the unnecessary size of the numbers involved. For me, it blights the concept! For each particle that might have a gravitational effect on the rest, there has to be a whole universe worth of gravitons. Mathematically, they cannot be shared, if only because I can find one situation where I can keep splitting an affected mass down to its atoms. Going this way, for me, feels nearly equivalent to when the ancients held that the entire celestial sphere moved around once a year.

Plus, who's to say every particle gets one graviton? Maybe it takes more than one particle to make a graviton? Who knows?

That makes the situation worse! It leaves not enough to go around. If I can find just one situation that is physically realizable, that has gravitational effects between more than one separated bodies, then I can go on to point to all other bodies and, even to their constituent atoms.

Anyway, he idea of gravitons is not conceptually difficult at all IMO. The problem is not how they work at _large_ distances, but how they work at _small_ distances.

This won't do either! We have quite compelling evidence that gravity effects do work over large (galactic?) distances as well as small. If we are to have any theory at all that addresses it, it cannot be qualified to be truthful only for small distances.However much we are intrigued by speed of light observations, we should not too readily start postulating light-like properties to the gravity phenomenon. Gravity waves?

When you try to quantize gravity using QFT, and thereby come up with a workable model for quantum gravity, you get absurd results that don't jive with experiments, to say the least.

I am not at all sure how one would "quantize gravity", or even if it deserves it.

 

[peter0302]

Ah, but you really didn't address my most important point, which was almost all that you just said is equally true of light. Do you have a problem believing in photons as well?

 

[humanino]

Anyway, he idea of gravitons is not conceptually difficult at all IMO.

Please define the graviton field for me

The problem is not how they work at _large_ distances, but how they work at _small_ distances.

Sure. Both QCD and a putative quantum theory of gravity are nonabelian, right ? The nonabelianity of QCD makes it asymptotically free and confined at the same time. Where are the nonabelian effects in gravity ? If they are at short distance, can you explain me how to reconcile with the fact that nonabelian effects act physically at large distance in QCD ?

 

[Antenna Guy]

Do you have a problem believing in photons as well?

Would a photon with a frequency of 0Hz interact with matter like gravity?

Regards,

Bill

 

[humanino]

a photon with a frequency of 0Hz

There is no such thing ! This is the limit where energy goes to zero, so this is also a limit where there is no coupling with the graviton.

 

[GTrax]

The only thing I can appreciate on light having properties in common with gravity is that both get to visit the farthest reaches of observable universe.

I can make a pulse of light, as a spacetime event, (various methods), and I can measure its speed. Harder it is to suddenly create a mass presence pulse, but even if I could, I have no good reason to think that the effects would spread at the speed of light. I know Mr. Newton thought that gravitational effects, such as changes when masses split in collision, would propagate instantly through the universe. I am even less inclined to accommodate that notion.

Gravity is a property of mass. We can destroy mass, and yield energy like light and its other wavelength friends. The light, it seems, can travel a bent path caused by a large mass. I am told the space was curved, and the light did continue in what was, for it, a straight path. For me, gravity is not light. It is not like light. We know so little about what it really is, we can only obliquely approach it by describing its effects with equations. Moreover, however successful is the Quantum Theory in providing us with a predictive model for most material behaviour (usually under high energy bombardment), it leaves out gravity.

Earlier I described the quantum model as "a conceptual fairy-story, a mixture of mathematical rules around little characters, yet put together to provide predictive results of astonishing accuracy - except for gravity"!
However much more I learn of it in the detail, and even, the more I appreciate the teachings of Mr. Feynman, it remains for me just that... a very useful predictive model just crying out for a better story to be put around it - one that includes gravity!

 

[JesseM]

The only thing I can appreciate on light having properties in common with gravity is that both get to visit the farthest reaches of observable universe.

I can make a pulse of light, as a spacetime event, (various methods), and I can measure its speed. Harder it is to suddenly create a mass presence pulse, but even if I could, I have no good reason to think that the effects would spread at the speed of light.

Well, in general relativity gravitational waves do move at the speed of light, and aside from a few weird spacetimes involving causal loops, I'm pretty sure all the realistic GR spacetimes do have a light cone structure where events can only be influenced by other events in their past light cone. We could also say that GR's picture of mass and energy curving spacetime is "a conceptual fairy-story, a mixture of mathematical rules around little characters, yet put together to provide predictive results of astonishing accuracy"--and the idea that gravitational effects propogate at the speed of light is one of the theory's predictions, so it seems likely to be true even if the conceptual model is revised by a theory of quantum gravity.

Also, instead of thinking about a pulse of light, how about thinking the continuous electromagnetic force created by a charged object? In quantum electrodynamics, even in situations where there are no real photons (associated with electromagnetic waves created by accelerating charges), the force between charges is modeled in terms of "virtual photons" (which only appear in calculations and are never measured directly, so they are another 'fairy-story' which makes useful predictions). So in principle the situation could be similar with gravitation, where real gravitons would only be associated with gravitational waves caused by certain types of acceleration, while in other situations the gravitational force could be modeled in terms of virtual gravitons. Of course it's possible a theory of quantum gravity won't work like this, but I don't see why the idea is inherently more crazy than the electromagnetic analogue.

 

[GTrax]

Another thought I have had ...

Consider small things falling about the Earth in orbit. There are a whole bunch of them all parked, giving Earth a unique equatorial ring. Big masses and little masses, it does not matter. The forces work out such that an astronaut leaving the space shuttle going for a spacewalk does not suddenly part company with the spacecraft and adopt a new orbit. It would seem he and his vehicle are in a state of uniform motion, in a space that is so curved the path is a circle. But the beam of photons from his flashlight does not agreeably follow the curved space. We know from viewing past solar eclipses that such beams are so affected.

So now - a light beam is tugged by a mass (or its space is curved!) Is the effect mutual, like it is between masses? Would two light beams pull toward each other? if so, then gravity is not at all something that can be conjured as another photon-like thing. We would have photons emitting gravitons, and both get along at the same speed. This whole notion is only temporarily convenient, and however much I am prepared to accommodate mathematically useful models, I feel we do not have to pander to the grossly bizarre.

 

[peter0302]

Yes, Jesse that was pretty much my point, said much more artfully. :)

Antenna Guy, what do you mean regarding a 0 Hz photon? Obviously there is no such thing. Are you suggesting there might be such a thing as a 0 Hz graviton?

 

[JesseM]

So now - a light beam is tugged by a mass (or its space is curved!) Is the effect mutual, like it is between masses? Would two light beams pull toward each other?

Sure, in general relativity all forms of energy contribute to the curvature of spacetime, not just rest mass energy. And since light produces its own gravity in GR, this is probably going to be true in any theory of quantum gravity as well (whether or not the theory involves gravitons).

if so, then gravity is not at all something that can be conjured as another photon-like thing. We would have photons emitting gravitons, and both get along at the same speed.

Why do you think this means "gravity is not at all something that can be conjured as another photon-like thing"? Gluons are the carriers of the strong force just like photons are the carriers of the electromagnetic force, and gluons are also massless and move at the speed of light; but as mentioned here, gluons can emit other gluons (at least in virtual particle diagrams), so there's nothing unprecedented in quantum field theory about particles which move at c emitting other particles which move at c.

 

[humanino]

@ peter : You have nicely ignored my request for a definition, despite your bold statement :

Anyway, he idea of gravitons is not conceptually difficult at all IMO.

So I'll add another question for you

When you try to quantize gravity using QFT, and thereby come up with a workable model for quantum gravity, you get absurd results that don't jive with experiments, to say the least.

You accept the idea of graviton but admit that you have no clue what quantum gravity is. Do you understand that the graviton is needed only in quantum gravity ?

 

[humanino]

It might be helpful to read :
I hear physicists saying that the "quantum of the gravitational force" is something called a graviton. Doesn't general relativity say that gravity isn't a force at all?
I suspect this is where the OP found his own answer.

 

[Antenna Guy]

There is no such thing !

What is the limit of red-shift? I thought it was 0Hz, but I may be wrong.

Regards,

Bill

 

[humanino]

What is the limit of red-shift? I thought it was 0Hz, but I may be wrong.

Red shift is what occurs when the frequency if shifted towards lower values, wavelength towards larger values, pushed near the infrared if you will. In the mathematical limit of 0 Hz as I told you I agree, yes, there is no coupling to the graviton, because there is no energy anymore and truth is : there is no photon anymore.

 

[Antenna Guy]

there is no energy anymore and truth is : there is no photon anymore.

Where did it go?

Regards,

Bill

 

[humanino]

Where did it go?

You want to take the limit, you tell me where all the energy went !? I don't understand your confusion.

Look, tell me what you think about the following imaginary conversation :
Q: Would an apple with 0 volume bend the Roberval Balance ?
A: No, it would not, since the mass would go to zero in this mathematical limit
Q: But isn't it the massless limit ?
A: Yes indeed, that's why it does not bend the Roberval Balance, and frankly there is no apple in this mathematical limit
Q: Where did it go !?
A: You tell me !

Sorry, I can't really help much better...

energy of the photon is related to its frequency[/url] via $$E=h\nu$$

 

[MeJennifer]

Photons coupling to gravitons?

Sorry folks but this seems more like speculation than science to me.

 

[turbo]

If you want to believe in the speculations of the heyday of particle physics, let me propose this test: If gravitons mediate the attraction of masses, and if the Higgs Boson grants masses to particles, then both of them must be extremely fine-tuned ( I can't even estimate the coincidence)! How can the universe be so consistent in its appearance as far back as we can see? If the graviton is the mediating particle of the gravitational field and if the Higgs Boson grants mass to particles, then the Higgs field and the gravitational field must have been in exquisite concordance throughout the whole history of the universe. This is a very strong piece of evidence against the existence of at least one of the presumed bosons, and likely against the existence of both.

 

[humanino]

Sorry folks but this seems more like speculation than science to me.

Good point. The bending of light is not what is understood to be graviton coupling. In the absence of an external EM field, photon-graviton coupling can only occur via higher order quantum effect, same as in so-called photon-gluon fusion, not first order.

 

[Haelfix]

"If gravitons mediate the attraction of masses, and if the Higgs Boson grants masses to particles, then both of them must be extremely fine-tuned "

This is not accurate. Gravitons are not fine tuned at all, they are on dimensional grounds exactly where they should be. The Higgs boson vev is otoh fine tuned. It should naturally be somewhere around the GUT scale and this is called the Hierarchy problem, and there are many ways around this.

Also gravitons couple to photons in the same way that GR does (not surprisingly b/c it *is* GR to first order). Photons gravitate, as they must. The conclusion is really the same as the Podolsky, Tolman, Ehrenfest's results in the weak field limit, and requires a loop calculation.

 

[humanino]

Also gravitons couple to photons in the same way that GR does (not surprisingly b/c it *is* GR to first order). Photons gravitate, as they must. The conclusion is really the same as the Podolsky, Tolman, Ehrenfest's results in the weak field limit, and requires a loop calculation.

Now I'm, confused

I was thinking in terms of QFT in curved space-time, where the gravitons would propagate over a background. Now I wonder what would be a first order spin1-spin2 coupling term.

 

[peter0302]

@ peter : You have nicely ignored my request for a definition, despite your bold statement :

So I'll add another question for youYou accept the idea of graviton but admit that you have no clue what quantum gravity is. Do you understand that the graviton is needed only in quantum gravity ?

Geeze, why is everyone picking on me lately?

I can't define a graviton for you. My point was that the *concept* of a graviton is no more troubling than the concept of a photon or a gluon. It's the fact that when you actually try to bring the graviton into the standard model you run into problems that is the difficulty. The OP was entirely concerned with the very notion of a gravity-carrying particle, not the details in working it into QFT.

BTW, do you have a clue what quantum gravity is? :)

 

[humanino]

My point was that the *concept* of a graviton is no more troubling than the concept of a photon or a gluon.

But my point is that I think it is ! The traditional way of defining the graviton field as the difference between the metric and the metric of the vacuum for instance does not allow you to couple to fermions, that is, no matter in your universe. So people come up with other prescriptions, and nobody knows which one is the best one, and then I see you come around and claim "there is no conceptual difficulty". I don't think that's fair. When you say

It's the fact that when you actually try to bring the graviton into the standard model you run into problems that is the difficulty.

Like what difficulties ? Lately people have been doubting whether N=8 supergravity is finite or not, and if it is finite it would not be due to supersymmetry but to gravitational symmetry, hinted towards the fact that, maybe, a correct QFT of gravity would be finite. So it is not clear to me what is difficult when you come around and claim "one run into difficulties". What are the difficulties faced by the various approaches, or at least, one ? Choose your favorite.

 

[JesseM]

But my point is that I think it is ! The traditional way of defining the graviton field as the difference between the metric and the metric of the vacuum for instance does not allow you to couple to fermions, that is, no matter in your universe. So people come up with other prescriptions, and nobody knows which one is the best one, and then I see you come around and claim "there is no conceptual difficulty". I don't think that's fair.

Perhaps the problem is that you and Peter are using "conceptual" differently. When I think of a "conceptual" difficulty in physics, to me that stands opposed to more technical/mathematical difficulties--if it's not a difficulty that can be explained in English to a reasonably educated layman, then I wouldn't call that a conceptual difficulty. There may be serious problems with incorporating gravitons into a quantum field theory (though I was under the impression that treating gravitons as strings rather than point particles solved most or all of these problems), but they don't involve the sort of simple plain-English arguments that GTrax was making.

 

[humanino]

Perhaps the problem is that you and Peter are using "conceptual" differently.

I can admit that easily, since I am not a native english speaker. That also partly explains why I only trust mathematical statements, the only which are unambiguous.

From http://wordnet.princeton.edu/perl/webwn?s=conceptual

being or characterized by concepts or their formation

Concept comes from "to conceive" (to take in), like in "formation", "creation" or "construction". If there is no accepted mathematical definition, then yes I admit I call that a "conceptual difficulty", because we do not know how to construct the object.

Yet in that precise case, the mathematical difficulties are not benign, like when one claims we don't know how to define rigorously a QFT path integral, which is true but anecdotal since the calculations provide us with predictions, which happen to work⁽¹⁾. In that case, we do not who to believe to construct the gravitational field, because we do not know which physical ingredients are relevant.

In the old theory of Sakharov popularized by Wheeler (induced gravity), the gravitational field arises from a phenomenological coarse-graining of more fundamental fields. The vacuum is a sort of perfect crystal over which the graviton is some sort of space-time phonon. These old ideas come back and forth, as in non-commutative geometry lately for instance. The action for the graviton in some class of models is written as the trace of the log of some differential operator depending on the metric, say over a compact euclidian space-time. Space-time in that case can be entirely constructed from the fields we think live on it ! I think this mere possibility is remarkable⁽²⁾. I do not want to claim that it is correct, I just want to point out that there are interesting stuff lying in the limbo of this no-consensual-definition situation.

(1) not to indicate there is nothing interesting there either
(2) this is made possible by the fact that the space-time geometry is constructed from a more fundamental algebra

 

[Haelfix]

Now I'm, confused
I was thinking in terms of QFT in curved space-time, where the gravitons would propagate over a background. Now I wonder what would be a first order spin1-spin2 coupling term.

It depends sort of on what you want to calculate really and the accuracy which you desire. The problem you run into for the deflection of starlight, is that you really 'morally' need to use the Schwarzschild solution, in which case you are entirely correct, the formal solution requires summing to all orders in perturbation theory around a curved metric b/c there is a distance scale inherent in the field theory. So care must be utilized in how you arrange your variables, and truncate to get an effective field theory around the massive body (this goes beyond me, but it has been done)

For a physicist who is not a specialist, it suffices to handwave it out. Take two lumps of stress energy that interact via the exchange of a graviton. You will have an interaction that clearly depends on both particles stress energy tensors. Then you simply take one particle to be the photon, and the other 'the sun'. Plugging their properites into the general tree level solution will yield the desired scattering amplitude, and that should be (up to a problematic constant) the same as the case for pure GR.

For the case of a photon - photon interaction with the exchange of a graviton, I think the solution is done explicitly in 'QFT in a Nutshell' by Zee, and its a real half a page quicky b/c all the indices contract.

 

[humanino]

For the case of a photon - photon interaction with the exchange of a graviton, I think the solution is done explicitly in 'QFT in a Nutshell' by Zee, and its a real half a page quicky b/c all the indices contract.

I realize what's going on now. I have read Zee's paragraph and need to find this Tolman Ehrenfest and Podolsky reference. I see that you can formally calculate photon-photon scattering in terms of one virtual graviton scattering, but then the question is what happens for real graviton / real photon scattering.

Thank you very much for the precision

 

[peter0302]

But my point is that I think it is ! The traditional way of defining the graviton field as the difference between the metric and the metric of the vacuum for instance does not allow you to couple to fermions, that is, no matter in your universe. So people come up with other prescriptions, and nobody knows which one is the best one, and then I see you come around and claim "there is no conceptual difficulty". I don't think that's fair.

Jesse's right, what I mean are the OP's philosophical problems with the idea of a particle that reaches across galaxies or seemingly unbelieveable numbers of them. I certainly do not mean to say that the numerous scientific/mathematical difficulties, many of which you mention, aren't legitimate.

When you sayLike what difficulties ? Lately people have been doubting whether N=8 supergravity is finite or not, and if it is finite it would not be due to supersymmetry but to gravitational symmetry, hinted towards the fact that, maybe, a correct QFT of gravity would be finite. So it is not clear to me what is difficult when you come around and claim "one run into difficulties". What are the difficulties faced by the various approaches, or at least, one ? Choose your favorite.

The main difficulty I was referring to was the inability to renormalize.

 

[humanino]

The main difficulty I was referring to was the inability to renormalize.

In the part you just quoted, that is exactly what I question : do we need to renormalize or not ? We are not even sure ! PF is not a peer-reviewed reference, but this discussion contains interesting stuff :
[thread=242473]Hints gravity finite[/thread]

 

[D H]

Perhaps the problem is that you and Peter are using "conceptual" differently.

Humanino is thinking in terms of conceptual as meaning "an accepted mathematical definition". Peter is using conceptual in the sense of a rather vague and not very-well-defined idea; i.e., lots of hand-waving (sorry for the idiom, Humanino).

 

[peter0302]

I should have said philosophical or metaphysical. :)