# Gravitons have gravity?

1. Aug 2, 2005

### nabodit

hi
i'm in a dielema.
i dont know if it is the right place to post the thread.
if every particle is destined by the universe to have some gravitionrl force then does graviton itself have gravity. there is no point of it not having if it is a particle as treaded by quantum physics
nabodit

2. Aug 2, 2005

### DaveC426913

I believe that gravitons have no mass, thus no gravity.

3. Aug 2, 2005

Staff Emeritus
But photons, which have no mass, do gravitate, as Eddington's observation of the eclipse (claimed to have) showed. Not jus mass, but all forms of momentum and energy gravitate under the Einstein equations, and the graviton is supposed to couple to matter in imitation of those equations.

4. Aug 3, 2005

### nabodit

if u look at photons u can say that it has created a disortion in space time as it is equivalent to matter*c^2. now there is no point on graviton having no curvature in the fabric causing its own gravity.

5. Aug 3, 2005

### ZapperZ

Staff Emeritus
Er... PHOTONS created a distortion in space time? Which nutty part of GR says that?!

Zz.

6. Aug 3, 2005

### Stingray

That nutty thing known as the Einstein equation ;). An electromagnetic field has a nonzero stress-energy tensor, so it does (in theory) 'gravitate.' By the way, the eclipse observations did not demonstrate this effect. They only showed that light is affected by gravity - not that it generates its own field.

Anyway, the original question cannot really be answered because there is no well-understood, accepted theory involving gravitons. Now, gravitons were originally hypothesized because the linearized Einstein equation looked a lot like a 'spin-2 field equation.' There is actually a self-consistent (as far as I know) quantum field theory that reduces to the linearized Einstein equation in the classical limit. The analogs of photons in this theory are called gravitons. Unfortunately, it has very little to do with the full Einstein equation, which is presumably the only thing that matters to nature.

Since GR is a very nonlinear theory, I guess you could say that gravitons must gravitate, but this really shouldn't be taken too seriously. For example, if you send two gravitational wave packets at each other, the result won't be their sum when they collide. Because of this, you might imagine that gravitons do not propagate 'freely' when other gravitons are around (even if all the matter fields vanish). This interaction could labelled as 'gravitons generating gravity,' but you should take that with a very large grain of salt.

7. Aug 3, 2005

### ZapperZ

Staff Emeritus
Ah, but the "stress-energy tensor" is created by a more general form, and a SUBSTANTIAL part of it is due to mass. This part has been verified. The "photon created distortion in space time" part hasn't and, frankly, I don't see us detecting even a glimpse of that within a few lifetimes. This probably won't be an issue if we're in String Theory, but we're not.

I, on the other hand, suspect a misunderstanding of some basic idea that caused that statement.

Zz.

8. Aug 3, 2005

### Stingray

What do you mean by a "more general form?" Anyway, radiation-generated gravity is generally considered to have a large effect in cosmology, and this has a direct influence on (for example) CMB observations. I'm not going to say that that interpretation is unique, but it is certainly a non-negligible component in the standard cosmological model.

I agree with you that detecting more direct effects would be extremely difficult.

9. Aug 3, 2005

### Nereid

Staff Emeritus
[controverial mode ON]TeV photons have been detected (albeit indirectly); PeV ones are (widely?) expected to be generated in the 'local' universe, and may too be detected within a decade; the gravitational effect of baryonic mass, travelling non-relativistically, has been detected, for masses as low as a microgram (and lower?); so 'all' we need to do is find a way measure the gravitational pull of a photon that isn't too much different, in magnitude, from that of a small lump of steel [/controversial mode]

10. Aug 3, 2005

### ZapperZ

Staff Emeritus

:)

Zz.

11. Aug 3, 2005

### nabodit

when matter is equivalent to energy then the net effect of gravity by matter is equal to that by energy.
-(reference)= faster than the speed of light by joao

12. Aug 3, 2005

### JesseM

But you asked "which nutty part of GR says that", not "which experiment has shown that". From a theoretical point of view, GR does predict that electromagnetic fields contribute to the curvature of spacetime, even if this prediction hasn't been experimentally verified.

13. Aug 3, 2005

### Antiphon

All non-zero mass-energy density gravitates.

This includes photons and gravitons.

It even includes mechanical potential energy. A wound-up spring
gravitates more than when it is loose.

14. Aug 3, 2005

### Stingray

Well, I'm pretty sure that a concentrated electromagnetic wave packet does not gravitate in any remotely Newtonian way. I don't off-hand know if it would be that easy to detect.

The much more serious experimental concern would be that any piece of apparatus near such a photon would be completely overwhelmed by electromagnetic forces - unlike with a lump of steel.

Antiphon, what's mass-energy? That doesn't mean anything outside of a particular physical theory. Each of them define it differently, and it doesn't work very well at all in exact GR, let alone whatever the theory of quantum gravity ends up as.

15. Aug 3, 2005

### pervect

Staff Emeritus
Eotovos type experiments done on Aluminum and gold also help confirm that electromagnetism contributes to the stress energy tensor. The electrostatic binding energy in these two elements are considerably different, .1% vs .4% of the total mass - but we can see that both materials fall at the same rate, nonetheless.

16. Aug 3, 2005

### Antiphon

I mean it in the context of GR where rest mass is related to rest energy
by $$E = m_0c^2$$. If an electron-positron pair anhilate one another,
the resulting gamma rays produce the equivalent gravitation.

But it's important to note that even the classical notions of kintetic and potential
energy are included in GR as source terms of gravitation. Spinning flywheels
gravitate more than stationary ones, the coiled sping I mentioned above, etc.

I'm not sure what you mean by this.

Last edited: Aug 3, 2005
17. Aug 3, 2005

### Stingray

That's special relativity. There are no good definitions of rest mass or rest energy in GR, except in very special cases (or in some approximations).

Electron-positron annihilations (a quantum mechanical process) are not technically handled in classical GR. So you'd have to go to quantum field theory in curved spacetime to discuss that, which I doubt you really want to do :). The concept of 'particle' actually loses its meaning, among other things.

18. Aug 3, 2005

### Nereid

Staff Emeritus
Well, maybe.

I suspect detection of the 'gravity' of photons won't come from a lab setup that resembles anything anyone reading this post would have familiarity with. For example, detection of TeV photons is, at present, via air showers - the cherenkov footprint of a photon can be distinguished from that of proton/electron/etc. Perhaps there's a signature in the CMBR that can be interpreted as photon's gravity? Perhaps some aspect of a certain class of GRBs will betray this GR aspect of photons?
Me, no (and saying any more would give you far too many clues as to my age!).

However, underestimating the ingenuity of experimentalists is all too easy to do; if detecting this effect were to become one of the hottest things in physics, period, I'd not be at all surprised if some group came up with a breath-takingly clever way to get a fix on it.

19. Aug 3, 2005

### Antiphon

I'll take your word for this, but I didn't think $$E=mc^2$$ had
trouble holding true in GR. Nevertheless, the equations of GR have
any energy or matter distribution as a gravitational source.

Although particle creation isn't handled directly by GR, I don't think this
would change the status of the gravitational field implied by the conservation
of energy.

20. Aug 3, 2005

### ZapperZ

Staff Emeritus
If you are going to play by the speculation rule of joao, then I should be able to also. For example, if the Higgs really is the mass field, then it doesn't couple to photons (nor gluons). So there IS a difference between "mass" and "energy" even though one can be CONVERTED to another.

Zz.