# Photon gravity

1. Oct 26, 2009

### blaksheep423

Do photons produce gravitational fields, even though they are massless? In other words, can a photon's gravity affect other photons or elementary particles?

2. Oct 26, 2009

### nicksauce

Well in General Relativity, energy/momentum cause spacetime to curve, and photons have energy/momentum, so yes you can say that photons produce gravitational fields.

3. Oct 26, 2009

### blaksheep423

ok, then the next question ive been wondering about is this:

if gravity propagates at c, and photons move at the exact same velocity, could a photon travelling in a straight line be constantly affected by it's own gravity at any single point? if so, wouldnt the gravity of each point of the photon's path add up continuously, causing a "gravity boom" (which, as the curvature of spacetime keeps adding up, would in effect be a black hole), for lack of a better term?
on the other hand, since i've never heard of this happening, wouldn't gravity have to propagate at either a speed less than c, or greater than c, to prevent this from happening?

please let me know if i am phrasing this in an unclear way.

4. Oct 27, 2009

### Ich

Only if the photon were emitting gravitytional waves - which it can't, as it cannot change with time.

5. Oct 27, 2009

### JesseM

Do photons accelerate in the weak-field approximation when their path is bent by a massive body? If so why wouldn't this result in gravitational waves?

6. Oct 27, 2009

### A.T.

Would a single electron emit http://en.wikipedia.org/wiki/Cherenkov_radiation" [Broken] if it moves exactly at the speed of light in the surrounding medium? Is this a valid analogy to a photon causing a "gravity boom"?

Also: Wouldn't the gravitational field of a photon be Lorentz-contracted to zero along the movement direction and eventually cancel out?

Last edited by a moderator: May 4, 2017
7. Oct 27, 2009

### Ich

Hey, I'm not on solid ground here. I mean, a sonic or cerenkov boom is indeed the addition of something generated by the respective particle. The particle does work. I can't see how a photon could generate something, as no proper time passes.

I dont't say that there are no discontinuities in the "field ofa photon", but they come from its point-like nature in this classical particle view. Maybe something more realistic can be learned from http://en.wikipedia.org/wiki/Monochromatic_electromagnetic_plane_wave" [Broken]. Or maybe others have something more specific to say.

Last edited by a moderator: May 4, 2017
8. Oct 27, 2009

### S.Vasojevic

Excellent question. From much hated, but always right anthropic principle answer is obviously that such thing can not happen. From theoretical grounds it should be looked at quantum level, but on the other hand we don't know gravity on the quantum level. My answer for short: I really don't know, I would be glad to learn.

9. Oct 27, 2009

### blaksheep423

Well put, S. Vasojevic. It obviously doesn't happen. And I don't know nearly enough about quantum mechanics (or, to be honest, relativity. im merely a physics undergrad) to try and comprehend what prevents this. It's just a question I couldn't answer myself and couldn't find an answer to.
So, of the myriad of possible solutions to this problem, most of which are well beyond my comprehension, could a potential answer be that gravity does not propagate at light speed? From what I've learned about it so far, there has never been any evidence or theory in which gravity propagates slower than light. I also know that the speed of gravity has been experimentally verified to within 1% of c.
Does anyone know if it is necessary in the Standard Model for gravity to propagate at c? Also, can anyone think of another solution to the gravity boom?

10. Oct 27, 2009

Staff Emeritus
No. The power emitted by Cerenkov radiation goes as: $$\left(1 - \frac{1}{\beta^2_{med}}\right)$$

which is clearly zero if it moves moves exactly at the speed of light in the surrounding medium.

Hard to say, as there is no power radiated in the EM case. Zero is zero.

A photon bends spacetime. It does not, by itself, emit gravitational radiation, although it might be part of a system that does.

Last edited by a moderator: May 4, 2017
11. Oct 27, 2009

### blaksheep423

So if a photon does not emit gravitational radiation, is this the reason that a "gravity boom" does not happen?

12. Oct 27, 2009

Staff Emeritus
"Gravity boom" appears to be your own creation. It's difficult to discuss such a thing.

13. Oct 27, 2009

### JesseM

So a photon whose path was bent by a gravitational source, or which could even be in orbit around a black hole, would not emit gravitational radiation? But a massive object orbiting or having its path bent by a gravitational source would emit gravitational radiation, correct?

14. Oct 27, 2009

Staff Emeritus
No single particle can emit gravitational radiation. You need a changing gravitational quadrupole moment, and that means you need multiple particles.

15. Oct 27, 2009

### JesseM

If you had a point particle orbiting a large gravitational source, could the combined system emit gravitational waves even if the point particle's mass was negligible compared to the source, so that the source itself was not being affected in any appreciable way by the point particle?

16. Oct 27, 2009

### Staff: Mentor

There is no quantum theory of gravity, so strictly speaking it is not really possible to answer the OP's question. However the spacetime for a brief and intense pulse of light is given by the Aichelburg–Sexl ultraboost. There is no gravity boom or event horizon.

17. Oct 27, 2009

### Phrak

This is one of the most excellent questions I've seen posted on Physics Forum. Questions like this are inspiring, and take us out of our comfort zones where we think we understand things.

Take your question and ask about pulsed lasers instead. With this, there is a particle-like region that is not a massive particle. Since it's light and not a 'massive particle' it doesn't bend space like electrons, protons, and atoms 'n stuff. Massive particles travel less than the speed of light. All massless particles travel at the speed of light.

For massive particles there couldn't be a shock wave like built-up gravity waves, what you call a 'gravity boom'. This is known because some very complicated mathematical derivations of general relativity about massive particles in motion. Light is not massive so we need someone who has some idea of how light bends space and time to answer this. That's not me!

Anyone?

Last edited: Oct 27, 2009
18. Oct 27, 2009

### Phrak

Wikipedia: "In general relativity, the Aichelburg-Sexl ultraboost is an exact solution which models the physical experience of an observer moving past a spherically symmetric gravitating object at nearly the speed of light."

I'm not sure this counts?? The object is massive so has a different stress-energy tensor...

19. Oct 27, 2009

### JesseM

Why do you say it doesn't bend space? Anything with energy should contribute to the curvature of spacetime, no?

edit: this thread has a short discussion of how light can curve spacetime...

20. Oct 27, 2009

### Phrak

But I didn't sat that... Or didn't mean to. Light doesn't bend space and time the same as massive particles. Good of you to bring it up.

Last edited: Oct 27, 2009