Is there direct evidence photons attract gravitationally?

[Drakkith]

Has anyone considered that photons may not "exist" until observation? IE the EM wave travels outwards from it's source as a normal wave, with the photons only appearing when the wave interacts with matter. In such a case I wonder how gravitation affects a propagating wave front, and if it's any different than "photons as little bullets". Especially once the flux is so low that the average distance between photons would be much larger than their wavelength.

 

[Low-Q]

Also, if light did attract itself, wouldn't light from far away stars and galaxies look...different after thousands, millions, or billions of light years?

Maybe that explains why we think that matter in the universe accelerate away from each other? The stars and galaxies appears to be located further away than they really are. The further away the observed star or galaxy is, the errors in estimated distance accelerates and not the matter itself?

EDIT:
So the redshift observed from a far away star might happen due to the longer distance the light has traveled - due to the photon-photon gravity(?). And also, if photons have gravity, they have mass, and therfor they does not travel at the "ultimate" speed...

Vidar

 

[TurtleMeister]

Not only is there no direct evidence that photons attract gravitationally, there is also no direct evidence that leptons attract gravitationally. This is an area that I have researched (mostly within the limits of classical mechanics), and the consensus seems to be that it is still an open question. I am assuming that by "attract gravitationally", the op is referring to active gravitational mass.

Newton's third law of motion is usually used as an argument that these particles must have active gravitational mass. But this is an argument that I have never understood, because in all other cases, other than gravity, the source that creates the force (one body, the other, or both) is irrelevant. Why it should make a difference in the case of gravity is a mystery to me. But gravity is unique, so who is to say one way or the other. There is no direct evidence to confirm or deny. However, one thing is for sure, if these particles (at least in the case of leptons) do not generate a gravitational field then the equivalence principle will be violated.

 

[Drakkith]

Hmmm, my multi-quote button won't work for some reason, so I'll have to do this the hard way.

Low-Q, a couple of things. First, light does not have mass, it has energy. When we say mass we mean "invariant mass", that is the mass something has when observed in it's stationary frame of reference. Light does not have an a frame of reference, so we cannot apply a mass to it. Since it does carry energy, and energy contributes to the stress-energy tensor of General Relativity, and that tensor determines gravity, we can say that light does in fact have gravity.

Second, EM radiation is everywhere, so light would not be red or blue shifted due to the approximately equal distribution of light in the universe. The same thing applies to the distribution of galaxies. (If it's pulling equally strong from all sides, gravity cannot red or blueshift light)

TutleMeister, what experiments have you done? Do you have a link to a paper or something? I'd love to see it. (Or do you mean something less professional when you say "research")

 

[TurtleMeister]

TutleMeister, what experiments have you done? Do you have a link to a paper or something? I'd love to see it. (Or do you mean something less professional when you say "research")

Here is an essay from The Gravity Research Foundation. I'm not sure if this meets your requirements, but it has references at the bottom of the page.

http://www.gravityresearchfoundation.org/pdf/awarded/2001/unnikrishnan_gillies.pdf

 

[cragar]

Lets say we have a black hole and then we have another black hole that is identical but made of antimatter and they collide, they will turn into photons and we will still have a black hole, the light can't escape. And this Black hole will cause other photons to be curved by it or drawn in.