Of course a photon doesn't really have mass in the traditional sense, but all energy behaves like mass in that it creates gravity as I understand it. This could very well be wrong so please do feel free to correct me (photons don't really get trapped in black holes, or there is another explanation?).

Energy doesn't create gravity, mass creates gravity.

fafalone: "Isn't the spin quantum number for a photon 1?

 

Yes. However, by "additive quantum number" I mean something that adds algebraically (like charge or strangeness). Spin adds like a vector.

 

fafalone: "Energy doesn't create gravity, mass creates gravity."

 

You are stuck on Newton. Look at the GR equations again, and you will see that it is energy-momentum that is the source term for spacetime curvature (and thus gravity).

 

Tom

It gets harder to accelerate to c as you approach c because momentum energy acts like mass and thus it requires more energy to make each successive increase in speed. Things get infinetely heavy at c I think, so is there something changing besides momentum energy???

 

Would such a body still act with the gravitational force it has at rest?

be a bit careful there with your wording, interpreting things like momentum energy and mass. The thing to do is look at the Lorentz Transforms to see what is going on, in terms of energy and so on.

 

I can't see wha an object travelling near c would have greater gravity, since it is still the same mass, and if you were in the same inertial frame as the object, its "mass" would be "normal" and as such the gravity too.

I don't doubt I word things poorly because i've learned all this stuff on my own rather than in a physics community like college. Sorry for anything that is unclear.

 

According to us, anything accelerated to c would have infinite energy. To them, it would seem 'normal.' It seems to me that by your logic from the last post, they wouldn't really have the energy that we perceive. I think that is incorrect. Rather, that they just don't realize it is a lot because it is normal to them. Likewise with gravity seeming normal to them, but very much increased to us.

umm... I suggest reading a book on special relativity, all the equations are rather easy to derive from a few basic premises and thought experiemnts, and looking at it from a mathematical point of view might help you see it more clearly

 

one of the things you can't have, is something with mass travelling at c.

I understand special relativity, and I know that you can't accelerate to c.

Use 99.9999999999999999999% of c if it helps you, rather than c, in my last post.

okay, ignore the rest of the universe for a minute.

 

two objects, when at rest have identical mass, are hurtling past one another at nearly c. which one has the energy and which one's gravity is greatest?