Antigravity Particle

13emandel

I'm just curious about this one right here. If there is such a thing as an anti gravity particle or just any entity that is not affected by gravity, wouldn't it just speed off into the sky relative to the observer? Some people associate anti gravity with things floating in air but if they're floating then that means they're still being held down by the Earth's gravity. Given that the Earth is spinning, orbiting the Sun, the Sun is orbiting the center of the Milky Way, and the Milky Way itself is moving through space, shouldn't it just fly off?

arunma

The photon is an example of this "anti-gravity particle" that you speak of (at least in the non-relativistic limit, since GR says that gravity can bend the photon's path). And yes, it does indeed behave as though it were oblivious to gravity.

Vanadium 50

There is no such thing as an anti-gravity particle, and the photon does not behave in the way arunma describes it.

13emandel

I guess I should rephrase that. I'm not talking about a particle in general but instead an entity itself. Just an anti gravity anything.

arunma

Then perhaps I'm misunderstanding 13emandel's question. He's asking about a particle that isn't affected by gravity, and neglecting the effects of GR which are present in stronger gravitational fields, this is indeed how the photon acts. Am I missing something?

matlaber

I support this view.

espen180

Of course the photon is "unaffected by gravity" in the limit as g->0. This is a misleading argument. If like saying that the electron behaves like a classic particle "if we ignore quantum effects".

You don't need "relativistic"* field strengths to observe how gravity affects photons. We can observe gravitational frequency shift of photons on Earth.

*I assume by "relativistic" you mean very large, like close to the Sun.

As for OP's question, theoretical "exotic matter" with m<0 would be subject to a repelling gravitational acceleration.
See http://en.wikipedia.org/wiki/Exotic_matter

Phrak

Do any of the hypothetical dark energy or dark matter models have negative energy?

Of course, for there to be a particle, the field would need to be quantizable...

The field about confined dark matter would repel normal matter, but would a mass of normal matter repel dark matter?

espen180

It would have to, wouldn't it? Momentum and energy are conserved in gravitational interactions.

diazona

Maybe I'm a little late but I was going to say, the photon is affected by gravity, just like any other particle. The difference is that since photons always travel at a high velocity (c), the deflection they experience in weak gravitational fields, like the Earth's or the Sun's, is nearly undetectable.

matlaber

I support this view.

jtbell

Even Newtonian gravitation was expected to bend the path of a light ray, going as far back as about 1800. See for example the following PF thread:

Does Newtonian gravity bend light?

Phrak

Yeah, hu? It seems like an obvous result... So how do the stress energy terms find this to be true or false?

Too bad Pervect doesn't show up except on the rarest occasions, since this is his forte, but we might get some hints from this: http://www.physicsforums.com/showthread.php?t=152662

SmashtheVan

bring me a non-relativistic photon, and I've got a bridge in brooklyn id like to sell you.

arunma

I can see my reference to relativity has caused some confusion. Please allow me to elaborate.

Obviously the effect of special relativity is important when considering the photon. I was referring, rather, to general relativistic effects. Since the original poster's question pertains to the effect of gravity on the photon, it normally wouldn't make sense to talk about any low-mass or low-coupling constant limit (I think this is what espen180 was referring to), except that gravity is noticable to humans even in cases when it is not strong enough to have any important effects on the photon. To use an example from my reseach, we usually don't need to consider general relativity when detecting high energy photons from distant galaxies. That's why I bring it up as an example of a particle that isn't affected by gravity (for the most part). But given the objections presented, perhaps it isn't such a great example.

milford30

has Einstein's idea of bending spacetime and not light been disproven? so gravitons do interact with photons which implies bosons interact with each other?
am i missing something? or is my logic wrong?

i think the problem at hand is if photons interact with gravitons?

pallidin

Until we can verify gravitons I think we should be somewhat careful with extensions of the implications.