Expert: Daniel Mazur - 6/30/2009

Question
Hi Dan,

I'm sorry for asking too many questions in a single day. Please take your time if it bothers you.

Thank you so much for the answer about the force. I got it, to some extent atleast, will get better with time.

I'm aware of curvature of spacetime and have also heard about the particles like gravitons, gluons...n all.

What I want to ask is that when we say that gravity results from exchange of gravitons between two objects, do we mean to say that there are actually some tiny particles that fly from one object to another??

Would be great if you could tell me briefly about the entire idea of such particles.

No need to get into too many details if it takes a lot of time, just a small introduction will do.

Thanks and I really appreciate your answering all questions in such a great a way!!

Cheers
Shikhin

Answer
Dear Shikhin,

this is really not my branch of physics, nor does this belong to the undergraduate curriculum that all physicists have in common, but I'll do my best.

I don't think it is the best explanation to say that "gravity results from exchange of gravitons between two objects", even if this is a popular statement. I would rather say that any acceleration and deceleration of mass is associated with absorption and emission, respectively, of gravitons. In a model situation, when there are only two massive objects in the universe, one could say that the only gravitons available in the universe are those originating from deceleration events of the two... which would at least look like an exchange of gravitons between the two.

However, this does not sound like photons at all, so I really doubt this would be the proper picture in case of gravitons. Since creation of Universe there have been plenty of photons around, which just keep on flying - cosmic radiation. Many of the original photons are long since absorbed by matter: Each time these photons encounter matter (an atom at least), they excite its electrons and then inside the matter the electrons loose the extra energy by means of collisions with the nuclei (results in warming of the material) or emission of new photons... But it doesn't matter to the electron, if the photon exciting him has been the "original photon" since the birth of the Universe, or if it had been emitted more recently by another electron near or far away. I think it must be the same case with gravitons: Huge amounts of them are probably flying around and get absorbed and emitted by matter.

Nature may work in different ways about the gravitational attraction. I can imagine, for example, that the universe is in fact full of gravitons flying in all directions. A single massive object in the Universe then would stay put in its place (no acceleration), because the total impulse delivered from all directions adds up to zero and because what energy the gravitons bring, the object's mass re-radiates (no change of temperature). On the other hand, if there is another massive object in the universe, the two objects would throw "gravitonic shadow" over each other. Instead of having the same density of gravitons hitting from all directions, there are suddenly LESS gravitons hitting from the direction of the other object - the shadowing effect. This would exactly correspond to the observation that massive objects always attract each other and never repell. Even matter and antimatter gets attracted together.

This model has of course the weakness, that it does not explain the origin of gravitons. On the other hand, physicists are currently trying to search for "dark matter" in the universe, which is supposed to take up 98% of the Universe's mass, so it may be that the "missing mass" is in fact contained in the graviton cloud. You see, as we are unable directly detect gravitons, we cannot tell, exactly how they behave. Try and have a look at http://arxiv.org/abs/gr-qc/0601043 , where authors are debating the implications of graviton being (perhaps) undetectable.

Okay, so this is as much as I dare say to the subject. It not much an answer as an opinion, because it seems that nobody really knows the answer. Please take it as such and forgive me the delay.

Cheers,
Daniel