# In which direction do gravitons travel? Do they originate from mass or are they collected by masses?

This is a kind of silly question, that mostly goes unanswered: are gravitons detatched from the objects they act on, like photons in free space.

Maybe gravity or gravitons are collected by mass (including black holes), as they are collected by the curve in spacetime caused by the mass. The origin of gravitons in such an arrangement seems phenominal in many respects, but could it be that space itself is just composed of evenly distributed graviton sources? these spill gravitons when a mass distorts it.

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Possible duplicates: physics.stackexchange.com/q/937/2451 and links therein. –  Qmechanic Jun 30 '13 at 22:29
Please use capital letters (only) where they are necessary. –  Wouter Jun 30 '13 at 23:08
@ChrisWhite with all due respect I think the duplicate link answers the information content, which is a meta level to the present rather naive question on gravitons. –  anna v Jul 1 '13 at 5:08
@annav Well in any event the rewording away from the black hole case differentiates, so I'm voting to reopen. –  Chris White Jul 4 '13 at 5:37
@ChrisWhite followed your lead –  anna v Jul 4 '13 at 5:56

In GR, gravity was considered to be the curvature of spacetime. Spacetime is curved due to a reigon of energy

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This curvature is described by a field called the spacetime metric $g_{\mu\nu}$. Now, in QFT, particles are considered to be excitations of quantum fields. Photons are excitations of the electromagnetic field $F_{\mu\nu}$, fermions are considered to be excitations of the fermion field $\Psi^\mu$, higgs bosons are excitations of the higgs field $\phi$ etc. This resulted in the standard model, a special quantum field theory of fermions, electromagnetism, the weak force, the strong force, and the higgs. Something missing? Of course! Gravity! Gravity is missing! Why is that? Gravity, like QED, QCD, etc. results in a divergent peturbation, but unlike QED QED and etc., it is non-renormalisable. Nobody likes infinites!

For gravity, of course, we have GR but that is a classical theory. There were also attempts to put the standard model on a curved spacetime (Semiclassical gravity; everything is quantum, but gravity is classical) and other results which used both QFT and GR, such as the Chandrashekhar limit, Hawking radiation etc. But, still, gravity was treated as classical.

It seems natural to consider $g_{\mu\nu}$ as a graviton field, and gravitons as excitations of it in a Quantum Field Theory. But as we said, it is not possible because it is non-renormalisable.

So, how do we solve the problem? String theories naturally predict the graviton field (Spacetime metric). For example, in Type IIB string theory, the graviton field, is found in the Neveu-Schwarz Neveu-Schwarz sector (no it is really two copies of "Neveu-Schwarz"), along with the dilaton field and the Neveu-Schwarz B-field (a string theoretical version of the Electromagnetic field, note the "a" and not the "the"). The dilaton field also appears in Kaluza-Klein, where it unifies its siblings; the gravitational field, and the electromanetic field.

Now, this only means that the curvature of spacetime is a field for the graviton. Fields, determine the probability of finding a graviton at a particular point. Don't imagine them as bumping into objects like classical projectiles aimed at near by objects. Instead, imagine them as excitations of fields which determine the motion of objects towards a reigon of energy. So a black hole won't "not allow gravitons to escape". Instead, the diagrams of them "warping" spacetime should be thought of as describing the probability amplitude of finding gravitons at a particular place. With this, you could think of objects being attracted by gravity as "Interacting with the field, getting information from this excitation, and moving in the direction it is told to.".

That said, there are gravitational waves, which describe gravitons being emitted away, never to ever return...

Also note that the information about theese gravitons is encoded completely on the event horizon so they don't need to escape.

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Gravitons are the hypothetical, as yet, exchange particles in a future quantum field theory of gravity in a unified mathematical model of all forces.

Of course theorists search for a unification of forces and the graviton is expected to be the exchanged virtual particle in gravitational interactions viewed as quantum field theories with similar properties to the photon,Z0 and gluon, the other exchanged virtual particles in interactions between particles.

Photons do not appear as real, carrying energy away from their source, except in transitions between energy levels of a potential model holding charges stable, as in an atom or molecule, or in acceleration of charged particles.In the expected analogy of behavior with photons bodies falling into a black hole might , through their acceleration in the fall, produce free gravitons, ( if they exist ),that would behave similar to photons, i.e. for example be trapped in a black hole inside the horizon.

Now the picture in the last paragraph is sketchy because there is no accepted unification in the quantum field theoretical framework ( except string theory seems to be well on the way of getting there). It is analogous to how the other three forces manifest in nature in the microcosm. In the macroscopic framework of General relativity it is the distortion of space around a mass that creates the "force" of gravity in the limit with Newtonian gravity, and gravitons will only be explainable in this framework once the unification of all four forces gets to a solid mathematical model.

but could it be that space itself is just composed of evenly distributed graviton sources?

Once a solid mathematical model of unified theories is attained this will be answered, i.e how gravitons and space distortions are related.

I expect virtuality will play a great role in this explanation: virtual gravitons will be exchanged between gravitational sources creating the general relativity energy-momentum tensor. They will not be evenly distributed but according to solutions of the equations, dependent on the strength of the source. This would be in analogy with the field lines created by electric or magnetic sources: they can be thought as exchanges of virtual photons between the sources distributed according to the solutions of the problem, as the fields are variable depending on geometry. The gravitons from gravitational sources building up space will be virtual.

At the moment gravitational sources appear as distortions of space, in GR, where there is no place for "gravitons".

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