(Submitted December 17, 2007)
General relativity tells us that mass warps spacetime and spacetime
"tells" mass/energy how to move. If this is so, then why do we need
gravitons (since gravity is an effect, not the cause)?
Thank you for your question. Indeed, from the point of view of pure
general relativity theory, there is no need for such a thing as
gravitons. However, in quantum field theory all fundamental forces are
mediated by the exchange of (virtual) particles (massless particles in
the case of long-range forces). Therefore, if the conjecture that all
of the forces of nature are unified at some energy scale is correct,
one must also be able to describe gravity in these terms. Presumably,
in this far-from-completed ultimate theory, the exchange of gravitons
and the curvature of spacetime would emerge as equivalent ways of
describing the same phenomenon.
-- Michael Loewenstein and David Chuss
for "Ask an Astrophysicist"