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The Question

(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)?

The Answer

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"

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