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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Is gravity the exchange of gravitons, or the curvature of space-time?

Theorists believe that both of these descriptions are valid, in much the same way that we can think of the force of electromagnetism as being either the product of a continuous field, or the exchange of numerous force-carrying particles called photons. For certain 'classical' calculations, the description of electromagnetism as a field is more workable than its 'quantum' description, and vice versa. The problem is that, although physicists have a workable theory of gravity that involves the gravitational field, and gravitational forces as a curvature of space-time, there is no currently believable quantum theory of gravity involving 'gravitons'. We do not even know, a priori, whether there are such things as gravitons even though we have isolated the particles responsible for the other three forces in nature. Just as photons are 'packets' of the electromagnetic field, gravitons would be considered 'packets' of the gravitational field or space-time curvature.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.