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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

How does gravity travel through space?

Mathematically, according to general relativity, it takes advantage of the fact that the curvature of space-time is mathematically defined by 10 more curvature terms than are constrained by the portion that is controlled by matter. In a previous question and others in the 'Physics -- Relativity' archive, I have mentioned how Einsteinian gravitational forces are DEFINED as the curvature of space-time. But, this curvature in 4-dimensional space-times is specified by 20 distinct terms. Only 10 of these are defined by the local distribution of matter which is mathematically found by solving Einstein's equation of gravity. The other 10 terms in the full 'Riemann Curvature' tensor define how the space-time outside the massive body respond to the presence of the mass, and define a 'source-free' solution for gravity. This only happens in space-times with more than 4 dimensions, which is why gravity does not exist as a force in space- times of dimensionality of 3.

That is the mathematical reason why gravity can 'travel' through space. In general relativity, gravity and space-time are EXACTLY THE SAME THINGS BY DEFINITION. It is impossible, within general relativity, to separate gravitational fields from the fundamental properties of space-time. This is like trying to define what a computer is without its software.

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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.