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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

If nothing can travel faster than light, why does gravity take affect instantly?

Woah! Who said that gravity operates instantaneously? There is no observation that justifies this 'Newtonian' statement, and lots of evidence from special and general relativity that gravity obeys the speed limit set by the speed of light. The dynamics of our solar system, with light travel times up to several hours, would be very different if gravity acted instantaneously. The calculation of planetary positions and their mutual gravitational influences would lead to different predictions for where the planets are in the sky at a specific time, especially for the planets beyond Jupiter where the light travel times are the longest.

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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.