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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Is it ever possible to determine whether a body is at rest?

No, because there are no universal absolute standards of rest in the universe. All velocity measurements must be made relative to some arbitrarily established standard of rest such as the Earth's surface if you are launching a satellite or batting a baseball, or the center of the sun if you calculating the orbits of the planets. The choice of the standard of rest you use in a particular problem is decided on the basis of which vantage point results in the simplest mathematical description of the process you are studying. With the possible exception of the cosmic background radiation, there is no 'medium' against which every observer in the universe can agree is perfectly at rest, in fact the very existence of the expansion of the universe means that no such measurement is possible.

Copyright 1997 Dr. Sten Odenwald
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