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The Question

(Submitted March 27, 1998)

I would like to know if it is true, that the theory of relativity has been proven by some scientists, who made satellite experiments?

What was there discovery and how did they prove it?

The Answer

This topic is fairly broad, so let me try to narrow it a little. "Relativity" is a rather general term that encompasses both special and general relativity. The former encompasses effects such as the changes in physical properties of objects with speeds approaching that of light, whereas the latter includes effects having to do with the bending of "spacetime" by massive bodies. There is no one experiment which "proves" relativity, and yet so many experiments have provided consistency with the "theories", that most scientists accept them as being extremely accurate in their descriptions of reality.

"Special Relativity": The strongest direct evidence comes probably from particle accelerators, in which subatomic particles such as electrons and positrons are accelerated to within a few inches per second of the speed of light. We can observe very clearly and accurately the changes in, for instance, the apparent masses of the particles. They are observed to increase dramatically, and in fact new and much heavier particles can be created by making counter-rotating beams of, say, electrons and positrons, collide head-on with each other. Special relativity has played a key role in the design and operation of particle accelerators for many decades.

"General Relativity": There have been a variety of experiments over the years which have supported general relativity in ever more detail. I would say the culmination was the awarding of the 1993 Nobel Prize in Physics to Russell Hulse and Joe Taylor for the discovery of the binary pulsar 1913+16. This binary star system consists of two neutron stars which are orbiting about their common center of mass about every 7.75 hrs. Over time, they are spiraling in toward each other, due to loss of energy via "gravitational radiation" - a prediction of general relativity. Other general relativistic effects are observed, such as the "precession of the periastron". That is to say, the stars are in elliptical orbits, and the "long direction" of each ellipse is precessing with respect to a distant observer. This effect is about 4 degrees per year. (In comparison, for Mercury going around the Sun, it is about 44 seconds of arc per century.)

There are a host of other experiments which confirm different aspects of both special and general relativity. I view those just mentioned as among the strongest examples.

J.K. Cannizzo
for Ask an Astrophysicist

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