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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

If light has traveled thousands of years, how could it possibly have aged according to relativity?

When astronomers say that light has taken 4.3 years to travel from Alpha Centauri to Earth, they mean that in our reference frame, this is the duration of the journey of this messenger relative to OUR clocks. The light itself was created at the surface of Alpha Centauri, and reached Earth 4.3 years later where it was annihilated by being absorbed by our photographic film or the human retina. To ask how old the light is is simply to ask how long has it been in our reference frame between when the light was created and destroyed. As to what you would experience if you were riding on the photon at exactly the speed of light is an interesting but completely non-physical question. Special relativity says that the time dilation factor is infinite which means that the 4.3 year journey is over in an instant. But the passage of time 'on' the photon is irrelevant to us on the Earth. What counts for us is the information that is conveyed by the light, and because light has a finite velocity, this information is always, for Alpha Centauri, 4.3 years out of date by the time we get it!

Gravity is also believed to travel at the speed of light, so gravitational influences in the universe are always delayed at the point of reception, from the moment when the were generated. This delay is important in determining precisely where the planets in our own solar system will be at a specific Earth time.

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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.