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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Why does time change when you travel near the speed of light?

No one really knows why Nature wants to do things this way, but experiment after experiment for the last 90 years has shown that this is exactly what happens. It all has to do with how we tell time. Two clocks, one stationary and one in motion, are physically NOT the same clock. The slowing down process has nothing to do with the physical mechanism in the moving clock suddenly moving like molasses. To someone moving with this clock, they would notice absolutely nothing strange about how this clock is measuring the passage of time. It is counter-intuitive, and a 'common sense' explanation for it will continue to elude us.

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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.