Motion and Its Causes


Do all relative velocities add?

Suppose you were riding on a spaceship moving at half the speed of light. If you were to point a laser in the direction the ship was moving, the person on the spaceship would be able to determine that the speed of the laser light was the speed of light, 300,000,000 meters per second. What speed would a stationary observer measure? Surprisingly, she would find that the light was traveling at the speed of light, not the sum of the speed of the spaceship and the speed of the laser light as measured by the traveling person. That is, the speed of light is the same in all frames of reference. This is another of the results of Einstein’s Special Theory of Relativity. It has been tested, not with spaceships, but with gamma rays emitted by subatomic particles moving near the speed of light.


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