| MadSci Network: Physics |
Both people will see the light traveling at exactly the speed of light. The speed of light is a constant, and always appears the same to any observer, regardless of her velocity.
The way this seeming paradox is resolved is with the realization that measurements of both time and space are not constants. From the Earthman's point of view, time runs more slowly on the spaceship, and the ship itself has been shortened in the direction of its travel. The Spaceman sees the same thing about the Earth, from his own point of view (or reference frame).
This all seems very strange, but it's true! These principles have been confirmed with myriad experiments over the years. It only seems counter-intuitive to us because we spend our lives at sub-relativistic (i.e. nowhere near the speed of light) velocities. If we commonly moved around at relativistic speeds, we might not think this odd at all!
-Aaron
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