Light in Outer Space
What vibrates as light travels through the emptiness of outer space?
This is a good question, and one that baffled scientists for quite some
time. In fact, the discovery that light apparently does not need a medium
to travel through, unlike other types of waves, was one of the driving
forces for the development of the theory of relativity. (It's a long
story.) The short answer to your question is that light is both like a
wave and like a particle. Particles don't need anything through which to
travel; in fact, you might expect particles to travel better when there's
nothing for them to interact with. Light is like that, too. Light travels
faster in a vacuum than through any material.
Richard Barrans Jr., Ph.D.
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Update: June 2012