Einstein was enthralled by light. Even as a teenager he pondered its
properties. At 16, he imagined what it would be like to chase, catch up with,
and ride on a light beam. When he started thinking more seriously about light,
he questioned the thinking of his day—that it traveled through a medium
dubbed "ether," and that its speed was determined by how fast its source moved
through this mysterious substance. Einstein realized that the speed of
light—about 186,000 miles per second—is constant whether it comes
from a moving source such as a speeding car's headlights or an unmoving source
such as a ceiling light. But here's the catch: The speed of light is constant
only in a vacuum, a place where there's no matter, like the vast emptiness of
space. Here on Earth, the speed of light can slow down. To see if you can
identify what ordinary objects in your house can kick
light into a lower gear, click on the image at left.—Karen
Hartley
Note: This feature originally appeared, in slightly different form, on NOVA's "Einstein Revealed" Web site, which has been subsumed into the "Einstein's Big Idea" Web site.
