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.
