It's always great to get a question like this, because not everybody
remembers the different things they've heard and understands them well
enough to know when they're contradictory. Of course, it's also a
little scary to give an answer to someone who will actually remember
what you say.
Anyway, the main answer here is really simple-
don't always believe what you're always told! When people say that
light doesn't bend, they mean that those gravitational effects aren't
noticeable under most conditions. When you do take them into account,
as Einstein did
in General Relativity, the situation gets even
worse than you think. There just aren't any old-fashioned straight
lines in our universe.
For simple Earthly experiments, it STILL
isn't quite true to say that light travels in straight lines. Everybody
is familiar with how light bends when it goes from air to water or
glass, etc. Of course, the people who told you that light travels in a
probably assumed you wouldn't apply that to such
situations. Even leaving those situations out, and ignoring gravity,
the statement that light travels in straight lines still isn't quite
Waves have a tendency to spread out. Even the straightest
light beam you can get, from a good laser, will gradually spread out.
If you look carefully at the shadows of light from objects with sharp
edges, you'll find that the shadow won't have a sharp edge, even if
light all came from a tiny region. Sometimes this effect- called
diffraction- produces amazing results, such as making a bright spot in
the middle of the shadow of a round object.
Of course if I'm
saying not to believe what people tell you, you might wonder why you
should believe me. You could do some of these experiments yourself.
Some of the easiest involve shining laser beams through little slits,
and looking at the interesting patterns produced.
(republished on 07/27/06)