Sound is our brain interpreting vibrations of the air that is in
our ears. A vibration is something moving back and forth or up and
down. The air vibrations that we hear come from something moving air
rapidly back and forth.
For example, I hear a car honk it's horn down the street because
a mechanism in the car vibrated the air around it back and forth. That
vibrating air caused the air that was around it to vibrate, and those
vibrations moved outward until they got to my ear. The key to me
hearing the car's horn is that everywhere between me and the car there
is air to vibrate.
Now, if I was in outer space floating in a space suit outside my
space ship, I would not hear anything if the ship's engines fired. That
is because, although there is air in my helmet, and there is air in the
ship, there is no air between me and the ship. Since there is no air,
there is nothing to carry the vibrations. Vibrations like sound need
something to move back and forth, or there is no sound. That is why you
can't hear things in space.
This is not an entirely obvious fact, expecially since many of
us have been fooled by Star Wars and Star Trek to think that a ship in
space should make a "whoosh" noise whenever it goes past us.
(republished on 07/29/06)