system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If sound doesn't travel in space, what happens to sound waves as they leave the earth? Is there some sort of invisible barrier around the earth where the atmosphere turns into space that sound can't travel through? If so, what happens to sound when it reaches that barrier? Does it bounce off?
gpt
Sound waves start when you compress a small region of air, say with a speaker.  The compressed region tries to expand, compressing the region next to it,  that region expands, compressing the region next to it, ... .  When you go away from the Earth, each chunk of air has a little less air in it.  That means that when the compressed air expands, it runs into fewer molecules.  That means less compression in the next region, and less expansion, and less sound energy in the next region.  Eventually, the region can expand without hitting any air molecules.  Then the sound ends.  All the energy was turned into a little more mass of air a little farther from the center of the Earth.  Gravity will pull those molecules back, eventually, but it's hardly a "bounce off" situation.