Humans cannot hear in space because sound cannot travel in a vacuum. Sound is a mechanical wave, which means it requires a medium to propagate through. Sound waves are created by the vibrations of particles, and these vibrations are transferred to the surrounding air molecules, which then bump into each other and transfer the sound wave onward. In space, there is no air or other medium for the sound waves to travel through, so sound cannot propagate.

For this reason, astronauts in space cannot hear each other talk unless they are wearing spacesuits that have built-in communication devices. These devices use radios to send sound waves through the air within the spacesuits, where they can be heard by the astronauts.