Why Is Space Silent?

Sound is a longitudinal pressure wave that propagates by compressing and rarefying molecules in a medium such as air, water, or solid materials. In the vacuum of space, the density of particles is extraordinarily low — roughly 1 atom per cubic centimetre in interstellar space compared to about 2.5 × 10¹⁹ molecules per cm³ in air at sea level. Without enough molecules to bump into one another, pressure waves cannot form or travel. Astronauts on spacewalks communicate via radio waves, which are electromagnetic and do not need a medium.