Space isn quite empty. Can it transmit sounds?
This question is more subtle than it appears. Sound is carried by pressure waves. Imagine the variations in pressure in a sound wave in air: a pressure maximum occurs where the density is highest. Due to molecular collisions, molecules tend to move from high to low pressure. That, plus the momentum of the molecules, produces the wave. Pressure, however, is a macroscopic concept. We don’t talk of the pressure of a few molecules, we talk about the forces that molecules exert during interactions. To talk of pressure, we need a significant number of molecules. In space, to have a considerable number of molecules/ atoms/ ions, you have to consider large volumes. First let’s consider just the atoms and molecules in space. The typical distance between them is about a centimetre. So we need a large volume to have a significant number. But it’s more subtle than that. A sound wave propagates by intermolecular (or interatomic) collisions. Molecules in air only travel a nanometre or so (their mean
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