Physics trivia

Why doesn’t sound propagate in space?

To understand why sound does not propagate in space, it is necessary to know that sound waves are of the mechanical type and need a propagation medium, which does not exist in a vacuum.

Sound does not propagate in space because sound waves are mechanical and need a propagation medium.
There are two types of waves : those that need a medium to propagate, called mechanical waves , and those that don’t need a medium to propagate, called electromagnetic waves . String waves, sea waves and sound waves are classified as mechanical. Light, x-rays , microwaves and radio waves are classified as electromagnetic.

Sound can not propagate in space, because in that environment there is a vacuum. Because there are no molecules in the environment, sound waves are not transmitted, and the result of an explosion in space, for example, would be silence. In addition to the existence of molecules in the medium, sound is also related to the arrangement of these molecules. Thus, the greater the proximity of these molecules, the faster the sound waves propagate, therefore, the speed of sound in solids is greater than in liquids and gases.

The condition of electromagnetic waves is completely different. They can propagate even if there are no molecules in the environment. Light from the Sun , for example, travels approximately 150 million kilometers through a vacuum to reach Earth.

Noisy vacuum from the movies

There are many film productions that tell their stories in outer space , and it is common to have films in which explosions, gunshots and collisions in space produce sounds as if they were happening on Earth. The famous Star Wars film series is a clear example of this: at all times, it represents sounds produced in space.

On the other hand, there are films that portray very well the impossibility of propagating sound waves in space. A good example of this is Gravity ( Gravity – 2013), in which the collision of various objects in space occurs without the production of characteristic sounds. Another possible highlight of the Gravity scenes is the representation of what happens to the sound in the passage of the characters between depressurized and pressurized environments.

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