Blue Sky Science: Why is light faster than sound?

Molly Torinus


Why is light faster than sound?

Light and sound are very different. Sound is actually a mechanical disturbance through air or another medium. Sound always needs a medium to travel through and the type of medium determines its speed.

Imagine a bunch of molecules bouncing around in the air. If you hit an object or make a fast motion, the molecules that you push are going to hit the ones in front of it. You’ll get this disturbance in the direction of travel of however you made the initial motion, and it will move through the medium. That’s how sound travels—as a pressure wave.

Light, on the other hand, is not a pressure wave—it’s a fundamental particle. One ray of light is typically called a photon, and it’s an electromagnetic disturbance. Light doesn’t need a medium to travel.

The speed of sound through air is about 340 meters per second. It’s faster through water and it’s even faster through steel. Light will travel through a vacuum at 300 million meters per second. So they’re totally different scales.

No information can propagate faster than the speed of light. If you have light that’s going through a media, it can travel slower than that. But the speed of sound and speed of light are totally incomparable.

You normally don’t notice this speed difference on a day-to-day basis. This speed difference does become apparent, for example, with lightning. You’ll always see lightning before you hear it, because typically lightning will be a mile away, two miles away. That’s a great enough distance that that speed difference becomes apparent to your brain.