AJ Kell, of Balclutha Primary School, asks :-

Why is light faster than sound?

John Campbell, a physicist at the University of Canterbury, responded.

Sound travels relatively slowly because it involves atoms and/or molecules colliding with each other. Solids and liquids are not very compressible so their atoms are touching. In them, sound energy is transported by an atom knocking its neighbour in the direction the sound is travelling in. But each atom is in random motion due to thermal energy and it looks like chaos with atoms moving in all directions. However, because the atoms are effectively touching the sound energy is transferred with minimal time delay.

In aluminium sound travels at about 5000 metres per second which is fast compared to the speeds of travel of cars or planes. Aluminium atoms are quite light so can travel faster than heavier atoms therefore the speed of sound in aluminium is faster that the speed of sound in lead, which is about 1300 metres per second.

When we turn a solid or liquid into a gas the volume occupied expands by about 700 times. That is close enough to 1000, or 10 times 10 times 10, so that atoms in a gas are about 10 diameters apart. That is why gases are compressable. It means that if we give some sound energy to a gas atom or molecule, it has to travel about 10 diameters before it can pass the sound energy to a neighbouring atom or molecule. Hence sound travel slower in a gas, for example about 330 metres per second in air. Lighter gas atoms travel faster between collisons so the speed of sound in hydrogen, as an example, is about 1300 metres per second, which is similar to that for a heavy metal.

Light is the transport of electromagnetic energy so doesn't involve having to shift energy by the collisons of atoms or molecules. For reasons we dont fully understand, light travels incredibly fast, about three hundred million metres per second in space. No matter where we measure the speed of light in space we obtain the same number and this is a cornerstone of Einstein's Theory of Relativity. Physicists have linked together all major forces except for gravity. Once there is a unified theory developed we may know why light travels so fast.

In contrast to sound, the speed of light in materials varies very little. In water it slows to three-quarters of the speed of light in space, and, at the other extreme, for diamond it slows to about half the speed of light in space.