Welcome to You Ask Andy

Marvin Craig, aged 11, of 'AJinnsboro, S.C., for his question:

Why can’t sound travel through a vacuum?

Place your fingers on your throat and say a long Ah. You can feel a steady vibration of the sound in your vocal chords. Sound is vibration. It moves. It takes movement to start it going. And movement requires energy. So sound is a form of energy. The musicians in a full orchestra exert 100 horsepower of energy per second to keep the music going at top pitch. But the sound produced from all this effort is equal to about only one horsepower per second.

A sound begins where some energy was spent in setting up these vibrations. A tree falls and bangs the ground. Someone strikes a drum. The noise spreads outwards on waves of sound, rather like ripples on a pond. It spends its energy as it goes. It is loudest closest to its source.

Most sounds travel to us through the air. But if you put your ear to the water pipes you trill discover that sound is also carried through solid materials. If you dive under water and bang two stones together you will hear sound carried by a liquid. So sound travels through the air, through liquids and through solids.

There is an experiment, however, which shows that sound cannot travel through a vacuum. An electric bell is placed under a glass dome. The air is syphoned out and the bell sits in an almost vacuum. The bell is then buzzed from an attached wire. You can see the clapper work. But you can hear no sound.

Sound then needs more than the energy which starts it going. It needs something through which to travel. We are told that it is carried by the minute particles of which air, solids and liquids are made. Energy strikes the nearest particles, sets them vibrating and they strike those next to them.

Imagine a row of tin soldiers. You knock over the first one. He falls against the second one and the second one falls against the third. The last fellow may not get a bump big enough to knock him down. For the original energy may have spent itself. This is very much how sound travels from particle to particle.

The more particles in a substance, the faster the sound can travel. It travels faster through water than through air. It travels fastest through densely packed solid substances. But sound cannot travel at all without the minute particles to carry it along.

In a vacuum few, if any, of these particles exist. Hence sound is impossible. There is no air to carry sound on the surface of the moon. So, we are told, the moon is noiseless. When we get there, we shall have to do our talking by shortwave walkie talkie radio.

 

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