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Does Sound Travel through Space

Space battles in the movies are noisy affairs with explosions and blasts hitting the ships. But would sound really travel through space? The simple answer is no. Yet there's more to it than that.

What is sound?
Sound is a kind of energy. It's produced when something vibrates. Whatever produces the vibration, such as your voice, is the source of the sound. The sound moves away from the source through the air or some other material.

Air molecules whiz around at high speed, so overall they're fairly evenly spread out. But what if you decide to play the guitar? The strings vibrate. As a string moves outward, it pushes the nearby air molecules together. This creates an area where the molecules are denser. When the string moves back, it leaves an area with fewer particles in it, so it's less dense.

The vibration spreads outward because the areas of alternating high and low density change the density of the molecules next to them and so on. The way the density changes as the sound wave travels is shown here. The differing densities cause tiny changes in air pressure and our ears are sensitive to them. Our brains interpret them as sounds.

The frequency of a sound tell us how often the waves arrive. The lower the frequency the lower the pitch. The lowest sound the human ear detects is twenty waves per second.

Space battles – noisy or silent?
Since sound needs particles to carry the vibration, it can't travel through a vacuum. This demonstration shows what happens to the ring of a bell in a jar when the air is pumped out. As the air goes out the sound gets fainter. They can't get all the air out, so you can hear a feeble sound which gets louder as they let the air back in.

If we were watching a space battle, we wouldn't hear an explosion when a ship was hit – unless we were in it! In that case the sound could come through the hull and the air inside would carry it further.

Astronauts
Since the Moon has no atmosphere, astronauts on the surface communicate by radio. Radio waves are electromagnetic radiation like light, so they don't need particles to carry them. If two astronauts were close to each other, they might be able to talk directly by touching helmets to transfer the sound. Underwater divers in helmets do this.

Noisy Sun
Vibration is sound and the Sun is vibrating all the time. These vibrations are created by convection just under the Sun's surface. Convection is the way heat travels in a fluid (liquid or gas). Hotter, less dense material rises, and cooler, denser material sinks. Convection is how water boils on the stove. You see big bubbles rise and break as they hit the surface, and the water gets very agitated.

Something similar happens in the Sun, but we can't hear it. The sound waves don't travel to us through space, and the frequency is too low for human ears. However the in-and-out motions of the vibrations can be detected by a special instrument on the SOHO (Solar and Heliospheric Observatory) spacecraft.

Is space a vacuum?
We know what sound is, so let's think now about what a vacuum is. A perfect vacuum would have no particles in it. We don't know of any of these. Even the best laboratory vacuum on Earth has a few hundred particles per cubic centimeter. That may sound like a lot, but remember these are extremely small particles. Each cubic centimeter of the air you breathe contains about thirty quintillion particles. (That's a 3 followed by 19 zeros!) Even in the space between the stars there are around five particles in each cubic centimeter, and there are more in nebulae.

The singing black hole
We've seen that the Sun's acoustic (sound) waves don't get very far, but the vibration itself can be detected visually. However in 2003 a team of astronomers from Cambridge, England observed pressure waves – essentially, sound waves – coming from a black hole in the Perseus Cluster of galaxies some 250 million light years away.

A black hole doesn't suck up matter like someone drinking through a straw. Gas and other material orbits it in an accretion disk and spirals into the black hole. Because of its strong gravity there is strong frictional heating which releases energy as X-rays. The Cambridge team was observing the region using the Chandra X-ray Observatory.

Energy from the black hole heats the nearby gas, making it less dense than the rest of the gas in the cluster. Occasionally a wave of energetic particles is released into the gas, causing the equivalent of a sound wave. These waves show up as enormous ripples in the gas - 30,000 light years across. You can see the ripples in the gas in this NASA image. The astronomers used the ripples to calculate the frequency of the wave. The black hole only sings one note: a B-flat that's 57 octaves lower than a piano's middle C. Its frequency is one per 10 million years, unimaginably far below our hearing threshold.

Can sound travel through space?
In summary, yes. There's sound in space in the form of very slow-moving acoustic waves. The particle density varies in space, but there's no perfect vacuum. We may detect the waves with telescopes.

But no, there isn't any sound if by sound we mean something we could hear or detect with a sensitive microphone. Space explosions would be silent.

Reference:
Niels Marquardt, “Introduction to the Principles of Vacuum Physics” http://www.cientificosaficionados.com/libros/CERN/vacio1-CERN.pdf

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