Contrary to conventional wisdom, sound can be transmitted through space. And it’s become an important research tool for astronomers at Harvard and beyond.

Eric Bland

Every space movie features sound in outer space. Guns blast, lasers twang, and bombs explode, making for great entertainment. The problem is that, according to classical physics, sound in space is impossible.

But astronomers have shown that sound—albeit extremely low-frequency sound undetectable by the human ear—does, in fact, exist in space. For example, researchers from the Harvard-Smithsonian Center for Astrophysics, working with the space-based Chandra X-ray Observatory, announced in 2006 the discovery of sound coming from M87, a giant elliptical galaxy in the Virgo cluster of galaxies. The sound, which originated from a black hole in M87, was discordant and complex, ranging from 56 to 59 octaves below middle C. Now they and astronomers around the world are using space sounds as a measure of energy to better understand the physics of black holes and stars, addressing mysteries such as why certain gas clusters never go on to form stars or planets.

“If you can see how much energy is coming and when it comes out, then you know what the object is doing,” says Bill Forman, a Harvard-Smithsonian astronomer involved in the research.

Scientists have detected sound waves coming from just outside the black hole at the center of this galaxy, known as M87. The red jets on either side of the galaxy generate the sound waves. (Credit: NASA/CXC/CfA/W.Forman et al.; Optical: DSS)

Even if the sound emitted from M87 was detectable by the human ear, it would be no symphony. “It would be like a rapid succession of gun blasts,” says Paul Nulsen, one of the astronomers who worked on the M87 project. “In fact, it would probably hurt your ears a lot.”

Can anyone hear me?

On Earth, sound is transmitted through mediums such as air or water, in which the individual atoms are close enough to physically bump into one another. In the vacuum of outer space, however, there is nothing to carry the sound waves.

But there is plenty of matter around black holes; an enormous amount of gas falls towards the black hole, building pressure and energy and creating a low-density atmosphere around it. During this process, some of the matter (through a process astronomers are still trying to figure out) shoots back out from the edge of the black hole as a gigantic cosmic burp, rippling through the galaxy’s atmosphere and jostling particles to generate both sound waves and high energy X-rays. The X-rays travel through the empty parts of outer space and are detected by the Chandra Observatory. Based on the frequency and energy of the X-rays detected, astronomers calculate the pitch of the sound.

The 2006 results helped answer an astronomical mystery: why the gas in some galaxy clusters never cooled enough to form stars and planets. The answer is that the energy put out by these galaxies is enough to heat and stir up the gas, stopping the formation of stars and planets, and also allowing for the generation and transmission of sound. The amount of energy being dumped into the gas is astonishing.

“M87 puts out the energy equivalent to 10 million supernovae,” says Nulsen. “One supernova alone puts out about as much energy as the sun does in its entire lifetime.”

Booming stars

The research from Chandra is part of a growing body of sound-in-space research. Teams from all over the world are studying star composition and formation (including our own sun) and the evolution of the universe using sound waves from space. For example, differences in the rate at which sound waves travel inside a star can provide clues to its elemental make-up.

So what did the most famous sound in the universe, that of the Big Bang, sound like? Nothing. The early universe was too hot and uniform for any pressure differences to form and transmit sound. It took about 380,000 years for the universe to cool enough for any sound waves to form.