Illustration: Chelsea Beck (Gizmodo)
Giz AsksIn this Gizmodo series, we ask questions about everything from space to butts and get answers from a variety of experts.  

The human tolerance for sound is, on a galactic level, puny. Volcano eruptions, jackhammer-intensive construction work, My Bloody Valentine concerts—these tinnitus-inducing phenomena are barely whispers besides the majestic, roiling bursts and collisions going on in outer space.

Of course, much of this activity is technically soundless—space’s atmosphere lacks the material that make sound waves possible. So for this week’s Giz Asks, we asked experts in astronomy and astrophysics what the loudest sound would be, if sound as we understand it existed up there. As it turns out, it sometimes does—and when it doesn’t, we can sometimes convert the relevant emissions to a sound tolerable to our tiny, earthbound ears.

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Greg Salvesen

National Science Foundation Postdoctoral Fellow, Astronomy & Astrophysics, University of California, Santa Barbara

As far as I’m aware, the Perseus galaxy cluster is the current record holder for the loudest sound discovered in the Universe. Generating sound requires two conditions. First, there must be a medium that the sound waves can travel through, like air or some other gas. Indeed, there is very hot gas that pervades the space between the thousands of galaxies that make up the Perseus galaxy cluster. This gas shines as X-ray light that we can observe with X-ray telescopes in space, like the Chandra X-ray Observatory. The second condition for sound is a source to actually produce the sound waves. A powerful black hole is at the center of one of these galaxies that make up the Perseus galaxy cluster. Periodically, this black hole ejects an enormous amount of energy into the hot surrounding gas, which transports the energy as sound waves traveling out through the cluster like expanding bubbles.

What makes the sound loud is the ability of the gas to efficiently carry away the energy released by the black hole, which amounts to an energy comparable to 100 million exploding stars! Although this sound from the Perseus galaxy cluster is very loud—that is, the amplitude of the sound waves is huge—we couldn’t actually hear it with our own ears. That’s because the sound corresponds to a B-flat some 57 octaves below middle-C on a piano. That means it takes about 10 million years for one sound wave to pass by, which is quite a bit longer than you’re likely to live even if you exercise regularly and eat healthy.

Puragra Guhathakurta

Astronomer and Professor at the UCO/Lick Observatory at the University of California Santa Cruz

Sound is really a form of energy transmittal, it’s vibration. The problem is the transmittal of that energy in the form of sound—there is no sound in space. But energy gets transmitted in other ways—a blast wave from an explosion, for instance. Gamma ray bursts are considered to be the most energetic events in the universe—they’re not fully understood, but they’re almost certainly explosions of stars, and they release more energy in 10 seconds than the sun will in its entire ten billion year lifetime.

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Robert Erdelyi

Professor, Mathematics and Statistics, University of Sheffield, whose research is focused on solar, space and plasma physics, MHD waves, linear and non-linear waves

Sound cannot really travel in empty space. For sound you need some medium—like gas, for instance, in the Earth’s atmosphere—and in space that material is very, very rare—maybe one atom per cubic kilometer, or less. But that doesn’t mean that a big explosion couldn’t generate acoustic waves.

Space is filled by plasma, which is the fourth state of matter, the others being (according to our current knowledge) the solid, the liquid and the gas. The universe itself is 99.9% in a plasma state. It’s only on Earth that we haven’t got so much plasma.

In space, there is magnetic field everywhere. The same is true of Earth, but we don’t really feel it. In space, if the magnetic field is not very strong, and there is plasma under these circumstances, sound could propagate.

Stars are continuously bubbling, you could say, through a process called convection. That type of disturbance in the plasma state generates a lot of acoustic waves—sound waves. The Sun itself does this. Sometimes these acoustic periods can last for hours, sometimes just a few seconds. You could interpret these kinds of acoustic waves as very loud sounds.

Musical sounds created by longitudinal vibrations within the Sun’s atmosphere recorded by the University of Sheffield.

The energies involved in the generation of these acoustic waves are billions of billions of billions of times the power of an atomic bomb. The explosions that produce these sounds are absolutely massive—you cannot imagine.

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Jim Fuller

Assistant Professor, Theoretical Astrophysics, Caltech

The loudest sound in the universe definitely comes from black hole mergers. In this case the “sound” comes out in gravitational waves and not ordinary sound waves. As long as the black holes are in the range of roughly 1-100 solar masses (which is the case for black hole mergers recently detected LIGO), the sound is indeed in the human hearing range! These mergers output something like 10^52 Watts of power. That’s about a billion billion times the energy output of the Sun. If translated to the decibel Watt scale, that equates to something like 520 decibels. That doesn’t sound too large but remember the decibel scale is logarithmic, so an increase by 10 decibels is a factor of ten in volume.

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Donald Gurnett

Professor, Physics and Astronomy, University of Iowa, whose research is focused on experimental space plasma physics

This isn’t a sound, it’s a radio emission—but you could convert it to sound.

The signal came back to us as a waveform, and then on the ground we converted it to a sound that you can listen to, and it is very, very loud.

It is something called a heliospheric radio emission. There is a very special radio receiver on the Voyager that covers the frequency range from about 10 kilohertz to 50 kilohertz—a very low frequency, well below a car radio, for instance. We detected an intense radio emission, produced out at the boundary between the solar wind (the wind that comes out from the sun, and flows at about a million miles per hour, expanding outward almost to infinity) and the interstellar plasma (called the heliopause) which eventually stops the solar wind.

So there were an intense series of explosions on the sun—often called solar flares—in 1991. These sent a shockwave out through the solar system. We detected this shockwave with four spacecraft: Pioneer 10, Pioneer 11 and Voyagers 1 and 2. We also detected it when it went by the Earth. It was moving at 600-800 km per second—several million miles an hour. I postulated that this radio emission was produced when the shockwave finally reached the the heliopause and ran into the interstellar plasma.

I think this is the most powerful radio emission we’ve ever detected. In 1995 I quoted the radiated power as 10^13 watts. As far as emissions detected anywhere near our solar system go, it is clearly one of the most intense.

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