Image: SXS

Music of the Spheres

When scientists detected gravitational waves, “astronomy grew ears.” /

"As the eyes, said I, seem formed for studying astronomy, so do the ears seem formed for harmonious motions: and these seem to be twin sciences to one another." — Plato, The Republic.

Galileo was wrong.

That was the famous judgment issued 400 years ago this month by the Inquisition. He was ordered to abandon his opinion about Copernican theory, that “the sun is the center of the world and completely devoid of local motion.”

Such an idea, the committee of theologians said, “is foolish and absurd in philosophy, and formally heretical.”

You’ve probably heard the story. Galileo had made the telescope famous just seven years earlier. He looked at the distant lights that had enthralled humans for millennia and found mountains on the Earth’s moon. Sunspots. Moons around Jupiter. Masses of stars forming clouds of nebulae. And he found that Venus has phases, like our moon. Which meant it orbited the sun, not the Earth.

And you’ve probably heard that Galileo wasn’t the first with a telescope. Wasn’t the first to see sunspots. Wasn’t the first to map the moon. You might know that the science-vs.-religion aspects of his fight have been dramatically overstated. That he was often a self-promoting braggart who treated his questioners as fools.

But give the man credit. Galileo was right. Venus orbits the sun. So does Earth.

Galileo was wrong, too.

In 1616, just days before the Inquisition’s declaration, Galileo wrote to Cardinal Alessandro Orsini, arguing his proof that the Earth moves. But rather than pointing to the heavens, Galileo pointed to the sea. The tides, he said, could only really be explained by Earth’s orbit around the sun and its daily rotation.

He was familiar with the argument that the tides were caused by a strange attractional force from the moon. German mathematician and astronomer Johannes Kepler had argued it in 1609. But Galileo dismissed it as conjecture. He also didn’t have much interest in Kepler’s argument that the planets moved around the sun in ellipses, not perfect and unchanging spheres.

Galileo was wrong. Because what he was missing, we now know, was gravity.

And gravity may now be the key to an entirely new kind of astronomy.

“Four hundred years ago Galileo turned a telescope to the sky and opened the era of modern observational astronomy,” David Reitze announced to the world last week. “I think we’re doing something equally important today. We’re opening a window on the universe.”

But Reitze, executive director of the LIGO laboratory that detected gravitational waves for the first time, suggested that we won’t be looking through that open window so much as listening through it.

“LIGO takes these ripples in space-time and records them,” he said. “You can actually hear them. It’s the first time the universe has spoken to us through gravitational waves. Up until now we’ve been deaf. Today we’re able to hear them.”

The LIGO scientists have described that first sound spoken by the universe in a gravitational wave as a “chirp.” It’s an understated noise given what produced it: 1.3 billion light years away, two black holes merged, forming a black hole 62 times the mass of our sun. But the two black holes had been, combined, 65 times the mass of our sun.

For a tenth of a second, the collision released 50 times more energy than all of the stars in every galaxy in the observable universe. Energy equivalent to three times the mass of our sun exploded out at the speed of light.

But the explosion was completely dark, unobservable with our previous optical and radio telescopes. The energy burst forth in waves of space-time itself.

“Everything else in astronomy is like the eye,” LIGO scientist Szabolcs Marka told The New York Times. “Finally, astronomy grew ears. We never had ears before.”

Except that can be misleading. A decade ago, NASA released the audio of radio emissions from Saturn. Voyager has been sending back similar recordings from other planets for two decades, and in 2013 sent back vibrations from plasma waves in interstellar space.

Take any waveform and run it through a speaker and you’ll get a sound. Granted, not all speakers are capable of producing all sounds, and our ears can’t hear all sounds. So sometimes you have to do a fair bit of translating to make them audible. This was true of LIGO’s chirp, too.

But gravitational waves are not like electromagnetic waves. Ultimately, the information they give us is more like sound. Electromagnetic waves are more like images.

The Atlantic explained it like this:

The sound waves that come out of your mouth when you speak are about as long in wavelength as you are tall. That makes locating you from your voice, much less figuring out your detailed shape, pretty challenging. Instead we use features like the tone, pitch, and rhythm of voices to make sense of social scenes around us (that’s Alice talking; now that’s Bob interrupting; silly Bob). On astrophysical scales, the same is true of gravitational waves. By listening for changes in the amplitude and frequency of gravitational waves, scientists … can literally hear the story the waves are telling.

They can hear the difference between two black holes merging and two neutron stars colliding, for example.

“We never had ears before” Marka said.

But how we’ve wanted to hear.

Consider, for example, Kepler, whose 1619 book The Harmony of the World became one of the most important in astronomical history. For him, the “music of the spheres” was no metaphor. He went looking for music in the planets. After having no success searching for musical ratios in the distances between planets and the sun, he began looking at the planets’ speeds, where they were fastest and slowest. And he found it! Earth, for example, varies by a ratio of 16:15, a musical halftone. “The Earth sings MI, FA, MI so that you may infer even from the syllables that in this our domicile MIsery and FAmine obtain,” he wrote.

That’s not very cheery. But he found it beautiful. Then again, the same is true of a black hole. Light cannot prevail against this region of space-time. But yet it sings, chirping like a songbird.

“That’s one of the beautiful things about this,” LIGO’s Gabriela González told reporters. “We are going to be listening to it.”

Ted Olsen (@tedolsen) is editor of The Behemoth.

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Also in this Issue

Issue 42 / February 18, 2016
  1. Editor's Note from February 18, 2016

    Issue 42: A surprise DNA test, an unexpected power plant, and a breakthrough chirp. /

  2. Around the World in 46 Chromosomes

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  3. Why Solar Power Might Get a Lot More Green

    It turns out that Popeye was right all along. /

  4. Little Blessing for My Floater

    “This tiny ruin in my eye” /

  5. Wonder on the Web

    Issue 42: Links to amazing stuff.

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