This morning, scientists from the Laser Interferometer Gravity Observatory announced that LIGO had detected gravitational waves from the collision of two black holes, an event so cataclysmic that it converted the mass of three solar systems into pure energy in about a tenth of a second.
Scientists have been trying to figure out how to “listen” to gravitational waves—and to prove their existence—ever since Einstein predicted them in 1915. But until September 14, 2015, when the colliding black-hole event was first detected, this was beyond our technical abilities. In the words of Scott Hughes, “Gravity is a weak force. Measuring these things is bloody hard.”
Hughes is a theoretical physicist at MIT who has been contemplating LIGO since its inception in 1992. He has struggled with a question at the heart of the observatory program: Once we do hear gravitational waves, how will we know where they come from? How can we use them to explore the mysteries of the cosmos? To this end, Hughes has modeled the “sounds” of a host of astrophysical events, including colliding black holes. (You can listen to his impressions of these sounds below.)
“LIGO acts a whole lot more like an ear than an eye,” Hughes explained this week in a colleague’s office in the physics department at MIT. (Although the discovery was by then an open secret among the 900 LIGO collaborators and their peers, the meeting took place behind closed doors and on condition of embargo.) Here’s what he means.
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 like Hughes can literally hear the story the waves are telling. “I like to think of it in a linguistic way,” said Hughes. “The vocabulary of the [event] is imprinted on the wave.”
For example, in the simplest kinds of collisions of two black holes, the sound you hear is a simple chirp; if the black holes are spinning rapidly, however, you get a warble on top. Hughes cautions that scientists are only just learning to understand this new language—the warbling that codes for spin could be inaudible if both black holes spin the same way. “There are homonyms,” he said, laughing. “Two sources can say different things but sound exactly the same.”
Which is one reason LIGO uses multiple detectors around the world to listen to these elusive waves. “It’s like listening to music. You might have the bass in the left channel, the snares in the right,” he said.
What this music will reveal is a set of stories about the universe that can only be heard in the language of gravitational waves; stories that would otherwise be lost in the cosmic noise of the Big Bang. That's the lasting importance of LIGO.
Listen below for Hughes’ moving account of what this discovery means to him personally and the astronomy community more generally—and the chirps and whizzes he has been investigating in his research.