When two black holes collide, they sing. The energy released by the cataclysmic merger of these infinitely dense bodies is so massive, it bursts outwards in a thunderous vibration that rattles reality itself—producing gravitational waves that actually ripple the very fabric of space-time. And now, humans are starting to hear the music.
In her book Black Holes Blues, Janna Levin imagines what it would be like to experience a collision of two black holes up close: “an astronaut floating nearby would see nothing. But the space she occupied would ring, deforming her, squeezing then stretching. If close enough, her auditory mechanism could vibrate in response. She would hear the wave. In empty darkness, she could hear spacetime ring.”
In late May, astronomers at LIGO, aka the Laser Interferometer Gravitational-Wave Observatory, announced in a report published in the journal Physical Review Letters that in January of this year they had for the third time, in fact, heard this very sort of space-time ring—this time produced by a smashup of two black holes from three billion years ago. The energy output of this collision is estimated to have been equivalent to one billion trillion suns, emitted in less than a second. After hurtling across the cosmos at the speed of light for three billion years, or roughly one-fourth the estimated age of the universe, gravitational waves from this event passed through LIGO at its two recording points in Louisiana and Washington, causing laser wavelengths to distort by less than the width of a proton. Next, LIGO translated this wavelength distortion into an audible soundwave.
The collision of two black holes —a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO—is seen in this still from a computer simulation. Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project
Chirp, said LIGO.
LIGO’s “chirp,” as the astronomy community calls it, is the closest humankind has come so far to actually proving the existence of black holes. Astronomy has long forced humankind to learn humbling lessons in scale, challenging us to orient ourselves in a universe where sizes routinely range from the sublimely large to the infinitesimal, and where extremes often cross paths in truly uncanny ways. LIGO’s meek little chirp is the briefest of songs, the trace of a frighteningly colossal cosmic event, its fingerprint borne to Earth after three billion years traversing the void. LIGO’s recordings of gravitational waves stretch our understanding of the cosmos further than ever before, and the very fact that humans have just recently been able to perceive these waves represents the birth of a revolution in astronomy.
In empty darkness, she could hear spacetime ring…
Black holes have long been lurking ghosts in space. A century ago, Einstein’s theory of general relativity predicted black holes, but Einstein himself disavowed them for being too radical—the universe couldn’t, he concluded, allow such extreme entities to exist. Over the years, as circumstantial evidence mounted and hypotheses on the roles of black holes in cosmic life expanded, the scientific community found itself in a quandary, forced to rely on the theoretical existence of something that no one had ever seen—because the monstrous gravity of black holes sucked all light into them, without any hope of escape, erasing them from the visible universe. Then the scientists behind LIGO—including influential physicists Rainer Weiss and Kip Thorne—decided to stop looking and start listening.
Einstein’s theory of general relativity also first predicted the existence of gravitational waves. If a stellar object is massive enough, Einstein’s theory claimed, its acceleration, such as that undergone by a black hole racing towards its phantomic counterpart, will actually distort space-time around it. The same way, for example, that a large cruise ship thrusts waves out into the ocean as it barrels forward.
Since gravitational waves, like ocean waves, decrease in intensity as they travel, scientists believed for years that gravitational waves might be occurring too far away from Earth to be detected—that our planet was simply an island too remote. Then, in September 2015, almost exactly one hundred years after Einstein’s prediction, LIGO heard its first sound from the void.
LIGO is the most expensive undertaking to date ever funded by the National Science Foundation (NSF), with costs exceeding one billion dollars. It is actually made of two research centers, and two separate interferometers, for verification purposes, one in Hanford, Washington, and the other in Livingston, Louisiana.
Each detector has an L-shaped antenna with arms 2.5 miles long, bearing ultra-precise mirrors at either end. A laser is sent to the end of both arms and bounces back to LIGO’s detector. When the two wavelengths are exactly the same, as is most often the case, the detector sees nothing. When a gravitational wave passes across the two arms, the two returning lasers differ in wavelength by the fraction of a width of a proton, causing the detector to light up rhythmically. Then LIGO turns the flicker into a chirp.
Artist’s conception shows two merging black holes similar to those detected by LIGO. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)
LIGO’s third recording of gravitational waves has made the NSF an even prouder parent. “This is exactly what we hoped for from NSF’s investment in LIGO: taking us deeper into time and space in ways we couldn’t do before the detection of gravitational waves,” France Cordova, the foundation’s director, said in a statement.
With every new “song” that LIGO sings, the foundation for a new branch of astronomy becomes more solid. “We are moving in a substantial way away from novelty towards where we can seriously say we are developing black-hole astronomy,” said David Shoemaker, a spokesman for the LIGO Scientific Collaboration, an international network of astronomers and physicists who use LIGO data. Black-hole astronomy, as Levin points out, takes a “recording device, not a telescope.” Its future playlist will be made not only of the songs of black holes, but potentially those from other stellar objects including neutron stars, pulsars, and as-yet unnamed astrophysical calamities. As LIGO probes further into space, the beat goes on.