Physicists detect gravitational waves, proving Einstein right
A team of physicists who can now count themselves as astronomers announced Thursday that they had heard and recorded the sound of two black holes colliding 1 billion light-years away, a fleeting chirp that fulfilled the last prophecy of Albert Einstein’s general theory of relativity.
That faint rising tone, physicists say, is the first direct evidence of gravitational waves, the ripples in the fabric of space-time that Einstein predicted a century ago. And it is a ringing (pun intended) confirmation of the nature of black holes, the bottomless gravitational pits from which not even light can escape, which were the most foreboding (and unwelcome) part of his theory.
More generally, it means that scientists have finally tapped into the deepest register of physical reality, where the weirdest and wildest implications of Einstein’s universe become manifest.
Conveyed by these gravitational waves, an energy 50 times greater than that of all the stars in the universe put together vibrated a pair of L-shaped antennas in Washington state and Louisiana known as LIGO on Sept. 14.
If replicated by future experiments, that simple chirp, which rose to the note of middle C before abruptly stopping, seems destined to take its place among the great sound bites of science, ranking with Alexander Graham Bell’s “Mr. Watson — come here” and Sputnik’s first beeps from orbit.
“We are all over the moon and back,” said Gabriela González of Louisiana State University, a spokeswoman for the LIGO Scientific Collaboration, short for Laser Interferometer Gravitational-Wave Observatory. “Einstein would be very happy, I think.”
Members of the LIGO group, a worldwide team of scientists, along with scientists from a European team known as the Virgo Collaboration, published a report in Physical Review Letters on Thursday with more than 1,000 authors.
“I think this will be one of the major breakthroughs in physics for a long time,” said Szabolcs Marka, a Columbia University professor who is one of the LIGO scientists.
“Everything else in astronomy is like the eye,” he said, referring to the panoply of telescopes that have given stargazers access to more and more of the electromagnetic spectrum and the ability to peer deeper and deeper into space and time. “Finally, astronomy grew ears. We never had ears before.”
The discovery is a great triumph for three physicists — Kip Thorne of the California Institute of Technology, Rainer Weiss of the Massachusetts Institute of Technology and Ronald Drever, formerly of Caltech and now retired in Scotland — who bet their careers on the dream of measuring the most ineffable of Einstein’s notions.
“Until now, we scientists have only seen warped space-time when it’s calm,” Thorne said in an email. “It’s as though we had only seen the ocean’s surface on a calm day but had never seen it roiled in a storm, with crashing waves.”
The black holes that LIGO observed created a storm “in which the flow of time speeded, then slowed, then speeded,” he said. “A storm with space bending this way, then that.”
The chirp is also sweet vindication for the National Science Foundation, which spent about $1.1 billion over more than 40 years to build a new hotline to nature, facing down criticism that sources of gravitational waves were not plentiful or loud enough to justify the cost.
Word of LIGO’s success was met by hosannas in the scientific community, albeit with the requisite admonishments of the need for confirmation or replication.
When Einstein announced his theory in 1915, he rewrote the rules for space and time that had prevailed for more than 200 years, since the time of Newton, stipulating a static and fixed framework for the universe. Instead, Einstein said, matter and energy distort the geometry of the universe in the way a heavy sleeper causes a mattress to sag, producing the effect we call gravity.
A disturbance in the cosmos could cause space-time to stretch, collapse and even jiggle, like a mattress shaking when that sleeper rolls over, producing ripples of gravity: gravitational waves.
LIGO’s antennas are L-shaped, with perpendicular arms 2.5 miles long. Inside each arm, cocooned in layers of steel and concrete, runs the world’s largest bottle of nothing, a vacuum chamber a couple of feet wide containing 2.5 million gallons of empty space. At the end of each arm are mirrors hanging by glass threads, isolated from the bumps and shrieks of the environment better than any Rolls-Royce ever conceived.
Thus coddled, lasers can detect changes in the length of one of those arms as small as 0.0001 (one ten-thousandth) the diameter of a proton — a subatomic particle too small to be seen by even the most powerful microscopes — as a gravitational wave sweeps through.
Even with such extreme sensitivity, only the most massive and violent events out there would be loud enough to make the detectors ring. LIGO was designed to catch collisions of neutron stars, which can produce the violent flashes known as gamma ray bursts.
Black holes, the even-more-extreme remains of dead stars, could be expected to do the same, but nobody knew if they existed in pairs or how often they might collide. If they did, however, the waves from the collision would be far louder and lower pitched than those from neutron stars.
On Sept. 14, the system had barely finished being calibrated and was in what is called an engineering run at 4 a.m. when a loud signal came through at the Livingston site. “Data was streaming, and then ‘bam,’” recalled David Reitze, a Caltech professor who is the director of the LIGO Laboratory, the group that built and runs the detectors.
Seven milliseconds later, the signal hit the Hanford site. LIGO scientists later determined that the likelihood of such signals landing simultaneously by pure chance was vanishingly small. Nobody was awake, but computers tagged the event.
Reitze was on a plane to Louisiana the next day. Weiss, on vacation in Maine, found out when he checked in by computer that morning. “It was waving hello,” he said. “It was amazing. The signal was so big, I didn’t believe it.”
The frequency of the chirp was too low for neutron stars, the physicists knew. Detailed analysis of its form told a tale of Brobdingnagian activities in a far corner of the universe: The last waltz of a pair of black holes shockingly larger than astrophysicists had been expecting.
One of them was 36 times as massive as the sun, the other 29. As they approached the end, at half the speed of light, they were circling each other 250 times a second.
The ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a second, Earth time.
The signal conformed precisely to the predictions of general relativity for black holes as calculated in computer simulations, Reitze said.
Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer.
In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up. There are hopes for more in the future, in India and Japan.
Michael S. Turner, a cosmologist at the University of Chicago, noted that astronomers had once referred to the search for gravitational waves as an experiment, not an observatory. “LIGO has earned its ‘O,’” he said. “That is, it will be an observatory, getting tens of events per year.”
Turner added, “The loudest things in the gravity-wave sky are the most exotic things in the universe: black holes, neutron stars and the early universe.”
The future for the dark side looks bright.