TECH PRIOR

A team of scientists announced on Thursday that they had heard and recorded the sound of two black holes colliding a billion light-years away, a fleeting chirp that fulfilled the last prediction of Einstein’s general theory of relativity.

Einstein on 1915

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.

Einstein was not quite sure about these waves. In 1916, he told Karl Schwarzschild, the discoverer of black holes, that gravitational waves did not exist, then said they did. In 1936, he and his assistant Nathan Rosen set out to publish a paper debunking the idea before doing the same flip-flop again.

According to the equations physicists have settled on, gravitational waves would compress space in one direction and stretch it in another as they traveled outward.

Hearing a Gravitational Wave 

As Predicted by Einstein’s general theory of relativity 100 years ago, gravitational waves have been directly detected for the first time. LIGO, the Laser Interferometer Gravitational-Wave Observatory, heard black holes colliding.

1. TWO BLACK HOLES
   About 1.2 billion years ago in a distant galaxy, a pair of black holes circled each other. The larger black hole was 36 times the mass of our sun, and the smaller one 29 times.
2. COLLISION
   The intense gravity accelerated the black holes to half the speed of light, pulling them closer and carving distortions in space and time. In a fraction of a second, the pair collided and merged into an irregular shape.
3. RING DOWN
   The unstable blob smoothed into a sphere, a process called ring down. Three solar masses’ worth of energy were vaporized in a storm of gravitational waves, distorting space and time and leaving a new black hole 62 times the mass of the sun.
4. GRAVITATIONAL WAVES
   The invisible waves rippled outward at the speed of light. But waves fade with distance, and when they finally reached Earth, the distortions were too small to be measured above the heat, noise and other vibrations of our planet. 
5. DETECTION
   LIGO is a pair of L-shaped observatories 1,900 miles apart. Ultra-pure mirrors at the ends of each arm are isolated from vibrations. Passing gravitational waves push and pull the arms, changing the length of tunnels by less than the width of a proton. 
6. A CHIRP
   On Sept. 14, LIGO’s detectors measured their first vibrations from a gravitational wave. Translated to sound, it was a short chirp, the billion-year-old echo of the collision of those two black holes.

Summary

Astronomers now know that pairs of black holes do exist in the universe, and they are rushing to explain how they got so big. According to Vicky Kalogera of Northwestern University, there are two contenders right now: Earlier in the universe, stars lacking elements heavier than helium could have grown to galumphing sizes and then collapsed straight into black holes without the fireworks of a supernova explosion, the method by which other stars say goodbye. Or it could be that in the dense gatherings of stars known as globular clusters, black holes sink to the center and merge.

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.”

Source: LIGO, Caltech, M.I.T., Simulating eXtreme Spacetimes project By Jonathan Corum