Signs of black holes merging arrive a century after Albert Einstein predicted them
TWO black holes circle one another. Both are about 100km across. One contains 36 times as much mass as the sun; the other, 29. They are locked in an orbital dance, a kilometre or so apart, that is accelerating rapidly to within a whisker of the speed of light. Their event horizons—the spheres defining their points-of-no-return—touch. There is a violent wobble as, for an instant, quintillions upon quintillions of kilograms redistribute themselves. Then there is calm. In under a second, a larger black hole has been born.
It is, however, a hole that is less than the sum of its parts. Three suns’ worth of mass has been turned into energy, in the form of gravitational waves: travelling ripples that stretch and compress space, and thereby all in their path. During the merger’s final fifth of a second, envisaged in an artist’s impression above, the coalescing holes pumped 50 times more energy into space this way than the whole of the rest of the universe emitted in light, radio waves, X-rays and gamma rays combined.
And then, 1.3 billion years later, in September 2015, on a small planet orbiting an unregarded yellow sun, at facilities known to the planet’s inhabitants as the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), the faintest slice of those waves was caught. That slice, called GW150914 by LIGO’s masters and announced to the world on February 11th, is the first gravitational wave to be detected directly by human scientists. It is a triumph that has been a century in the making, opening a new window onto the universe and giving researchers a means to peer at hitherto inaccessible happenings, perhaps as far back in time as the Big Bang.
Finger on the pulsar
The idea of gravitational waves emerged from the general theory of relativity, Albert Einstein’s fundamental exposition of gravity, unveiled almost exactly 100 years before GW150914’s discovery. Mass, Einstein realised, deforms the space and time around itself. Gravity is the effect of this, the behaviour of objects dutifully moving along the curves of mass-warped spacetime. It is a simple idea, but the equations that give it mathematical heft are damnably hard to solve. Only by making certain approximations can solutions be found. And one such approximation led Einstein to an odd prediction: any accelerating mass should make ripples in spacetime.
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