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The black-hole collision that reshaped physics

A momentous signal from space has confirmed decades of theorizing on black holes — and launched a new era of gravitational-wave astronomy.

The event was catastrophic on a cosmic scale — a merger of black holes that violently shook the surrounding fabric of space and time, and sent a blast of　space-time vibrations known as gravitational waves　rippling across the Universe at the speed of light.

But it was the kind of calamity that physicists on Earth had been waiting for. On 14 September, when those ripples swept across the freshly upgraded　Laser Interferometer Gravitational-Wave Observatory　(Advanced LIGO), they showed up as spikes in the readings from its two L-shaped detectors in Louisiana and Washington state. For the first time ever,　scientists had recorded a gravitational-wave signal.

Gravitational waves: How LIGO forged the path to victory

“There it was!” says LIGO team member Daniel Holz, an astrophysicist at the University of Chicago in Illinois. “And it was so strong, and so beautiful, in both detectors.” Although the shape of the signal looked familiar from the theory, Holz says, “it's completely different when you see something in the data. It's this transcendent moment”.

The signal, formally designated GW150914 after the date of its occurrence and informally known to its discoverers as 'the Event', has justly been hailed as a milestone in physics. It has provided a wealth of evidence for　Albert Einstein's century-old general theory of relativity, which holds that mass and energy can warp space-time, and that gravity is the result of such warping. Stuart Shapiro, a specialist in computer simulations of relativity at the University of Illinois at Urbana–Champaign, calls it “the most significant confirmation of the general theory of relativity since its inception”.

But the Event also marks the start of a long-promised era of gravitational-wave astronomy. Detailed analysis of the signal has already yielded insights into the nature of the black holes that merged, and how they formed. With more events such as these — the LIGO team is analysing several other candidate events captured during the detectors' four-month run, which ended in January — researchers will be able to classify and understand the origins of black holes, just as they are doing with stars.

Still more events should appear starting in September, when Advanced LIGO is scheduled to begin joint observations with its European counterpart, the Franco–Italian-led Advanced Virgo facility near Pisa, Italy. (The two collaborations already pool data and publish papers together.) This detector will not only contribute crucial details to events, but could also help astronomers to make cosmological-distance measurements more accurately than before.

“It's going to be a really good ride for the next few years,” says Bruce Allen, managing director of the Max Planck Institute for Gravitational Physics in Hanover, Germany.

3D simulations of colliding black holes hailed as most realistic yet

“The more black holes they see whacking into each other, the more fun it will be,” says Roger Penrose, a theoretical physicist and mathematician at the University of Oxford, UK, whose work in the 1960s helped to lay the foundation for the theory of the objects. “Suddenly, we have a new way of looking at the Universe.”

A matter of energy

Physicists have known for decades that every pair of orbiting bodies is a source of gravitational waves. With each revolution, according to Einstein's equations, the waves will carry away a tiny fraction of their orbital energy. This will cause the objects to move a bit closer together and orbit a little faster. For familiar pairs, such as the Moon and Earth, such energy loss is imperceptible even on timescales of billions of years.

But dense objects in very close orbits can lose energy much more quickly. In 1974, radio astronomers Russell Hulse and Joseph Taylor, then of the University of Massachusetts Amherst,found just such a system: a pair of dense neutron stars in orbit around each other. As the years went by, the scientists found that this 'binary pulsar' was losing energy and spiralling inwards exactly as predicted by Einstein's theory.

The two black holes detected by LIGO had probably been losing energy in this way for millions, if not billions, of years before they reached the end. But LIGO did not register the gravitational waves coming from them until 9:50:45 Coordinated Universal Time on 14 September, when the wave's frequency rose above some 30 cycles per second (hertz) — corresponding to 15 full black-hole orbits per second — and was finally high enough for the detectors to distinguish it from background noise.

Gravitational waves: a three-minute guide

But then, in just 0.2 seconds, LIGO watched the signal surge to 250 hertz and suddenly disappear, as the black holes made their final 5 orbits, reached orbital velocities of half the speed of light and coalesced into a single massive object (see　'What made the wave').