Observation of Gravitational Waves from a Binary Black Hole Merger
Two black holes collided a billion years ago, and we heard spacetime itself ring.
Scientists directly detected gravitational waves for the first time — ripples in spacetime set off when two black holes crashed together over a billion years ago.
The idea, unpacked
A century ago, Einstein predicted that violent events in space — like two black holes colliding — should send ripples through spacetime itself, stretching and squashing space as they spread outward, like waves on a pond. But the ripples reaching Earth would be fantastically faint, and for a hundred years no one could measure something so small.
Where it came from
LIGO is the machine built to catch them: two enormous L-shaped detectors with arms four kilometres long, using laser beams to measure those arms' lengths to almost impossible precision. When a gravitational wave rolls past, it lengthens one arm and shortens the other by a hair's breadth — far less, actually, than the width of an atom. On 14 September 2015, both detectors, far apart, twitched in exactly the same way at almost the same instant. They had felt the tremor from two black holes merging more than a billion light-years away — a century, almost to the year, after Einstein's prediction.
Why it mattered
This confirmed the last untested major prediction of Einstein's general relativity, a hundred years after he made it. More than that, it gave astronomy a brand-new sense: until now we'd only ever “seen” the universe with light. Now we can “hear” it through gravity, letting us detect things — like colliding black holes — that give off no light at all.
Why it's called a “chirp”
As the two black holes spiral closer, they whirl around each other faster and faster, and the gravitational wave they send out climbs in pitch and volume — exactly like a bird's rising chirp. Turn the signal into sound and that's what you hear: a swoop upward, then a thump as they merge. How fast the pitch rises gives away how heavy the black holes are. Set their masses below and watch the chirp change shape.
What came next
GW150914 was just the first. LIGO and its partner detectors have now caught dozens more black-hole collisions, and in 2017 they heard two neutron stars merge — an event telescopes also saw as a burst of light, revealing it as a cosmic forge where gold and platinum are made. Each detection is a note in a kind of music from the dark side of the universe, the part that gives off no light at all.
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal.
This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.