gravitational-wave detection (LIGO)
On 14 September 2015 the two LIGO observatories in the United States caught a faint shudder in spacetime, the first ever direct detection of gravitational waves. It came from two black holes, each about thirty times the Sun's mass, spiralling together and merging more than a billion light-years away. In the final fraction of a second they radiated more power as gravitational waves than all the stars in the visible universe shine as light combined.
LIGO works by splitting a laser beam down two long arms set at right angles, each four kilometres long, and bouncing the light back to compare the two paths. A passing wave stretches one arm and squeezes the other by an unimaginably small amount, shifting the laser light just enough to register. The required precision is like measuring the distance to a nearby star to within the width of a human hair, which is why it took decades of engineering to reach.
The detection mattered far beyond confirming Einstein once more. It opened an entirely new way to observe the universe, one that does not rely on light at all. Telescopes see electromagnetic waves; gravitational-wave detectors feel the shaking of spacetime, letting us 'hear' violent events like black hole and neutron star collisions that emit little or no light, and revealing parts of the cosmos that were previously invisible.
In the first detection, about three solar masses of energy left as gravitational waves in a fraction of a second.
Indirect evidence existed since the 1970s from binary pulsars, but 2015 was the first direct catch. Detectors do not 'see' an image; they record a waveform, and locating the source on the sky needs several detectors working together.