Legacy · Direct Prediction
Gravitational-Wave Astronomy
Since 2015, humanity has had a second way to observe the universe: not by its light, but by the trembling of spacetime itself.
A New Sense
Everything astronomers knew about the universe before 2015 came from light of one kind or another, plus a few particles. Gravitational waves are different in kind. They are not carried through space; they are vibrations of space. They pass through gas, dust, and whole galaxies almost undisturbed, and they are produced most strongly by events that may give off no light at all.
The Instrument
The Laser Interferometer Gravitational-Wave Observatory consists of two detectors 3,000 kilometers apart, in Hanford, Washington, and Livingston, Louisiana. Each sends laser light down two perpendicular arms four kilometers long and compares the returning beams. A passing wave lengthens one arm and shortens the other.
The change is absurdly small. LIGO is designed to detect a shift in arm length of about one ten-thousandth of the width of a proton. LIGO's own comparison is measuring the distance to the nearest star to within the width of a human hair.
The First Signal
On 14 September 2015 both detectors recorded a brief rising chirp. Two black holes, roughly 36 and 29 times as massive as the Sun, had orbited each other faster and faster and merged, about 1.3 billion light-years away. For a fraction of a second the event radiated more power than all the stars in the observable universe combined.
It was the first direct detection of gravitational waves, the first observation of two black holes merging, and a confirmation of general relativity under the most violent conditions ever tested. The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish, and Kip Thorne.
Gravity and Light Together
On 17 August 2017 the detectors, now joined by the Virgo instrument in Italy, caught a longer signal from two neutron stars spiraling together 130 million light-years away. About two seconds later, satellites recorded a burst of gamma rays from the same patch of sky. Some 70 observatories on the ground and in space turned to look, and found the fading glow of the explosion.
That one event showed that such collisions forge heavy elements, including gold and platinum, and confirmed that gravitational waves travel at the speed of light to within a tiny margin. It was the beginning of what astronomers call multi-messenger astronomy.
Since Then
Detections are now routine, with hundreds of mergers cataloged. They have revealed a population of black holes heavier than anyone expected, and are being used to make an independent measurement of how fast the universe is expanding.
How Direct Is the Link?
Direct. The waves are a prediction of Einstein's equations, and the shape of every signal is compared against solutions of those equations. He would have been astonished: he thought the effect too small ever to be measured.