Observatory · the chirp
Gravitational waves
Two black holes in orbit shake space itself, and the shaking spreads outwards at the speed of light. It carries energy away, so they spiral closer, orbit faster, shake harder, until they merge in a last fraction of a second. Turned into sound, that last stretch is a rising whoop: a chirp. On 14 September 2015 two detectors in the United States heard one for the first time, from 1.3 billion light years away. Everything below is worked out in your browser from one line.
The inspiral
Fig. I. Two black holes in their last orbits, the ripples of space, and the wave as a detector records it.
- The pair
- Masses
- Chirp mass
- Tone now
- Time
- Heard from 20 Hz for
- Peaks at
The pattern around the pair is the wave itself, the stretch of space, worked out pixel by pixel: two spiral arms, because a pair looks the same after half a turn, so the wave goes up and down twice every orbit. Each ring left the pair a little earlier, travelling out at the speed of light. The line below is what a detector records, zoomed in towards the end, where the tone rises to a few hundred hertz in a fraction of a second; then the merged hole rings like a struck bell and falls silent.
Light pairs take far longer. The two neutron stars of GW170817 sang in the detectors' band for well over a minute before they collided, and 1.7 seconds after the waves, gamma rays arrived from the same place: the same speed, after 130 million years, as the speed limit shows. Telescopes then found the glow of the debris, a kilonova, lit by freshly made heavy elements; by the models, several Earth masses of gold among them.
A ring of space
Fig. II. A ring of free particles as the wave passes, and the two arms of a detector.
A wave coming straight at you stretches space one way while it squeezes it the other, and swaps over every half cycle. A ring of free particles becomes an oval, then the other oval. A detector is two long arms at right angles with a laser bouncing along each; when one grows and the other shrinks, the light coming back no longer lines up. The drawing shows the stretch billions of billions of times too big: the real one, about one part in 1021, changes each four-kilometre arm of LIGO by roughly four billionths of a billionth of a metre, a four-hundredth of the width of a proton.
Who worked it out
Where it already matters
Before anyone heard one. In 1974 Russell Hulse and Joseph Taylor found a pulsar in orbit with another neutron star. Over the years the orbit shrank exactly as much as the waves it must be giving off should make it shrink: the first proof that they exist, and the Nobel prize of 1993. The 2017 prize went to Rainer Weiss, Barry Barish and Kip Thorne for hearing them directly.
A catalogue of collisions. The detectors in the United States, Italy and Japan have since caught black holes and neutron stars merging by the hundreds; the third catalogue alone, in 2021, held 90. Each one weighs its pair from the chirp, and tells how far away it is from how loud it was, without needing anything else: a ruler for the universe.
The slow hum. In 2023 pulsar timing arrays, which watch dozens of pulsars like clocks across the galaxy, reported evidence for a background of waves years long, most likely from supermassive black hole pairs in the centres of galaxies. A space detector, LISA, is planned to listen in between, from the 2030s.