Gravitational waves

Throw a pebble into a pond and ripples spread across the water. Something like that happens to space itself.

When very heavy things move violently, they send out ripples that stretch and squeeze space as they pass. These are called gravitational waves. They travel at the speed of light.

Albert Einstein worked out that they should exist, a hundred years before anyone caught one. He doubted that anyone ever would, because by the time a wave has crossed space to reach us it is unbelievably faint.

On 14 September 2015 two machines in the United States felt one for the first time. It came from two black holes that had circled each other, closer and closer, and then joined into one. That happened 1.3 billion light years away, so the ripple had been travelling for 1.3 billion years.

Everything people knew about the stars before then came from light. Gravitational waves are a second way to find out what is out there, and they can tell us about things that give out no light at all.

In Einstein’s theory, gravity is the bending of space and time by mass. If the mass moves, the bending has to change, and the change cannot reach everywhere at once. It spreads outwards at the speed of light as a wave. As the wave passes, it stretches space a little in one direction and squeezes it in the direction across, then the other way about, over and over.

What makes them. Any mass that speeds up, slows down or changes direction makes gravitational waves. A spinning ice skater does. But space is very stiff, and only the most violent events shake it enough to measure: two black holes or two neutron stars whirling round each other, or a star exploding.

The slow spiral. Two heavy bodies in orbit send out waves all the time. The waves carry energy away, so the two draw closer. Closer means faster, and faster means stronger waves, so the whole thing runs away with itself. For millions of years almost nothing seems to happen. The last few turns take a fraction of a second.

The first sign of this came long before any wave was caught. In the 1970s Joseph Taylor and Russell Hulse watched a pulsar in orbit with another dense star and found the two drawing closer by just the amount the theory said waves should cause. That was indirect proof. It won the Nobel Prize in Physics in 1993.

The first one caught. These are the measurements from 14 September 2015.

What happened	Two black holes merged
Their masses	About 36 and 29 times the Sun’s
The black hole they made	About 62 times the Sun’s
Turned into waves	About 3 Suns’ worth of mass, in a few tenths of a second
Distance	About 1.3 billion light years (410 megaparsecs, though it could be 180 less or 160 more)
The signal	Rose from 35 to 250 vibrations a second, then stopped

36 and 29 make 65, and the black hole left behind has 62. The missing 3 Suns of mass became the energy of the waves. For that short moment the merger was putting out many times more power than all the stars in the visible universe give out as light.

The vibrations happen to fall in the range the ear can hear. Played through a speaker, the signal is a short rising note, and so it is called a chirp.

How faint it was. Like light, the waves weaken as they spread. When this one reached the Earth, it changed lengths by one part in a thousand million million million. Over a 4 km arm that is, by our sum, 4 millionths of a millionth of a millionth of a metre, hundreds of times smaller than a proton. The Nobel committee compared it to measuring the distance to a star ten light years away to within the width of a hair.

How the detector works. The detector, LIGO, is shaped like a letter L, with two arms each 4 km long and a mirror hanging at each end. A laser beam is split in two at the corner and sent down both arms. The beams bounce back and meet again. The lengths are set so that the two returning beams cancel each other exactly and no light comes out. A passing wave stretches one arm while it squeezes the other. The beams then no longer cancel, and a little light gets through to a sensor. An instrument that compares two beams of light like this is called an interferometer.

There are two of these, about 3,000 km apart, one at Hanford in the north-west of the United States and one at Livingston in Louisiana. A lorry on a distant road or a falling leaf can shake a mirror. A real wave shows up in both places. The 2015 signal reached Livingston first and Hanford 7 thousandths of a second later, nearly the same in each.

Waves and light together. On 17 August 2017 a different kind of signal arrived, from two neutron stars spiralling together about 130 million light years away (40 megaparsecs). A burst of gamma rays from the same part of the sky was picked up 1.7 seconds after the waves. Within 11 hours telescopes had found a new point of light in a galaxy called NGC 4993, and over the following days and weeks it was watched in ultraviolet, visible light, infrared, X-rays and radio.

It was the first event seen both ways. The waves gave the masses and the distance, and the light let astronomers watch what followed. Collisions like this make some of the heavy elements, gold and platinum among them. And the waves and the gamma rays, after travelling for 130 million years, arrived less than two seconds apart, which shows how closely the speed of the waves matches the speed of light.

What they are good for. Black holes give out no light. Before 2015 nobody had direct evidence that two could orbit each other and merge, or that black holes of 30 to 60 Suns existed at all. Gravitational waves are a second sense, and most of what it will find is still ahead.

What the 3D view shows. The view of the cosmic web draws more than 43,000 real galaxies. Nearly all of them lie within about a billion light years, and a few of the brightest are drawn out to more than two billion. The 2017 neutron stars met about 130 million light years away, deep inside the crowded part of that map. The 2015 black holes were about ten times further off, at 1.3 billion light years, out where the map has thinned to a scattering of the brightest galaxies. The view does not mark either place, and no wave is drawn. It is there to show how much space the ripples crossed.