First observation of gravitational waves

The first direct observation of gravitational waves was on September 14, 2015.^[2] The LIGO and Virgo groups announced it on February 11, 2016.^[2][5] Before this, scientists had only seen the effects of gravitational waves indirectly.^[2] They saw these effects in how some stars, called pulsars, moved.^[2]

The wave that was found came from two black holes.^[2] The black holes were spiraling inward and joined together.^[2] The signal was seen by both of the LIGO detectors.^[6] It looked just like what general relativity said it would look like.^[7][8][9] The signal was named GW150914.^[10] The name comes from "Gravitational Wave" and the date it was found, 2015-09-14.^[2]

This was also the first time a binary black hole (two black holes orbiting each other) was seen merging.^[2] It proved that systems with two black holes of that size exist.^[2] It also proved that they can merge in the lifetime of the universe.^[2]

Finding the waves was a very big success.^[2] Scientists had tried to find them directly for over fifty years.^[11] The waves are so small that Albert Einstein did not think they would ever be found.^[11] The wave from GW150914 was a ripple in spacetime.^[10] It changed the length of a part of the LIGO detector by a thousandth of the width of a proton.^[10][12] The energy the wave released was huge.^[2] For a few moments, the power of the wave was greater than all the light from all the stars in the known universe put together.^[2][13]

This discovery was the last time a prediction of general relativity was proven right after not being directly seen.^[2] It began a new type of astronomy called gravitational-wave astronomy.^[14] This new type of astronomy lets scientists see events in space that were impossible to see before.^[15] It might let them see the very beginning of the universe.^[16]

Albert Einstein first predicted gravitational waves in 1916.^[11] He came up with the idea from his theory of general relativity.^[11] General relativity says that gravity is caused by mass bending spacetime.^[11] When masses move or speed up, they can make ripples in spacetime.^[11] These ripples travel out from the source at the speed of light.^[11]

Einstein thought this was interesting, but he knew the ripples would be much too small to find with the technology of his time.^[11] When two objects in orbit, like stars or black holes, give off gravitational waves, they lose energy.^[17] This makes them slowly spiral closer to each other.^[17] This effect is also usually very small.^[17]

The waves are strongest when two very dense objects, like neutron stars or black holes, merge.^[18] In the last moments before they join, a large part of their mass can be changed into gravitational energy.^[18] This makes the waves easier to find.^[18]

The first proof of gravitational waves was found in 1974.^[19] It came from studying a pair of stars called PSR B1913+16.^[19] In this pair, one star is a pulsar.^[19] A pulsar sends out radio waves at very regular times.^[19] Scientists Russell Hulse and Joseph Taylor saw that the time between pulses got shorter over the years.^[19] This meant the stars were spiraling toward each other.^[19] The amount of energy they were losing matched what Einstein's theory said would be lost to gravitational waves.^[19] Hulse and Taylor won the 1993 Nobel Prize in Physics for this work.^[20]

Finding gravitational waves directly was hard because the effect is so small.^[2] In the 1960s, a method called interferometry was suggested.^[21] The technology got better, and it became possible.^[21]

LIGO uses interferometers. Here is how they work:

 * A laser beam is split in two.[22]
 * The two beams travel down long, separate paths. Then they are brought back together.[22]
 * If a gravitational wave passes by, it changes the length of the paths.[22]
 * This change causes the two light beams to no longer line up perfectly when they come back together. This creates a pattern that can be measured.[22]

This method is very sensitive.^[22] An interferometer with arms that are 4 km long can find a change in spacetime that is a tiny piece of the size of a proton.^[2] To be sure it is a real wave, there need to be at least two detectors far apart.^[22] A real gravitational wave will be seen by both, but other shaking and noise will not.^[22]

The Laser Interferometer Gravitational-Wave Observatory (LIGO) project was started in 1992.^[23] LIGO has two observatories that work together.^[24] One is in Livingston, Louisiana, and the other is at the Hanford Site in Washington.^[24] They are 3,002 km (1,865 mi) apart.^[24] The first LIGO search from 2002 to 2010 did not find any gravitational waves.^[25] After that, the detectors were shut down and made much better.^[26] The new, better version was called "Advanced LIGO."^[26]

On September 14, 2015, the new detectors were being tested.^[27] At that time, the instruments saw a possible gravitational wave.^[27] This event was named GW150914.^[28]