First observation of gravitational waves

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

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

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

Finding the waves was a very big success. Scientists had tried to find them directly for over fifty years. The waves are so small that Albert Einstein did not think they would ever be found. The wave from GW150914 was a ripple in spacetime. It changed the length of a part of the LIGO detector by a thousandth of the width of a proton. The energy the wave released was huge. 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.

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

## Gravitational waves

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

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

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

### Indirectly

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

### Directly

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

LIGO uses interferometers. Here is how they work:

- A laser beam is split in two.
- The two beams travel down long, separate paths. Then they are brought back together.
- If a gravitational wave passes by, it changes the length of the paths.
- 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.

This method is very sensitive. 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. To be sure it is a real wave, there need to be at least two detectors far apart. A real gravitational wave will be seen by both, but other shaking and noise will not.

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

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

## Infobox (astronomical event)

- **name**: GW150914
- **event type**: Gravitational wave
- **image**: Warped Space and Time Around Colliding Black Holes (Courtesy Caltech-MIT-LIGO Laboratory, produced by SXS project).webm
- **caption**: A computer video showing how space-time was bent by the two black holes. It also shows the gravitational waves that were made.
- **detected by**: LIGO
- **discovery**: about 1.4 billion years ago
 (found on 14 September 2015, 9:50:45 UTC)
- **duration**: about 200 milliseconds
- **progenitor**: 2 black holes
- **distance**: 1.4 billion Light-year, ly
- **energy**: × _c_<sup>2</sup>
- **constellation**: Southern celestial hemisphere, Southern hemisphere