# Gravitational waves (LIGO)

### Gravitational waves (LIGO)

Massive accelerating or orbiting bodies cause ripples in spacetime. These are called gravitational waves.

Physics

Keywords

gravitational wave, LIGO observatory, black hole, gravitation, interference, laser beam, spacetime, Einstein, gravity, theory of relativity, relativity, neutron star, wave, interphase, speed of light, astronomy, astrophysics, observatory, Newton, wavelength, light, physics

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## The LIGO network

The LIGO's measuring instruments are so sensitive that they can detect even the weakest terrestrial vibrations, which may be caused by earthquakes, ocean waves crashing on shores, explosions, etc. Therefore, in order to distinguish true, cosmic signals, from terrestrial noises, it is necessary to compare the signals detected by several observatories located thousands of kilometers away from each other.

Another benefit of performing parallel measurements in several observatories is that it makes it possible to determine the direction of the source of cosmic signals based on the time difference between the detection of signals.

Initially, there were only two LIGO observatories, one in Louisiana, the other in Washington state. To determine the direction of the source of signals accurately, these two observatories were not sufficient. In 2017, the Virgo observatory in Italy joined the LIGO project, and thereby more accurate measurement of the direction of signals became possible.

## How do measurements work?

The operation of LIGO (Laser Interferometer Gravitational Wave Observatory) is based on the fact that lengths change for a brief moment when a gravitational wave passes through a point in space. Lengths stretch in one direction, and shrink in the other direction.

The change in the lengths of the two arms of LIGO is measured with the help of an interferometer. During its operation, a laser beam is split into two identical beams by a semi-transparent mirror. The two beams travel down the two perpendicular arms. When they reach the mirrors at the end of the arms, they are reflected back towards the splitter mirror, where they merge into a single beam. Depending on the difference between the paths the two beams travel, they either intensify or weaken each other. If the difference between the paths they cover is exactly half the wavelength, they cancel each other out, while if the difference is equal to the wavelength, the amplitude of the resulting beam is double of that of the original beam.

This method can be used to show path length differences that are in the range of the wavelength of light. The longer the path the light beams travel, the greater the sensitivity of the instrument. In the Washington and Louisiana interferometers, the laser beams travel in 4-km-long (2.49-mi-long) arms. A gravitational wave passing through the arms changes this length only by a thousandth of the diameter of a proton, but the instrument is able to detect even this small change.

## The merger of black holes

The LIGO’s first verified detection took place on September 14, 2015. The two observatories detected similar signals, and the waveform purified from noise matched exactly the waveform predicted by computer models, which occurs when two black holes, with 36 and 29 times the mass of our Sun respectively, merge.

The detection lasted only 0.2 seconds. The signals clearly showed how the intensity and the frequency of the gravitational waves increased as the two black holes orbited each other with a gradually shortening radius until eventually they merged and the signals faded.

Merging black holes are the most powerful energy sources in the universe known to science so far. The power of the gravitational waves peaked at about 3.6×10⁴⁹ Watts, which means that it was 50 times greater than the combined power of all light radiated by all the stars in the observable universe.

By comparing the strength of the detected signals and the predicted signals, scientists could estimate the distance of the source of the waves. In this case, the distance was about 1.4 billion light years, that is, the event detected by the instrument occurred about 1.4 billion years ago, and its gravitational waves reached the Earth only now.

### Narration

In Newtonian physics, the structure of space does not change, regardless of what physical phenomenon occurs in it. Therefore absolute time and space exist independently of any perceiver. According to Einstein's theory of general relativity, however, gravity is the curvature of spacetime caused by massive bodies. Consequently, absolute space does not exist, bodies change the space around themselves.

Massive accelerating or orbiting bodies cause ripples in spacetime, which travel at the speed of light. These are called gravitational waves.
When a gravitational wave passes through a certain point in space, it causes lengths there to shorten for a very short time, then to lengthen. These changes are very small, their detection was impossible before LIGO.

The strongest gravitational waves are formed when black holes merge. In such a case, the energy released in the form of gravitational waves is enormous, it is the most intense release of energy in the Universe known to science so far.

The LIGO's measuring instruments are so sensitive that they can detect even the weakest terrestrial vibrations, which may be caused by earthquakes, ocean waves crashing on shores, explosions, etc. Therefore, in order to distinguish true cosmic signals from terrestrial noises, it is necessary to compare the signals detected by several observatories located thousands of kilometers away from each other.
Initially, there were only two LIGO observatories, one in Louisiana, the other in Washington state. In 2017, the Virgo observatory in Italy joined the LIGO project too.

The central building houses the most important parts of the observatory: the laser unit, the mirrors, the detectors and computers for processing signals. There are two 4-km-long (2.49-mi-long) arms, perpendicular to each other, extending from the central building, encasing two vacuum tubes, in which the laser beams travel. At the end of each tube there is a mirror.

The operation of LIGO is based on the fact that lengths change for a brief moment when a gravitational wave passes through a point in space. Lengths stretch in one direction, and shrink in the other direction. The change in the lengths of the two arms of LIGO is measured with the help of an interferometer. During its operation, a laser beam is split into two identical beams by a semi-transparent mirror. The two beams travel down the two perpendicular arms. When they reach the mirrors at the end of the arms, they are reflected back towards the splitter mirror, where they merge into a single beam. Depending on the difference between the paths the two beams travel, they either intensify or weaken each other.
This method can be used to show path length differences that are in the range of the wavelength of light. The longer the path the light beams travel, the greater the sensitivity of the instrument. A gravitational wave passing through the 4-km-long (2.49-mi-long) arms of the observatory changes their length only by a thousandth of the diameter of a proton, but the instrument is able to detect even this small change.

The LIGO’s first verified detection took place on September 14, 2015. The detected signals were emitted during the merger of two black holes, with 36 and 29 times the mass of our Sun respectively. The detection lasted only 0.2 seconds. The signals clearly showed how the intensity and the frequency of the gravitational waves increased as the two black holes orbited each other with a gradually shortening radius until eventually they merged and the signals faded.
By comparing the strength of the detected signals and the predicted signals, scientists could estimate that the distance of the source of the waves was about 1.4 billion light years, that is, the event detected by the instrument occurred about 1.4 billion years ago, and its gravitational waves reached the Earth only now.

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