Black Holes and Gravitational Waves

Alexander Marks

On the 14th August 2017, the fourth set of gravitational waves were detected. Although the first waves were recorded in early 2016 by LIGO (Laser Interferometer Gravitational-Wave Observation), this time three different observatories detected the gravitational waves. A pair of black holes caused these waves by violently merging together.

Three Scientists at LIGO, Rainer Weiss, Kip Thorne, and Dr Barry Barish have just been awarded the Noble Prize in Physics, for the first detection of gravitational waves and it was these three scientists who designed and ran the two LIGO observatories, situated in Washington and in Louisiana. In the most recent detections a new observatory in Italy called Virgo also measured the same waves.

Why are three detection’s better than two? Three detection’s allow scientists to better pin point the origins of the signals, 20 times more precisely than just two. This is key for follow up observations. It also provides more information about the object that made them, such as the angle they are tilted at compared to Earth.

Gravitational waves where first predicted by Einstein’s theory of relativity back in 1916. This theory was ground breaking and combines space and time to form the space-time continuum. His theory states that any object with mass warps the space-time continuum, the more massive the object the bigger the warp. It is these warps in space-time that cause gravity.

The famous equation of general relativity is incredibly hard to solve, and requires super computers to find solutions. One of the solutions predicts gravitational waves.

Gravitational waves are caused by all objects as they move through the space-time continuum.  Every object makes gravitational waves, meaning that even a tiny snail moving in the grass produces them. They ripple through space-time much like ripples caused by throwing stone in a still pond.  Gravitational waves were the last part of Einstein’s theory to be proven.

The equation predicts that gravitational waves would travel at the speed of light and carry information of the objects causing them. But most of the gravitational waves are too weak to be measured. It requires a massive object to create the large enough ripples in space time to be measured.

Enter black holes and neutron stars. Black holes are the most massive objects in the known universe. Their mass is so large that light cannot escape their gravity. When two black holes orbit very quickly around each other and eventually merge, they create immense distortions in the space-time that can be measured on Earth.

By measuring the gravitational waves and using Einstein’s theory of relativity, scientist can learn a lot about the darkest parts of our universe. Scientists can predict the mass, rotation and how powerful the event was.

Neutron stars are the remains of stars that have collapsed in on themselves, and are also very massive and could theoretically be detected as well. Yet, there has been no detection of gravitational waves caused by them but, there is promise that these will soon be detected as well.

Even the largest ripples in space time are very difficult to measure. LIGO and Virgo are carefully designed to detect these ripples. Each of the observatories is shaped like an L. Each arm of the L is a long tunnel that are vacuum sealed. At the end of each tunnel there is a mirror, and a split mirror where the two meet. (A split mirror can split laser light in two, and send it in different directions).

Lasers are sent down the tunnels at the same time, without the presence of gravitational waves both lasers would return at the same time. When gravitational waves are present the space-time is warped in such a way that one mirror gets closer and the other gets further away. This results in the laser beams returning at different times, allowing scientists to measure the amount the mirrors were warped. This measurement is very small, about 1000 times smaller than a proton.

This means the bigger the gravitational waves the larger the time gap between the lasers returning. As the time gaps are so small, only very massive object can produce waves big enough to be detected.

The black holes that created the most recently detected gravitational waves had masses of 25 and 31 times the mass of our sun. They were orbiting each other 1.8 billion light years away and merged into a black hole of 53 times the mass of our sun. This is a supermassive black hole, and is bigger than ever expect to be found.

This is the third black hole to be bigger than expected. Black holes of this size appear be more common than originally thought and the rate at which they occur will soon be figured out.

The observatories are currently being upgraded and will become even more sensitive. Scientist hope that when they are turned back on in Autumn 2018 they will detect up to ten of these events each year. There is also hope of detecting gravitational waves from neutron stars as well.

With observatories planned in Japan and India, it can be expected to find new phenomena occurring in the universe that may have been thought impossible.

#Space #AlexanderMarks

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