More than a century ago, Albert Einstein predicted the existence of gravitational waves. However, its existence will not be proven until 2015. A two-kilometer-long vacuum tube was used as the detector.

 

Just as the stone is thrown into the water and there is a ripple on the water, the gravitational waves stretch and compress the space. This spatio-temporal fluctuation is caused by a cosmic catastrophe, such as an exploding star or a colliding black hole. Then, the gravitational wave propagates at the speed of light. At least this is what Einstein predicted when he proposed general relativity in 1915.

However, before 2015, the existence of mysterious gravitational waves was a purely theoretical assumption because they were very difficult to measure. Gravitational waves change the space between the blinks and pass only a small fraction of the atomic diameter. One hundred years after the prophecy was presented, the collision of two giant black holes provided direct evidence of gravitational waves.

Several kilometers of measuring equipment

These gravitational waves fluctuated in time and space for 2 billion years, and until September 14, 2015, they only slightly distorted the time and space patterns on Earth. These gravitational waves can be measured in two identical LIGO detectors (LIGO = Laser Interferometer Gravitational Wave Observatory) that are 3,000 km apart in the United States.

They consist of two vacuum tubes at an angle of 90 degrees. One is two kilometers long and the other is four kilometers long. In the place where the two sides meet, the laser beam is emitted and split into two by the beam splitter. Then, the two halves of the beam enter the two tubes separately. The energy recovery mirror ensures that the light is reflected back and forth multiple times until it reaches the beam splitter with a total distance of 1120 km. If the gravitational wave passes through the space, one arm of the interferometer will stretch and the other will contract. This results in a measurable change in the intensity of the laser beam.

Vacuum and measurement accuracy

In order to ensure that the measuring equipment can work completely trouble-free, these vacuum tubes must be at a pressure of one-tenth of a billionth of the sea level pressure. To this end, the tubes are initially heated for 30 days and then the remaining air is extracted by a high performance vacuum pump. Finally, an ion pump is used to extract the remaining gas molecules. In the ultra-high vacuum generated, no air molecules can deflect the laser beam, vibrate the mirror, and there is no dust that may cause light to diffuse.

In February 2016, after a huge amount of calculations, scientists involved in the work announced that they were actually able to measure gravitational waves and confirm Einstein's theory. With his work on this project, Rainer Weiss, Barry C. Barish and Kip Thorne won the 2017 Nobel Prize in Physics.