What are these famous gravitational waves?
They are spacetime distortions or, explaining this enigmatic definition further, they are waves that ripple across the universe. Gravitational waves are manifested in local compression and expansion of the component parts of the universe. Much like the waves in an oscillating spring or ripples propagating on the surface of the water after a stone was thrown in, gravitational waves propagate in the universe and have an effect both on us as well as on our environment, and globally, on our planet.
What causes these waves?
Well, not unlike in the case of laser where light is emitted (electromagnetic wave) as a result of a change in the direction of elementary particles, gravitational waves are caused by a change in the motion of gargantuan objects such as black holes. For example, when a black hole slows down, it releases energy, which causes gravitational waves.
What does Einstein have to do with it?
Einstein predicted such a phenomenon over one hundred years ago. It intrigued physicists for years but no technology was available that would allow them to detect it. The source of gravitational waves may be billions of light-years away from Earth (luckily, there are no black holes in our proximity as this might mean an imminent end of the planet). These waves dampen too and once they reach Earth they are capable of atomic scale displacements of matter.
How do you measure waves?
In 1970s, Reiner Weiss of MIT started analyzing interference that could hinder the detection of gravitational waves. He designed a measurement system based on the phenomenon of interference. LIGO (Laser Interferometer Gravitational-Wave Observatory) experiments use just this interferometer that is an instrument to measure interference of electromagnetic waves generated with lasers. The interferometer is in the shape of the letter L. Light is generated at the point of the intersection of the two arms with light beams traveling along each arm. At the end of each arm, there is a mirror to reflect the light beams. If no gravitational waves are present, the light travels the same distance along both arms and it is dampened to zero. However, when a gravitational wave ripples through Earth, it hits the arms with a microscopic delay, which causes a displacement in the electromagnetic wave (light beam), which is detected by the interferometer.
However, to actually measure gravitational waves, another 40 years of intense scientific and engineering effort had to follow. On the road to make this great dream of theirs come true, they had to tackle a number of challenges. One of them was the development of an optic system that would ensure the stability of the light beam. In addition, the entire system needed to be well isolated from any environmental interferences (a truck passing by or seismic activity could obstruct measurements) yet sensitive enough to detect mirror displacement caused by gravitational wave propagation. Eventually, new materials had to be developed, one of the world's largest installation with vacuum had to be constructed, and algorithms for the filtration of interferences or for data analysis exploiting machine learning techniques had to be designed.
A breakthrough moment
As it usually happens, one experiment is not enough. To verify measurements, a twin experiment needed to be created that should also detect spacetime distortions with a certain delay. Thus, a LIGO observatory in the West coast (Hanford, in Washington state) and another one in the East coast (Livingston, in Louisiana), 3 000 km apart from each other, were built.
Finally, on September 14 2015, gravitational waves rippling through Earth were detected. The Livingston laboratory detected them first and 7 milliseconds later (gravitational waves travel at the speed of light), they could be 'heard'. 'Heard' because the frequency of the waves is within the range of human hearing. Try it here.
Einstein was right
This discovery confirmed calculations made by Einstein over one hundred years before. Even though at the time, he himself doubted whether gravitational waves really existed or whether they were just a mathematical artifact.
Gravitational waves are another tool for space exploration, especially of phenomena related to large-scale objects. It may well be, as Barry Barish suggests in the CERN Courier, that they will help us better understand how the universe began or the Big Bang theory. I will talk again about the cooperation of scientists and engineers in my next blog post.
In writing this post, I refered to information available from the Noble Prize website, Wolfram Blog and CERN Curier.
