This February 11th was a great day for science! Not only was it the UN International Day for Women and Girl’s in Science, but it was also the day that a team of scientists announced that they had recorded the sound of two black holes colliding billions of years ago.
This sound bite (which you can listen to here) is not only an amazing demonstration of today’s technology (more on that later), it also confirms Einstein’s theory of General Relativity which was created over one hundred years ago.
The Theory of General Relativity describes gravity geometrically as a property of space-time. Space-time refers to a method of defining our universe in four dimensions, three dimensions of space with the fourth dimension being time. Einstein’s theory, suggests that large, massive objects curve space-time and that these curves are where gravity comes from. Accordingly, large acceleration of such massive objects could cause space-time to “stretch, collapse and even jiggle, like a mattress shaking when that sleeper rolls over, producing ripples of gravity: gravitational waves”. These gravitational waves, according to equations developed using Einstein’s theory, would squeeze space in one direction while stretching it in another as the waves passed through . This means that gravitational waves have the potential to be observable.
This potential prompted the design and construction of the LIGO, the Laser Interferometer Gravitational-Wave Observatory, in the early 2000s. Recently the LIGO has undergone renovation to increase its sensitivity. The LIGO is made up of two L-shaped antennae that are each 4 kilometers in length. There are two detectors, one in Hanford, Washington and the other in Livingston, Louisiana. Here’s how it works:
Inside the LIGO a laser is shot towards a beam splitter. The beam splitter splits the laser light in two, so an equal amount of lights goes through each of the antennae. At the end of each of the antennae there is a test mass (a mirror) of “40-kg fused silica substrates with low-loss dielectric optical coatings… and are suspended with fused silica fibers from the stage above” . The laser light hits these test masses and reflects back down the antennae and towards the beam-splitter once again. When the LIGO is not detecting any gravitational waves (or other vibrations) the test masses are equal distances away from where the beam splitter is. This means that when the laser light is reflected back, the light from each antennae meets and cancels out. If either of the test masses have moved (say, for example, from being hit by the vibrations of a gravitational wave) the reflected light does not completely cancel out and light hits a photodetector. Because the source of any gravitational waves detected by the LIGO would be far away (weakening the vibrations), each component of the LIGO (save the laser source) is mounted on vibration isolation stages and kept in a vacuum. This isolates the LIGO from the vibrations of Earth that would interfere in the measurements of any potential gravitational waves.
Gravitational waves were finally detected on September 14, 2015. Through careful search, it was determined that the source of these gravitational waves had come from “the merger of two stellar-mass black holes” .
In other words, billions of years ago, in a galaxy far, far away, two massive black holes (the largest of which was 36 times the Sun’s mass) collided to form one even more massive black hole, 62 times the Sun’s mass . This merger of cosmic giants produced gravitational waves that travelled through the universe, passing through our relatively tiny Earth, where a team of scientists (a subset of the species Homo sapiens, found on Earth) recorded the event as “the first direct detection of gravitational waves and the first observation of a binary black hole merger” .
A final recap:
“There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.
There is another theory which states that this has already happened.”
― Douglas Adams, The Restaurant at the End of the Universe 
 Overbye, D. (2016, February 11). Gravitational Waves Detected, Confirming Einstein’s Theory. Retrieved February 11, 2016, from http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html?_r=2
 Abbott, B. P., Et. al. (2016). Observation of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters, 116(6). Retrieved February 12, 2016.
 Adams, D. (1981). The restaurant at the end of the universe. New York: Harmony Books.