"Matter tells space how to curve, space tells matter how to move", said John Archibald Wheeler describing Albert Einstein's General Theory of Relativity which he put forth in 1915. The theory states that space and time are intertwined as four dimensional spacetime. This was essentially also a theory of gravity describing it differently from what we understood from Isaac Newton. According to Einstein, a massive body such as the Earth, distorts spacetime around it and other bodies that move through this distorted spacetime move differently. Unlike Isaac Newton, Einstein did not think of gravity as a force but rather as a disturbance or distortion in spacetime, caused by bodies as they moved through it. It is this distortion that provides the attraction between large objects.
This is how much of a genius Einstein was, that for a hundred years this theory has passed almost every test. Except for one. That is, the direct proof for gravitational waves – ripples or fluctuations in the fabric of spacetime – caused by accelerating masses – which transport energy as gravitational radiation. While they have been observed indirectly before, there has never been a direct detection of these waves.
And almost 100 years later, in 2016, this final test also has been passed. At the LIGO (Laser Interferometer Gravitational wave Observatory), intense laser beams are passed through two perpendicular, 4 km long, vacuum tubes and light reflected off mirrors at each end is analyzed. The analysis helps to detect tiny changes in the distance between the mirrors. So in the eventuality that a gravitational wave happens to pass through the tubes, the distance light had to travel between the mirrors increases and decreases as space expands and contracts. This experiment is extremely delicate and its effects are so miniscule that it shakes the mirrors through a distance less than a millionth of the size of an atom. This is the reason that they have two similar detectors separated by nearly 2000 miles, to look for events that show up in BOTH detectors.
On February 11, 2016, it finally happened. Scientists (who have published a paper) were able to detect gravitational waves cause by two merging black holes, over one billion light years away. They were able to hear and record this sound, which came out as a chirp. And they are almost exactly the same as the predictions made at the sites of the two mirrors. Listen to it here.
This is not the only way gravitational waves are being searched for. At the end of last year the European Space Agency (ESA) launched the LISA Pathfinder to test the technologies for detecting gravitational waves in space. Here is what LISA is going to do according to ESA: "it will put two test masses in a near perfect gravitational free fall, and control and measure their motion with unprecedented accuracy". The aim of this exciting feat of engineering is to test the potential of building a gravitational wave space observatory in space and enable us to understand the cosmos very shortly after the Big Bang.
The waves have already shown that they travel at the speed of light and that black holes do exist. But there is a whole lot else they can answer and potentially change our understanding of the universe. They could tell us how fast the universe is expanding and what makes stars explode as well as help us discover new celestial bodies and phenomena.
This for me is the discovery of the century. We have finally detected that disturbance in the force and who knows where it may lead us.