On the 14th September 2015, an event occurred which many scientists have since called, without hyperbole, one of the most ground-breaking in recent memory – the discovery of gravitational waves.
About a billion years ago, two black holes (each of mass approximately equal to 30 stars the mass of our Sun, crushed into a sphere only tens of kilometres across) collided with each other. These black holes had been drawn into other’s gravity and had been orbiting around each other for a very long time, getting closer and closer to each other every day. Eventually they became so close that they collided and merged, forming a single black hole of around 60 solar masses.During this collision event, something incredible happened. Akin to how two objects colliding here on Earth might release energy in the form of sound, these colliding black holes released unimaginably huge amounts of energy, but in the form of gravitational waves.
To understand what these are requires a basic understanding of general relativity, the cornerstone of Einstein’s legacy. His proposal, along with other collaborators, was that space (i.e. the three physical dimensions we feel around us) and time, are actually just different manifestations of the same thing. This joint quantity was dubbed, perhaps a little unimaginatively, ‘spacetime’, and is described with such beautiful and neat mathematics that, for the past 100 years or so, physicists have been assuming Einstein was correct in his proposal, without any real tangible proof.
One of the key tenets of Einstein’s theory was the existence of gravitational waves. Spacetime is the fabric of the Universe in which stars and planets exist, just like a cotton sheet is the fabric of clothes, or the surface of a lake is the ‘fabric’ of the water. Dropping a pebble into this lake would, as you might expect, create ripples which spread outwards, away from the where the pebble dropped and along the surface of the surrounding water. Now, instead of a lake, imagine we have the Universe, and instead of a pebble dropped into the lake, we have two black holes combining. Both events create ripples which permeate away, whether they be water waves or gravitational waves. These ripples aren’t ‘visible’ in the way we are used to seeing things – instead they are rippling through spacetime itself, the intangible and fundamental fabric of what makes up the Universe. As they do so, they disrupt the and perturb the matter which lies on this fabric.
For over a billion years, these ripples travelled on their way towards Earth, and it was only through sheer luck that humans were listening at the right time to detect these waves; they are so incredibly weak that only phenomenal releases of energy (such as that created by the collision of black holes) are detectable with current equipment.
The detection happened near-simultaneously at two LIGO (Laser Interferometer Gravitational-Wave Observatory) facilities in the U.S., and involved measuring the minuscule deflection of matter as these ripples passed through the Earth. Indeed, even despite the incomprehensible power that these waves carried, the LIGO instruments had to be sensitive to changes in the size of their equipment down to a tiny fraction of the width of an atom.
So, around a century after Einstein produced his infamous theory, he has finally been vindicated. Now that we’re sure they exist, in the future, we might even be able to use gravitational waves as a new method of astronomical observation – one that does not rely on light. It’s like we’re now finally able to listen, as well as see, out into the Universe.