This article was originally written for Popular Astronomy magazine.
A little over a billion years ago, back when complex life on Earth was just getting started, two black holes were colliding in a far-off galaxy. Each as massive as thirty Suns, these two behemoths spiralled towards each other and ultimately merged so violently that space itself buckled and twisted beneath them, creating a shockwave that rippled out across the universe.
When the ripples arrived at Earth on 14 September 2015 they were so weak as to be barely noticeable. They passed straight through our planet without causing much fuss. A few lasers wobbled almost imperceptibly, but little else happened. The effect might have gone entirely unnoticed, except for the fact that these particular lasers had been waiting patiently for this moment for over a decade.
On the 11 February 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the unambiguous detection of these ripples in spacetime known as gravitational waves. A prediction of Einstein’s General Theory of Relativity, long sought after but never before seen, the detection of gravitational waves marks the beginning of a new era of observational astronomy.
Gravitational waves occur when a massive object accelerates through space. Loosely speaking, the heavier the object and the faster it moves, the bigger the wave it causes. Even so, gravity is the weakest of the four fundamental forces and gravitational waves are extremely hard to detect.
The first indirect evidence for the existence of gravitational waves came in 1974 when radio astronomers Russell Hulse and Joseph Taylor, Jr. discovered a pair of pulsars (small, dense, rapidly rotating neutron stars) orbiting one another. Astronomers observed this ‘binary pulsar’ for years and found that the neutron stars were drawing closer together. For them to be getting closer, they had to be emitting energy somehow. The only conclusion that fit the observations was that they were emitting Einstein’s elusive gravitational waves.
Taylor and Hulse were awarded the 1993 Nobel Prize in Physics for this discovery. The previous year, the US National Science Foundation had approved funding for a US gravitational waves observatory: LIGO. The aim of LIGO was to directly detect gravitational waves through the squeezing and stretching effect they have on space.
LIGO currently consists of two detectors, one in Louisiana and the other in Washington. Each detector consists of two 4 kilometre-long perpendicular arms. A laser beam is split in two, reflected back and forth along each of the arms and then recombined at the heart of the detector where its intensity is measured. Any change in the relative length of the arms – due to the way space is distorted by a gravitational wave – will change the measured brightness.
This setup is essentially the most accurate ruler in human history. If one of the 4 km long arms were to change in length by even a hundred-trillionth the width of the full stop at the end of this sentence, LIGO would detect it.
Construction of LIGO was completed in 2002 and the world waited for signs of gravitational waves. For years, nothing happened. Then, on 14 September 2015, shortly after LIGO came back online after an upgrade, a candidate signal was finally detected. The data from both LIGO sites is shown in the image accompanying this article.
Lasting only 0.2 seconds in total, this ‘chirp’ perfectly matched the signal predicted by Einstein’s equations for two black holes spiralling towards each other and eventually merging. The signal starts as a slow oscillation that increases in pitch as the black holes spiral closer together, peaking at the moment they merge and fading out as the resulting single black hole stabilises. Not only was this the first direct detection of gravitational waves, it was also the first confirmation of stellar-mass black holes existing in a binary system, the first observation of a black hole merger and indeed the most direct measurement of black holes themselves.
Gaining so much information from a single measurement is an incredible feat, but it’s only the beginning of what astronomers hope to achieve with gravitational waves. “The next stage is to build more detectors and to improve the sensitivity of the existing ones even further,“ Rebecca Douglas of the University of Glasgow’s Institute for Gravitational Research tells Popular Astronomy. “This will allow us to create a global network of detectors able to judge where in the sky a source comes from. As our detectors get better and better we expect to see entirely new phenomena, and that’s what makes gravitational wave astronomy so exciting.”
The detection of gravitational waves marks the beginning of a new way of looking at the universe. LIGO’s incredible discovery has shown us that gravitational waves exist and that we can detect them. Future detections may shed light not only on black holes and the nature of gravity, but perhaps even on the beginnings of the universe itself.