The direct detection of gravitational waves has been hailed as the discovery of the century, with the potential to revolutionise the way we study the Universe. But what are gravitational waves? Where did they come from? And why is their discovery so important?
To understand gravitational waves, you need to understand gravity. And to understand why they are important you need to know a little about Einstein’s General Theory of Relativity. Albert Einstein published his theory in 1916, and in it he showed that time and space are inextricably linked. Together they form the fabric on which the Universe is built – space-time.
One way of thinking about space-time is to imagine it as a two-dimensional sheet (in reality, there four dimensions of space-time, but a sheet is much easier to visualise!).
According to Einstein, gravity is consequence of the warping of space-time. Any object with mass warps the space-time in its vicinity, effectively making a dent in the space-time into which objects with less mass will ‘fall’. This is the object’s gravitational field. We think of gravity as the acceleration of objects within this gravitational field.
All objects are made of matter. Matter tells space-time how to bend and space-time tells matter how to move. Imagine a bowling ball making a dent in our imaginary sheet – if you roll a marble close to the bowling ball, it will fall towards it and accelerate. Although it might look like the marble is attracted to the bowling ball, really it is just following the only path that the sheet will allow it to.
If mass bends space-time, then mass under acceleration must create ripples, or waves, in space-time. It is a bit like moving the tips of two fingers together then apart in a bowl of water, and creating waves that ripple out across the water’s surface. A massive object moving through space-time will create waves that ripple out across the Universe. These are gravitational waves.
Any object with mass under acceleration will generate gravitational waves – the Earth does as it is accelerated by the Sun’s gravity and so do you as you run down the street, but these are far too feeble to be detected.
Despite the way it appears to us on Earth, in universal terms gravity is an extremely weak force. In fact, it is the weakest of the fundamental forces – so it takes an extremely massive object to create a gravitational wave signal strong enough to be measured here on Earth. Nothing has more mass than a black hole.
The strength of a gravitational wave signal also increases with the amount acceleration an object experiences (think back to the bowl of water; if you move your finger through it more quickly, you’ll create bigger ripples). So it is not surprising that our first direct detection of gravitational waves came from two rapidly accelerating black holes locked in a death spiral.
LIGO Hanford in Washington State is one of two identical detector sites.
(Credit: Caltech/MIT/LIGO Laboratory)
The two black holes were detected by the Laser Interferometer Gravitational-wave Observatory (LIGO); two identical facilities in Louisiana and Washington State.
The black holes were 29 times and 36 times the mass of the Sun, and were locked together by their mutual gravitational attraction, orbiting very quickly around a shared centre of mass.
As they orbited, they ploughed through space-time, throwing out gravitational waves. But gravitational waves can’t be made for free – it takes energy to churn up the fabric of the Universe – and, with each orbit, the black holes lost energy, which was carried away by the gravitational waves, and their orbit shrank, drawing the two black holes closer and closer together.
As their orbit shrank, the black holes accelerated, which created more powerful gravitational waves, which caused their orbit to shrink more, which caused the black holes to accelerate even more. This vicious circle could only end one way: the black holes collided.
With this collision, the black holes merged together and formed a single, even more massive black hole.
This newly formed black hole was 62 times the mass of the Sun, which means that three whole Sun’s worth of mass were lost in the collision. All of this missing mass had been converted into gravitational energy that, like a rock thrown into a pond, sent gravitational waves crashing outwards at the speed of light. It was this series of steadily-increasing gravitational waves (caused by the black holes spiralling together), followed by a huge peak and gradual decline (as they collided and the new black hole settled down), that LIGO detected.
You can’t see the effects of gravitational waves, but you can measure how they affect an object they pass through.
As a gravitational wave travels through space-time, it causes it to stretch in one direct and compress in the other (think of a wave ripple through a caterpillar as it moves). This, in turn, causes any object that occupies that region of space-time to also stretch and compress as the wave passes over them. So, when a gravitational wave passes the Earth, it will cause the planet to be ever so slightly squashed and stretched, and this is what LIGO is designed to detect.
LIGO’s two four-kilometre-long arms are arranged in an L-shape, so, as a wave passes through, one arm is lengthened and the other shortened. Lasers travelling up and down the arms can measure the smallest change in length that would indicate that a gravitational wave has passed through. This is exactly what happened.
A LIGO technician inspects one of LIGO’s mirrors for contamination by illuminating its surface.
(Credit: Caltech/MIT/LIGO Laboratory)
Although the prediction was made 100 years ago, it is only recently that equipment sensitive enough to detect them (and the techniques needed to isolate it from outside interference) has been developed.
This is because by the time gravitational waves have travelled the tens of millions of light years to Earth, they are so weak that it takes extremely sensitive equipment to detect them.
To put this into context, the amount distortion created by the two black holes and measured by LIGO’s detectors was less than the width of a proton (the tiny particle that, along with the neutron, makes up the nucleus of an atom!).
Not only is the effect tiny, the signal is so weak it could be overwhelmed by something as trivial as a person dropping a cricket ball near of LIGO’s mirrors. There are lots of things that can interfere with detection or create a false signal – the movement of the mirror surfaces caused by the heat energy of their molecules, the movement of the tides, or the rumble of distant traffic – and each erroneous signal has to be identified and removed. There is so much potential for interference that it took five months of painstaking analysis and reanalysis before scientists were confident enough to announce the discovery.
Gravitational waves were one of the last unconfirmed predictions of Einstein’s theory. Despite a century of searching, they had never been directly detected.
Before now, the strongest evidence of gravitational waves came from observations of superdense, spinning neutron stars called pulsars. In 1978, Joseph Taylor and Russell Hulse discovered a pulsar orbiting a neutron star. Later, it was observed that the stars’ orbit was shrinking at exactly rate predicted by General Relativity if losing energy in the form of gravitational waves.
But this was only an indirect detection of gravitational waves – it’s kind of like the difference between finding a footprint left by Big Foot and actually catching the beast on camera.
The discovery opens up an entirely new way of studying the cosmos. For all of human history, we have depended on the electromagnetic spectrum (from radio waves, through infrared, visible and ultraviolet light, to gamma rays) to provide our window to the Universe.
But there are great parts of the cosmos we cannot see with electromagnetic waves – that are either blocked (like during the first 300,000 years after the Big Bang) or lost in a swirling vortex of gravitational attraction (caused by black holes). Because gravitational waves move through the very fabric of the Universe, they can travel unimpeded and unaffected by anything that may lie in their way.
Although the signal detected by LIGO was very small, in theory the field of gravitational wave astronomy that will emerge from the discovery could mean there will be no part of the observable Universe that will be invisible to our gaze – including the first moments of the Universe’s existence after the Big Bang.
Find out more about gravitational waves, and how the UK contributed to their historic detection.