Gravitational waves: Everything you need to know
The first direct detection of gravitational waves back in September 2015 was 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 should we care?
What is gravity?
To understand gravitational waves, you need to understand gravity.
In 1915 Albert Einstein published his General Theory of Relativity in which he showed that space and time are inextricably linked and form the fabric on which the Universe is built. This fabric is called 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 (three of space and one of time), but a sheet is much easier to visualise.
Einstein showed that gravity is an effect of the warping of space-time — any object with mass distorts space-time in its vicinity — effectively making a dent into which an object with less mass will ‘fall’. It is this acceleration of an object in a gravitational field that we think of as gravity.
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 — it might look like the marble is attracted to the bowling ball, but really it is just following the only path that the sheet will allow it to.
What are gravitational waves?
If mass bends space-time, then mass under acceleration must create ripples, or waves, in space-time. You can think of it as being a bit like moving the tip of your finger through bowl of water — you will create waves that ripple out across the water’s surface. A massive object moving through space-time will create waves that ripple out across the 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 you will create your own gravitational waves as you run down the street (but they are far too feeble to be detected).
If they are so common, why did our first detection come from two black holes 1.3 million light years away?
Despite appearances, gravity is actually an extremely weak force — in fact, it is the weakest of the fundamental forces — so it takes an extremely massive object to create a signal strong enough to be measured here on Earth. Nothing has more mass than a black hole. Also, the greater the amount of acceleration the object experiences, the stronger the gravitational waves it will throw out. So it is no accident that our first direct detection came from two rapidly accelerating black holes locked in a death spiral.
How did the black holes create the signal that LIGO detected?
The two black holes had masses of 29 times and 36 times the mass of the Sun and were locked together by their mutual gravitational attraction. They orbited around a shared centre of mass at a significant percentage of the speed of light (which, by the way, is very fast indeed) and, as they ploughed through space-time, they threw 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 orbital energy, which was carried away by the gravitational waves, and their orbit shrank.
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. It was a vicious circle that could only end one way: the black holes collided.
When they collided, they merged together and formed a single, even more massive black hole, but the new black hole was less than the sum of its parts.
The newly formed black hole had 62 times the mass of the Sun, which means that three whole Sun’s worth of mass had been 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, (as the black holes spiralled together), followed by a huge peak and gradual decline (as they collided and the new black hole settled down), that LIGO detected.
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 2015, the strongest evidence of gravitational waves came from observations of superdense, spinning neutron stars called pulsars. In 1974, 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 — despite this, the discovery won Taylor and Hulse the 1993 Nobel Prize for physics.
What exactly did LIGO detect?
You can’t see the effects of gravitational waves (Taylor and Hulse saw what happens to an object emitting gravitational waves, not the waves themselves), 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 (a bit like a rubber ball).
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.
Why did it take so long to detect gravitational waves?
No matter how dramatic the process the process that made them, by the time the 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 give you an example of how weak the signal is, the amount distortion 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 walking too close to one of LIGO’s mirrors. The number of things that can interfere with a detection, or create a false signal are legion — heat generated by the equipment, the movement of the tides, or the rumble of distant traffic can all cause the mirrors to vibrate. Each erroneous signal has to be identified and removed from the data.
Although the prediction was made 100 years ago, it is only recently that equipment sensitive enough to detect them (and the techniques required to isolate it from outside interference) has been developed.
There is so much potential for the result to influenced by external sources that it took five months of painstaking analysis and reanalysis before scientists were confident enough to announce the discovery.
That’s all very interesting, but what difference will the discovery really make?
It might seem that, at best, a barely detectable wobble that vindicates a 100-year-old theory is little more than a scientific curiosity, but that couldn’t be further from the truth. There’s a very good reason that the discovery has been hailed as the ‘discovery of the century’.
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. Unfortunately there are great parts of the cosmos that are opaque to electromagnetic waves — where they are either blocked (like during the first 300,000 years after the Big Bang) or lost in a swirling vortex of gravitational attraction (black holes). Because gravitational waves propagate through the very fabric of the Universe, they can travel unimpeded and unaffected by anything that may lie in their way. In theory, the field of gravitational wave astronomy that will emerge from the discovery will mean that there will be no part of the observable Universe that will be invisible to our gaze — including the very first moments of the Universe’s existence after the Big Bang.
Story: Ben Gilliland
Related stories:
The UK: Taking gravitational wave detection to infinity (and beyond)
It’s not just massive gravitational objects that have been making waves in the field of gravitational wave research, the UK has also been making waves for more than 40 years (now that’s how you strain a metaphor).
