How British scientists played a crucial role in gravitational waves breakthrough

A long time ago, in a galaxy far, far away, two massive collapsed stars – known as black holes – collided. The resulting explosion created ripples in the fabric of spacetime. Known as gravity waves, those ripples reached Earth on 14 September last year, where they were picked up by a pair of detectors known as the Laser Interferometer Gravitational Wave Observatory (Ligo).

It was an astronomical first. After decades of effort, scientists had finally detected gravitational waves, whose existence Albert Einstein had predicted, with some hesitation, in 1915 in his Theory of General Relativity. Not surprisingly, the detection has been hailed as a scientific milestone and made headlines round the world. If nothing else, this was surely the final vindication, a century on, of the great man’s genius.

But there is more to the discovery, for this is also a story of remarkable scientific pig-headedness and resolve, one that has much in common with Samuel Johnson’s acerbic view of remarriage being a triumph of hope over experience. Certainly scientists could not be accused of being faint-hearted. For the past 30 years, new gravitational wave detectors were built, but failed to pinpoint any hint of these extra-galactic signals. So scientists and engineers simply returned to the drawing board and refined their instruments – to try again, only to fail once more.

In 1998 I interviewed Professor Jim Hough of Glasgow University’s physics department about his work on the Geo-600 gravitational wave detector, the UK-German collaboration that had just been built outside Hanover. Hough has been hunting gravitational waves since 1970 and at the time was quietly confident that this latest machine would soon hit pay dirt. “We should spot gravitational waves by 2001,” he confidently told me at the time. It transpires that he was 15 years out.

“At the time, we thought that supernovae – exploding stars – would produce gravitational waves that would be fairly easily detectable,” Hough says today. “Then the theorists decided they probably didn’t produce detectable gravitational waves. So we had to make our instruments even more sensitive so that we could pick up waves from more esoteric events, such as collisions of neutron stars or black holes.”

This proved to be a distinctly tricky task, however. Gravitational waves may be generated by enormously energetic events but these are also incredibly remote. As a result, a wave’s energy is dissipated to a tiny fraction of its original magnitude by the time it reaches Earth and produces a shift of a only few hundred billion-billionths of a metre in a detector’s instruments.

It sounds an improbably tiny measurement to make. In fact, the process is relativity straightforward. A detector has tunnels running along two long arms (on Ligo they stretch for 4km) and an identical beam of light tuned to a single wavelength is shone down each of them. At the end is a mirror that reflects the beams back down the tunnel to a central detector, where they are recombined so that their wavelengths are kept in step. In other words, the beams’ light waves peak together, producing a bright spot.

However, if the mirrors’ positions are moved very slightly – by, for instance, a gravitational wave changing the length of one arm more than the other – a peak on one beam will coincide with a trough in the other beam and the image will be darkened.

The trouble is that gravitational waves are not the only types of vibration that can affect a detector. If a person walks close to one of its mirrors, his or her mass will be sufficient to exert a tiny gravitational influence on it, causing the mirror to shift. In the case of Geo-600, there is the problem of the North Sea. The coast is more than 100km away. Nevertheless, the waves pounding on that distant shore create a clear signal for its delicate instruments. Fortunately that signal is a highly rhythmic one and can be removed from the machine’s output.

Far more pernicious is the problem of thermal noise – the heat of the apparatus itself. This causes mirrors and mountings to vibrate slightly, again creating spurious signals. Dealing with this issue has pushed the ingenuity of gravitational wave scientists to its limits and has involved the development of delicate systems of mirrors and pendulums made from pure silica. This system eliminates most of the thermal vibrations that have bedevilled detectors in the past.

The silica set-up developed at Geo-600 uses four platforms suspended from each other in layers like a giant, shiny mobile. This eradicates virtually all vibrations. “It is the scientific equivalent of the best crystal glassware,” says Hough’s colleague Professor Sheila Rowan.

The system was developed with input from a team of scientists from Glasgow, Strathclyde and Birmingham universities and from the Rutherford-Appleton Laboratory near Didcot in Oxfordshire, and was the final component of the sensationally successfully upgrade of Ligo. Scientists at the Ligo labs – one based at Hanford in the US’s Pacific north-west and the other in Livingston, Louisiana – had begun hunting for gravitational waves in 2002. The two detectors found nothing and were switched off in 2010. A $200m upgrade involved fitting them with new features that included the silica mirrors that British scientists had helped develop.

Within weeks, the machines hit pay dirt. In September last year, both detected gravitational waves that have since been shown to come from two massive black holes colliding 1.3 billion light years from Earth. “We did it. This was truly a scientific moonshot. We landed on the Moon,” said David Reitze, executive director of Ligo.

It should also be noted that, this time, Hough had been remarkably prescient in anticipating the discovery. “We should see gravitational waves, probably in an American machine fitted with our devices, by around 2015,” he told me in a 2012 interview. And that, of course, is exactly what has occurred.

The elation with which this discovery has been greeted is understandable though we should be clear how it came about. The theoretical framework that explains gravitational waves is old: it was outlined by Einstein in 1915 in his Theory of General Relativity. Neither is the basic detection method new: it was in use 20 years ago.

What changed the state of the playing field was the involvement of teams of scientists from different disciplines and several nations, including Britain, Germany and the US, who combined their talents to find ways to eliminate all the spurious signals that were blocking detection of gravitational waves. This massive clearing-up exercise involved lots of people: materials scientists, who developed those delicate silica mirrors that removed the thermal vibrations; geologists, who played a key role in the understanding of seismic events that affected detectors; and laser scientists, who created the special stable beams now used in these machines.

The commitment, the level of collaboration and the diligence and determination displayed by these hunters of gravitational waves is startling, and a gratifying indication of what can be achieved by big science. For decades, new headaches and problems beset the quest for gravity waves, but, with remarkable singlemindedness, they have been swatted aside.

We should record a note of caution, of course, one sounded by astronomer Gerry Gilmore of Cambridge University. “Gravitational waves are ‘classical’ physics that are related to the mere 5% of the universe that is made of ordinary matter,” he points out. “We still have the nature of reality to discover. So let’s be excited but remain humble. We do indeed know more and more, but it is still all about very little. There is the issue of discovering the make-up of dark matter, for example.”

The Ligo result is still a remarkable discovery, of course, but what comes next? There are many answers to this question, it transpires. For a start, it is now clear that astronomers have created a new type of astronomy: gravitational wave observation.

Until now, everything we have learned about the universe has been based on studies of electromagnetic radiation – from infrared to visible light to gamma rays. Gravitational waves are going to give us a new way of looking at the universe. Indeed, they have already thrown up one unexpected finding. That burst of gravitational waves detected by Ligo shows they were produced by a collision of one black hole – a collapsed star that was 35 times the mass of our sun – with another that was slightly smaller.

“We were confident about the existence of small black holes a few times the mass of the sun, and about very, very large ones,” says Hough. “But this confirms the existence of a range of intermediate black holes, about which we had theoretical doubts. Now those have gone. We’re already going places.”