by Marcus Chown
Infeld, leaning one way and then the other in a vain attempt to find a clear line of sight to his boss on the other side of the desk, said he had been telling Robertson about the paper Einstein had written on gravitational waves with his former assistant, Nathan Rosen. Robertson had spotted a flaw, which Infeld proceeded to describe nervously, not knowing how Einstein would respond. He need not have worried. ‘Ah,’ said Einstein. ‘I too have recently become aware of that error.’
As soon as Infeld left his office, Einstein located the manuscript of the paper he had written with Rosen and began to revise it. The subject of gravitational waves had turned out to be far more problematical than he had expected. In 1916, within only a few months of presenting his revolutionary theory of gravity, the general theory of relativity, it had seemed blatantly obvious to him that it was possible to jiggle space–time and in doing so create ripples that propagated outwards at the speed of light. Such gravitational waves were completely analogous to the waves that rippled through the electromagnetic field and which, in 1862, Maxwell had triumphantly recognised as light.
However, Einstein had found it impossible to extract exact predictions from his full theory of gravity. This was not too surprising; in order to describe the gravitational field of any distribution of energy, he had been forced to replace Newton’s one equation by ten. And if that was not enough of a complication, all forms of energy – not just mass-energy – have gravity, including gravitational energy itself. Gravity creates gravity! To avoid his theory’s calculation-defying ‘non-linearity’, Einstein had no choice but to consider only the case where gravity was weak and responsible for creating essentially no extra gravity. Not only did it cut down his ten equations to a manageable one, but it yielded a ‘wave equation’, just like the one found by Maxwell. Gravitational waves, it seemed, must exist.
The truth was that Einstein could be sure only that gravitational waves existed in this special instance of his theory when gravity was weak, but that would turn out to be no more than a mirage if they did not exist in the general case when gravity was strong. Proving that they were a feature of his full theory was a nightmare; the theory was so complex that all his intuition went out of the window. Nevertheless, the need to prove definitively that gravitational waves must exist had been nagging at the back of his mind for two decades. When he fled Nazi Germany and ended up at Princeton’s Institute for Advanced Study following a sojourn in Southern California, it had come to the forefront of his thoughts once more.
Remarkably, he and Rosen – his first American student – had found a gravitational wave ‘solution’ to the full equations. It should have been a triumph, but unfortunately it contained a ‘singularity’, a place where the description of the wave blew up to infinity and so made no sense. Having set out to prove that gravitational waves exist, it appeared that he and Rosen had inadvertently proved that they did not exist after all.
Einstein was far from dismayed. Given that every other known ‘field’ – the electromagnetic field, air, water, and so on – could be rippled by waves, he viewed it as a remarkable finding that the gravitational field could not.* He had written to his friend, the quantum pioneer Max Born, back in Germany: ‘Together with a young collaborator, I have arrived at an interesting result that gravitational waves do not exist.’ He and Rosen had then co-written a paper, under the title ‘Do Gravitational Waves Exist?’, which they had submitted to the American journal Physical Review at the end of May 1936.
Einstein had been taken aback when, on 23 July, the editor of the journal had returned the manuscript. John Tate’s covering letter said that an anonymous ‘referee’ who had been sent the paper had pointed out that it contained an error. The referee claimed that the troubling infinity, which Einstein and Rosen had taken as proof that gravitational waves did not exist, was no more than a mathematical artefact that could easily be removed. Tate said he would be glad to have Einstein’s reaction to the referee’s comments and criticisms.
Rosen, who had left Princeton to take up an academic post in the Soviet Union, was spared the sight of his supervisor blowing his top. Back in Germany, journals such as Annelen der Physik published everything Einstein sent them, without question. If he had slipped up in a calculation, someone would simply publish a follow-up paper pointing out the error. This was the German way of doing science. Einstein could not believe the effrontery of Physical Review in rejecting the paper. He was not a prima donna. It was not a matter of pride. It was just not the way things should be done.
Einstein wrote a stinging letter back to Tate:
27 July 1936
Dear Sir,
We (Mr Rosen and I) had sent you our manuscript for publication and had not authorised you to show it to specialists before it is printed. I see no reason to address the – in any case erroneous – comments of your anonymous expert. On the basis of this incident I prefer to publish the paper elsewhere.
Respectfully,
Einstein
P.S. Mr Rosen, who has left for the Soviet Union, has authorised me to represent him in this matter.
Einstein was true to his word. Stung by his first encounter with ‘peer review’, he decided to send the paper to a small publication he had used before: the Journal of the Franklin Institute. Fortunately, he had not yet done so when Infeld relayed to him his conversation with Robertson; if he had, he would not have had the chance to correct his error.
The whole point of his equations of the gravitational field was that they were universal, or ‘covariant’. Everyone in the universe must see the same physics and it should not depend on anyone’s vantage point, technically known as a ‘co-ordinate system’.† But the lucky flip side of this was that, if a calculation proved difficult or impossible in one co-ordinate system, it was necessary only to switch to another co-ordinate system in which the calculation might be easier to carry out. That was precisely what Robertson had suggested to Infeld that Einstein do.
By coincidence, the anonymous referee at Physical Review had suggested the same way out. At the time, Einstein had been too blinded by rage to take any notice, but when Infeld relayed his conversation with Robertson, he was happy to own up to his error.
He sat for an hour in his office, scribbling corrections in the margin of the paper. When he had finished, he leant back in his chair, lost in thought. So now he was saying that gravitational waves existed. He crossed out the title ‘Do Gravitational Waves Exist?’ and replaced it with ‘On Gravitational Waves’.
He contemplated the intellectual journey that had led him to this point. In Berlin, in 1916, he had been sure that gravitational waves existed, but the fact that he was not completely certain had been nagging in the back of his mind for two decades. This year, 1936, he and Rosen had convinced themselves that gravitational waves did not exist after all, and now, finally, he was sure that they did. However, the only plausible astronomical source he could imagine in 1918 – two stars locked in mutual embrace and spiralling together – would create gravitational waves of such mind-boggling weakness as to be impossible to detect in practice.‡
He took his paper to Helen Dukas, his long-standing secretary, who had come with him from Germany. While she was retyping it, he returned to his office and wrote a covering letter to the editor of the Journal of the Franklin Institute.3 In the early evening, as he left the Institute, he dropped it off with Dukas.
On Olden Lane, he passed a yellow bulldozer that was parked in the rutted earth in front of a house that was under construction. He squinted at it for a moment in the early-evening sunshine, wondering what it would be like to drive such a machine. He resolved that next time, if the workman was around, he would ask whether he could have a go. Fame, despite its quite considerable drawbacks, also had its perks.4
As he turned right into Mercer Street, heading for his house at number 112, he saw a figure come out of Marquand Park on the left, cross the road ahead and disappear down Springdale Road. It was the unmistakable figure of a pipe-smoking Howard Roberts
on. How peculiar, he mused, that the man’s suggestion had been so similar to that of the anonymous referee.5
This thought occupied his mind only briefly; the important thing was not how he had come to correct his paper with Rosen but that he had corrected it. Gravitational waves, he was now certain, must exist.
Hanover, September 2015
The weeks and months after 14 September 2015 were hard for Drago and everyone else on the LIGO–Virgo team. There was a long checklist of equipment and software to work through, and each item had to be examined to confirm that it had not been malfunctioning. ‘Our first priority was making sure we weren’t fooling ourselves,’ said Keith Riles, a physicist at the University of Michigan and member of the LIGO Detection Committee.
There were no shortcuts – it was painstaking work that took a lot of time and effort. Could something on Earth have mimicked a cosmic signal? It was necessary to check seismic records across the world, to rule out the possibility that what had been detected was actually a small earthquake. Could something have happened simultaneously at each site, at the precise time the signal was registered? Had someone ridden past on a bike? Had a car hit a bump on a nearby road? In order to get identical signals at the two detectors, something identical would have to have happened at both Hanford and Livingston, which was stretching things a little. Nevertheless, extraordinary claims require extraordinary levels of confidence. All kinds of logs had to be consulted, microphone recordings listened to and CCTV video watched, in order to rule out the possibility of something like two doors being slammed at the same time.
The conclusion at the end of all this work was that, on average, random noise would generate two simultaneous and identical signals like the ones seen at Hanford and Livingston less than once every 200,000 years. The reality of the signal appeared to be beyond any doubt, but there was one final possibility that caused many members of the collaboration to have sleepless nights: that the signal they were seeing was malicious.
Could somebody have hacked the computers at each LIGO site and injected the signals? Such a possibility was difficult to disprove, but the team came to the conclusion that for a hacker to break into the computers and leave a false signal and no trace whatsoever that they had done so would have required a tremendous knowledge of many complex systems. ‘Mission Impossible would be easier,’ says Drago.
By the end of 2015, LIGO itself bolstered the team’s confidence that the 14 September signal was genuine. There had been two more trigger signals from the giant rulers at the two sites – one on 12 October and one on 26 December – and each was exactly what was predicted for the merger of black holes. Both signals were weaker and more difficult to discern than the first one. What was the chance that mundane terrestrial problems with the instrument would conspire to create not one but three signals? ‘We became a lot more confident,’ says Drago.
Secrecy was paramount, and nobody in the collaboration was permitted to tell their friends or families what LIGO had discovered. For Drago this did not prove too difficult. The urge to tell people about the discovery was enormous, but he was so busy that everything except the work at hand was pushed from his mind.
Drago and his colleagues were in the rare position of knowing something nobody else in the world knew, and that nobody in the history of the world had known. The British biographer Peter Ackroyd has speculated that Isaac Newton may have savoured such a feeling, which might explain why he did not tell anyone about his universal law of gravity and his laws of motion until twenty-five years after his discovery. But such thoughts were far from Drago’s mind. He was too busy to feel anything but exhausted. And, anyhow, he was sure the public would not have the slightest interest in the discovery. How wrong he was.
Although everyone was sworn to secrecy, it is very hard to keep a lid on things when around one thousand people from sixteen countries are involved. The task had been made even harder because of the need to inform people outside of the team. The time delay between the arrival of the signals at Hanford and Livingston did not allow the location of the source in the sky to be deduced precisely, but it narrowed it down to a relatively narrow band across the heavens. Astronomers at major observatories around the world were notified and asked to scan the band with their telescopes to see if they could spot anything unusual in the sky on or around 14 September.
Inevitably, rumours of a major discovery circulated within the scientific community. On 25 September, Lawrence Krauss of Arizona State University in Tempe tweeted: ‘Rumor of a gravitational wave detection at LIGO detector. Amazing if true. Will post details if it survives.’ The tweet caused journalists to begin phoning people on the collaboration. ‘It was a little upsetting for everyone,’ says Drago.
Gabriela González, the Argentinian–American spokesperson for LIGO, was dismayed. As early as 16 September, the day after Drago first saw the signal, she and four colleagues had emailed the LIGO–Virgo collaboration. ‘We want to remind everyone that we need to maintain strict confidentiality,’ they said. Premature publicity was a big worry for González because it is essential to be sure of a scientific result before announcing it. Nobody wants to have to retract an incorrect claim at a later date and get egg on their face. When journalists enquired, they received the official reply: ‘We take months to analyse and understand foreground and background in our data, so we cannot say anything at this point.’
In early 2016, the signal first seen by Drago on 14 September 2015 received a name: GW150914. In the outside world, excitement that something important had been discovered was steadily mounting. On 11 January, Krauss upset everyone further, this time by tweeting: ‘My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!! Exciting.’ Not surprisingly, many on the LIGO team saw this as a brazen attempt to steal its thunder.
Excitement was reaching a crescendo when, on 8 February, a press conference was called for Thursday 11 February. It was timed to coincide with the publication of the paper announcing the discovery in Physical Review Letters.6
The press conference was held at the National Press Club in Washington DC. It kicked off at 10.30am Eastern Standard Time, with a brief introduction and video about the project. Kip Thorne, a theorist from Caltech in Pasadena, was on stage, as was Rainer Weiss from MIT. Although over the decades more than a thousand people had been involved in the project, Weiss and Thorne were widely considered to be the founding fathers of LIGO.
David Reitze, deputy director of LIGO, stood up, while behind him a TV screen showed a simulated picture of two black holes. He surveyed the expectant audience, paused for effect, and then spoke. ‘Ladies and gentlemen. We have detected gravitational waves. We did it!’
‘Many of us on the project were thinking if we ever saw a gravitational wave, it’d be an itsy bitsy little tiny thing; we’d never see it,’ said Weiss. ‘This thing was so big that you didn’t have to do much to see it. I keep telling people I’d love to be able to see Einstein’s face right now!’
Public interest exceeded Drago’s wildest expectations. The discovery of gravitational waves, almost exactly one hundred years after Einstein had predicted them, was a huge international story, and the public were right to be excited. Science had gained an entirely new sense. Imagine if you had been deaf since birth and suddenly, overnight, were able to hear. This is what it was like for physicists and astronomers. Throughout history, they had been able to ‘see’ the universe with their eyes and their telescopes; now, for the first time, they could ‘hear’ it. Gravitational waves are the ‘voice of space’.
Scientific discoveries are often over-hyped by the media, but a good case can be made that the detection of gravitational waves on 14 September 2015 was the most significant development in astronomy since Galileo turned his new-fangled telescope towards the heavens in 1609.
Drago was the first person in history to see the signature of gravitational waves. Before the moment he opened that email, they had been travelling for
1.3 billion years across space, and no human being knew of their existence. ‘I could so easily have gone to lunch,’ says Drago. ‘Someone else would have been the first to see the signal, not me.’
Drago happened to be in the right place at the right time, something he freely acknowledges. ‘When Christopher Columbus arrived in the Americas, there was obviously one person who happened to be the first to spot land,’ he says. ‘But everyone knows that it took a lot of people – in the case of Columbus, the crew of an entire ship – to get to that point. As it was with the discovery of the Americas, so it was with the discovery of gravitational waves.’
*
The two black holes whose merger created the gravitational waves which buffeted Earth on 14 September 2015 were the relics from two massive stars that exploded as ‘supernovae’. Paradoxically, when such a star explodes, its ‘core’ implodes – in fact, the implosion is believed to drive the explosion. As the core undergoes runaway shrinkage, its gravity intensifies until it is so strong that nothing, not even light, can escape, and a black hole is born.
By its very nature, a black hole is black and tiny. Until 2015, evidence of the existence of black holes was necessarily indirect: commonly, the observation of a star, or stars, whirling around an invisible celestial object at unfeasibly high speed. However, the gravitational waves picked up on 14 September 2015 changed everything. Since their signature was precisely that predicted by Einstein’s theory of gravity for merging black holes, it proved beyond doubt that black holes really do exist.7
The irony here is that black holes – a prediction of general relativity that Einstein did not believe – were confirmed by gravitational waves – a prediction that he did believe (or that he believed, did not believe and then believed again!).
The gravitational waves intercepted by LIGO on 14 September 2015 came from two black holes that were extraordinarily massive – twenty-nine and thirty-six times the mass of the Sun respectively. In the explosion of a supernova, most of the matter is blown into space and only a relatively small amount ends up sucked into a black hole. In fact, according to astrophysicists’ estimates, the precursor stars must have been at least 300 times the mass of the Sun. Such stars are so rare as to be pretty much non-existent. However, there are strong theoretical reasons to believe that the first generation of stars that formed after the Big Bang – our Sun is a third-generation star spawned from the debris of two earlier generations – were far bigger than today’s suns.8 If LIGO’s black holes really are relics of the very first stars, it would be like walking down London’s Oxford Street and spotting two Roman legionaries among the crowds of shoppers who have miraculously survived in the city since the day the Empire’s soldiers departed in AD 410.