Five years ago today, the LIGO and VIRGO gravitational wave detectors made what is still their most fruitful detection: two neutron stars during their last minute or so of orbiting about one another and then merging together, exploding spectacularly, and forming … well, we still aren’t sure whether they formed a black hole (probably) or a more massive neutron star. We call the event GW170817.
This is what the waves from the last minute of the orbit and the merger looked like and sounded like, as inferred from the GW data. Listen carefully at the very end! The other signals in this video are black hole mergers, which last much much less time.
GW170817 was fruitful scientifically because the explosion — something you don’t get when two invisible black holes merge into a bigger invisible black hole — was visible from Earth and was observed by astronomers using not only optical telescopes, but also radio telescopes and satellites detecting X-rays and γ-rays. We listened to it with GWs and watched it with light. This combination of different ways of observing has led to thousands of research publications (Google Scholar today lists over 13,000, though this may include much second-hand reporting) on the physics and astrophysics of neutron stars.
GW170817 also established gravitational wave detection as a full partner to the other branches of astronomy. And it effectively created a new form of astronomy called multimessenger astronomy, the combining of information brought to us by different messengers (GWs, electromagnetic waves, cosmic rays, neutrinos).
I wrote about this event in a previous blog post on the first anniversary of the detection in 2018, so I am not going to repeat all that. What is remarkable is that, just since 2019 there have been over 1200 research papers which mentioned GW170817 in their abstracts. The event clearly continues to stimulate research across many areas of astrophysics!
And yet the event itself has remained unique. We have detected a few further mergers of neutron stars with other neutron stars and even with black holes, but these events have been so far away that it has not proved possible for astronomers to identify the counterpart. That’s because an event that is further away is weaker, so we can’t pinpoint its location on the sky very well, so astronomers have to search over too large an area. What is more, no gamma rays have been detected from these, presumably because the narrow jet emitted by the merger, which radiates the gamma rays, has been directed away from our line of sight. We have come to appreciate how lucky an event GW170817 really was!
As we look forward to the re-starting of observations by LIGO, Virgo, and now the Japanese KAGRA (with LIGO-India under construction), by the end of this year or early next, we have great hopes that we will finally get to do joint observations again with other astronomers on a handful of merger events involving neutron stars. The detectors will have improved their reach by a factor of 1.5 or more, which opens up an observing volume more than (1.5)3 = 3.4 times larger than before. Meanwhile, optical astronomers have also been improving their ability to survey the skies for short-lived explosions.
The improved sensitivity during the upcoming one-year-long observing run O4 will produce an abundance of events. Black-hole binary mergers may be detected once every one or two days, and the science from them may well answer many questions left by previous observations. How are these black hole binaries formed? Do they constitute a large part of the so-far invisible dark matter of the Universe? Can we use them to measure the expansion rate of the Universe (the Hubble-Lemaître constant) accurately enough to resolve the tension among current determinations? And, of course, we will be waiting with fingers crossed to see more GW170817’s!
Today my good friend and colleague Jim Hough (otherwise known as Professor James Hough FRS of the University of Glasgow) goes to Buckingham Palace to be knighted for his immense contributions to gravitational wave research, and of course to the first detection that LIGO made in 2015. I thought I would permit myself to ramble on a bit here about how deserving he is of this award, how his life and work essentially trace out the whole story of gravitational wave detection so far, and how we really need more Jims in science!
Jim won’t like this post, because he is one of the most modest people you could meet. Give him a compliment on what he has done, and he is always at pains to remind you how important the contribution of his students, collaborators, and other colleagues has been. And how it was not that great anyway, since we still have much more to do.
At the same time, Jim’s standards for himself and others are extremely high. His research group at the University of Glasgow, now known as the Institute for Gravitational Research (IGR), pioneered many of the key technologies that made the LIGO and Virgo detectors the most sensitive displacement-measuring instruments ever built and allowed LIGO to make the Nobel-Prize-winning first detection of gravitational waves.
Big science needs a special kind of leadership
Watching Jim guide others’ research has always been a pleasure for me. He’s invariably full of praise for a good step forward. Should a correction be needed, he will offer advice almost with diffidence, yet in a way that allows everyone to understand immediately why he must be right: “Well, I’m not so sure that is the right way to go about it …” he will say, thoughtfully and slowly, as if he needs to think it through himself (even though of course he knows exactly what is wrong right away); and then “Why not think about it this way …” He has mentored several of the top people in our field, and they all seem to inherit his exacting standards for their own work alongside a wonderfully cooperative approach to working with others.
Working together is a key aim. If you work for him in his group, you soon get to feel you work with him. There is mutual respect, teamwork rather than internal competition. This, I think, partly explains another striking feature of Jim’s group: he has succeeded in training many talented women scientists, and he was doing this long before achieving gender balance became a priority in physics. An outstanding example is Prof Sheila Rowan FRS, an international leader in our field (she chairs, for example, the Gravitational Wave International Committee), who is currently Scotland’s Chief Scientific Advisor.
Too often, the public image of physicists is of either the ego-driven ruthlessly competitive scientist (name your favourite!) or the nerd (as in television’s “Big Bang Theory”). But big science, such as LIGO, works best when people work together with mutual respect and constant communication, sharing the ego rewards when they come, and using their social skills as much as their mathematical and technical ones in order to move the project forward. Jim provides a great example of this kind of leadership, and so by the way do the three LIGO Nobelists of 2017: Rai Weiss, Kip Thorne, and Barry Barish.
One of the founders of GW physics
Jim was in gravitational wave detection from the start. He got his PhD under the supervision of Ron Drever, then a professor at Glasgow, and later one of the founders of LIGO. It was just at the time (1970) that Joe Weber in the USA was claiming to have made the first detections of gravitational waves with his bar antennas. This was exciting, but Joe’s claim looked extravagant — he said he was detecting events roughly once a day, and the sensitivity of his detector meant that each one had to be carrying a huge energy flux, at least 105 times brighter (for a few milliseconds) than the Sun. So Ron asked Jim (and a handful of other young Glasgow scientists, most still in the field today) to work with him to build their own pair of antennas, to check the claim. Several other groups around the world, including in Munich and Rome, embarked on similar experiments.
Remarkably, Ron and Jim’s twin detectors registered one significant coincident event that could not be explained away, but because it could also not be checked against any other detectors – they were the only ones on the air at the time – it remains an historical enigma. (I am reminded of Blas Cabrera’s 1982 magnetic monopole, another unverifiable one-off.) On the whole, however, the ensemble of new bar detectors registered nothing else of significance, certainly nothing like once a day, and this led the physics community to reject Weber’s claims.
So then the question for all these physicists who had just made their bar detectors irrelevant was: what next?
Rai Weiss at M.I.T. was telling people that laser interferometers would do better than Weber-like bars at catching these weaker signals, and Ron and Jim had been led by their own thinking in a similar direction. The Munich Max Planck group, led by the great Heinz Billing, also agreed with Rai and also embarked on this route. Out of these small beginnings in the 1970s, out of just three groups with a few people in each, grew today’s mammoth 1000+ LIGO-Virgo collaboration.
Time to think big
I won’t go into the whole history, but a few facts are important here. Ron accepted Kip’s invitation to go to Caltech in 1979, leaving Jim in charge at Glasgow. Kip wanted Ron because he had already made several creative innovations, which have since proven their worth as standard features of today’s interferometers. Jim was of course determined keep up this innovative tradition in Glasgow after Ron left.
The field rapidly began thinking big. In the mid-1980s the Munich group made the first proposal, for a 3-kilometre-scale detector. Ron and Kip got together with Rai to propose what is now the 4-km LIGO. Groups in Italy and France new to the field formed Virgo and got their 3-km proposal out. And Jim decided to put in a UK proposal for a more modest, but easier and faster to build, one-kilometre detector in Scotland, not far from St Andrews.
This proposal got me into the field, and gave me the pleasure of working with Jim. In order to write a convincing proposal to the UK science funding body (then called SERC), Jim needed a science case: what could such a detector observe, what could be learned from observations? I was in Cardiff (University of Wales, Cardiff, as it was then known), and I had worked on the theory of neutron stars and binary systems, so Jim invited me to write that part of the proposal. I wasn’t sure I wanted to get into big science. He was persuasive! I joined.
That first proposal in 1986 wasn’t successful, but it led SERC to suggest we cooperate with the Munich group and propose something bigger that could be funded jointly by Germany and the UK. This was a far-sighted idea and it led to what we call the GEO collaboration, which is still going strong, now as a part of LIGO.
The 1989 joint GEO 3-km proposal initially got plenty of positive reaction in both countries, but in the end it did not get funded, partly because of UK science politics and partly because German reunification happened in 1990 and shifted the spending priorities in Germany. It was hard to argue against the German agency’s point of view: reunification had to be done right if Europe was to remain peaceful after the Berlin Wall came down.
A setback leads to a brilliant recovery
But for us this was an immense and unexpected setback. To my mind, the measure of Jim Hough is how he held his research group and research aims steady despite this. It was clear by this time that the LIGO and Virgo collaborations were soon likely to get approval for their detectors, so how could Glasgow survive in this field?
Some group leaders would have given up and done something else; the leader of our German partner group did just that. But Jim kept his group together, and kept morale up, by deciding that they were going to stay at the leading edge of gravitational wave technology. Gravitational wave science was going to have a long future, marked by successive generations of more and more sensitive detectors, and Glasgow was going to invent the technologies they required.
Jim’s reasoning was that building these big detectors — something nobody really knew how to do yet at the level of detail that was going to be needed — would absorb so much of the effort of the US, French, and Italian scientists in our field that they would not have much spare effort to devote to looking ahead at the technologies that would be needed to keep improving the detectors, after their first versions had been built and operated.
We knew enough in 1990 about likely gravitational wave sources to know that the expected sensitivities of these first big detectors were not guaranteed to yield detections, and indeed history proved this to be correct. The LIGO proposal had explicitly envisaged a second, advanced stage of sensitivity, which would all but guarantee detections. Again, history proved this to be correct. But much of the technology for Advanced LIGO existed only on paper. Or not at all.
So Jim saw the opportunity and went for it. The Max Planck Society also showed resolution and vision: they kept our German partner group together by inviting a young Stanford scientist, Karsten Danzmann, to take over the Munich group and move it to the Leibniz University, Hanover.
Karsten immediately began thinking creatively about the future himself, and he secured funding for an intermediate-scale instrument: a 600-m interferometer that would be an excellent development platform for moving the new technologies out of the laboratory and getting them ready for the advanced versions of the 4-km detectors. And as a bonus, GEO600 (as we call it) would employ the advanced technologies that would be created in Glasgow and Hanover to become a kind of mini-advanced detector, able to compete with first-stage LIGO in sensitivity.
This insight, that the future of the field needed technology labs, that there were advantages to being a group that wasn’t building a big detector, was brilliant. It led Jim to Buckingham Palace today.
Getting ready for tomorrow’s detectors
Jim’s group in Glasgow and Karsten’s in Hanover certainly had more fun than anyone else in the field during the 1990s. Experimental physicists typically enjoy time in the lab more than anything else in their jobs. Building the immense LIGO and Virgo detectors, by contrast, was disciplined hard work, where ground-breaking, ultra-high-precision engineering was followed by painstaking and often tedious trouble-shooting.
Getting the big instruments to work as designed pushed the limits of what was possible, technically and organisationally. After 8 difficult years, Barry Barish took charge of LIGO in 1994 and made the project work. Along the way, LIGO lost Ron Drever, the creative genius who simply couldn’t leave the lab, couldn’t fit into the business suit needed to build a big detector. (Ron still deserved, and received, huge credit for the success of LIGO. Sadly he died before the Nobel Prize was decided, and that gave Barry a chance to receive his due recognition. The restriction of the Nobel Prize in Physics to three individuals produces strange anomalies for big-science projects.)
Out of Glasgow came, among other advances, the technique of signal recycling, devised by the late Brian Meers, followed by the monolithic suspension system, a project led by Sheila Rowan. Barry Barish, understanding the importance of these technologies and others developed by Glasgow and Hanover to LIGO’s future, brought the GEO collaboration into LIGO before the end of the 1990s.
Signal recycling was tested in GEO600 in the period 2005-10 and then implemented in Advanced LIGO, ready for the first detection, GW150914. Sheila Rowan’s suspension system uses glass fiber suspensions for mirrors and other optical components rather than wire, and it was also demonstrated in GEO600 during the 2000s and then implemented in Advanced LIGO. It contributed to the first detection by helping reduce the amount of noise in the frequency band where that signal had most of its power. Both technologies will be standard in all big detectors for a long time to come.
And while on the subject of key technologies, let’s note Glasgow’s important contribution to the optical measurement package that was part of the LISA Pathfinder mission (LPF). Launched by ESA soon after GW150914, LPF was designed to test the new technologies needed for the future LISA space-based gravitational wave detector, and it did so with flying colours, beating not only the performance goals set by ESA but also the much more stringent nominal measurement accuracy required for the individual LISA spacecraft themselves. The Glasgow group’s contribution was led by Harry Ward, who has been part of the Glasgow gravitational wave effort since the 1970s.
LISA is scheduled for launch in 2034, so here again Glasgow is developing the technologies of the future. And it is worth remarking that Jim was part of the early LISA proposal team back in 1995; so was the equally visionary Karsten, who has led the LISA project ever since.
Seeing ahead, far ahead, is one of Jim’s most notable characteristics. Another is that he sees both the wood and the trees. Jim placed Glasgow at the core of the LIGO experimental effort by combining his people skills with his vision of the future. He has the ability to understand what is needed long-term, and the amiability to motivate his team with this vision. His knighthood is a splendid recognition of this, and I hope that younger scientists will be inspired by the honour to study his example and emulate it. It can lead, after all, to the very top.
P.S. If you would like to see the man himself talking about his science, and besides that driving his red sports car, he gave a very interesting interview in the Scienceface series. It was filmed before LIGO was converted to Advanced LIGO using new technologies, key ones from his group. He predicts here that the first detection will happen in 2016 or 2017, so he was a little (but not much) on the conservative side. Also on the same webpage is an interview with Sheila Rowan explaining in simple terms the monolithic suspensions.
Last Friday we celebrated the one-year anniversary of an event that those of us who were involved will never forget. The Virgo gravitational-wave detector had joined the two LIGO instruments on August 1, 2017, and the three detectors had since then been patiently listening out together for gravitational wave sounds coming from anywhere in the Universe. On August 17, the deep quiet was interrupted by a squeal, a chirp lasting much longer and going to a much higher pitch than the GW150914 chirp that had launched the field of gravitational wave observational astronomy two years earlier. We named it, prosaically, GW170817.
This one-minute-long squeal was followed by an incredible explosion that radiated intense gamma-rays, X-rays, light, radio waves — right across the whole electromagnetic spectrum. What came first was a burst of gamma-rays, just 2 seconds after the end of the squeal. Then it began brightening up at other wavelengths. The explosion itself did not register in LIGO and Virgo, because as it rushed out in all directions it was too smooth to generate gravitational waves. But astronomers at their telescopes saw it: a kilonova, a new type of cosmic explosion.
You may like to think of the chirp as the sound of a fuse, the fuse that triggered a giant explosion. The chirping sound was produced by two neutron stars orbiting one another at nearly the speed of light. The pitch of this gravitational-wave sound is at twice the frequency their orbit. Their orbital motion stirs up gravity near them, and this creates gravitational waves. These waves take away energy, and this friction brings the stars closer together. The binary that became GW170817 had probably formed billions of years earlier, with widely separated stars. At first the weak gravitational waves carried away energy so slowly that the system hardly changed from one millennium to the next. But eventually, inevitably, the stars drew so close to one another that the waves had frequencies of around 20-30 Hz, and they entered the observation frequency band of LIGO and Virgo. The emitted waves were very strong by then, and so the system was evolving quickly, shrinking noticeably with every orbit, the gravitational wave pitch rising rapidly. Within another minute or so, the stars made contact, triggering the kilonova. I mentioned in my post of June 9 last year that we were hoping to detect this kind of event with neutron stars, and ten weeks later it happened.
You can listen to the reconstructed sound of this death-spiral, this fizzling fuse, in the video here, although you need patience, because for most of the one minute its pitch is too low for normal hearing. LIGO and Virgo can go to deeper pitches than the human ear. But the end is worth waiting for, because of what it says about the stars: as the pitch approaches about 1 kHz (two octaves above middle C), the stars are orbiting one another 500 times each second!
What blew up after the chirp ended?
Previous gravitational wave detections had also recorded chirps, but these fuses had finished without igniting explosions. That is because they were the gravitational wave sounds made by orbiting black holes, which are strongly curved but empty space. So when they came together, they formed a bigger black hole, but it was still just empty space. There was nothing to explode. But last year, the things that came together were neutron stars, and they smacked together at something like 20% of the speed of light. Perfect for creating an enormous splash.
Neutron stars are not ordinary stars like our Sun. They are stars formed from the remains of a normal star that has run out of its nuclear fuel, so it can no longer maintain its original size against the pull of gravity. Astronomers know about two kinds of such “remnant” stars: white dwarfs and neutron stars. Our Sun will form a white dwarf in the end (some 5 billion years from now). It will shrink down to the size of the Earth after slowly boiling away maybe half of its gas (and incinerating the Earth in the process). Much bigger stars than the Sun end much more dramatically in what we call supernova explosions, and these usually leave behind a neutron star, which is typically a little heavier than our Sun but is only the size of a big city, about 20 km across. Neutron stars are sometimes described as “dead” stars, or as “cinders” left by the supernova, but in reality they are balls of tightly packed explosive, just waiting for something else to set them off.
What kind of explosive material is a neutron star made of?
Some of the densest things we normally deal with in everyday life are made up of atoms that fit very tightly together, packed right up against one another: we call them crystals, like diamond. But a diamond the size of a city (what an image!) will not have anything like the mass of the Sun; it will only have roughly the mass of a city. So if you want to pack all 1057 of the Sun’s atoms into a ball the size of a city, there will be no room for the atoms to be ordinary atoms. Instead, they get squashed together until their nuclei bump together and merge — the neutron star becomes basically one big atomic nucleus, an element with atomic weight 1057.
Now, a nucleus is unstable if its atomic weight is more than 240 or so, i.e. above uranium. So a nucleus with atomic weight 1057 is not something that would normally stay together for more than a fraction of a second, let alone millions of years. It is intrinsically very unstable, very unhappy being so big and dense. It only stays together as a star because of the tremendous force of gravity holding it in.
To get a feeling for how strong gravity is on a neutron star, imagine trying to stand on one. Your body would weigh several trillion kilograms — except that it wouldn’t, because that much weight would squash you, and gravity would just merge the few atoms that are you into the vast pool of nuclei already there. In fact, the whole neutron star is pretty close to being pulled in too much by gravity — it is just on the edge of collapsing into a black hole. It manages to resist this pull because the nuclear matter hates being compressed so tightly. The matter pushes back, and this just barely manages to keep the star in balance.
So how do neutron stars explode?
Given this tricky balance, neutron stars are okay if they are left alone, or if a bit of stuff just gradually falls onto them. But smashing two of them together at close to the speed of light is a different story. That will knock a lot of the stuff off of the surface; and once that material is freed from its confinement, it goes wild. All the energy that was put into compressing it in the first place is now available to help it escape the neutron star’s gravity.
The result is a huge kilonova explosion, a cloud of matter expanding away from the site of the collision initially at close to the speed of light, incredibly hot material, full of twisted magnetic fields, giving off radiation across the spectrum. It is a display to entertain astronomers of every persuasion. When GW170817 happened, it is said that something like 10% of all the world’s professional astronomers rushed to their telescopes.
But in fact only part of the neutron star stuff exploded away. When the stars collided, their combined gravity actually got stronger, because there was now nearly twice as much mass. This immense gravity squashed the two stars into one, holding onto most of the original matter. And although a single neutron star can generate enough pressure inside itself to resist collapse, a neutron star with twice the normal mass probably can’t. So the final merged remnant in GW170817 most likely collapsed to a single black hole.
The gamma-ray burst that happened just 2 seconds after the stars merged would have been triggered by this collapse. A beam of this radiation pierced through the already expanding cloud of escaping matter and announced the explosion to us. The material that blew off was the lucky stuff: the rest of our two neutron stars has fallen into the black hole that the stars themselves created, disappearing forever.
Why did everyone get so excited by a couple of neutron stars colliding, so far away?
Normally astronomers don’t know about it when an explosion like this happens. They may chance across evidence of it long afterwards, but by then it is too late. So when the gamma-ray and gravitational-wave astronomers quickly spread the word that this explosion was just starting, astronomers rushed to monitor it. In fact some of them are still watching it, using telescopes in the radio and X-ray parts of the spectrum.
One of the reasons for wanting to watch it happen is that kilonovas like this are rare, occurring maybe once every 100,000 years in any one galaxy. The gravitational waves from GW170817 told us that the explosion had happened in a galaxy 120 million light-years away. This may seem like an enormous distance, but for LIGO and Virgo this was exceptionally close: they could have seen it even if it had been three times more distant. And for astronomers this is also pretty close: even modest-sized professional telescopes could study it.
But the interest in GW170817 is not just its rarity. What is even more interesting is that neutron-star mergers have played a surprisingly important role in enabling life on Earth, and even for human evolution. This is because of what the stuff that is blown off in the kilonova turns into. It starts out, remember, as part of one big nucleus. As it expands away, its nuclear instability reasserts itself, and it breaks up into chunks. This is nuclear fission on a grand scale, and the kilonova event is nothing less than a gigantic atomic bomb, something like a 1035 megaton explosion.
The initial chunks are still huge, so they keep breaking up into smaller and smaller chunks, until the chunks are small enough to be stable atomic nuclei again. At this point the nuclear fission stops. The expanding cloud ends up made up of stable atoms, but its composition is strongly biased toward the heavier elements of the Periodic Table.
Now here is the important bit: astronomers have come to the conclusion that this is the main way that these heavy elements are made throughout the Universe. Most of the ordinary matter in the Universe (and in our Sun) is just hydrogen and helium, the two lightest elements. So essentially everything else has to have been made by stars. Astronomers’ observations of GW170817 have confirmed that the expanding cloud of gas was indeed heavily loaded with elements from the upper half of the Periodic Table.
GW170817 and the origin of life
You may remember the newspaper stories about GW170817 when it was made public in October last year; they enjoyed talking about how all the gold and platinum in our jewelry must have been made long long ago in another similar explosion.
Now, although the extraordinary history of the gold in our rings is truly awe-inspiring, this actually misses the most important point. At least two of these heavy elements are essential to human life: molybdenum and iodine. As shown in the figure, iodine is produced almost exclusively from neutron-star binary mergers, and molybdenum (involved among other things in producing the body’s energy source ATP) comes from neutron stars and low-mass stars in roughly equal amounts.
And then there are thorium and uranium, which may have played a key role in the evolution of life itself. That is because the gradual decay of the long-lived uranium and thorium isotopes in the Earth’s mantel provides roughly half of the heat that, in flowing outwards through the solid Earth, drives volcanism and plate tectonics. In particular, the movements of the continents seem to need the extra heat generated by these ultra-heavy elements. These geological phenomena have played an important role in the evolution of life on Earth, by challenging species with a constantly changing environment that drives continued evolution. It does not seem unreasonable to speculate that, if the Earth had been geologically quiet, some well-adapted distant ancestor of ours might just never have needed to change, and evolution might have slowed dramatically or even stopped.
The presence in our planet of all the elements in the heaviest half of the Periodic Table tells us that, sometime before the Sun formed 5 billion years ago, a neutron-star merger — driven by the loss of orbital energy to gravitational waves — happened in our own Milky Way galaxy, triggering its version the GW170817 kilonova. Its expanding cloud of heavy elements polluted a cloud of hydrogen and helium gas, and that cloud later collapsed to form the Sun. This gas cloud had also evidently been polluted with the lighter elements of the Periodic Table that had been formed in other much more frequent nearby events, such as in supernova explosions or in the gas blown away at the end of the life of stars like our Sun. The expanding cloud of a supernova or of the kilonova may even have triggered the collapse of the polluted gas cloud, which led to the formation of the Sun, and hence of the planets, and eventually of living things.
We are, as has often been remarked, stardust. But there might just be one particular long-ago gravitational-wave-driven neutron star merger, probably very like GW170817, to which we owe our very lives.
I mentioned here but did not explain how the gravitational wave signal told us how far away the source was. This is a whole story in itself, and I will come back to it in a future post. It led to a milestone in gravitational-wave astronomy when we were able to use GW170817 to make a measurement of the expansion rate of the Universe, the Hubble constant.
Last week the international gravitational wave collaboration* announced its third very secure detection. Our new acquaintance GW170104 — named for its arrival date — passed through our two LIGO detectors and whispered to them about the coalescence of a pair of black holes in a binary system that had taken place almost three billion years ago. And then it raced on, hardly affected at all by its encounter with us and our planet. In that respect it was much like its two predecessors, GW150914 and GW151226. But every detection is special, and this one is very special indeed.
One reason is simple: it brought a big smile to all of us in the collaboration, and allowed us a big sigh of relief — our first two detections were not just a lucky fluke, nor a super-mysterious instrumental malfunction. The dates of the detections tell the story: the first two in 2015, this one at the start of 2017. The big gap is there because the LIGO detectors had been shut down for most of 2016 in order to improve their sensitivity. Once we started up again, with significantly modified detectors, Nature provided us with helpful reassurance: the waves keep coming, and they look just the same as they did in the previous version of the detectors. (Goodness knows what we missed during 2016!)
The second important thing about GW170104 is that it has started our transition from mainly doing fundamental gravitational physics to primarily doing gravitational-wave astronomy. The first detection GW150914 was the science event of the decade because it showed how right Einstein was with general relativity: his amazing fundamental theory of gravity had predicted gravitational waves and black holes, and in just one event, lasting only about two tenths of a second, both predictions were spectacularly confirmed. But now, as we continue to observe more such events, we become more and more focussed on what they tell us about the Universe. We will of course use all new data to further test our confidence in Einstein’s general relativity. But the new detections are going to explore more and more of what Kip Thorne, one of the founders of LIGO, famously called the Dark Side of the Universe.
A hint of spin
But the really special aspect of the new detection is the hint it gives us about the way the black holes may have been spinning. Black holes spin, as do all astronomical bodies. And the normal expectation is that the spins in a binary system of black holes will be in a consistent direction, just like the spins of planets in the solar system. The Sun and the planets all formed together out of a rotating cloud of gas, so it is natural that they inherited their spins from that cloud, so that these spins are all roughly in the same sense, which is also the same as the sense of the orbital rotation of the planets around the Sun, and indeed of the spin of the Sun itself. In our previous detection, GW151226, we found evidence that the more massive of the two holes was spinning, and the evidence was that its spin was probably aligned with the sense of the orbital rotation. But in GW170104, there is just a hint that the black holes’ spins were either anti-aligned with each other, or they were anti-aligned with the sense of their orbital motion about one another before they merged.
if this was the case, then it would suggest that the two black holes had not been formed together from a single primeval cloud of gas. Instead, they were probably unrelated single black holes, which just by chance linked up into a binary system. This ought to sound bizarre to you, wildly improbable. After all, black holes, even the rather massive ones in this system, are only something like 100 km across, maybe twice the size of London. How could two such small objects ever encounter one another if they were wandering through the vast emptiness of space between the stars in their galaxy? And you would be right: that just wouldn’t happen. But it would be much more probable if it happened inside the big dense clusters of stars that we call globular clusters.
These globular clusters are roughly spherical micro-galaxies, and in them heavy things like black holes sink, over millions of years, down toward their centers, where there can be thousands of stars in a volume that would contain just one star in neighbourhood of our Sun. In these conditions, near encounters between black holes do sometimes happen. What’s more, if a third object, even a normal star, is nearby, then sometimes the third object can be a kind of catalyst, helping the black holes to form a binary system by stealing and speeding off with some of their energy. So the hint that the spins in GW170104 don’t match up is a hint that this system might have been formed in this haphazard way.
The magnetic side of gravity
But here’s the amazing part: how did we get this hint in the first place, about how things spin in a system that is totally in the Dark Side? What did we read in the signal we call GW170104 that tells us about the spins? The answer lies again in the fundamentals of Einstein’s theory. Einstein predicted that spin creates a different form of gravity, a magnetic form. Think of a spinning charge, or of the electric current winding round and round through the coil in an electromagnet: this creates magnetism, which we know is the partner of the electric force in the theory of electromagnetism. Magnets exert forces on other magnets and on moving charges. In a closely analogous way, a spinning mass creates a form of gravity that we call gravitomagnetism, through which spinning masses exert gravitational forces on other spinning masses and on moving masses. So our spinning black holes are gravitomagnets, which interact with one another and with their orbital motion, and this slightly changes the orbital periods of the last few orbits before the holes merge together. We always look in our data for these changes, and in this case we found a hint not only that the spins were significant, but that they might be anti-aligned.
But it is only a hint. The odds are only 4-1 in favor of the spins’ being anti-aligned rather than their being consistently aligned. It could be simply a distortion in the signal caused by a slightly improbable level of noise in our detectors. But since we will never get any more information about this particular system, we will never be able to say for sure whether this was how GW170104 in fact formed. So GW170104 is not only special, it is also frustrating!
What sort of physics are we doing?
This frustration has pushed the global gravitational wave collaboration to confront the question of whether we are still primarily doing fundamental physics or whether we have started the transition to astronomy. One of the differences between these two disciplines is their expectation of how much uncertainty they can tolerate in a measurement. The observation of gravitomagnetism is of course very fundamental, even though this prediction of Einstein has already been verified by an experiment in orbit around the earth  and by observations of the orbits of satellites themselves . A fundamental physicist wants to pin down the laws of physics, and that needs a great deal of certainty. As a measurement of gravitomagnetism, our hint of spin doesn’t do as well as the previous experiments have done, and is certainly not up to our standard for deciding we have detected something in the first place. While the actual detection of GW170104 has very high significance (odds of better than 70,000 to 1 that no chance noise-generated event this strong would have happened in coincidence between the two detectors in a whole year’s worth of data), the information we can glean about spin from the slight spin-induced nuance in the orbital motion has only 4-1 odds of being real. So a number of physicists in the collaboration did not want to see the spin result highlighted strongly in the announcement of our observation.
Astronomers, by contrast, have learned to live with uncertainty: you can never do a controlled experiment on a star, so you have to be content with whatever data it sends you. Some of the astronomers in our collaboration felt that the possible implications of this hint of spin for our understanding of the formation history of this system was interesting enough to deserve prominence in our announcement, even at this low significance level. The philosophy is to put the data out there so that other astronomers can judge whether they find it interesting or not. This clash of scientific cultures led to a rather lively discussion inside the collaboration on how to report the tantalising but rather uncertain measurement on the spins of the black holes, and it even delayed the announcement of the detection by some weeks. What you can read in the paper we wrote and the publicity we put out is, of course, our compromise. As you might guess from this post, I find myself on the side of the astronomers.
But it was already 5 months ago…
The detection we announced last week happened 5 months ago. Has anything else interesting been detected since then? Well, by the confidentiality rules of our collaboration, I can’t answer that question. But what I can say is that the increases in sensitivity that we looked forward to a year ago are taking more time than we hoped. These incredibly complicated detectors, with their astonishing sensitivity to such weak gravitational waves, are also immensely sensitive to all kinds of disturbances, from outside the detectors and especially from inside. The two LIGO detectors are indeed more sensitive now than they were before they shut down at the beginning of 2016, but they will need much more work before they reach the ultimate goal that we call Advanced LIGO. The Italian-French detector Virgo, near Pisa, has also had a number of challenges in improving its sensitivity. We are hoping it will start observations soon, finally giving us the three-detector network that is required in order to get accurate information about the location of events on the sky and their distances.
We can expect a further planned shutdown of LIGO, and probably of Virgo, late this year or early next year, and another long wait for data as the sensitivity is further improved. But this kind of wait should be worthwhile. A factor of 2 increase in sensitivity corresponds to a factor of 2 increase in the range of our detectors, in the maximum distance to which we can detect events. This implies an increase in the volume we can survey by a factor of 8, so it would bring us more events in 2 months than we presently get in a year.
Gold from gravitational waves
And higher sensitivity might also get us a new kind of event: mergers of neutron stars from binary orbits. They are less massive than black holes, so their signals are weaker, but they are no less interesting. Whereas black holes merge, well, blackly, neutron stars merge in a burst of color. They should produce a spectacular display of light, radio waves, X-rays, and gamma-rays, easily detectable by telescopes on Earth.
Besides all the physics and astronomy that we will learn from neutron star mergers, here is the really spooky part: when we finally observe such an event, we may be observing a re-run of our own cosmic history. Astronomers believe that most of the common heavy elements on Earth — gold, silver, platinum, mercury, uranium and many more — may have been created in just one such event, the merger of two neutron stars. They were ejected by the explosion triggered by the merger, polluting the nearby gas cloud that would eventually condense into our Sun and solar system. In fact the explosion might even have triggered that cloud to condense, setting off the long chain of events that led to our own existence. The rings on our fingers, and perhaps even our fingers themselves, may have come from a nearby gravitational wave event that happened long long before we were able to build detectors to observe it!
So there is much more to come, with patience.
* The collaboration consists of the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration. Altogether there are over 1000 scientific members as authors to the papers.
1. Everitt; et al. (2011). “Gravity Probe B: Final Results of a Space Experiment to Test General Relativity”. Physical Review Letters. 106 (22): 221101.
2. Ciufolini, I.; Pavlis, E. C. (2004). “A confirmation of the general relativistic prediction of the Lense–Thirring effect”. Nature. 431, 958–960. Ciufolini, I., et al (2016): “A Test of General Relativity Using the LARES and LAGEOS Satellites and a GRACE Earth’s Gravity Model”.Eur. Phys. J. C. 76, 120.
Just a year ago today, after travelling some 1.4 billion years, the gravitational wave chirp we christened GW150914 passed through Earth. It disturbed the two gravitational wave detectors of the LIGO observatory enough for us to notice it, to get excited about it, and to get a large fraction of the general public excited about it! But GW150914 just kept on going and is now one further year along in its journey through the Universe. And it will keep going, spreading out and getting weaker but not otherwise being much disturbed, forever. Literally forever.
And GW150914 hardly noticed us! When we observe the Universe with our telescopes, detecting light or radio waves or gamma rays from the enormous variety of luminous objects out there, we capture the energy that enters our telescopes. The photons from a distant star terminate their journeys in our telescopes, leaving a tiny hole in the ever expanding cloud of photons that we didn’t catch. We simply eat up the ones we catch. But GW150914 transferred an absolutely minuscule amount of its energy into the LIGO detectors. We and the famous chirp enjoyed a brief handshake, and then it was gone.
Not that GW150914 had little energy to give: quite the opposite. At its peak, it was 20% as “bright” as the full moon! For the few milliseconds of its passage, GW150914 outdid any star in the sky. Of course, its energy wasn’t in the form of light, so it wasn’t visible to anyone who by chance happened to be looking straight at it. But the energy was there: the gravitational wave energy going through that lucky stargazer’s pupil was 20% of the light energy that would have gone in, had the stargazer turned to gaze at the full moon. The difference, as I noted above, is that the moon’s light energy would have been deposited in the stargazer’s retina; the gravitational wave energy didn’t stay around but just kept going through, leaving almost nothing behind.
It was the same story with all the other objects that GW150914 had encountered before it reached Earth. And it will be the same in the future, which is why the chirp will keep going, forever.
This seeming lack of engagement on the part of GW150914, its reluctance to share its energy with us, comes basically from the extreme weakness of gravity itself. Light and other forms of electromagnetic radiation connect to electric charges, and the coupling between them is strong because the electric force is strong, much stronger than gravity.
There is a simple way to get a feeling for the big disparity between these two forces. Pick up a tennis ball and you are demonstrating the immense superiority of the electric force over gravity. The weight of the ball is the result of all the atoms in our entire planet pulling back on it with their gravitational attraction. The electric force governs the structure of atoms and molecules, and regulates chemistry and the structure of materials. Your arm muscles’ chemistry easily defeats the total gravitational attraction, even though the muscle mass doing the work is less than one part in 10^24 of Earth’s mass. (That is, Earth has one million million million million times more mass than the muscles of one of your arms!) So when GW150914 passed through you (as it did one year ago), it was too weak to disturb you, so of course almost no energy was transferred to you.
How is this weakness consistent with the fact that it was carrying such a huge amount of energy? Here the best way to understand this apparent contradiction is to go back to Einstein’s basic picture of gravity, that gravity is the warping of space and time. It should be no surprise that it is exceedingly difficult to warp space. Before Einstein, nobody even thought it might be possible. A measure of how hard it is to bend space is that the waves of space that carry this warping, the gravitational waves, travel at the speed of light. Now, think about waves in other materials, and how stiffness of the material is related to the speed of the waves. Sound, for example, travels pretty fast through air but much faster through steel. Water waves travel rather slowly, but a crack in an ice sheet can streak across the sheet in no time flat.
By this measure, space is the stiffest medium we know, because its waves go at the speed of light, the fastest speed possible, a speed that is immensely faster than that of waves in any other material we know. But bending a stiff thing is hard, so bending space is hardest of all. To get GW150914 going required a huge energy input, even for a wave with such a weak effect on us. The chirp, as it started out, carried as much energy in total as one would get by converting the mass of three Suns into pure energy via Einstein’s famous E = m c². That was a blast equivalent to 10^34 Hiroshima-scale nuclear explosions. (That’s ten thousand million million million million million bombs!) All this energy came out in a fraction of a second. If you added up all the energy (in light, mainly) that all the stars and other objects in the entire Universe were putting out during that fraction of a second, you would come to a number that is 10 to 100 times smaller.
So our friend GW150914 was a messenger, giving us notice of an almost unimaginable event that was briefly more luminous in gravitational wave energy than the luminosity in light of the entire rest of the Universe put together. And that brings us to whose birthday today really is: that of the black hole that was formed in that inconceivably large gravitational-wave explosion. It was formed by the merging together of two pretty hefty black holes, one about 35 times as massive as the Sun and the other about 30 times. The black hole that was born on that day 1.4 billion years ago ended up with a mass of 62 solar masses. That is 3 less than the sum of 35 and 30: the deficit is the 3 solar masses that got converted into gravitational wave energy and set out across the Universe.
We know all this about GW150914’s pedigree because we were ready for this kind of message. We already knew how to read the information encoded in the message, encoded by the dynamics of Einstein’s gravity. That is a story for another time, for a future entry in my blog. For today, I and many of my colleagues in the gravitational wave collaboration are just going to raise a glass and wish GW150914 a very happy birthday, and many returns of the day! 🍾🎉