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!
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.
[Credit: NSF/LIGO/Sonoma State University/A. Simonnet]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.
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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.
A couple of weeks ago I helped kidnap Physics Nobel Prize winner Kip Thorne. He was at one of those glitterati Hollywood banquets that he goes to from time to time. There were 45 of us. We broke into the banquet room, surrounded the guests, and — after singing to him — hustled Kip out the door and off to Pasadena, where we gave him a meal instead in an Indian restaurant.
Well, it was almost like that anyway. It all started when some of Kip’s former PhD students conceived the brilliant idea of throwing a surprise party for Kip, to celebrate with him his sharing in the Nobel Prize in Physics in December. The team was led by Richard Price, editor of the American Journal of Physics. Carolee Weinstein, Kip’s wife, was on the team, as were Carlton Caves and Sándor Kovács. It was a simple plan: we all turn up in Pasadena on Saturday January 13, Carolee delivers Kip, we shout “Surprise!”, and we all have a great time.
At first the planning went perfectly. The word circulated discreetly, and something like 45 people signed on, a few even coming from Asia, South America, and Europe. And best of all, word of it never reached Kip’s ears. The event known as “Kip’s Spawn Reunion” was looking good!
But — it was such a well-kept secret that Kip got himself double-booked. That same weekend, Caltech was hosting a Physics Summit. The Summit series has traditionally attracted top thinkers in physics and related fields, and what could be more appropriate than to invite the two new local Nobel Physics Laureates, Kip and Barry Barish, to the Gala on Saturday January 13? And to invite local members of the LIGO project too, in order to recognize their contribution to the huge success of the gravitational wave detection enterprise. Kip, among the founding members of the series, happily accepted the invitation to deliver remarks at the Gala, despite having just returned from India the day before. And why not — there was nothing else in his calendar.
But unfortunately there was something else in all of Kip’s spawn’s calendars! And changing the date wasn’t an option: air tickets had been booked, hotels had been arranged. Happily, what to some people might be a disaster, to Richard Price and his team was an opportunity. They enlisted the help of the Caltech professor of physics in charge of the series, and through her they got the cooperation of the Gala venue staff. Together they evolved an audacious plan that required secrecy, complex coordination, the devious cooperation of many people, and luck!
Something unlike anything that had happened to a Nobel Prize winner before was going to happen to Kip on the 13th of January. He was going to be abducted by some of his closest colleagues.
If if worked, then something unlike anything that had happened to a Nobel Prize winner before was going to happen to Kip on the 13th of January. He was going to be abducted by some of his closest colleagues.
Late Saturday afternoon, about 45 people rendezvoused at the New Delhi Palace restaurant in Pasadena and then piled into a bus and headed for Hollywood. With military precision timing — or at least as close to that as Los Angeles traffic would allow — the bus arrived at the venue. We waited a block away until we were sure that Kip and the others had gone inside, and then we advanced. The venue staff, grinning conspiratorially, guided us to the service elevator that took us to the rooftop level, where the banquet room was. Four elevator-loads later, we were clustered near the swimming pool, out of sight of the event guests, admiring a pretty spectacular nighttime view of the Hollywood hills. Then word came by text from inside the dining room: Kip was speaking.
Single-file, with serious demeanor, we made our way through the kitchen and into the dining room, winding our way around the walls to encircle the guests. Kip looked astonished, remarked “What is going on?” — but, trooper that he is, he carried on, not missing a word of his speech paying tribute to his LIGO colleagues. His audience, of course, was more than a little distracted, but when Kip asked the LIGO team to stand, the rest of the guests pitched in with plenty of applause. Kip was followed at the lectern by J. Nolan, the acclaimed screenwriter, producer, and author, who paid a very warm tribute to Kip, with whom he had worked on the film Interstellar. Then, on cue from the professor who had become our co-conspirator, we took over.
First, four of us took turns loudly scolding Kip for not responding to our emails and phone calls because he was too busy with travel, poetry, Hollywood, … .
Richard then tried to reassure Kip by reminding us all that we had always found him to be a stable genius, and kind of, like, you know, really smart.
Richard then tried to reassure Kip by reminding us all that we had always found him to be a truly stable genius, and, like, you know, really smart.
The proceedings finally reached their nadir as all 45 of us burst into a rendition of that great Bernie and The Gravitones hit, Wise OldAdvisor From Pasadena (which had been created on the occasion of Kip’s 60th birthday), singing along to the original, which the venue was playing over the their audio system.
By the end of the second verse we had already long overstayed our welcome, so it was time for our final move: Richard and Carolee got Kip to stand up and lead us, single-file again, out of the room and into the elevators. Down we went and onto the bus and back to Pasadena. We returned to the New Delhi Palace, where an excellent buffet awaited us, and where Kip, beaming all the time, wandered the room, making sure he caught up with each of his former students.
Well, that is the true story of how we kidnapped Kip Thorne. And I think he enjoyed it!
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[For those of you who might want an official record of the proceedings, here is our script, with many thanks to Richard Price —
The voices of the Former Students are shouted by four Spawn in diverse locations around the room.
Student #1 (Cliff Will): Kip, I was your student. Kip, I’ve written the first draft of the paper we talked about. I’ve been trying to get in touch with you. I tried email. I tried calling. When I called I was told you are away, in Stockholm or India. They weren’t sure.
Student #2 (Bill Press): Kip, I was your student. Kip I would like you to be on a panel I am organizing, but I haven’t been able to get in touch with you. I tried email. When I called I think that they said that you were at a poetry reading, but I probably heard wrong.
Student #3: (Bernie Schutz) Kip, I was your student. Kip, I need to submit my grant proposal, and I was hoping for a letter of support from you. I’ve sent you email. I tried calling but I was told that you were on a movie set.
Student #4: (Saul Teukolsky) Kip, I was your student. Kip, I need a letter of recommendation for my tenure decision. I tried sending email. Then I tried calling. They had no idea where you were.
Richard (to Kip):
Kip we are your academic spawn. Kip.. Why hast thou forsaken us? You are our mentor; we are your mentees. We are the products; you are the producer. In unstable times, and in our unstable lives you have been a stable genius. And kind of, like, you know, really smart. But now… now you are a cinema celebrity, a poet of some renown, Kip-
We would hate to embarrass you; the last thing we would want to do is to embarrass you, but we are reclaiming you because to us, you will always be our wise old advisor from Pasadena. (Music blares.)]
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 [1] and by observations of the orbits of satellites themselves [2]. 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.
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* 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.
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1. Everitt; et al. (2011). “Gravity Probe B: Final Results of a Space Experiment to Test General Relativity”. Physical Review Letters. 106 (22): 221101.
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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! 🍾🎉
The Rumbling Universe 