5 years ago: GW170817 proved the value of Gravitational Wave Detection

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!

Eddington in Cardiff 100 years ago today: the first proposal that stars are powered by fusion

“… fulfilment [of] our dream of controlling this latent power for the well-being of the human race – or for its suicide.”

One hundred years ago today, on 24 August 1920, with over 1000 people gathered in Cardiff for the annual meeting of the British Association, Arthur Eddington gave his address as the incoming president of the physical and mathematical sciences section. He elected to speak on the subject of the “Internal Constitution of the Stars”. When I first came across the text of the address last year (published in Nature in 1920), I was amazed to find as early as this such an insightful proposal that stars are powered by the synthesis of helium from hydrogen. But what really brought me up short was this sentence:

If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfilment our dream of controlling this latent power for the well-being of the human race – or for its suicide.

Twelve years before Chadwick discovered the neutron, twenty-five years before Hiroshima, thirty-seven years before the nations of the world agreed to cooperate to make peaceful fusion power a reality, Eddington basically saw the whole package. 

What Eddington had learned in 1920 — and it was enough to open the whole vista to him — was the rest-mass deficit:

The nucleus of the helium atom, for example, consists of four hydrogen atoms bound with two electrons. But Aston has further shown conclusively that the mass of the helium atom is less than the sum of the masses of the four hydrogen atoms which enter into it; and in this, at any rate, the chemists agree with him. There is a loss of mass in the synthesis amounting to about 1 part in 120, the atomic weight of hydrogen being and that of 1·008 and that of helium just 4.

Francis W. Aston in fact presented this (Nobel-Prize winning) result at the Cardiff meeting the following day, as recorded in the meeting’s archives. So Eddington the relativist had what he needed: energy would necessarily be released if four hydrogen atoms could be persuaded to combine to form a helium nucleus. How they might decide to do this, and then how the helium nucleus might contrive to stay together against the electrostatic repulsion that the four protons exerted on one another, offset by only two electrons: these were not things that Eddington even talked about. Eddington knew that answering these questions wasn’t necessary at this point. All that mattered was that the mass deficit, converted into energy, was ample to power the stars:

If 5 per cent. of a star’s mass consists initially of hydrogen atoms, which are gradually being combined to form more complex elements, the total heat liberated will more than suffice for our demands, and we need look no further for the source of a star’s energy.

Notice that Eddington is not stopping just with helium. He expects this process will synthesise even heavier elements, although he observes that the energy payoff is not so dramatic as for the conversion of hydrogen into helium.

Apparently most people in 1920 still seemed to believe in Kelvin’s hypothesis that stellar contraction and the liberation of gravitational energy should power the stars. To prepare for his nuclear fusion hypothesis, Eddington earlier in the article ruthlessly destroys this idea, particularly pointing out that the short lifetime of stars implied by Kelvin would already have produced observable changes in the pulsation periods of some Cephied variable stars. Anyway, he says, most scientists no longer take Kelvin’s idea seriously, even though they have nothing to replace it with:

Lord Kelvin’s date of the creation of the sun is treated with no more respect than Archbishop Ussher’s.

Mindful that, even so, his audience may not be prepared to switch to the nuclear hypothesis so readily, he acknowledges

I should not be surprised if it is whispered that this address has at times verged on being a little bit speculative … 

and then he goes on to defend the role of speculation in theoretical physics, including pointing out that even wrong speculations can help advance a field if they motivate experiments that clarify a subject.

Eddington’s address covers much more than just how stars shine. He starts by trying to establish the track in the Hertzsprung-Russel diagram that stars follow as they evolve. This is not what we understand today, although it was a hugely important step in his time: he thinks giant stars are young stars that evolve into a Sun-like stage and then become dwarfs. It would take a good number of years before the theory of nucleosynthesis in stars would lead to the more complicated evolutionary tracks we are familiar with today. And speaking of dwarfs, the story is well-known of how, fifteen years later, Eddington arrogantly rejected the perfectly sound calculations of a young Chandrasekhar, who had had the temerity to suggest that dwarfs had a maximum mass, so that evolutionary tracks of very massive stars had to lead elsewhere — Eddington was not having anything to do with what we now call black holes. Maybe speculation had lost its lustre in those fifteen years!

Eddington of course published a book with the same name as the lecture, the first edition appearing in 1926 and covering much the same material, but more quantitatively and extensively. So his Cardiff presidential address was just the taster. But what a taster! 


Eddington’s address was printed as a news item in Nature a week later: vol 106, p. 14. It is freely available from Nature here. The book of the same name was published by Cambridge University Press, which now has an online edition available, with an introduction by none other than Chandrasekhar. Eddington in his book also credits J. Perrin for independently coming to the same conclusion about the fusion of hydrogen to helium, also in 1920: Revue du Mois 21, 113.

Black Lives Matter to Physics

A racist attack on a black person is an attack on the core of America. White Americans need to understand that it is an attack on them too.

Today I am joining the Strike for Black Lives at the urging of the particle physics movement Particles for Justice. Here I want not just to show my support publicly, but to share my concern that this issue, which I thought way back in 1965 we were well on the way to eliminating, continues to poison American society. Let me summarise where I am going by saying that thinking about this struggle as just an issue of failed justice, as the particle physicists’ webpage name suggests, is not going to be enough to save black lives in the future.

We all need to understand that what happens to black people happens to America. Killing black people harms the whole society. I am afraid that the demonstrations of the past days won’t accomplish their aim if the struggle is framed as one of protecting and bringing justice to a minority. The frame must be bigger. Americans need to begin to see black people as a cross section of America. Simply, black people embody America.

Before explaining my point of view, I want to start with the situation in physics. I started my PhD in 1967, and there were no black Americans in my cohort. There was also only one woman. Today there are significant numbers of women physicists (although in the US it is far short of 50%), but many fewer black Americans. There are also plenty of immigrants, many with dark skin from South Asia. Now, physics may be esoteric to many but it carries high prestige. When physicists are interviewed on TV or give advice to government, what they say is important, but so is the metadata: these white men, these women, these immigrants are making important contributions to society. The paucity of black Americans in these forums certainly does nothing to change the minds of people who think that black lives don’t much matter. Physics is failing here. And this doesn’t even mention how we are failing the young blacks who – just as we ourselves were – are turned on by the subject, but who – unlike us – won’t get the chance to go for it.

The disparity in the progress of women and black Americans in physics between 1967 and today contains a bitter irony. The women’s movement for equal treatment took huge inspiration from the black civil rights movement of the 1960s. The Civil Rights Act of 1968 prohibited discrimination on the basis of race, religion or national origin. It wasn’t until 1974 that it was amended to include sex in that list. Protection against discrimination for people with disabilities and for gay people followed even later.

This takes us to the core of my concern. In the words of Pulitzer Prize winner Nikole Hannah-Jones in last year’s podcast series 1619 (which I highly recommend) on American slavery, “It is black people who have been the perfectors of democracy in this country.” They haven’t taken democracy for granted they way most of us whites have (and now realize that we shouldn’t have). If there were two things that America has, at least until recently, been admired abroad for, they were its dedication to democracy and its creative, ebullient musical culture. America owes both to its black population.

Black Americans have roots in their country that go much, much further back in time than do those of most white Americans. The ancestors of any black American, taken together, probably did more to build America than my ancestors did, more than the ancestors of almost any other American who claims German or Irish or Scandinavian or Italian or Polish heritage. If any group could be said to own America, black Americans have the second-best claim, after native Americans. But of course neither group runs the place.

And then there is the question of what we mean when we talk about black Americans. There is no well-defined distinction between black Americans and the rest of us. Their ancestors’ heritage in Africa was just as culturally diverse as that of the later European immigrants. And as geneticists have learned, their ancestors in Africa were genetically more diverse from one another than any European is from any other non-African, let alone from other Europeans. And then of course most black Americans have European genes as well, from a long history of rape. Uncomfortable as that may be to whites, what it means is that black Americans are the best example America has of the melting pot. Slaves were brought to America and thrown together with greater differences than our European ancestors had in languages, customs, skin colors, stature, lip shapes, nose shapes — you name it.

From all this variety, black Americans built much of America, gave it its music, led the defence of its democracy. We white Americans must understand, no – we must have respect for, how central to the core of America our fellow black citizens are. There is a political battle going on in America right now over who defines the country, and very clearly democracy and the rule of law are under threat. Science is also clearly under attack by the same forces, already partly forced to toe the political line. No people in America has a greater interest in preserving America’s core values than the black population. It is not enough for us whites or us physicists just to want to help people who are beleaguered. A racist attack on a black person is an attack on the core of America. White Americans need to understand that it is an attack on them too.

Black people embody America. Unless we whites understand that, I fear that there won’t be enough momentum to change once and for all the deeply racist institutions that over the years have kept their power, that have kept denying the important fact that black lives matter. BLM matters to democracy, to science, to everyone.

(Image of Washington DC and the White House courtesy of Planet.)

25 Years of the AEI

Today is the 25th anniversary of the opening of the Albert Einstein Institute (AEI) in 1995. It was a day that changed my life — for the better! — and that has affected the science of Einstein’s general relativity in lots of ways, also I believe for the better! So I hope it will be interesting to some readers if I ramble and reminisce today about the foundation of the AEI 25 years ago. Since today much of the world is shut down because of the SARS-CoV-2 pandemic, what could be a better time for a virtual history dig into this important institution? Our real silver anniversary celebration is planned for October this year, COVID-19 willing!

The AEI belongs to the Max Planck Society (MPS), and its formal name is the Max Planck Institute for Gravitational Physics, or even better das Max-Planck-Institut für Gravitationsphysik. I will talk about how it got two names in a minute, but the first thing is how the AEI got started. 

Jürgen Ehlers’ one political idea

The AEI exists because of a political event — the fall of the Berlin Wall in 1989 followed by the reunification of Germany in 1990 — and a political idea — the proposal by Jürgen Ehlers to the MPS that it should set up a new research institute for general relativity.

(The early history is much more complicated than this brief statement might suggest, since there were different suggestions by a number of others as to how to re-establish relativity research in Germany in this period, where reunification seemed to open up many possibilities. See the essay by Hubert Gönner for a very different perspective.)

Jürgen by 1989 had become one of the world’s leading experts in general relativity theory, a gentle mathematician who ran a small but world-class research group in the Max Planck Institute for Astrophysics in Garching, near Munich. He was at home with the most difficult mathematical challenges of Einstein’s theory, and to address them he liked working alone or with a small group of collaborators. His work had sometimes led him into controversy, such as with the eminent Nobel Prize winning astrophysicist Chandrasekhar, but he was not a mover and shaker, not someone who wanted to organise the world of science. But he could recognise an opportunity when he saw it, and he saw it in 1991. 

The Max Planck Society had decided, with the support of its funder, the German government, that it needed to expand into the states of the former East Germany by setting up new institutes. So it was looking for suggestions. Jürgen decided to pursue what he later called the one political idea of his life: one of them should be dedicated to general relativity. This wasn’t just a scientific proposal, it was also a political one, because after all Einstein had developed general relativity in Berlin, and then in the Nazi times his work had been vilified in Germany. So the MPS ought to do this not only for the science, but also as a public affirmation of the importance of this science and an acknowledgement of its German roots. The force of this argument was strong, and Jürgen soon found himself the nominal leader of the project to shape a formal proposal and get it approved by MPS. He recruited his two closest collaborators and group members, Bernd Schmidt and Helmut Friedrich, to help him.

From idea to actuality

Bernd Schmidt took me aside as early as December 1991, when were were both attending the ICGC-91 conference in Ahmedabad, to ask if I would be interested to be one of the founding directors if the proposal succeeded. This is how setting up an institute works — in MPS, an institute’s management is focused on a small number of directors, and MPS likes to stand back once it is started and let the directors run the show. So a proposal for a new institute is partly to do with the scientific need and partly to do with who will run it. 

The scientific case was going to include the growing impact of general relativity in astrophysics as seen in the early 1990s: ultra-relativistic neutron stars were being monitored as pulsars by radio astronomers, black holes almost certainly existed in many of the systems being studied by X-ray astronomers, the famous Hulse-Taylor binary pulsar had already proved that Einstein was right about gravitational waves, and it was looking like large-scale gravitational wave detectors were going to be built to look for such waves directly. I was involved in the theory behind a lot of these issues, so Bernd said that I might be a possible candidate for a director. 

Was I interested? You bet!

Between the Wall coming down in 1989 and the opening of our doors in 1995 is only six years, so in retrospect things moved quite fast! The proposal took shape rapidly, for an institute with three directors for its three divisions: mathematical relativity plus the two burgeoning research applications of Einstein’s theory, astrophysical relativity and quantum gravity. To start up quickly it might go with two directors (Jürgen and possibly me), expanding soon afterwards into string theory and quantum gravity. A small symposium was held in 1993 where I and a number of other people working at the interface between relativity and astrophysics were invited to speak, and where the most important part of the audience was a committee of senior scientists whose job it was to oversee the scientific decisions involved in founding the institute. After this committee ratified the idea that I should be the second founding director, it was time to inform myself more about the MPS, which works in a very different way from that of the universities I was accustomed to.

I began visiting the headquarters in Munich. In meetings with various MPS people, I was being informed but also recruited. One meeting stands out vividly in my memory, with the then MPS Vice President Herbert Walther. At one point in our amiable conversation he smiled and said that, if I became a director, that would mean that MPS would have such faith in my scientific judgement that if I were to decide to switch my division’s work from relativity to, say, chemistry, they would not do anything to stop me, as long as the work I did continued to be world-class! I understand that this is still said to new prospective directors today. It reflects the immense independence that directors have, which in my view is one of the reasons that research in the MPS is so strong.

I also began visiting Potsdam and Berlin: the association with Einstein made it a no-brainer that this was where the institute was going to be. Some of those visits included Jürgen and Bernd. We were scouting for start-up locations for the institute, discussing research priorities, and reaching understandings about how to run such an institute. Jürgen and I saw eye-to-eye on that one; we both wanted our scientists to work in a supportive and relaxed atmosphere. In early 1995, when we interviewed for our support positions, we explicitly looked for (and found) people who would help our scientists focus their time on science. On one of the visits, sitting with Bernd near the circular fountain beneath Sans Sousci palace, I remember first talking about the idea that later became the online open-access journal Living Reviews in Relativity: using a part of the financial resources we would control to develop a scholarly resource for our graduate students and for the whole community. It was a creative period, one I feel privileged to have been lucky enough to experience.

How we got our name

The whole process of founding an institute in MPS culminates with the final meeting with the President, which is called a negotiation because your terms of employment will be settled, as well as the budget for your institute. In 1994, Jürgen and I did the institute part together, meeting with then President Hans Zacher in Munich. Zacher wanted to discuss an important question that was on our minds: besides the long name Max-Planck-Institute für Gravitationsphysik, should the institute have a secondary name, the Albert Einstein Institute? 

This would be unusual: few Max Planck institutes have a secondary name, and few are named after individual scientists. On the positive side, putting Einstein’s name on an institute in Potsdam would be a way for MPS, representing the German academic community, to repudiate the way Einstein had been treated by Germany, to affirm Germany’s recognition of his huge importance to science. On the other hand, Zacher, who was a scholar of law, was concerned that we did not have the moral right to do that, that Einstein had said he didn’t want anything more to do with Germany, and that using his name might be misconstrued as wilful ignorance of the history of Germany’s dealings with Einstein. 

In the end, the three of us agreed that this affirmation was past due, that the positives were important enough to take a risk with the negatives, and so the AEI officially got its name. We were good to go!

The doors opened on 1 April 1995 to our rented accommodation in the new office block called Haus der Wirtschaft (Commerce House). We had only a handful of staff, all of whom could sit around a single table to drink tea in the afternoon. I wasn’t even officially on board: my contract with Cardiff University wouldn’t release me until after exams, on 1 June. But I was nevertheless present for the first day, taking these photos. A small and slightly bewildering but also exciting beginning, all of us wondering how we would learn to work together, where we would go!

A worldwide home for general relativity

Younger scientists today may not be aware that general relativity research always had a political side, at least starting from the mid-1950s, one that explicitly aimed to protect research in the field from the global divisions of that era. Ehlers had grown up with this, and was very keen that part of the mission of the AEI would be to maintain this global view, to be a place that scientists around the world could visit and feel welcome, and that would assist relativists around the world if possible. Most research institutes, like most university science departments, are keenly aware of the competition with other places around the world. But Jürgen took the view that, because the AEI would be the only institute in the world dedicated to research across all of general relativity, it had a responsibility to keep its doors open to all.

A bit of an aside might be in order on what I called the political nature of this research field. This had its roots in three circumstances: (a) the field in the 1950s was very small; (b) there were very good relativists on both sides of the Iron Curtain; and (c) the subject desperately needed reviving. The field had almost died of neglect from the 1930s onwards, partly because quantum theory and then quantum field theory were the hot topics that attracted top theorists, and partly because the war had put the focus on nuclear physics. Einstein, who died in 1955, hadn’t helped by frequently asserting that gravitational waves were not real, nor were black holes. 

But in the 1950s and ‘60s, people like Wheeler in the US, Bondi and Pirani in the UK, Trautman in Poland, and Zel’dovich in the USSR were turning toward relativity, and there were big problems to solve: was gravitational radiation real, were black holes real, how could one separate coordinate effects from real ones, could general statements be made about solutions even when exact solutions were lacking? This was not the time to splinter apart because of international rivalries. 

So in the mid-‘50s, relativists organised the International Committee on General Relativity and Gravitation (now the International Society of General Relativity and Gravitation). This was in itself unusual. Normally, subject-specific scientific societies exist within single countries: the American Physical Society, the Royal Astronomical Society, and so on. Physics societies then acquire international links by adhering to the International Union of Pure and Applied Physics (IUPAP) through subject-oriented commissions. But in 1957 the ICGRG exceptionally became itself a commission of the IUPAP, recognising its inherently international scope. The ICGRG began publishing a newsletter to keep its members informed of research around the world; this later evolved into the General Relativity and Gravitation (GRG) Journal. And the ICGRG encouraged exchanges across the Iron Curtain. When the political winds blew cold and national scientific societies felt that they had to represent their own national interests, the ICGRG steadfastly remained apart, working to make sure that lines of communication remained open, that scientists could still visit one another and exchange ideas across the Iron Curtain. (It helped, of course, that even the most rabidly nationalistic politicians couldn’t find anything about research in general relativity that might have any strategic importance to their countries!) 

This was all very recent history when the AEI opened. The Iron Curtain had lifted only 6 years earlier. Jürgen wanted the AEI to actively help to continue this spirit, which he felt was going to be needed because the growing importance of general relativity for astrophysics and in quantum gravity was inevitably going to change the nature of the field into one with more competition, more rivalry. The AEI should be a place where good scientists did their own work, but where visitors could come and bring in different points of view, where rivals would want to come to put their own views.

To this end, Jürgen had two big priorities in our negotiation with President Zacher: a well-funded library (especially attractive to visiting relativists from small universities or underdeveloped countries), and a big dedicated fund to support visits. He got what he wanted. The new building in Golm included huge space for the library. And for many years the visitor money supported something like 60 visitors a year, and it was well used by all three divisions (Hermann Nicolai had joined the AEI to direct the quantum gravity division very soon after we started up). A big help was the Max Planck guest house on the Golm campus, which the AEI initially managed on behalf of the MPS, because so many of our visitors used it. I think that Jürgen supported the unusual enterprise of publishing the Living Reviews journal because he saw it serving this function of making the AEI into a focus for world research.

Growth and impact

This is the end of my reminiscences about the founding of the AEI, but I can’t close without reflecting on the enormous changes we have seen. By the time we moved from our office block to the Golm campus in 1998, we were three active divisions. By the time the Golm building was officially dedicated in 1999, we were already overflowing it. We also were developing our plans for the new branch in Hannover, taking into the MPS the existing university group of Karsten Danzmann and expanding further. In 1999 we also hosted the annuals international Strings conference, attended by Stephen Hawking, and got on the cover of the important German news weekly, Der Spiegel. We had arrived! 

By now we have over 300 staff on two sites, big hardware projects for ground-based and space-based gravitational wave detection in Hannover, big supercomputing projects in both Hannover and Golm, and enormous presence in the worlds of string theory and of the theoretical support of gravitational wave detection. The gravitational wave enterprise has moved from expectations to enormous success, and the AEI has contributed massively to that. Jürgen, who saw part of this development but who passed away in 2008, would I think have been astonished by how far the AEI has come, and I certainly hope he would have loved it. The AEI still owes a huge debt to his one political idea ever! 

Sir Jim


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 Hough
Jim Hough (credit: Glasgow University)

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. 

GEO600 optical components.
GEO600 optical components (AEI).
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. 

GEO600 monolithic suspension close-up.
Monolithic suspension: close-up of the glass fiber bonded to the glass “ear”, which is bonded to the body of the GEO600 mirror. The bonding is glass directly to glass, with no intermediary layer of glue, which would create friction and make the mirrors noisier at observation frequencies. (AEI, H. Lück)

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.

Jim being interviewed in the Scienceface series.
Jim being interviewed by Annalie Schutz in the Scienceface series (AEI)

The merger of two neutron stars, one year on: GW170817

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. 


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.


The Kidnapping of Kip Thorne

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 Old Advisor 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!


[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.)]

Another wave, and a smile

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.


* 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.


!Happy Birthday GW150914!

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! 🍾🎉

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