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)
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