## Remembrances

Several of Andrew's friends and colleagues who were unable
to come to Louisville sent in written contributions, which were read aloud
during a session of the symposium entitled *Remembrances*. The
contributions collected here are listed below. Many other reflections and
reminiscences by Andrew's friends and colleagues may be found in Clifford
Johnson's
blog .

### Raphael Bousso, University of California, Berkeley

Andrew was one of the first people I met in Cambridge as a clueless beginning graduate student. He was incredibly welcoming, warm, and open, and helped me find my orientation (whenever he wasn’t helping me lose it by being so much fun to party with). I wrote two papers with Andrew in 1998. Admittedly, they are not among his most cited. But for me, working with Andrew was very valuable. I was just out of grad school, so it was a time of growing up as a physicist. He was inspiring, and he knew how to build a young person's confidence. He also introduced me to some beautiful physics I hadn't heard about. (For example vacuum decay by membrane nucleation, which later came in handy.) Though we did not collaborate again, we had many more physics conversations. He was quite nonconformist and would often find unusual, interesting angles on a problem. Andrew knew the most wonderful runs near Cambridge (he’d wait patiently while I schlepped my tired limbs back into the college, then announce, beaming, that there’s still plenty of time for a good long gym session). Not to mention the best runs in Santa Barbara. (At the bird refuge, he introduced me with some amusement to the “least sandpiper”.) Andrew understood the importance of listening to the Grateful Dead while driving around the American Southwest. And how to nucleate membranes. There may be others who can converse about the Texas Chainsaw Massacre, or about the Goldberg variations, but few can do it in the same sentence. Andrew was a rare combination: intellectually sparkling, personally warm, and simply a joy to be with. Thanks for making our lives richer.

### Roberto Emparan, University of Barcelona

Andrew and I collaborated in seven papers, all of them during our first (and crucial) postdoc years, which I think provides a good measure of how we influenced each other's careers. Somehow we shared an interest in subjects that others would deem obscure, but which more than once have later turned out to become important in unexpected ways. In this respect, I would like to describe here how Andrew played a decisive role in what I regard as one of my finest pieces of work (with Harvey Reall): the discovery of black ring solutions in five-dimensional gravity – even if Andrew himself never published anything on this subject.

Our first joint paper ("Bubbles in Kaluza-Klein theories with space- or time-like internal dimensions," Phys.Rev.D55:754-765,1997) sprouted from conversations in which Andrew explained to me a result he had discussed in his Ph.D. thesis but had not published in a journal. It concerned "bubble solutions" in Kaluza-Klein theories with an internal time direction. Such theories are pathological, but Andrew's construction of a "two-timing monopole" was so ingenious and beautiful that we could not resist studying it further. Thus we dived into an exploration in earnest of the general properties of KK-bubbles. Following an earlier suggestion by Gibbons and Rasheed, we discovered that such bubbles would naturally lead to the spontaneous decay of the vacuum of KK theory with internal time. As a byproduct, we also found a solution (in conventional KK theory, with spacelike internal dimension) that described a pair of electric KK bubbles accelerating apart. This rather obscure-looking solution, it turned out five years later, was a double Wick-rotation of a rotating black ring. Looking back at the way Harvey Reall and I actually identified the solution, it is unlikely that we would ever have found it other than through its connection with the solutions in my paper with Andrew.

Andrew's role in finding the black ring also had a different but no less crucial side. As all his friends know, Andrew was an extremely social person and so was, in the academia, a catalyzer for collaborations. This is certainly one of the things that I'm most grateful to him for, since without him it is unlikely that I would ever have collaborated with people like Rob Myers, Clifford Johnson, or Harvey Reall. My collaboration with Harvey began in Spring 2001, a few days after I sent to the arxives a paper on some supergravity solutions with a ring-like structure. I received an email message from Andrew (at that time visiting Queen Mary University), as cheerful, optimistic, and full of suggestions as usual, telling me that he and his colleagues at Queen Mary, Harvey Reall and Fay Dowker, had spotted my paper and were thinking about how to explore further similar solutions (Andrew had earlier introduced me to Harvey in one of the times he invited me to DAMTP). Very quickly the four of us began working together, but Andrew and Fay decoupled shortly afterwards – Andrew to finish another project he had already been engaged in, Fay to deliver a paper and a baby within just a few days. With Harvey, things advanced quick and far, and a few months later we published two papers that provided the starting points for much of my research in recent years, and which I think are significant contributions to our understanding of black holes in higher dimensions.

It is all too clear to me that, without Andrew's participation at key points, none of this research would ever have occurred. I can't help wondering what other opportunities for unexpected work I'm going to miss now that he's gone.

### Gary Gibbons, University of Cambridge

One of the greatest of all Theoretical Physicists, Paul Dirac, once said that Theoretical Physicists divide into those with an algebraic imagination and those with a more geometric imagination. In the case of Andrew, there can be no doubt that he was in the latter category. After a brilliant undergraduate career in mathematics at Rice, where he came under the influence of Ronnie Wells, Andrew set off for Oxford, home of that High Priest of Geometry and Physics, Roger Penrose to study relativity, spacetime and and Twistors. Provoked by an incautious conjecture of myself and Stephen Hawking about the possibility of time travel in highly kinked spacetimes, Andrew and Roger were able to show that we were wrong. The argument was a type which I was to recognise as characteristic of Andrew, clear and intuitive; it could be followed completely in one's head, with in Andrew's case a bright smile on his face.

For Roger, the topic was one in which he had excelled, one can almost say invented, but it no longer had the fascination it once had. For Andrew it was a gateway from mathematics into physics, a gateway which lead him from Oxford to Cambridge and a Phd in DAMTP and Dirac's old college, St Johns. For most of his PhD, Andrew continued to work on the topics which had drawn him to Oxford, but from a different, more quantum mechanical perspective. The dominant theme was the global, geometric structure of spacetime, singularities, time machines, and above all the strange and mysterious properties of spinor fields, such as describe the electron or the neutrino, in spacetimes where there is no distinction between left and right or between past and future, or where there are two times and only two space dimensions. It sounds like science fiction but tempered by his more practically minded contemporaries it had a serious purpose: to understand the physics which us behind the Big Bang, possibly using Stephen Hawking and James Hartle's proposal that time has no beginning, that our past has no boundary. In the process he was able to correct some ideas of Nobel Prize winner David Politzer.

This led Andrew into all the complexities of Quantum Gravity, a subject he tackled with characteristic enthusiasm bringing to bear his great geometric gifts.After a Postdoc at Santa Barbara he returned to Cambridge, gaining a Junior Research Fellowship at Pembroke which allowed him to continue and deepen his technique and also allowed him to broaden the range of his collaborators. From instanton calculations of quantum mechanical pair production of black holes or inflationary instantons for cosmology to vacuum stability in higher dimensions, Andrew was by now a master of the the semi-classical expansion in quantum gravity, and much else in general relativity as well.

But he couldn't stop there. Supergravity theory and superstring theory were undergoing an explosive and revolutionary period of development in which the sort of non-perturbative viewpoint Andrew had been developing coming to fore, with D-branes and supergravity solitons playing an essential role, resulting in Juan Maldacena's daring and profound AdS/CFT conjecture. Like many of his generation this, and the realisation that string theory and supergravity were possibly all part of a unified whole called M-Theory, had an enormous influence on the direction of Andrew's work. No longer working slightly on the periphery, his his work become more central and much more physical. It still rested on his vivid geometric imagination, but a wider and wider range of physical ideas were being brought to bear.

Nothing illustrates this better than the speed with which he picked up on the brane world idea and and developed it. In the words of Dan Freedman, a notoriously strict judge of physicists both young and old, when he arrived at MIT from Pembroke, he hit the ground running.

Obviously this move to centre ground was to Andrew's taste, and could have been anticipated, but his last major work went even further, and gives us glimpse of what he might have achieved had he lived longer. I am referring to his work on the possibility of producing microscopic black holes in collider experiments at the LHC in Geneva. It is not surprisingly that he, like many others should have picked up on this. It represents, after all the culmination of his early dreams about the exotic spacetime geometry,black holes, quantum gravity, higher dimensions brane worlds, and so forth. His whole career had been a preparation for a development like this. What is less obvious is that he would throw himself so completely into the the nitty gritty details.

Sadly, Andrew will not see the results of the LHC, and will never know whether the dreams and aspirations of his youth will mature into the certain experimental realities of his old age. As only a partial compensation he is also spared the possible desolation of having cherished hopes shattered.

For those of us who survive him, he leaves behind, not only his legacy of work and ideas for us all to call upon but also the memory of the joy with which they were achieved. To be blessed with a geometric imagination is to be blessed with a very palpable vision of the physical world as place of order, harmony, and beauty. All who knew Andrew also knew of his love of beauty manifested in his love and talent for musical performance and his delight in the out-doors and the magnificent scenery of his native land. All who knew him will also recall his almost ever present smile.

Those who knew him as a physicist will recall it as the sweet smile of reason gazing at a reality governed by the laws his life work it was to unravel.

### Andreas Karch, University of Washington

I am sorry I can not be there with you to commemorate our friend and collaborator, Andrew Chamblin. I knew Andrew mostly as a colleague, but he was also a dear friend. We started working together during our joint time at MIT and helped each other to cope with the environment in this scientifically truly outstanding but sometimes also very intimidating institution. One collaboration we initiated back then, about the Hawking Page transition on an AdS braneworld, we only finished a little more than a year ago and it is still hard for me to accept that this is going to be the last time we heard Andrew speak about science. His energy and vision paired with his deep knowledge of GR were always a great source of inspiration for me and he will be deeply missed.

### Malcolm Perry, University of Cambridge

I don't really know what to say on occasions like this and the more I examine what is in my mind, the more numb I become about the unfairness of it all. Andrew was a wonderful warm person who was the life and soul of all of the events that surrounded him wherever he went. It did not matter if it was a physics event or a dinner someplace or hike into the wilds. He would always have a different take on the events and find something original and unexpected. My primary interaction with him was as a physicist and it is cruel that we will no longer have his insights and have been robbed of what he would have discovered had he lived. The world without him is a sadder and chillier place.

### Lisa Randall, Harvard University

I had the good fortune to meet Andrew Chamblin about five or six years ago when I was helping organize a Santa Barbara physics workshop and he wrote to me to ask to be considered. His note was peculiarly polite. In it he told me about some of the work he was doing and also about his interest in my research. One gets notes of this sort at times but his was different. Clearly this person cared about physics and was talented to boot. I was very pleased when he came to Santa Barbara and later to MIT as a postdoc. And I was also delighted when I saw how expansive and warm and funny he was--especially when compared to what I expected on the basis of that first email. On top of that Andrew was indeed very bright with a unique approach to physics questions and a real dedication to the subject.

Andrew was a spectacular individual. I'm sure we'll never meet anyone like him again. How many Texans are there with British tastes and values who feel completely at home in Santa Fe? I remember hanging out with him in Santa Fe one summer. At the time, I also had the good fortune of meeting his mother and her friends-all of whom were strong admirable women. It was a kick to meet them. And a kick to realize this is Andrew's heritage.

And Andrew got a kick out of others. He enjoyed their company and he was keenly observant. Andrew in turn was great fun to be around and was also a great friend. And he made people feel important. I know he did that for me. One friend I know characterizes people by whether they say surprising things. Andrew was full of surprises. I know I'm speaking not only for myself but for a whole community of people when I say that I'll miss him a great deal.

### Harvey Reall, University of Nottingham

I first met Andrew in Cambridge in 1997. I was starting my Ph.D. and he was a postdoc. He seemed to dominate the Relativity Group in DAMTP with his infectious enthusiasm for everything. After about a year, we started working together. It was an ideal collaboration because he was full of ideas, and as a young graduate student, I was keen to calculate anything.

Our first paper together concerned brane-world cosmology. The large extra dimensions craze was in full swing, and there had been some papers on Horava-Witten cosmology. Andrew realized that the branes in such models can be regarded as domain walls. He knew a lot about domain walls following a collaboration with Rob Caldwell and Gary Gibbons, and another with Raphael Bousso. In fact, a colleague at Cambridge once remarked to me that Andrew was "obsessed" with domain walls! Fortunately, this obsession paid off because it led to our paper, which was the first to employ the Israel junction conditions to describe the gravitational backreaction of a codimension one brane world. Shortly after, the Randall-Sundrum models appeared and codimension one brane worlds became very popular. We also wrote a follow-up paper with Malcolm Perry employing the same techniques to obtain solutions describing moving D8-branes in supergravity.

Following the appearance of the Randall-Sundrum papers, Andrew became interested in the problem of describing a black hole on a Randall-Sundrum brane. We quickly realized that one could recover the Schwarzschild metric on the brane by taking the bulk solution to be an infinite black string. However, Andrew pointed out that such strings typically suffer from the Gregory-Laflamme instability. (This was the first I had heard of this instability, which I later wrote a paper on.) It turned out that Stephen Hawking had also been thinking about the problem of black holes on branes and with him we studied the instability in more detail, leading to our proposal that the string would fragment, leaving behind a black hole bound to the brane. This qualitative picture is still the best available description of black holes on Randall-Sundrum branes. Andrew and I wrote another paper with Tetsuya Shiromizu and Hisa-aki Shinkai exploring the effects of charge on such black holes.

My final collaboration with Andrew followed up a paper he wrote with Gary Gibbons studying a nonlinear version of the Randall-Sundrum model in which the graviton is described by a pp-wave on a thick domain wall. They argued that this configuration typically leads to curvature singularities in the bulk. Together with Dominic Brecher, Andrew and I investigated the AdS/CFT interpretation of such singularities. We found that they arise in the near-horizon limit of infinitely boosted branes and should therefore admit a dual interpretation in terms of a CFT in the infinite momentum frame.

It is clear that collaborating with Andrew was of great benefit to my career. But more important than that are the many happy memories that I have of time spent with Andrew, in particular the delight he took in showing me the American South-West. I will miss him dearly.

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