Early one morning in October 1933, Fred's mother and father, and his twelve-year-old sister Joan, gave the young scholar a hearty send-off. He began the long journey to Cambridge at Bingley railway station. As the local train chugged along the valley, Hoyle looked back at the forest of mill chimneys clouding the air with smoke. On the hillsides the trees were already showing their autumnal colours. His travelling companions included half a dozen of his classmates from the grammar school. At the first stop, Shipley, they hauled their bags and suitcases to another local train, which brought them to Leeds. Here they bought tickets for the express to Peterborough, about thirty-five miles north of Cambridge. From there a local train clattered across the bleak fenland. Hoyle must have found this flat landscape strikingly different from the moors and dales of Yorkshire. From his window seat he could see farmers ploughing with horses, an extensive drainage system of ditches and dykes, and the fourteenth-century octagon tower of Ely Cathedral soaring over the fens. After some eight hours of travel, Fred and a throng of students descended from the packed train at Cambridge, where they found themselves on the longest railway platform in the country.

The returning undergraduates and the freshers dispersed either to colleges or to lodging houses. Emmanuel College had assigned young Fred a room in a shared house about a mile from both the college and the railway station. Of course, he had no money to spare for a cab.

Since the advent of the Hubble Space Telescope (HST), the progress in studying and understanding black holes has been impressive. Early questions regarding the very existence of black holes have been replaced by questions regarding the role that they play in the formation and evolution of galaxies, particularly at early epochs in the universe. However, the apparently well-established relationship between the mass of the black hole and the mass or luminosity of the galactic bulge rests on a relatively small number of direct observations, and while very few doubt that this relationship exists, it is essential to actually measure the properties of a number of black holes over a range of masses and host galaxies. The direct methods adopted to measure black holes in the nearby universe use gas or stellar kinematics to gather information on the gravitational potential in the nuclear region of the galaxy. The stellar-kinematical method has the advantage that stars are present in all galactic nuclei and their motion is always gravitational. The drawbacks are that it requires relatively long observation times in order to obtain high-quality observations, and that stellar-dynamical models are very complex and potentially plagued by indeterminacy. Conversely, the gas-kinematical method is relatively simple; it requires relatively short observation times for the brightest emission-line nuclei, even if not all galactic nuclei present detectable emission lines. However, an important drawback is that noncircular or non-gravitational motions can completely invalidate this method.

By early 1971, the problem of continuing funding for the Institute of Theoretical Astronomy (IoTA) had become critical. As we have seen, the Nuffield Foundation and the Science Research Council (SRC) had agreed to bear the lion's share of the running costs (80 per cent) until 31 July 1972, at which date it was assumed the University of Cambridge would take full responsibility for the salaries. Of course, there would have to be a limit as to how much the university could afford for the support of astronomy and astrophysics.

From the teaching point of view, the radio astronomers in the Cavendish provided physics lectures and laboratory classes for the natural sciences tripos, while the teaching officers at the Department of Applied Mathematics and Theoretical Physics (DAMTP) carried a large load for the mathematical tripos. The permanent staff of the observatories had graduate students but played no role in undergraduate teaching. This was not due to a lack of willingness on their part but simply reflected the fact that observational astronomers worked at night and in observatories overseas. Most of the Institute staff were on short-term research contracts and therefore were unavailable to support undergraduate teaching, although they had graduate students. In any case, the whole point of founding the Institute had been to free the best minds for research, which is partly why George Batchelor had opposed its foundation in the 1960s.

Following completion in California of the 1957 nucleosynthesis paper, Hoyle developed a strategy for spending some weeks each year at Caltech. In general, he did not reside there during the university summer vacations, which he could have done without seeking permission from authorities at Cambridge. There were good reasons for staying in Britain in the summer months. Barbara had taken a dislike to Los Angeles and Fred absolutely detested its smog, which drove him indoors when it was bad. Fred, Barbara and the children enjoyed old-fashioned seaside holidays in their caravan, rather than expensive visits to California.

When Fred returned in 1957, the family had arranged to move from their country house in Little Abington to a fine home being constructed at 1 Clarkson Close, on land belonging to St John's College. However, the builders were running two months late, which forced the family to live in their caravan for several weeks. As soon as the school holidays started, Fred's Humber Hawk hauled them to Scotland up the old A1 road (which was not then a dual carriageway). From there, they crossed to Northern Ireland, then travelled over the border to the Republic of Ireland. For six weeks of glorious weather, they parked in a narrow lane hemmed in by stone walls, sited 400 feet above the sea.

Over the years, Fred used caravan trips of this kind to compose much of his science fiction.

Introduction

General properties of black holes were studied extensively in the early 1970's, and the basic theory was developed. One of the key results was Hawking's proof that the area of a black hole cannot decrease (Hawking 1971). This led Bekenstein (1973) to suggest that a black hole should have an entropy proportional to its horizon area. This suggestion of a connection between black holes and thermodynamics was strengthened by the formulation of the laws of black-hole mechanics (Bardeen et al. 1973). In addition to the total mass M, angular momentum J, and horizon area A of the black holes, these laws are formulated in terms of the angular velocity of the horizon Ω, and its surface gravity k. Recall that the surface gravity is the force at infinity required to hold a unit mass stationary near the horizon of a black hole.

An immediate consequence of his resignation was that Hoyle was no longer the holder of a mainstream university post, and therefore he would no longer have research students, postdoctoral research assistants or research staff alongside him. Barbara Hoyle would have to function as his typist, his diary manager and his publishing agent. The professor who believed so strongly in high-quality support for research had suddenly cut himself off: at the mundane level, he no longer had the right to use a photocopier free of charge and, more seriously, he could no longer apply to the Science Research Council (SRC) for research funding. Worse still, he had resigned ten years before the retirement age and this would have a devastating impact on his future income.

Did he really have to do this? If instead of resigning the chair, he had simply resigned the remaining months of his directorship of the Institute of Theoretical Astronomy (IoTA), he could still have made his point. Though his Plumian chair was assigned to the new Institute of Astronomy, he could have worked from St John's College or from home. So long as he had carried out any teaching duties required of him, the university would have left him alone. He would have been free to travel to Caltech, to write books and to give acclaimed public lectures. Perhaps he could have developed a sideline as an antiestablishment pundit with a radical newspaper column.

Cosmology kept hoyle in the public spotlight for decades. In professional circles, however, he became more respected for his work on nucleosynthesis – that is, the origin of the chemical elements – an interest that preceded his cosmology. To appreciate his Olympian contributions to our present knowledge of the history of matter in the universe, we need to revisit Cambridge in the 1920s and 1930s.

Physicists at the Cavendish Laboratory had made stunning progress in probing the atomic nucleus, for which they won a string of Nobel Prizes. By the time Fred commenced research, all the components for major advances in understanding the relation of nuclear processes to the structure of stars were present in Cambridge, but no person or group seemed quite able to put all the pieces together. This was a consequence of the organization of the university in those days, which led to the dichotomy between astronomy and physics. Astronomy, a subject in the curriculum for centuries, had a higher prestige than physics, then an upstart less than fifty years old as a separate subject in the university.

In the eighteenth and nineteenth centuries, chemists isolated the elements to the point where John Dalton (1766–1844) could put together a plausible atomic theory. In St Petersburg, Dmitri Mendeleyev (1834–1907) noted similarities and patterns among the sixty-three elements then known, charting his findings in 1869 as the Periodic Table of the Elements.

The Space Telescope Science Institute Symposium on Black Holes took place during April 23–26, 2007.

These proceedings represent a part of the invited talks that were presented at the symposium. They cover many aspects of black hole physics and astrophysics, regarding stellar-mass, intermediate-mass, and supermassive black holes. Topics range from black hole entropy and the fate of information to supermassive black holes at the centers of galaxies, and from the possibility to produce black holes in collider experiments to the measurements of black hole spins. Since these articles were written by world experts in their respective disciplines, this volume represents an extremely valuable collection for researchers and students alike.

The ST ScI Symposium on Black Holes attempted to capture all the aspects involved in the astrophysics of black holes.

We thank Sharon Toolan of ST ScI for her help in preparing this volume for publication.

Even if there had not been a war looming, 1939 would have been a year of crisis, challenge and change for Fred Hoyle. He lurched from one supervisor to another under circumstances that might have caused a weaker student to give up. He had to decide whether to continue in nuclear physics or switch to astronomy. And he and his wife hardly knew each other when they married. Thankfully, however, he had finished up with Dirac as his supervisor and had secured very generous funding from the 1851 Exhibition and St John's. From now on, he could pick and choose what research to do.

For many scientists, the most creative years come after winning a postdoctoral fellowship, when they start to work with some degree of independence. To build a solid career it is important to choose the right place and the right people. Location and one's immediate colleagues are more important than choosing which intellectual puzzles to attack. Science has a social dimension and it is almost impossible to do research in isolation. Today, it would be very unusual for an ambitious scientist to do undergraduate study, postgraduate work, and postdoctoral research all at one university – even one as prestigious as Cambridge – as Hoyle had done.

Fred Hoyle chose to remain in Cambridge, when no doubt he could have gone to Birmingham or Manchester. Cambridge was a centre of excellence in astronomy, and he was determined to add to the towering achievements of Eddington.

Fellows of the Royal Astronomical Society (RAS) of a certain age will agree that the most abrasive relationship in British astronomy in the second half of the twentieth century was that between Fred Hoyle and Martin Ryle. Their academic arguments, conducted in the most public manner imaginable, lasted nearly three decades. Until both became older and wiser in the mid-1970s, astronomy at the University of Cambridge went through a period of strong polarization as a result of these two prima donnas failing to coexist more harmoniously. The destructive and competitive forces that they unleashed harmed the standing of astronomy and cosmology at Cambridge, particularly when their public disagreements became widely reported. To put this dramatic and highly significant interlude in context, it is essential to understand the background and personality of Martin Ryle.

Ryle engaged fully with all his students. Always available and seldom travelling, he admired the laboratory, his staff and his students. His door was always open; invariably, he took his morning coffee and afternoon tea with the rest of his group. Unlike Hoyle, Ryle regularly added his name to his student's research papers, not to grab personal credit, but to give weight to their future curricula vitae. Hoyle's immense output includes only a handful of papers authored jointly with students, and he seldom read or corrected their drafts. Ryle was the complete opposite in this respect.

A yawning gulf separated the social and family backgrounds of Hoyle and Ryle.

Introduction

Black holes are natural products of stellar evolution in massive stars, and may also result from dynamical interactions in dense stellar systems, such as star clusters and galactic nuclei. They can significantly influence the dynamics of their parent cluster, and may also have important observational consequences, via their x-ray emission, the production of gravitational radiation, and their effect on the structural properties of the system in which they reside.

Globular clusters offer particularly rich environments for the production of black holes in statistically significant numbers. Direct evidence for black holes in globulars is scarce, although several independent lines of investigation now hint at their presence.

In the mid-1960S, relentlessly taunted by his opponents, Hoyle was a caged bird. Devious and envious colleagues, desperately searching for a final stitch-up, flapped and screeched at the gaudy parrot who had so much to say. They were blackbirds defending their territory. After Hoyle withdrew his resignation in October 1964, he still felt trapped by the pecking order in Cambridge politics. In an effort to free his mind of turmoil, he decided to graduate from hill walking in the Lake District to mountain climbing in the far-flung regions of the Scottish Highlands, 500 miles north of Cambridge. Here his mind could soar as a free spirit. The Scottish mountains are dangerous places, not to be explored alone under any circumstances, so he engaged the services of one Dick Cook, president of the Lake District Fell and Rock Climbing Club, whom he knew.

In spring 1965, they drove to Inverness, accompanied by a third climber, Norman Baggaley. A blizzard that blanketed the eastern Highlands in deep snow marked their arrival. Out west, the snow was not so bad, so, the following morning, they headed southwest along the already ploughed road through the Great Glen. After much skidding and sliding, Fred's Humber Hawk nosed along the shores of Loch Duich, which he had visited in the summer of 1935 on his first Highland hike. His mentor Dick Cook wanted a mountain he and Norman had not tried before.

Early in the war, Lyttleton and Hoyle had approached the Meteorological Office to see whether it had any use for the services of Cambridge mathematicians, but nothing resulted from this enquiry. They were concerned that compulsory military service might be introduced, though they figured they might be able to avoid it by supporting the war effort as civilian research scientists. Fred's call away from Cambridge eventually came from the Admiralty in Whitehall. A recruitment officer, Fred Brundrett, followed up a recommendation from Maurice Pryce, who had been hired by the Admiralty only a couple of months earlier. He set up an interview with Hoyle: the only offer on the table for him was a research position working on the development of radar. Brundett offered a salary worth about one-third of the emoluments from Hoyle's fellowships, which would be suspended. Fred accepted and agreed to be posted to the Signal School in Portsmouth.

The importance of radar technologies in combat had come to be appreciated soon after Hitler became a threat in 1933. The British government began to assess the possibility that Germany might launch an air assault on England and, in 1934, a large-scale air defence exercise was held to test the defences of southern England. Mock raids were carried out on London. Even though their routes and targets were known in advance, well over half the bombers reached their targets without opposition. Prime minister Stanley Baldwin's statement, ‘The bomber will always get through’, seemed true.

Introduction

Black holes (BHs), as physical entities, span the full range of masses, from tiny BHs predicted by string theory, to monsters weighing by themselves almost as much as a dwarf galaxy (massive black holes, MBHs). Notwithstanding the several orders of magnitude difference between the smallest and the largest BH known, we believe that all of them can be described by only three parameters: mass, spin, and charge.

Prolog: The classical view of accretion disks

It has been understood for decades that accretion through disks can be an extremely powerful source of energy for the generation of both photons and material outflows. When the central object is a black hole, the gravitational potential at the center of the disk is relativistically deep, so that the amount of energy that might be released per unit rest-mass accreted can be a substantial fraction of unity. If the central black hole spins, an additional store of tappable energy resides in its rotation.

Aberration of starlight. As light does not move infinitely fast, but at a rate of practically 300 000 km s–1, and as the Earth is moving round the Sun at an average velocity of 25 km s–1, the stars appear to be shifted slightly from their true positions. The best analogy is to picture a man walking along in a rainstorm, holding an umbrella. If he wants to keep himself dry, he will have to slant the umbrella forward; similarly, starlight seems to reach us ‘from an angle’. Aberration may affect a star's position by up to 20.5 seconds of arc.

Ablation. The erosion of a surface by friction or vaporisation.

Absolute magnitude. The apparent magnitude that a star would have if it could be observed from a standard distance of 10 parsecs (32.6 light-years).

Absolute zero. The coldest theoretically possible temperature: –273.16 °C.

Accretion disc. A disc structure which forms round a spinning object when material falls on to it from beyond.

Achromatic object-glass. An object-glass which has been corrected so as to eliminate chromatic aberration or false colour as much as possible.

Aerolite. A meteorite whose main composition is stony.

Airglow. The light produced and emitted by the Earth's atmosphere (excluding meteor trains, thermal radiation, lightning and auroræ).

Albedo. The reflecting power of a planet or other non-luminous body. The Moon is a poor reflector; its albedo is a mere 7% on average.

Alfvén wave. A low-frequency travelling oscillation of the ions and the magnetic field of a plasma.