Fred Hoyle (1915 - 2001)
Fred Hoyle died on Monday the 20th of August 2001.
Sir Fred Hoyle, who died though still working at age 86, applied field theory to cosmology and began new astronomical disciplines. A national hero, he was knighted by the Queen in 1972 for a large number of distinguished contributions to astronomy and to the UK: he worked on radar during WWII, created Cambridge's Institute of Astronomy, and chaired the Science Research Council's astronomy committee for creation of the Anglo-Australian Telescope. By creating and challenging our human view of the universe for more than half the century, Hoyle demonstrated his creative genius. His name became well known to the public following his BBC broadcasts, "The Nature of the Universe (1950)." His 1955 book, Frontiers of Astronomy, inspired a generation of astronomers and public alike.
Nucleosynthesis in Stars: After World War II, it was already popular to envision the beginnings of the universe as an expansion from a very dense state. But attempts to create the elements in that setting, which was the initial goal, were unsuccessful; so the picture languished. Hoyle changed the nucleosynthesis paradigm in 1946 by showing that the interiors of evolved massive stars should eventually reach very high temperature and density. In that setting, the natural dominance of iron in the iron abundance peak could be understood as a consequence of statistical equilibrium, provided the neutron/proton ratio was properly chosen. Hoyle and collaborators called this "the e process," with "e" standing for equilibrium. If explosive disruption of the star followed, the interstellar gas would be enriched in iron. This paper shifted attention to nucleosynthesis in the stars and created the field of galactic chemical evolution. In 1954 Hoyle published an ApJ paper detailing not only the "e" process but also the synthesis of all elements between carbon and nickel as a series of successive thermonuclear epochs in which the ashes of one stage became the fuel of the next. Hoyle's most accepted theoretical innovation, this process would dominate the next three decades of theoretical astrophysics. An irony is that because the neutron/proton ratio does not reach the value that Hoyle initially suggested, it is not properties of iron nuclei that account for its high abundance, but rather those of radioactive nickel, which was demonstrated by others in the late 1960s to be the radioactive parent of iron, of the radioactive power for supernova light, and the source of a realistic test of the theory through detection of the gamma-ray lines.
Steady-State Cosmology: Hoyle is perhaps more widely known as creator of the steady-state theory of the universe during 1947-1948. Bondi and Gold also published a discussion of this idea in 1948 from a more philosophical point of view. Hoyle's approach, however, went straight to the need for a field theory of gravitation that included a field for creating matter. Hoyle invented and introduced a scalar field for that purpose. A large number of publications during the next 15 years, many with Jayant Narlikar, explored the mathematical implications of this (and other) fields in cosmology. Their work and book on time-symmetric quantum electrodynamics was a Herculean effort of theoretical physics, which they saw as supporting the steady-state theory. They also introduced a new theory of gravity itself. These established Hoyle as founder and champion of the concept of creation in the universe, and field equations that achieve this will forever be associated with his name. In all of these, the influence of Dirac and of Hoyle's training in mathematical field theory shows through. Hoyle's field equation led to the exponentially expanding, but spatially flat, metric that he discovered, and that reappeared in similar form in the inflation epoch of the big-bang theory. Philosophical beauty was not his only guide, however; the conviction from work by Ambartsumian and others in the 1960s that violent cosmic objects represented ejection of matter rather than infall of matter strongly motivated Hoyle.
The steady-state theory makes strong predictions. Hoyle's reaction to poorly documented attacks on the steady-state theory was to demolish the "disproofs." Almost against his will this reaction placed Hoyle in the position of seeming a sore loser in a scientific debate, a perception that persisted until his death. But in 1964, Hoyle pioneered calculations of nucleosynthesis in a big-bang cosmology with Tayler by arguing that a hot big bang was the source of a uniform cosmic density of helium, though he and Tayler differed on whether the big bang was necessarily of a primordial object (which Tayler favored) or could have been a cumulative result of a series of smaller events involving miniature oscillating universes (which Hoyle himself favored.)
In 1967, with Wagoner and Fowler, Hoyle demonstrated a source for both isotopes of H and both isotopes of He as well as of 7Li in the big bang. The latter calculation set the standard for big-bang nucleosynthesis. Nonetheless, the common image is that Hoyle gave the big bang its name in sarcasm. But following accurate spectral measurements of the microwave background of the universe, Hoyle acknowledged that it was a possible knockout blow to the simple steady-state model. Still, he showed with Wickramasinghe the capacity of a modified steady-state picture for thermalizing starlight with carbon whiskers formed in stellar outflows. This effort contributed to his monograph, A Different Approach to Cosmology, with G. Burbidge and J. Narlikar, (2000: Cambridge University Press) presenting an alternative to the big bang based on an oscillating but otherwise steady state. Cosmology was led toward its modern empirical state by the galvanizing role of the steady-state theory in arguments about evolutionary cosmology.
Red Giants and Supernovae: Hoyle's great contributions to stellar evolution involved both physical models of stars becoming red giants and of their exploding as supernovae. The former occurred in 1953 during his visit to Princeton University. The issue was physical interpretation of the Hertzsprung-Russell diagram of globular cluster stars, which Hoyle had already addressed with Lyttleton in 1949. Hoyle and Schwarzschild constructed numerical models of the evolution of stars from the main sequence that not only explained the physical nature and cause of red giant stars but also introduced many physical ideas that now seem as if they must always have been known. This was done not with a Pentium™ processor, mind you, but by hand integration of the dimensionless q, t, P variables that Schwarzschild later used in his book on stellar evolution. Innovations included an isothermal helium core, a thin hydrogen burning shell (on the CN cycle), and a deepening surface convection zone owing to the failure of the radiative boundary conditions at the surface. Practitioners of stellar evolution reread this ApJ paper and marvel at the concepts that were argued into existence but that are now taken to be evident.
When Hoyle visited Kellogg Lab at Caltech for the first time in 1953, he argued that the triple-alpha rate would be inadequate for both red giants and nucleosynthesis unless 12C were to have an excited state with zero spin and positive parity at 7.7 MeV excitation. This pronouncement was incredible, because 12C has very few excited states; but it was soon shown to be true. Hoyle's prediction of the energy of this state was the most accurate that had ever been achieved, and it had relied on astrophysics rather than nuclear physics. Hoyle had anticipated the anthropic principle by arguing that because we are here, this 12C excited state must exist.
In 1960 and 1964 Hoyle published with Fowler physical interpretations of spectroscopically-defined supernovae of Types I and II. They argued that Type I were degenerate dwarfs that explode nuclear fuel and that Type II were implosion-explosion sequences within massive stars. These are today our paradigms, although they did not see the role of neutrino transport in the Type II rebound, arguing instead that centrifugal barriers to further collapse would allow thermonuclear power to eject matter. The physical pictures that Hoyle constructed of supernovae and of red giants remain mental images carried by virtually all astronomers.
Stardust and Panspermia: Hoyle's foray into interstellar biology began innocently enough. With Cambridge student Wickramasinghe he published, in the 1960s, papers on the condensation of refractory dust in winds from carbon stars and within the interiors of supernovae as they expanded and cooled. Within ten years a new branch of astronomy could be envisioned after others argued that such dust would be isotopically anomalous and could perhaps be found within the meteorites. The first such stardust in meteorites was isolated in 1987 and has enormously enriched astronomical knowledge.
Hoyle's adventure into interstellar biology grew from an indication that the absorption spectra of bacteria resembled interstellar extinction, plus his conviction that some driving principle would be needed to process interstellar matter into such forms with high efficiency. For this they boldly suggested reproductive chemistry. Hoyle saw this as a novel scientific argument, to be addressed by the usual scientific methods. After biologists attacked it in public comments, rather than in published scientific arguments, Hoyle's back stiffened. His subsequent foray into publishing books directly to the public rather than to scientists was unfortunate. Hoyle had written for the public brilliantly in his 1957 novel The Black Cloud, in which he imagined cold molecular clouds that developed a nervous system and consciousness that controlled their environments in an astrophysical "Gaia." The physical notion stayed with him. Writing to the public, Hoyle and Wickramasinghe argued in Lifecloud (1958), Diseases from Space (1979) and Space Travelers: The Origins of Life (1980) that comets carried the basic chemicals of DNA replication, and even of influenza epidemics. The scorn of the biochemical world was total. But it must be added that today the role of comets in delivering biochemically active matter is an open science topic, as is the question whether life emerged first on Earth or on another planetary body (Mars, say).
Hoyle's childhood and manhood: "The child is father of the man," wrote Wordsworth, and Hoyle has himself described the truth of that sentiment in his own case. In his autobiography, Home is Where the Wind Blows, Hoyle focused much attention on his early war with education in Gilstead, a village near Bingley in Yorkshire, where Hoyle had been born in 1915. His family was far from the privileged classes that gave England so many science superstars. His mother was a big influence. She had worked in the Bingley textile mill but later studied music at the Royal Academy and became a professional singer before she married. Through age nine Hoyle was at war with the educational system. Rebellious truant and foe of ignorant authority, Hoyle quit school after being slapped by a teacher. His mother strongly supported him in the confrontation with local educational authorities. After transferring schools, Hoyle eventually won a scholarship to Bingley Grammar School, to and from which he walked four miles daily. From there he managed to gather financial support to enter Cambridge University's Pembroke College in 1933. There he won a half share of the Mayhew prize in the mathematical tripos. Later he became a research student of Dirac because, Hoyle explained, Dirac could not resist the circular logic of a supervisor who did not want a research student who didn't want a supervisor!
During the 1960s and 70s, Hoyle organized climbing trips of "the Munros" of Scotland (peaks of more than 3,000 ft) for those that accepted his passion for talking day and night of the universe and its problems. (See "With Fred on Slioch" from my memoir, The Dark Night Sky). Yielding to a separate summer madness, Hoyle would terminate work and invite colleagues to his house to watch the cricket Test Matches while he explained its intricacies to Americans. He once exclaimed, "Is there not somewhere a cricket team that can beat the Australians!"
Three of Hoyle's papers were selected for the AAS ApJ "centennial volume" featuring the most influential research of the twentieth century published in AJ and ApJ. (This is a record equaled only by Chandrasekhar and Baade.) I would argue that his 1964 paper with Tayler on big-bang nucleosynthesis might also have been included. Most of his publications were in Monthly Notices of the Royal Astronomical Society, however, including the field theoretic steady-state model. These earned the international Crafoord and Balzan Prizes, and many felt that Hoyle might have shared Fowler's Nobel Prize but for Hoyle's embarrassed status over exobiology. Many relevant photographs are available on a web site for the history of nuclear astrophysics (http://www.clemson.edu/ces/astro/nucleoarchive/).