Malcolm Longair embarks on a voyage of serendipitous discovery.
Astronomy, astrophysics and cosmology are enjoying a golden age. The opening of the electromagnetic spectrum for observations in all wavebands by ground-based and space telescopes has revolutionised the scope for undertaking quantitative astronomical study. Not only highly publicised missions such as the Hubble Space Telescope, but also projects such as the Compton Gamma Ray Observatory, the Solar and Heliospheric Observatory, the Infrared Space Observatory, Chandra and the many advanced ground-based facilities for radio, millimetre, infrared and optical astronomy have opened up quite new areas of astrophysical endeavour.
In parallel with these observations, theory and the advance of computational astrophysics have made essential contributions to the new understanding. The subject is continuously evolving, and textbooks have a limited shelf-life. Furthermore, the subject is increasingly extensive so that new disciplines and ways of doing astronomy expand the vision of what can rightly be considered to be astronomy and astrophysics.
A splendid and important example in this encyclopedia is the space devoted to solar physics, solar-terrestrial relations and heliospheric studies. Galactic and extragalactic astronomers can learn a great deal from recent discoveries in these areas, though they are normally considered the province of a quite separate community.
This beautifully presented set of four volumes can be considered an authoritative survey of astronomy, astrophysics and cosmology at what is conventionally considered to be the turn of the millennium. And yet, during 2000, already the subject moved on in significant ways. The results of the Boomerang and Maxima experiments published in 2000 and the measurements of the power spectrum of the fluctuations in the cosmic microwave background radiation have produced some of the best estimates so far of key cosmological parameters. In March 2001, the first convincing estimates of the total amount of gravitating matter on the very large scale in the universe were published in Nature . The number of known extra-solar planets continues to increase unabated. Similar advances could be reported in many other fields. This is an obvious problem for any encyclopedia in such rapidly evolving fields, but the editor-in-chief and publishers hope to overcome this with an online edition, to which I shall return.
To give some impression of the excellence of these volumes, I will start with some of the basic statistics. The four volumes encompass 3,670 pages and include 700 main articles by named authors. This is a mouthwatering list of some 600 astronomers - an extraordinary number - all of them acknowledged experts in their fields. There are roughly 8,000 members of the International Astronomical Union, but it is fair to say that not all of them are research active. There are at least twice as many people involved in astronomy professionally, between 15,000 and 20,000 according to the entry on the profession, and so probably between about 5 and 10 per cent of the world's active research astronomers have contributed to this volume. With major articles by authorities such as John Bahcall, Roger Blandford, Jurgen Ehlers, Therese Encrenaz, Michael Hoskins, Eric Priest and Bernard Schutz, to mention only a few of the many international stars, this is a feast indeed. The editors are to be congratulated on persuading such a distinguished team to support this project. The 700 articles are outstanding and can be thoroughly recommended as definitive accounts of their fields.
The editors have taken the wise decision not to seek uniformity in the major essays. Some of them are quite mathematical, for example, the articles by Joseph Silk and Rychard Bouwens on "Galaxy formation" and by Chun Ming Leung on "Radiative transfer". John Peacock's article on "Cosmological models" is a complete lecture course on the standard cosmological models on its own. At the other end of the scale, the article by Blandford on "Astrophysical jets" is, perhaps surprisingly, devoid of all use of mathematics.
The articles are all well illustrated; most of the diagrams that I would have expected in such a publication are present. After all, one of the most useful functions of an encyclopedia is to make available illustrations and diagrams for teaching. One of the few missing is the energy spectrum of cosmic rays up to energies of 1021 eV, as is the distribution of the highest energy events over the sky; the Auger ultra-high energy cosmic ray experiment is also not mentioned. There is the occasional slip where an author is perhaps a little outside their area of expertise. The editor-in-chief attributes Adam Elsheimer's Flight into Egypt to Albrecht Altdorfer and does not mention the intriguing fact that the painting shows the resolution of the Milky Way into stars in the year before the publication of Galileo's Sidereus Nuncius in 1610.
The major essays complement each other nicely and there is an excellent balance between observation, theory and instrumentation. A very useful innovation, highlighted in the preface by Paul Murdin, is the essay by Eric Priest titled "Solar and solar-terrestrial physics". This sets in context all the entries on the Sun, solar physics, the interplanetary medium and solar-terrestrial physics. The success of this essay in enabling the reader to find the best place to begin any topic within this very broad field might be extended to other major areas of astrophysics and cosmology. Indeed, I could imagine such essays forming an introduction, or "propedia", to the encyclopedia as a whole.
These splendid longer articles are sup-ported by smaller entries: 100 on space vehicles and missions, 250 about observatories and 650 short biographies - none of which has a named author. These are somewhat less successful than the main articles. They have less authority and a few are not a proper reflection of the care and precision of language over which professionals agonise. An example of the type of error that might put off the reader concerns Newton's second law of motion, which, according to this volume, states that acceleration is proportional to force. Many of us labour the point ad nauseam to our students that Newton's second law states that "the rate of change of momentum is proportional to the impressed force". This encyclopedia and its readers deserve better than this.
In the definition of absolute zero, there is no reference to the thermodynamic temperature scale. In the entry on black body radiation, the words thermodynamic equilibrium are not used and it would have been helpful to give the expression for energy density per unit bandwidth - it would have been simple to extend the last paragraph to provide a definition of effective temperature, rather than the loose wording with which it concludes. In the entry on Olbers' paradox, no mention is made of the fact that Olbers was not the first to use the darkness of the night sky in a cosmological argument, nor that his thermo-dynamic argument was incorrect.
The same problem affects some of the brief biographies, which give the impression of being written by those who do not have a particularly deep understanding of the real achievements of those highlighted. For instance, in the biography of Fred Hoyle no mention is made of what many of us consider to be his greatest stroke of genius: the discovery of the triple-a process, by which carbon, can be synthesised from three helium nuclei. His extraordinary prediction of the resonance that permits the beryllium-8 isotope to exist for sufficient time to allow the addition of a third a-particle to form carbon solved the problem of stellar nucleosynthesis. The resonance was found soon after in laboratory experiments carried out at his suggestion. It is perhaps surprising that, in the article on Nobel prize-winners, Patrick Moore has not included Hoyle's name among those who have fully deserved to be honoured, but have not been accorded that accolade.
Errors of fact are bound to crop up in these biographies. For example, Johann Jakob Balmer was a Swiss schoolteacher with an interest in numerology rather than a German physicist.
Equally perplexing is the tone in which some of the biographies are written. In the article on Martin Ryle, following a discussion of the radio source counts, we find the statement: "This was the first of several near-fatal blows to the credibility of the steady state model, championed by his disputative Cambridge colleague Fred Hoyle, before the theory was finished off by the discovery of the microwave background radiation." This is scarcely the type of academic assessment expected in a scholarly publication. In fact, as Sir Fred remarked on the occasion of his 80th-birthday celebrations, his big mistake was to call the field responsible for the creation of matter C rather than Y - in the latter case, he would have been credited with the discovery of inflation.
A few other examples will highlight the types of improvement that I hope can be included in future editions. Ralph Alpher and Robert Herman are not given credit for making the first detailed estimates of the temperature of the cosmic microwave background radiation in the 1940s, many years before James Peebles who is credited with making these estimates in 1964 (they were also made by Igor Novikov in the same year). Indeed, Herman is not included in the index. The article on James Clerk Maxwell refers to his four equations, whereas Maxwell's version actually involved many more equations - Oliver Heaviside reduced these to their familiar four-equation form.
In the biography of Einstein, no mention is made of his introduction of the cosmological constant to create a stationary universe. The fact that Einstein realised that, for the first time in history, he had the theoretical infrastructure, general relativity, within which a fully self-consistent model of the universe could be created, is a remarkable intellectual achievement. Certainly, the authors should do better than state that Einstein "described the photoelectric effect".
Lyman Spitzer should be recognised as the figure behind the whole of the space astronomy programme in the United States and, in particular, as the one person who should be regarded as the father of the Hubble Space Telescope. Why is Margaret Burbidge, but not Geoffrey Burbidge, included in the biographies? Although many, including this reviewer, regard a number of his views as misguided, no one can deny that his early work in high-energy astrophysics was of central importance, and much of Margaret's work has been in collaboration with Geoffrey. Thomas Gold is given an unnecessarily rough ride. I shudder to think what the brief biography writers would have made of Kepler and Newton had they not been in the safe hands of the professionals.
Where do we go from here? Despite these niggles about a few of the briefer articles, this encyclopedia is a very important achievement. In his preface, Murdin explains that the intention is to update quarterly each of the items published in the electronic version. In his words: "We estimate that we will update 20 per cent of the material every year, so that the whole work will be refreshed on a time-scale typically of five years, quicker for rapidly changing areas." This is a laudable and ambitious goal and I wish him and the publishers luck. I hope his 600 distinguished authors will react to this proposal with equal enthusiasm.
It is a matter of debate what is the most effective means of making the material included in the encyclopedia accessible to those who need it. Nowadays, most of us use the web as an integral part of our lives as researchers and teachers. If we need a new image for a lecture or a publication, our first instinct is to look on the web: normally the image and the information we need will be there with a little bit of judicious browsing. If one selects suitable sites such as the Hubble Space Telescope website, we know that the information can be used free of charge under the US Freedom of Information Act, so long as we make due acknowledgement. It is hard to compete with that type of access to information. To take an example from this encyclopedia, there is an old picture of the quasar 3C 3 showing its optical jet. And yet, in a matter of seconds, I know how to retrieve the spectacular Hubble Space Telescope image of 3C 3 on John Bahcall's website, as well as images of many other quasars. Sensibly, most of the major articles in the encyclopedia include web addresses where further information can be gleaned.
Personally, I much prefer to read a long article in a book than on screen, or on a download from the web. So I am convinced that the hard-copy encyclopedia has a bright future, if its major articles are of the standard of those in these volumes, and provided it is supplemented by web addresses where more illustrative material can be accessed. There is no substitute, in my view, for the systematic, carefully prepared presentations of major areas of research by the leading experts in the field. While the business of cross-referencing can be carried out by computer, I would doubt if, in the end, it is any more efficient than searching this encyclopedia.
More important, one of the joys of an encyclopedia, and indeed the printed journals, is the serendipitous information that comes from flicking through apparently unrelated articles. It is often from these unexpected directions that new ideas arise. An example in this case was my delight in finding that three of the subjects of the 650 brief biographies were born in my home town of Dundee, in Scotland - Thomas Henderson, James Ivory and Williamina Fleming. I doubt if I could have come across this unexpected treasure by web browsing. Although the example is trivial, many of our best new ideas in research come from randomly accessing material in somewhat unrelated fields. We should also recognise that these volumes have historical importance as a comprehensive record of astronomical understanding at the turn of the millennium.
I therefore welcome strongly this major achievement by Murdin and his team of editors and expert writers. It is a massive enterprise and I enjoyed it thoroughly. All libraries should have a copy and all serious students of astronomy and astrophysics, who can afford it or persuade a benefactor to sponsor their studies, will derive great benefit from it. My mild concerns about a few of the shorter articles and biographies should be read as "friendly amendments", since I would not wish readers to be put off by these minor infelicities. And, please, do not discontinue publication of the hardback copy in favour of the online version. Some of us certainly need the lifeblood of serendipitous discovery to set us off in new directions.
Malcolm Longair is professor of natural philosophy and head of the Cavendish Laboratory, University of Cambridge.