Forget the Polyfilla

There is something ghoulishly fascinating about black holes. It is much more than the possibility that they are portals to parallel universes, and gateways to time travel; it is the fact that they are so unidirectional. You can go in, but you cannot come back. The black hole is a cosmic jail that imprisons both light and matter. And we have no idea what this matter is like when it gets inside a small black hole.

To produce a black hole you have to take the whole sun and squeeze it until it is less than four miles across. Or, staying closer to home, you must place the mass of a typical mountain into the volume of an atomic nucleus. These hypothetical objects then have such a huge gravitational field at their respective surfaces that nothing can escape. To get away from earth a rocket needs a velocity of only 25,000 mph. But to escape from the surface of a black hole the rocket needs to be travelling faster than the velocity of light, and nothing (yes nothing) can do that.

What is healthy about the black hole craze (and it is a craze) is that it constantly reminds us just how little science knows. The material inside a stellar black hole is in a mysterious form about which we have everything to learn, because at the moment we know absolutely nothing. What is unhealthy about black holes is that people seem to think they act like giant vacuum cleaners, sucking in any material that happens to approach too close. In fact if the sun became a black hole, the gravitational force that it exerts at earth's orbit would not change at all. We would still move round it at the same speed and stay at the same distance. The sun would have gone out, and it would get extremely cold, but that is all.

What is reassuring is the fact that modern cosmologists insist that we are actually living inside a huge black hole. If we follow Arthur Eddington and take the total mass of the universe as being about 1056g, and if we have the big bang taking place a reasonable 15 billion years ago, then the expansion of the universe is such that light cannot escape from it. And we have been living happily inside this black hole all along.

Do not worry too much about the sun becoming a black hole. The solar geriatric form is that of a stable, earth-sized, white dwarf star, in which the random motion of carbon and oxygen nuclei support it against gravitational collapse. All stars with masses less than about one-and-a-half solar masses will suffer a similar fate. Between that mass and a mass of about two and a half solar masses the stellar end-point is a neutron star. The first neutron star was discovered in 1968 and nowadays many are known. Stars with even more mass become black holes. No theoretical arguments suggest that stellar evolution should permit the existence of white dwarf and neutron stars while forbidding the existence of remnants so massive that they become black holes. But where are they? How can we identify something that does not even emit light? Here the theoreticians and the empirical astronomers had to get together. It was soon realised that a close binary star system held the answer, and especially one in which the black hole was capturing matter from its companion. As this matter fell into the hole it would be accelerated so much that its temperature would reach a hundred million degrees Kelvin and the material would be a prominent x-ray source. These have been picked up.

The other main black hole candidates are at the centre of galactic discs. Here the concentration of stars can be so high that collisions are frequent and once such a cloud of colliding stars begins to contract under its own gravity, collapse to a black hole is almost inevitable. In many cases quasars and active galactic nuclei are the result.

I have absolutely no hesitation in recommending Gravity's Fatal Attraction by Mitchell Begelman and Martin Rees to anyone who wants to start learning about black holes. The two authors are not only at the top of their profession but also have an enviable ability to explain extremely complicated topics in a way that anyone with the willingness to spend a little time and effort can understand. Their first-class text is augmented by a host of superb illustrations. There is no deja vu here. The pictures are new, thoughtfully produced, carefully coloured and in the very best tradition of the Scientific American team that has published the book. It is clear that no expense has been spared in producing the very best. Instead of having to struggle with a verbal description of, for example, the effects of orbiting stars crashing through a black hole's accretion disc, or gravitational radiation retarding orbiting black holes, or the warped disc of the spiral galaxy NGC 4258, or the form of the SS433 binary star, we are presented with beautiful images each of which clarifies the concept enormously. This book is a classic example of what a popular astronomy book should be like.

Clifford A. Pickover approaches the subject from a completely different angle. You can easily imagine him standing on a soap box, haranguing a crowd, and enquiring if you want to "take off on a mind-boggling journey to the ultimate frontier of fact-based scientific speculation"? Do you want to imagine yourself in an intergalactic spaceship with an extremely chatty first officer called Mr Plex, the aforesaid gentleman being "a scolex, a race of creatures with diamond-reinforced exoskeletons that allow them to explore outer space with little consequence to their health'' and who has a prediliction for screaming "Mon Dieu! My body simply won't withstand those kinds of tidal forces.'' Do you want to calculate for yourself things like the gravitational blue shift, the gravitational lens effect, the radiation received from a collapsing star and the cross-section of a traversable worm-hole? If the answer is yes, then this is the book for you. You travel with Mr Plex to the most amazing spots. The book is fun, zany, in-your-face and refreshingly addictive.

Kitty Ferguson brings us down to earth again. Her book Prisons of Light shows a somewhat earnest approach to the subject. It is enlivened by a rather quaint series of typewritten and pen-drawn diagrams. And we are told in no uncertain terms that black holes are "one of the most fascinating topics in modern science''. You then read pages and pages about what they are, how they work, what it is like to be close to one, whether we could survive a visit and what is inside. And then there are even more pages about whether a real black hole has yet been discovered, and you begin to wonder - surely, if they are going to qualify for being "one of the most fascinating topics in modern science'', the very least we should have done is found a few. And then Ferguson continues with even more weighty topics such as the lessons we learn from black holes about the mysteries of the universe and why angular momentum is such a worry, and the exciting consequences of the chaotic evolution of quantum gravity. And you become more and more convinced that you are dealing with the really exotic. And you begin to yearn for normality and the commonplace. You begin to appreciate your local neck of the galaxy, where stars are not black and holes do not exist, and planets are wet. And you even begin to have some sympathy with Walt Whitman. You imagine the person who stood there "In the mystical moist night-air, and from time to time,/ Look'd up in perfect silence at the stars".

And you think, never mind black holes; we haven't yet explained where earth comes from, or its life, or why the sentient life form should worry about the stars at all. And you find black holes slipping down the league of scientific topics, being relegated from the premier "most fascinating'' division, through the lesser divisions of fascinating, less fascinating, even less fascinating ... until you realise that they hardly influence your existence at all.

David Hughes is reader in astronomy,University of Sheffield.