Reflections on relativity

In the late 1960s, I began to gather material for a biography of Albert Einstein. I talked to the late Helen Dukas, who had been Einstein's secretary from the 1920s and had emigrated with him to the United States. At that time I knew that Einstein had had an unhappy first marriage with a fellow student in Switzerland named Mileva Maric. But I did not know much about Maric. I recall asking some sort of question as to whether they might have lived together as students. Dukas was shocked and offended and assured me that, at that time, that sort of thing was unthinkable. Two decades later we learned that not only was "that sort of thing" thinkable, but that Einstein and Maric had had a premarital daughter who subsequently disappeared.

Since that revelation the floodgates have opened. The fact that Einstein was the greatest physicist of this century, perhaps of any century, gets lost. Recently I watched an Einstein "biographer" on a TV books programme. The interviewer asked something about relativity. The poor author, who clearly did not have a clue, proceeded to read from a sort of prepared document. It was as if a biographer of Mozart were to be tone deaf and to focus on the off-colour letters Mozart exchanged with his cousin.

Tony Hey and Patrick Walters have written a fine popular book on Einstein, named after one of his famous paradoxes, in which Maric does not appear, even in the index. But if you are looking for a solid, attractive, up-to-date account of relativity theory and its consequences then by all means give their book a try.

In a companion book, The Quantum Universe, Hey and Walters treated Einstein's contributions to the quantum theory, which were equally monumental although, in the end, controversial. Einstein did not like quantum mechanics with its probabilities and uncertainties. He was in a deep sense a classical physicist and the theory of relativity is classical physics. If Einstein could have sat down for a couple of hours with his great classical predecessor, James Clerk Maxwell, I believe he could have explained the whole theory to him readily. Isaac Newton would have been a more obdurate case since Newton's commitment to absolute time and space - which relativity theory denies - was part of a profound metaphysical and theological infrastructure. In the end, Maxwell would have found Einstein's 1905 paper on relativity just as lucid as we do (though I am doubtful about Hey and Walters's statement that the paper can be understood "without any advanced mathematics").

In this part of the book, on "special" relativity, I would have emphasised a couple of things given only a mention by Hey and Walters. In the first place, there is a mistake in the 1905 paper. Einstein predicted that a clock placed at the North Pole would tick at a slightly faster rate than an identical clock at the equator owing to the fact that the latter is in motion with respect to the former. He even said by how much. This prediction is false. If both clocks are at sea level then they tick at the same rate. There is an effect of gravity that just compensates the special relativity effect; the cancellation has in fact been confirmed experimentally.

Then there is the matter of the contraction of objects in motion that is predicted by special relativity. Hey and Walters, like myself, seem to have grown up on George Gamow's popular science books. In these his Mr Tompkins lives in a world in which the speed of light is not much faster than that of a bicycle so that the effects of relativity impinge on daily life. The illustrations in Gamow's book - a couple of which are reproduced in Einstein's Mirror - show poor Tompkins in the process of contraction. We all agreed with this until 1959 when a Los Alamos physicist named James Terrell showed that it was completely wrong. Hey and Walters hint at the correction when they say things would not look "quite as simple'' as Gamow's illustrations. "Simple'' is not the issue. The images would not be contracted but rather rotated. This does not contradict relativity but is predicted by it.

The book's treatment of "general" relativity again calls for comment. The existence of black holes comes out of Einstein's general theory, as clearly shown in 1939 by Robert Oppenheimer and his students. Hey and Walters say Einstein was "suspicious" of the work that led up to this. This does not do justice to the situation. In 1939, Einstein published a paper - written in English, rather than his usual German - that purported to show how Oppenheimer et al's work contradicted the theory of relativity. In fact, the way out of the paradox Einstein constructed is just to have his system collapse into a black hole. After the war Oppenheimer became director of the Princeton Institute for Advanced Study. In vogue was the theory of elementary particles, in which Einstein had no interest. Oppenheimer had no interest in relativity and it is unlikely that the two men ever talked about black holes.

Hey and Walters discuss at some length nuclear weapons, which have little to do with relativity apart from the use of the equation E=mc2. It is true that Einstein signed the letter to President Roosevelt in August 1939 urging him to pay attention to the German nuclear physicists. But it is unlikely that Einstein could have successfully managed the calculations done by Rudolf Peierls and Otto Frisch in 1940 that showed the making of an atom bomb to be a real possibility. In fact, before learning about fission, Einstein once remarked that being able to use nuclear energy was as likely as a blind man finding a bird in a country where there were very few birds.

Jeremy Bernstein is emeritus professor in physics, Stevens Institute of Technology, New Jersey, United States.