A question gave Rudolf Peierls his place in history. He was so brilliant and so thoughtful he would certainly have found his way there by another route, but that question was enough. It was asked in Birmingham in early 1940 by Otto Frisch, one of the discoverers of nuclear fission, and it concerned certain properties of the element uranium. The answer, ultimately, was the atomic bomb.
Peierls, like Frisch, was a refugee from Hitler, a physicist, and concerned about the implications of the latest discoveries about uranium. By the spring of 1940, the prevailing scientific view was that a uranium bomb was impossible, because it would be too enormous, too unwieldy to be useful.
What if, Frisch asked, you did not use ordinary uranium? What if you used a refined lump of the rare type known as U-235? Would that be more practical?
Peierls had already developed a mathematical formula model for a calculation of this kind and the two set to work. They found that the "critical size" of the uranium weapon could be measured in pounds, not tons. This was something that could be dropped from an aeroplane.
Could enough U-235 be made? Between them they determined that it could. Their discovery set in motion the British atomic effort, code-named first Maud and then Tube Alloys, which in turn provided the vital stimulus for the American Manhattan Project. The bomb dropped on Hiroshima used U-235, as Frisch and Peierls had suggested. The bomb dropped on Nagasaki, which used plutonium and followed a quite different design, also owed a great deal to Rudolf Peierls.
The nuclear age had many fathers, and Peierls's place among them is beyond dispute. To those inclined to think this a dubious distinction, Peierls's later life offered an answer. From 1945 to within a few weeks of his death on Tuesday, he was among the most intelligent, informed and dynamic critics of nuclear weapons and the nuclear arms race.
Peierls was born in Berlin in 1907, the son of an engineering factory manager. Although his father's forebears were Jewish and his mother a Roman Catholic, he was baptised a Protestant. "My father," Rudolf wrote much later, "thought this would allow us to make our own choices when we grew up." This pragmatism, and the innocent spirit of subversion that went with it, were to rub off on the boy.
His pre-war career in science made him the embodiment of the old international physics of discovery, open exchange and free debate. He toured Europe, studying in almost every significant centre of research - Berlin, Munich, Leipzig, Zurich, Odessa, Leningrad, Rome, Cambridge, Manchester - and befriending all the "greats" of the period. On his travels he married a Russian physicist, Genia Kannegiesser.
He abandoned Germany just before Hitler took power and settled in Britain, becoming a professor at Birmingham in 1937. When he and Frisch had their conversation that day in 1940, Peierls was still not a British citizen but an "enemy alien", although this was very soon put right.
The "Frisch-Peierls Memorandum", setting out their findings, was the first practical blueprint for the atomic bomb. Central to its argument was the warning, which the writers were well qualified to issue, that German physicists were sufficiently able to think of this too, and that Hitler might already be working on the bomb.
Soon the bomb work transferred to the United States, and here Peierls made two distinct contributions. First, he advised on the complex technology required for separating U-235 from natural uranium. Then he moved to Los Alamos, the famous laboratory established in the New Mexico mountains under Robert Oppenheimer to design and manufacture the finished bombs.
At Los Alamos, this little man with bottle-end spectacles and a pipe clamped between his teeth became a popular fixture. His wife joined him, and their little house - one of the few with a bathroom - became something of a social salon. Peierls led the small but distinguished British team and was also in charge of an important theoretical research group known as the hydrodynamics group. This was remarkable in itself - not only was he neither American nor British, he was a German.
But Oppenheimer worked by merit alone and Peierls combined scientific ability of the first order with unusual gifts of managerial and political judgement. He was patient and kind, yet practical and quick-thinking. Progress reports he wrote to the British scientific mission in Washington were so thorough and yet so succinct that the US military authorities began to ask for their own copies.
Peierls's scientific contribution, particularly to the plutonium bomb which became the model for early post-war nuclear weapons, was considerable. A number of patents (subsequently to prove meaningless) were taken out in his name and they betray his extraordinary versatility, relating as they do to several quite distinct aspects of the design. He saw the first weapon tested at Alamogordo, New Mexico, in July 1945.
If Peierls later campaigned against nuclear weapons, this was not the result of guilt, or of some Damascene conversion. His views before and after 1945 were remarkably consistent. At first, he believed, it was necessary to build a bomb in case Hitler was doing so too. When the Germans surrendered, he continued because there was a bloody war going on in Asia which the bomb might shorten. The decision to drop it on a city may have been wrong, he believed, as its power could have been demonstrated in other ways. To drop it on two was "unnecessary". But he was certain that neither decision should or could have been made by scientists such as himself.
That he thought deeply about these issues from the start can be seen from the 1940 Memorandum, which included the observation that "the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country".
After the war, Peierls was president of the Atomic Scientists' Association, pressing in vain for a better understanding of nuclear issues both among politicians and the general public, and campaigning for some form of international control of nuclear weapons as a means of forestalling the Cold War.
More recently he was involved in Pugwash, the East-West scientific forum for disarmament, and he was among the many distinguished scientists publicly to express opposition to Star Wars. As recently as this spring, he was one of the authors of a Pugwash pamphlet, Does Britain Need Nuclear Weapons? The answer was no.
In 1963 he moved to Oxford, as Wykeham Professor, where he worked until his retirement in 1974. He loved Britain, praising the "reasonableness" of its people and their gift for rubbing along with one another despite differences. This gift, he admitted to me in a conversation in March, was less evident now than it was in the 1930s.
His affection for this country was tested more than once down the years. During the war, Peierls recruited to the bomb project the German-born physicist Klaus Fuchs, who later turned out to have been a Soviet spy. No one was more stunned when Fuchs was unmasked in 1950. The connection, his own family link with Russia and his activities in the Atomic Scientists' Association led to suggestions in the press that his loyalty was in doubt. On each occasion, he took care courteously to rebut the claim, and in 1979 he successfully sued the author of a book containing a similar implication.
Genia Peierls used to classify scientists as either "golfers", pursuing a lone quest for a known goal, or "tennis players", whose strengths are brought out in exchanges with others. It was no accident that "Rudi" was drawn into the making of the atomic bomb by a question, for he was the tennis player par excellence. He avoided specialising in any field of physics, and his gift was to spot flaws or openings in the work of others and then to turn them into new ideas.
Aside from his research, which he continued to pursue well after retirement, his principal pleasure was to foster the careers of others, a task which both he and his wife pursued with devotion and pleasure.
It is said that he once overheard another scientist saying: "Did you know that two of Rudi's former students are now lords?" The professor observed: "I have had more than 200 research students. I cannot be blamed if one or two go to the bad."
Rudolf Peierls's life has ended in the 50th year of the nuclear weapons age. He re- mained to the last a patient, lucid and generous spokesman for the bomb-makers and also for that remarkable generation of scientists who taught him or worked beside him in the golden years before the bomb.