Nuclear weapons no longer dominate headlines but they cannot be disinvented. Hitler never acquired the science to make the bomb - today's megalomaniacs have no such handicap. Moral responsibility for nuclear weapons is now in the hands of politicians, not scientists - or is it? Three distinguished scientists reflect on the dilemmas, past, present and future.

The bomb on Hiroshima, which heralded the nuclear age, was the creation of scientists, but it went sour on them from the very beginning. In total disregard of the basic tenets of science - openness and universality - it was conceived in secrecy, and usurped, even before birth, by one state to give it political dominance. With such congenital defects, and being nurtured by an army of Dr Strangeloves, it is no wonder that the creation grew into a monster; a monster with 70,000 heads, nuclear warheads; a monster that breathed fear and mistrust, and has threatened the continued existence of humankind on this planet ever since. We scientists have a great deal to answer for.

What is so sad about all this is that our intentions were good. The scientists in Britain who initiated the atom bomb project were convinced the bomb was needed to prevent the Nazi regime from dominating the world.

The British and American scientists feared that German scientists would develop the bomb. When our calculations had shown that an atom bomb was feasible, it was natural to assume that the German scientists had reached the same conclusion.

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It was not until five years later that we learned that the fear was groundless; owing to erroneous theoretical calculations and faulty experiments, the atom bomb project in Germany had, for all practical purposes, folded up as early as 1942 - even before the Manhattan Project started in earnest in the United States. But we did not know this at the time. We feared that if Hitler got hold of this weapon, it would enable him to win the war. It was this appalling prospect that convinced us to start the project, and to work out a rationale for making the bomb ourselves: the only way to deter Hitler from using the bomb against us would be if we too had it and threatened retaliation.

We needed the bomb so that it would not be used.

But as it turned out, we were wrong: the bomb was used; it was used as soon as it was made; it was used against a civilian population.

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Fifty years after these momentous events the question is still before us. Was the destruction of Hiroshima and Nagasaki necessary?

For most people in the West the answer to the question is a clear yes. The simple fact is that a few days after the atom bombs Japan surrendered and the war came to an end, thus saving many American lives. The deaths in Hiroshima and Nagasaki were a necessary sacrifice sparing the lives of much larger numbers of others.

But this simple answer has been challenged by some historians, notably Gar Alperovitz, who assert that the war could have been ended earlier, without the use of the atom bomb. My experience in Los Alamos, admittedly rather limited, led me to support the thesis that other considerations, including the need to demonstrate to the Soviets the newly acquired military might of the US, contributed to the decision to use the bomb on the Japanese cities.

This issue surfaced in connection with the commemoration of the 50th anniversary of Hiroshima. The Smithsonian Institution planned to prepare a comprehensive exhibition of the history of the development of the atom bomb, with an evaluation of its use, including the dissenting views of the "revisionists". The draft of the proposed contents became known to the public and was immediately attacked by the American Legion as unpatriotic. The Senate denounced the script as "offensive" to second world war veterans. The Smithsonian caved in, removed the controversial items, leaving only a shadow of the original, presenting the official view.

This shameful episode makes it even more necessary for all relevant information to be brought to public notice. There is another aspect of the story that is even more important: irrespective of the reason for the decision to use the bomb, how much has this decision contributed to the nuclear arms race that nearly brought our civilisation to an end? Could the arms race have been avoided?

The great Danish physicist Niels Bohr was one of the first to foresee the danger of the arms race that he believed was bound to ensue from the use of the bomb. My talks with Bohr at Los Alamos greatly influenced my thinking, and led me to the conviction that while the use of the bomb marked the end of the war, it was also the first step in the nuclear arms race. This view is strongly denied by those who say "Thank God for the bomb". In this respect, Dark Sun by Richard Rhodes is a most timely, and conclusive contribution to the debate.

Dark Sun is the sequel to The Making of the Atomic Bomb, the monumental work that dealt mainly with the history of the development of the atom (fission) bomb in the US. Dark Sun, despite the subtitle The Making of the Hydrogen Bomb, is to a large extent concerned with the same topic, but in relation to the Soviet Union. Since writing the first book, Rhodes acquired a great deal of information about the Russian bomb, much of it provided by the KGB itself in an attempt to prove the historical importance of its activities.

It is fascinating to read about the Soviet scientists building their bomb, stage by stage, on information provided by British and American spies. The normal way for science to progress is for scientists to publish their findings in a scientific journal. These are then taken up by other scientists, either by inspiring new ideas or by repeating experiments with better equipment, leading to new findings, and so on. The world scientific community thus collaborates in pushing forward the frontiers of science.

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The outbreak of war in 1939 brought this process to a halt. Scientists in the US, the United Kingdom and France voluntarily agreed not to publish papers that might help the German scientists in their atom bomb project. The Soviet scientists, though our allies, were also cut off and so had to resort to spying.

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Spying activities were part and parcel of the Soviet regime, and an elaborate spying net had been established long before the atom bomb project. These activities were much more effective than, say, the German ones, because of the leftist tendencies of the scientific community in the prewar period, plus the fact that the Soviet Union was our ally during the war. Some scientists felt that the withholding of information was harmful to the Soviet effort in the common fight against Germany. This was probably the motivation of amateurs like Alan Nunn May or Bruno Pontecorvo, as distinct from the professionals, such as Donald Maclean and Kim Philby. Klaus Fuchs is somewhere in between.

The important fact is that the Soviets knew about the UK atom bomb project from the very beginning. The information about the Maud Committee, which was set up in 1940, was provided not by scientists but by someone in the civil service, probably John Cairncross. But later on came the steady supply of secret information delivered by Klaus Fuchs and American spies.

No doubt, Soviet scientists of the calibre of Igor Kurchatov or Yuli Khariton were capable of making the bomb without the aid of spies, but they might have made the sort of mistake that thwarted the German scientists in their project. But by receiving the exact dimensions of Fat Man (the plutonium bomb) from Fuchs, the Soviet scientists were able to detonate their first bomb - an exact replica of the Nagasaki bomb - only four years after Hiroshima. General Leslie Groves, the head of the Manhattan Project, estimated it would take 20 years, and President Harry Truman believed that the Russians would never be able to make the bomb.

The loss of nuclear monopoly, after the first Russian test in 1949, triggered the crash programme to develop the hydrogen bomb in the US; the arms race, that was to rage for 40 years, started in earnest.

In describing the genesis and evolution of the crash programme, Rhodes gives a fascinating account of the step-by-step development of ideas about the Super Project, ending with a very detailed technical description of the first version of the thermonuclear weapon tested at the Eniwetok atoll. Much has been written about these events, but to me the new aspect was the role of Edward Teller. The mention of his name is usually followed by the epithet "the father of the hydrogen bomb", and he is always credited with personally persuading President Truman to start the crash programme, against the strong opposition of Robert Oppenheimer. It comes therefore as a surprise to learn that Teller actually delayed by some years the introduction of the H-bomb, by his insistence on an unworkable concept.

Teller is generally disliked, even loathed by a large section of the scientific community. Dark Sun quotes a number of episodes to justify this attitude, the most important being Teller's vendetta against Oppenheimer, culminating in the damning testimony during Oppenheimer's loyalty investigation. But, according to Rhodes, Teller was also disliked by the bomb makers at Los Alamos, for his obdurate stand that resulted in the long delay in developing the H-bomb. Rhodes places much weight on this, claiming that the ugly division in the US political and scientific communities might have been avoided if it were not for Teller's idee fixe. I doubt, however, whether this has made any difference to the really important issue, the nuclear arms race.

Scientists played a vital role in maintaining the momentum of the arms race. This applies to scientists in Los Alamos - distrusted by Teller - as well as to those in the Livermore Laboratory, Teller's child. It equally applies to Soviet scientists in the Chelyabinsk and Arzamas laboratories. As seen by Theodore Taylor, once chief designer of fission bombs at Los Alamos, or by Herbert York, the first director of Livermore, the arms race was often sustained by the addiction that often results when a scientist has the resources to explore and bring into reality completely new technical concepts, whether they are needed or not.

Rhodes tots up the cost of the arms race: four trillion dollars (about Pounds 2500 billion) in the US. Soviet costs were comparable and were decisive in the decline of the Soviet economy: "The expense of the arms race contributed to US decline as well, decline evident in an oppressive national debt, in decaying infrastructure and social and educational neglect."

It is more by good luck than by good management that the nuclear arms race has been brought to a halt before it led to the inevitable end - a nuclear holocaust. But the nuclear powers still cling to the deterrence policy, claiming that the possession of nuclear weapons is essential for national security. There is no evidence to bear out this claim, but it is likely to be taken up by other nations as an excuse for acquiring nuclear weapons.

In concluding his magnum opus, Rhodes says: "As instruments of destruction [nuclear weapons] have long been obsolete." This may be true if it refers to the role of nuclear weapons in enabling any side to win a military conflict, but not as instruments of destruction per se. Nuclear weapons cannot be disinvented: they will forever pose a threat to the continuing existence of the human species. Only the abolition of all war can prevent such a catastrophe.

Joseph Rotblat is president, Pugwash Conferences on Science and World Affairs, and winner of the 1995 Nobel peace prize.

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