It is now almost fifty-seven years since the atomic bombings of Hiroshima and Nagasaki forced an end to a long and terrible world war, the most destructive war in history, a war that obliterated fifty-five million human lives. If the killing has not ceased in the decades since 1945, neither has it raged in anything like the murderous intensity of those horrific six years. Wars have claimed an average of about one million lives annually in the years since 1945; but during the Second World War the annual toll was nearly eight times that average. The most fundamental reason for this historic reduction in man-made death, I believe, was science’s discovery of how to release nuclear energy. The Second World War was the first nuclear war. And that made all the difference.

Niels Bohr foresaw the changes that would come to the world with the discovery of nuclear fission. Fission was discovered in Berlin in late December 1938. In the next two years, Bohr contributed significantly to understanding how the extraordinary new reaction worked. Fortunately for the world, exploiting nuclear fission in a weapon turned out to be a complicated and expensive process. Natural uranium is a mixture of several isotopes, and only one of them, U235, turned out to be good bomb material. U238, the predominant isotope of uranium, actually poisoned the reaction. The U235, the bomb material, was only one part in 140 of natural uranium. The other 139 parts were U238. So the U235 would have to be separated from the U238 if it was to be used in a bomb. And separating the two kinds of uranium would require huge factories on the scale of major oil refineries to accomplish. Army agents, late in the Second World War, picked up a week’s output of U235 from the vast isotope-separation facility at Oak Ridge, Tennessee, and carried it to Los Alamos, in a small briefcase.

Difficult as isotope separation would prove to be, Bohr in 1940 had thought it would be even more difficult, and had doubted that an atomic bomb could ever be built. By the time he escaped from Nazi-occupied Denmark in the autumn of 1943 he knew better, as we will hear. His mother was Jewish, and he left his homeland only one step ahead of the Gestapo. He had been warned that the Nazis were preparing to round up the Danish Jews and ship them to a death camp. He crossed the Øresund with his family at night in a small boat. His first act after he reached neutral Sweden was to go to the king and convince him to broadcast an offer of asylum to the Danish Jews. Only when that mission was accomplished did Bohr travel on to England, carried famously in the bomb bay of a British Mosquito fighter-bomber like some new secret weapon, which he was. In England, Bohr was fully briefed on the Anglo-American atomic bomb program. He was surprised and shocked at how far the program had advanced. He immediately began thinking about the future consequences should atomic bombs be built.

When uranium fissions, a small amount of mass is converted into energy in the form of heat, a process that is several million times more energetic than chemical burning. Albert Einstein had first quantified this mass-energy conversion in his famous formula E = mc². Since the c in Einstein’s formula designates the speed of light, a very large number, and that very large number is squared, the formula emphasizes that even a small amount of mass, when it fissions, will release a stupendous amount of energy. The bomb that destroyed Hiroshima, for example, was a crude first-generation weapon, handmade at Los Alamos, with an efficiency of less than one percent. It contained about 150 pounds of U235; it exploded with energy equivalent to about 13,500 tons of TNT; but 13.5 kilotons means that less than one gram of U235 was completely converted into energy.

Bohr understood Einstein’s formula and quickly worked out the numbers, as physicists in many other countries had already done. But Bohr, who was not only one of the great physicists of the 20th century but also one of the great philosophers, also understood what the numbers meant. In late June 1944 we find him in residence at the Danish Embassy here in Washington, D.C., sweltering in the summer heat—for working in Washington in those days, Danish diplomats are said to have received a tropical hardship allowance, and perhaps they still do—preparing a memorandum on nuclear energy for Franklin Roosevelt, hoping to convey his visionary understanding to the President of the United States.

By then Bohr had already survived a disastrous encounter in London with Winston Churchill. Preoccupied with the approaching Normandy invasion and with England's declining fortunes, Churchill was not prepared to listen to advice on the highest affairs of state from a Danish academic. "I cannot see what you are talking about," the Prime Minister had scolded Bohr impatiently. "After all, this new bomb is just going to be bigger than our present bombs. It involves no difference in the principles of war."

Bohr knew better. He knew about not only atomic bombs; at Los Alamos that spring he had learned from Edward Teller about the possibility of hydrogen bombs as well, weapons with essentially unlimited destructive potential. These possibilities were worrying his younger Los Alamos colleagues. Bohr’s insight had brought them a measure of comfort, as the Austrian emigré theoretician Victor Weisskopf would remember. "In Los Alamos," Weisskopf said later, "we were working on something which is perhaps the most questionable, the most problematic thing a scientist can be faced with." Weisskopf meant they were working on what we today call weapons of mass destruction—a new experience for physicists, who up to then had thought of their discipline as almost theologically otherworldly. "At that time," Weisskopf continued, "physics, our beloved science, was pushed into the most cruel part of reality and we had to live it through. We were, most of us at least, young and somewhat inexperienced in human affairs, I would say. But suddenly in the midst of it, Bohr appeared in Los Alamos.

"It was the first time we became aware of the sense in all these terrible things," Weisskopf concluded, "because Bohr right away participated not only in the work, but in our discussions. Every great and deep difficulty bears in itself its own solution . . . . This we learned from him." Bohr was characterizing the complementarity of the bomb, its potential not only for devastation but also, as he saw, its potential for limiting war. The principle of complementarity had been central to his formulation of quantum physics; he had scolded Heisenberg for introducing it only in limited form in quantum mechanics as the Uncertainty Principle, because Bohr understood complementarity to be one of the deep organizing principles of the natural and human world.

So here in Washington at the Danish Embassy, occupying his hands sewing on buttons and darning socks, with his teenage son Aage at his side taking dictation, Bohr dictated draft after draft of his Roosevelt memorandum, lavishing as much attention on its logic as he had lavished on his papers in theoretical physics, struggling to explain the inevitable consequences of the discovery of how to release nuclear energy.

"The whole enterprise," Bohr told Roosevelt, "constitutes...a far deeper interference with the natural course of events than anything ever before attempted, and its impending accomplishment will bring about a whole new situation as regards human resources. Surely, [Bohr went on] we are being presented with one of the greatest triumphs of science and engineering, destined deeply to influence the future of mankind."

Bohr told of "enormous energy sources which will be available" that would "revolutionize industry and transport." (He was alluding to the development of nuclear power for generating electricity and for driving ships and submarines.) But of more immediate concern, he cautioned Roosevelt, was the creation of what he called "a weapon of unparalleled power" which would "completely change all future conditions of warfare." He said that better in 1957, after the nuclear arms race had begun between the United States and the Soviet Union. He said, "We are in a completely new situation, that cannot be resolved by war."

Bohr warned Roosevelt in 1944 of what he called "the terrifying prospect of a future competition between nations"—he meant a nuclear arms race—unless those nations negotiated what he called "a universal agreement in true confidence." He understood the necessity of transparency between nuclear powers to prevent (in his words) "a competition prepared in secrecy." He expected that there would have to be, as he put it, "such concessions regarding exchange of information and openness about industrial efforts, including military preparations, as would hardly be conceivable unless at the same time all partners were assured of a compensating guarantee of common security." Openness proved elusive, as we know, and the United States and the Soviet Union eventually came to rely on so-called national technical means of verification — that is, on satellite surveillance. Bohr had imagined that all sides would judge openness to be a fair exchange for security, but until recently the two superpowers resisted that conclusion. They preferred extended deterrence — preferred, that is, a dangerous and expensive arms race. Now in the aftermath of that arms race, Bohr's argument for openness remains no less valid than it was in 1944. Common security against nuclear danger requires transparency; a world free of nuclear weapons will have to be completely transparent where nuclear technology is concerned, each side able to inspect factories and military installations on the other side’s territory whenever it has reason to do so. "Bohr was clear," his colleague and admirer Robert Oppenheimer would say later, "that one could not have an effective control of...atomic energy...without a very open world; and he made this quite absolute.... In principle, everything that might be a threat to the security of the world would have to be open to the world."

Bohr hoped that openness achieved for nuclear security might have a complementary outcome as well. "What it would mean," he told Secretary of State George Marshall in 1948, after the Iron Curtain had clanged down, "if the whole picture of social conditions in every country were open for judgment and comparison, need hardly be enlarged upon." If peoples could see each other, that is, they could observe each others' form of government and way of life and use that information to improve or change their own. Bohr would seem to have modeled his vision of the world on the experience of his homeland. The Scandinavian countries had fought each other and the rest of Europe bloodily for centuries before coming to accept the futility of such conflicts. "An open world," Bohr told the United Nations in 1950, "where each nation can assert itself solely by the extent to which it can contribute to the common culture and is able to help others with experience and resources, must be the goal to put above everything else." And then, most generally and profoundly, Bohr concluded: "The very fact that knowledge is itself the basis for civilization points directly to openness as the way to overcome the present crisis."

Roosevelt listened, but Churchill continued to refuse to do so, and Bohr's lonely initiative went unheeded. Indeed, Churchill almost succeeded in having the Danish physicist thrown in jail. Britain and the United States did not sit down with the Soviet Union before the end of the Second World War to confront the common problem of controlling nuclear weapons. Instead, the Western nations tried to keep secrets of technology that were already being worked out independently by Soviet scientists and had already been lost to Soviet espionage. Britain and the United States agreed to use the atomic bomb against Japan without warning or demonstration, hoping to end the Pacific War sooner, to limit Soviet participation and to save American lives. When Japan had attacked China in the 1930s, the United States had publicly condemned what it called Japan's "inhuman bombing of civilian populations." The atomic bombings of Hiroshima and Nagasaki by two American B-29s made starkly clear the change in the scale of destructiveness that nuclear weapons initiated: in Hiroshima alone, of 76,000 buildings, 70,000 were damaged or destroyed, 48,000 totally. Ninety percent of all Hiroshima medical personnel were killed or disabled. Up to September 1st at least 70,000 people died. More died later from the effects of exposure to ionizing radiation. All from one bomb dropped from one plane early on an August Monday morning.

The tragedies of Hiroshima and Nagasaki would be unique; theirs would be the only atomic bombings. In 1945, the "completely new situation" Bohr foresaw that could not be resolved by war had not yet become reality; while there was yet only one nuclear power on earth, that nuclear power could dare to use its weapons against an opponent not similarly armed. For four years after the war, the United States continued to hold such a monopoly, and then the Soviet Union tested a plutonium bomb identical to the one that had destroyed Nagasaki, the superpower arms race began with a tumult of atmospheric testing in 1951 and the confrontation moved rapidly to stalemate.

Not deadlock, as in Korea, nor even defeat, as in Vietnam and Afghanistan, would ever again justify escalation from conventional to nuclear weapons. The danger was too great. McGeorge Bundy, national security adviser to Presidents John Kennedy and Lyndon Johnson, put that danger in perspective in a 1969 essay in the journal Foreign Affairs. "In light of the certain prospect of retaliation," Bundy wrote, "there has been literally no chance at all that any sane political authority, in either the United States or the Soviet Union, would consciously choose to start a nuclear war. This proposition is true for the past, the present and the foreseeable future.... In the real world of real political leaders [Bundy went on]...a decision that would bring even one hydrogen bomb on one city of one's own country would be recognized in advance as a catastrophic blunder; ten bombs on ten cities would be a disaster beyond history; and a hundred bombs on a hundred cities are unthinkable."

In the expert judgment of at least one experienced national security adviser, that is, one hydrogen bomb, guaranteed deliverable, has been for many years a sufficient deterrent. The vast gulf of numbers between that minimum deterrent and the tens of thousands of nuclear weapons that the superpowers stockpiled in the years of the Cold War and continue to maintain despite laudable reductions, reveals the extent to which nuclear arms have served purposes other than self-defense. Among those purposes have been political warfare, economic warfare and domestic economic stimulus. The truth is, through much of the Cold War, the United States and the Soviet Union dangerously and opportunistically contended for hegemony by building and testing nuclear weapons, risking all our lives.

Yet, though they bristled with genocidal armaments, paradoxically, across the past five decades, with reluctance and often with ill will, every nuclear power, large and small, has felt compelled to limit its power to make war—compelled, that is, to limit the exercise of its national sovereignty. Who or what drove that unprecedented compellence?

Science compelled that limitation. Knowledge—the knowledge that in Bohr's phrase is "itself the basis of civilization"—compelled that limitation. Nuclear energy was deliberately released for the first time minutely on a laboratory bench in Berlin in December 1938. Four years later, in December 1942, a small experimental nuclear reactor at the University of Chicago, the first in the world, increased that manifestation of the new knowledge to half a watt of power. In 1945, a plutonium implosion bomb tested in the New Mexican desert increased nuclear energy's compass further to the equivalent of eighteen thousand tons of TNT. Then bombs that exploited nuclear energy destroyed one Japanese city and tens of thousands of lives and then another Japanese city and more thousands of lives. Then the process bifurcated and began to manifest itself simultaneously as a source of energy and a source of destruction, but the destructive potential was encapsulated and went latent and only the energy was expressed. Nuclear energy today meets the annual electrical needs of more than one billion people throughout the world, our newest and cleanest form of energy, the only major source that does not depend directly or indirectly on sunlight. At the height of the Cold War, the encapsulated destructive potential of nuclear weapons reached at least 10,000 megatons, two tons for every man, woman and child on earth. This remarkable chronology charts an enlargement of influence across only six decades—a dispersion of knowledge, if you will—of orders and orders of magnitude.

Most of us were taught that the goal of science is power over nature, as if science and power were one thing and nature quite another. Bohr observed to the contrary that the more modest but relentless goal of science is, in his words, "the gradual removal of prejudices." By "prejudice," Bohr meant belief unsupported by evidence. It was a prejudice, based on incomplete knowledge of the natural world, that the earth is the center of the universe. It was a prejudice, based on incomplete knowledge of the natural world, that humankind is a separate creation rather than a product of evolution. One enduring prejudice resulting in immense human suffering has been the belief that in an anarchic world there are no limits to national sovereignty except those that armed conflict might determine. Knowledge of how to release nuclear energy, knowledge that only science was structured to perceive, has now come to define a natural limit to national sovereignty. The authority of the institution we call science, that is, has taken precedence, at least in this extreme arena, over the authority of the nation-state. Science has fielded no armies in order to do so and is indeed pacifist; rather, it has gradually removed the prejudice that there is a limited amount of energy available in the world to concentrate into explosives, that it is possible to accumulate and deliver more such explosives than one's enemies and thereby militarily to prevail. Science, revealing that matter, properly arranged, is all energy, available to even a small nation in essentially unlimited quantities, has revealed total war—world war—to be historical, not universal, a manifestation of destructive technologies of limited scale. The solution that the great and deep difficulty of the bomb bore within itself has been the prevention of large-scale war. In the long history of human slaughter, that is no small achievement.

*

Bohr hoped to convince the Allied leaders to move to negotiation against the common danger of nuclear weapons before the weapons were used and a nuclear arms race began. That hope was forlorn, probably because trust had eroded across the long years of war and openness could not yet be guaranteed. The price of containing the 20^th century’s epidemic of man-made death by the route of an arms race has been that we have all been held hostage—the earth itself has been held hostage—to the threat of omnicidal slaughter and the destruction of the human world. But Bohr’s fundamental assessment still applies. We are still in a completely new situation that cannot be resolved by war. Since late in the Cold War both we and the Russians have been reducing our nuclear arsenals. A diminished third wave of proliferation is playing itself out in South Asia and the Middle East, but it is difficult to imagine any nation or pair of nations wasting much of their national treasure on an extended nuclear arms race, given the record of its futility. The United States and Russia have dismantled more than twenty-five thousand nuclear weapons so far and have destroyed hundreds of delivery systems, and the two nations are now preparing further reductions. The fact that nuclear weapons are useless for war-fighting means that they burden military budgets, a truth that has not been lost on the Pentagons of the world. The September terror attacks brought that truth home, and the recently-completed Bush administration Nuclear Posture Review supports large reductions in the U.S. nuclear arsenal while proposing new generations of precision conventional weapons to replace them.

Nevertheless, we will not be safe from disaster so long as there are nuclear weapons in the world. Sooner or later, by accident or deliberately, weapons that are held in national arsenals will be used. The recent very frightening confrontation between India and Pakistan gives urgency to this point. In the long run, we will not be safe, and the world will not be safe, from devastation and horror on a scale far beyond the Holocaust, far beyond the two world wars, unless nuclear weapons arsenals are abolished. The end of the Cold War opened up a millennial opportunity to move in that direction. So far the opportunity has not been seized. One result is that India and Pakistan have become full nuclear powers. Iran may soon follow. National missile defense, whatever its merits, does not address the fundamental inequity of the insistence by the nuclear powers that nuclear weapons are vital to their security but that other nations should forego nuclear arms.

Abolition—which is what Bohr in 1944 hoped could be achieved preemptively—has seemed utopian: in a world without nuclear weapons, what if someone cheats? But even if an appropriate enforcement authority proved insufficient to dominate an outlaw entity; even if the conventional military forces of nations threatened by such an outlaw came to stalemate as well; the act of moving to build a clandestine nuclear arsenal would be an act of war. And since knowledge of how to release nuclear energy¾ knowledge of how to build nuclear weapons¾ will always be with us so long as we maintain it and the human world continues, such an act of war could always be countered¾ deterred¾ by reverting to nuclear weapons production. The knowledge that 20^th-century science extracted from the silence of the inanimate is part of our scientific and technological heritage, which means that nuclear deterrence will continue to influence international relations and limit national sovereignty even when there are no longer any actual nuclear weapons in the world.

Or think of it this way: early in the nuclear arms race, when the only delivery system available was intercontinental bombers, delivery time from base to target was perhaps twenty hours. Today delivery time by ICBM is thirty minutes, by forward-positioned nuclear submarine perhaps fifteen minutes, meaning national leaders have at best ten minutes during which to assess intelligence about a possible attack and decide to respond. In a world without nuclear weapons, in contrast, delivery time from factory to target would be perhaps three months, greatly extending the grace period available to make a decision, to negotiate, to intervene. This way of conceiving nuclear abolition¾ not as resolving the nuclear dilemma (because the nuclear dilemma is permanent and cannot be resolved short of ending human civilization) but rather as extending delivery times to give nonviolent means of resolution space to do their work¾ moves abolition from the realm of the utopian into the realm of the real.

Bohr located it in the realm of the real at the very beginning. "They didn’t need my help in making the atom bomb," he told a friend later. He came to America, came to Washington, to another purpose. He left his wife and children and work and traveled in loneliness to America for the same reason he had crossed the Æ resund and hurried to Stockholm in a dark time to see the King: to bear witness, to clarify, to win change, finally to rescue. His revelation was a vision of the complementarity of the bomb—a vision as clear, as fresh, as magisterial and as hopeful today as it was in 1944. Every great and deep difficulty bears in itself its own solution . . . . This we learned from him.

Thank you.