Bohr Talk

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Richard Rhodes

609 Summer Hill Road

Madison CT 06443 USA

“A Great and Deep Difficulty”: Niels Bohr

and the Atomic Bomb

(Delivered at the Copenhagen Symposium, Washington, D.C., Saturday,March 2, 2002)

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

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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-

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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.

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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."1

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

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deep difficulty bears in itself its own solution . . . . This we learned from

him.”2 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."3

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

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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."4

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

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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."5

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."6 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."7 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."8

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

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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."9 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.

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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."10

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.

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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

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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."11 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.

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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

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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.12

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 energy knowledge of how to build nuclear

weapons will always be with us so long as we maintain it and the

human world continues, such an act of war could always be

countered deterred by reverting to nuclear weapons production. The

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knowledge that 20th-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 abolition 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 work 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 ∅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

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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.

NOTES

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1. Jones, R. V. 1966. Winston Leonard Spencer Churchill. Biog. Mem. F. R. S. 12:35, p. 88ff.2 Quoted in Moore, Ruth. 1966. Niels Bohr. New York: Knopf, p. 330. Emphasis added.3. Niels Bohr to Franklin Roosevelt, 3.vi.44; J. Robert Oppenheimer Papers, Box 21, U.S.Library of Congress.4. Quoted in Nielson, J. Rud. 1963. Memories of Niels Bohr. Physics Today. Oct., p. 30.

Emphasis added.5. Oppenheimer, J. Robert. 1963. Niels Bohr and his times. Three lectures, unpublished

MSS, J. Robert Oppenheimer Papers, Box 247, U. S. Library of Congress. Lecture III, p. 9.6. Quoted ibid.7. Rozenthal, Stefan, ed. 1967. Niels Bohr. North-Holland Publishing Company, p. 350.8. Ibid, p. 351.9. Quoted in Markusen, Eric, and David Kopf. 1995. The Holocaust and Strategic Bombing:

Genocide and Total War in the Twentieth Century. Westview, p. 97.10. Bundy, McGeorge. 1969. To cap the volcano. Foreign Affairs 48:1 (Oct.), pp. 9-10.11. Bohr, Niels. 1958. Atomic Physics and Human Knowledge. John Wiley, p. 31.12 For a full discussion of this point cf. Butler, Richard. 2001. Fatal Choice: Nuclear Weapons

and the Illusion of Missile Defense Cambridge MA: Westview Press.

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