“Of course I do,” said the lad, and he pointed the way.
Szilard and Wigner were hot, tired, and impatient by the time they found the two-story white cottage. By contrast, the sixty-year-old Einstein was relaxed and genial; he had spent the early morning sailing in a small dinghy and now greeted his former colleagues wearing a white undershirt and rolled-up white trousers. Einstein bowed courteously as they met and led his visitors through the house to a cool screened porch that overlooked a lawn. There, speaking in German and sipping iced tea, Szilard and Wigner told Einstein about their recent calculations. They explained how neutrons behave, how uranium bombarded by neutrons can split or “fission,” and how this process might create nuclear chain reactions and nuclear bombs.
“Daran habe ich gar nicht gedacht,” Einstein said slowly, pondering what he had just heard. “I haven’t thought of that at all.”
Until that summer day, Einstein had believed that atomic energy would not be released “in my time,” that it was only “theoretically possible.” Einstein had not followed recent discoveries in nuclear research for years and sought only the “time for quiet thought and reflection” needed to unravel his unified field theory of the universe. Einstein had published his famous equation E=mc2 in 1905, but only now was that simple statement’s ultimate significance clear. For even a small mass the potential energy released could be immense. Fission is the most efficient way to fulfill Einstein’s equation because it releases the energy that gives matter its form—the binding energy holding the atomic nucleus together.
Einstein’s next thought about the chain reaction was philosophical. If it works, he said, this would be the first source of energy that does not depend on the sun. Wind and solar energy are created by the sun’s heat. And fossil fuels—oil, natural gas, coal—were once created from the carbon made by the sun’s energy through photosynthesis. But releasing the binding energy of atoms was something new.
Einstein’s third reaction was political. Although he was an avowed pacifist, he agreed to sound the alarm about atomic bombs, even if it proved to be a false one, in order to beat Nazi Germany to this awesome weapon. It took a scientist of Einstein’s stature and personal conviction to take this risk, Szilard later noted. “The one thing most scientists are really afraid of is to make a fool of themselves,” Szilard reflected on the day in 1955 when Einstein died. “Einstein was free from such a fear and this above all is what made his position unique on this occasion.”
When the three agreed that they should warn the Belgians, they sat around the dining-room table as Einstein dictated to Wigner in German a letter to the Belgian ambassador in Washington. Einstein warned that it might be possible to make bombs of unimaginable power from the uranium mined in the Belgian Congo and that Germany, which at first offered uranium for sale after taking over mines in Czechoslovakia, had recently banned all exports.
Wigner wondered whether the U.S. government should also be notified, and into the afternoon Einstein and Szilard drafted a similar letter, also in German, to the secretary of state. That afternoon, they agreed to send the State Department a copy of Einstein’s letter to the Belgian ambassador, giving the department two weeks to object if they opposed the letter.
When an “uneasy feeling about this approach” led Szilard to “talk to somebody who knew a little bit better how things were done,” he called on Dr. Gustav Stolper, a Viennese economist and publisher whom he first knew in Berlin. Stolper quickly understood Szilard’s situation and suggested approaching a friend, Dr. Alexander Sachs, who was a vice-president of the Lehman Corporation, a large Wall Street investment bank. Sachs had worked privately since 1933 as an adviser to Roosevelt’s New Deal and would surely know how to approach the government.
Szilard telephoned Sachs and soon called on him in his office at the corner of South William and Broad streets. A serious-looking man with wavy hair and thick glasses, Sachs listened intently to what Szilard said. Sachs needed little persuading; he was familiar with popular reports about uranium fission and fearful of German aggression. Einstein’s letter should not go to the Belgian royal family or a U.S. government department, Sachs said; they wouldn’t know what to do with it. It should go, instead, directly to President Roosevelt. Sachs boasted about his easy access to the White House and joined with Szilard in planning strategy. If Einstein would sign a letter, Sachs promised he would deliver it, in person, to the president. Szilard must have loved the idea.
As Szilard began to dictate, in his crisp Hungarian-German accent, a letter to “F.D. Roosevelt, president of the United States,” [the typist Janet] Coatesworth glanced up in disbelief. And when Szilard mentioned “extremely powerful bombs,” she recalled, “that convinced me! I was sure I was working for a ‘nut.’” Amused by her reaction, Szilard dictated more and more dramatically, his face beaming with mischief and merriment. He took special glee in closing the letter “Yours very truly, Albert Einstein.” That convinced her that Szilard was deranged, a judgment he confirmed by dictating a second, even longer letter. To Roosevelt. From Einstein. Only years later did Coatesworth learn the truth about this historic session.
“March of Time” Photo © Time Life Films, Inc., courtesy of SFM Entertainment
Albert Einstein and Leo Szilard reenact the drafting of the 1939 letter that alerted President Franklin D. Roosevelt to the urgency of an atomic bomb project.
Albert Einstein to F. D. Roosevelt
Albert Einstein, the world’s most renowned physicist and a Nobel Prize winner, had fled Germany in the 1930s. At the urging of Hungarian refugees Leo Szilard and Eugene Wigner, Einstein warned President Franklin D. Roosevelt about a possible German atomic bomb. On October 19, 1939, the President responded to Einstein, explaining that he had created a committee to investigate making an atomic bomb.
Albert Einstein
Old Grove Rd.
Nassau Point
Peconic, Long Island
August 2nd, 1939
F. D. Roosevelt,
President of the United States,
White House
Washington, D.C.
Sir:
Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations:
In the course of the last four months it has been made probable—through the work of Joliot in France as well as Fermi and Szilard in America—that it may become possible to set up a nuclear chain reaction in a large mass of uranium by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.
This phenomenon would also lead to the construction of bombs, and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.
The United States has only very poor ores of uranium in moderate quantities. There is some good ore in Canada and the former Czechoslovakia, while the most important source of uranium is Belgian Congo.
In view of this situation you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America. One possible way of achieving this might be for you to entrust with this task a person who has your confidence and who could perhaps serve in an inofficial capacity. His task might comprise the following:
a) to approach Government Departments, keep them informed of the further development, and put forwa
rd recommendations for Government action, giving particular attention to the problem of securing a supply of uranium ore for the United States.
b) to speed up the experimental work, which is at present being carried on within the limits of the budgets of University laboratories, by providing funds, if such funds be required, through his contacts with private persons who are willing to make contributions for this cause, and perhaps also by obtaining the co-operation of industrial laboratories which have the necessary equipment.
I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.
Yours very truly,
(Albert Einstein)
President Roosevelt replied two months later with a brief note to Einstein:
The White House
Washington
October 19, 1939
My dear Professor:
I want to thank you for your recent letter and the most interesting and important enclosure.
I found this data of such import that I have convened a Board consisting of the head of the Bureau of Standards and a chosen representative of the Army and Navy to thoroughly investigate the possibilities of your suggestion regarding the element of uranium.
I am glad to say that Dr. Sachs will cooperate and work with this Committee and I feel this is the most practical and effective method of dealing with the subject.
Please accept my sincere thanks.
Very sincerely yours,
(Franklin D. Roosevelt)
A Practically Irresistible Super-Bomb
This memorandum, written in Britain in March 1940 by two leading physicists, Otto Frisch and Rudolf Peierls, was an important assessment confirming the feasibility of an atomic bomb and its impact. It also recognized that “as a weapon, the super-bomb would be practically irresistible,” and argued for developing a bomb as a “counter-threat,” even if it was not intended for attack.
From Memorandum on the Properties of a Radioactive Super-bomb
BY OTTO R. FRISCH AND RUDOLF PEIERLS
The attached detailed report concerns the possibility of constructing a “super-bomb” which utilizes the energy stored in atomic nuclei as a source of energy. The energy liberated in the explosion of such a super-bomb is about the same as that produced by the explosion of 1000 tons of dynamite. This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city.
In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effect of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.
Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.
In order to produce such a bomb it is necessary to treat a few hundred pounds of uranium by a process which will separate from the uranium its light isotope (uranium-235) of which it contains about 0.7%. Methods for this separation of isotopes have recently been developed. They are slow and they have not until now been applied to uranium, whose chemical properties give rise to technical difficulties. But these difficulties are by no means insuperable. We have not sufficient experience with large-scale chemical plant to give a reliable estimate of the cost, but it is certainly not prohibitive.
It is a property of these super-bombs that there exists a “critical size” of about one pound. A quantity of separated uranium isotope that exceeds the critical amount is explosive; yet a quantity less than the critical amount is absolutely safe. The bomb would therefore be manufactured in two (or more) parts, each being less than the critical size, and in transport all danger of a premature explosion would be avoided if these parts were kept at a distance of a few inches from each other.
The bomb would be provided with a mechanism that brings the two parts together when the bomb is intended to go off. Once the parts are joined to form a block which exceeds the critical amount, the effect of the penetrating radiation always present in the atmosphere will initiate the explosion within a second or so.
The mechanism which brings the parts of the bomb together must be arranged to work fairly rapidly because of the possibility of the bomb exploding when the critical conditions have only just been reached. In this case the explosion will be far less powerful. It is never possible to exclude this altogether, but one can easily ensure that, say, one bomb out of 100 will fail in this way, and since in any case the explosion is strong enough to destroy the bomb itself, this warrant is not serious.
We do not feel competent to discuss the strategic value of such a bomb, but the following conclusions seem certain:
1. As a weapon, the super-bomb would be practically irresistible. There is no material or structure that could be expected to resist the force of the explosion. If one thinks of using the bomb for breaking through a line of fortifications, it should be kept in mind that the radioactive radiations will prevent anyone from approaching the affected territory for several days; they will equally prevent defenders from reoccupying the affected positions. The advantage would lie from the side which can determine most accurately just when it is safe to re-enter the area; this is likely to be the aggressor, who knows the location of the bomb in advance.
2. Owing to the spreading of radioactive substances with the wind, 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. (Use as a depth charge near a naval base suggests itself, but even there it is likely that it would cause great loss of civilian life by flooding and by the radioactive radiations.)
3. We have no information that the same idea has also occurred to other scientists but since all the theoretical data bearing on this problem are published, it is quite conceivable that Germany is, in fact, developing this weapon. Whether this is the case is difficult to find out, since the plant for the separation of isotopes need not be of such a size as to attract attention. Information that could be helpful in this respect would be data about the exploitation of the uranium mines under German control (mainly in Czechoslovakia) and about any recent German purchases of uranium abroad. It is likely that the plant would be controlled by Dr. K. Clusius (Professor of Physical Chemistry in Munich University), the inventor of the best method for separating isotopes, and therefore information as to his whereabouts and status might also give an important clue. At the same time it is quite possible that nobody in Germany has yet realized that the separation of the uranium isotopes would make the construction of a super-bomb possible. Hence it is of extreme importance to keep this report secret since any rumour about the connection between uranium separation and a super-bomb may set German scientists thinking along the right lines.
4. If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realized that no shelters are available that would be effective and could be used on a large scale. The most effective reply would be a counter-threat with a similar bomb. Therefore it seems to us important to start production as soon and as rapidly as possible, even if it is not intended to use the bomb as a means of attack. Since the separation of the necessary amount of uranium is, in the most favourable circumstances, a matter of several months, it would obviously be too late to start production when such a bomb is known to be in the hands of Germany, and the matter seems, therefore, very urgent.
5. As a me
asure of precaution, it is important to have detection squads available in order to deal with the radioactive effects of such a bomb. Their task would be to approach the danger zone with measuring instruments, to determine the extent and probable duration of the danger and to prevent people from entering the danger zone. This is vital since the radiations kill instantly only in very strong doses whereas weaker doses produce delayed effects and hence near the edges of the danger zone people would have no warning until it was too late. For their own protection, the detection squads would enter the danger zone in motor-cars or aeroplanes which would be armoured with lead plates, which absorb most of the dangerous radiation. The cabin would have to be hermetically sealed and oxygen carried in cylinders because of the danger from contaminated air. The detection staff would have to know exactly the greatest dose of radiation to which a human being can be exposed safely for a short time. This safety limit is not at present known with sufficient accuracy and further biological research for this purpose is urgently required.
As regards to the reliability of the conclusions outlined above, it may be said that they are not based on direct experiments, since nobody has ever yet built a super-bomb, but they are mostly based on facts, which by recent research in nuclear physics, have been very safely established. The only uncertainty concerns the critical size for the bomb. We are fairly confident that the critical size is roughly a pound or so, but for this estimate we have to rely on certain theoretical ideas which have not been positively confirmed. If the critical size were appreciably larger than we believe it to be, the technical difficulties in the way of constructing the bomb would be enhanced. The point can be definitely settled as soon as a small amount of uranium has been separated, and we think in view of the importance of the matter immediate steps should be taken to reach at least this stage; meanwhile it is also possible to carry out certain experiments which, while they cannot settle the question with absolute finality, could, if their result were positive, give strong support to our conclusions.
The Manhattan Project Page 5