Engines of Our Ingenuity

No. 1127:
GERMANY AND THE ATOM BOMB

by John H. Lienhard

Click here for audio of Episode 1127.

Today, we ask about Germany and the atom bomb in WW-II. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Physicist Jonothan Logan tells a strange tale that begins in 1945, soon after Germany surrendered. Fifteen of Germany's greatest physicists have been taken to an English country house and asked to write an account of German science during the war.

Suddenly, a radio announcement: The BBC says an atomic bomb has fallen on Hiroshima with a blast equal to 2000 ten-ton bombs.

Werner Heisenberg, of Heisenberg Uncertainty Principle fame, is there. He doesn't know the English have hidden microphones in the room. He blurts out that such a bomb would be impossible. No one had enough uranium to do the job. Some dilettante in America, who knows very little about it, has bluffed them, he grumbles.

The transcripts of those conversations were only declassified in 1992. As we listen in, a half-century later, we learn the real reason why Germany never produced an atom bomb.

The idea of an atomic bomb had been born seven years before -- in February, 1939. That's when Lisa Meitner published a paper with her nephew Otto Frisch explaining the release of energy by the fission of uranium. Seven months later Germany invaded Poland and full war began. Meanwhile, both Meitner and Frisch got out of Germany.

Meitner wanted nothing to do with bombs. She wanted to create peaceful nuclear power. But in 1940, Frisch wrote the English military to tell them it'd take only one kilogram of uranium 235 to make a bomb. U-235 is a hard-to-separate isotope that makes up less than one percent of natural uranium. Frisch underestimated how much of it we'd need, but only by a factor of ten. Heisenberg also made the calculation and got 13,000 kilograms. The huge difference in estimates has to do with the way chain reactions work:

A neutron is so tiny and fast moving that it travels a long distance in uranium before it chances to hit an atom and knock more neutrons loose. If it has to travel, say, a full meter, you need a huge chunk of U-235 to get a chain reaction. If it only has to travel, say, one centimeter, then a small block of U-235 will do the trick.

Heisenberg, brilliant theoretician, overestimated the path of travel. Experimentalists Meitner and Frisch did far better; and Frisch's note sent America on the way to building a bomb. Heisenberg's estimate so discouraged the German High Command that they never did undertake serious bomb-building.

Back in that English country house, Heisenberg heard about the second bomb over Nagasaki. So he quickly figured out how his calculation should've gone in the first place. Then he told the English he could've built a bomb all along, but he and his colleagues had been anti-Nazi. They'd kept Germany from bomb building and steered her into a slow program of nuclear power development.

That was the story for the next forty-seven years. Then we finally opened those hidden microphone records -- and history changed.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Logan, J., The Critical Mass. American Scientist, Vol. 84, May-June, 1996, pp. 263-277.


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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