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Energy loss: bridging classical and quantum formula


The usefulness of my Habilitationschrift to physics
Hans Bethe Scientist
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It turned out when I came to Cambridge, England in the fall of 1930, Patrick Blackett found this very interesting because the experimenters in nuclear physics had always determined the energy of a particle from the range of a particle in air. And so he said 'Now, we have an old formula for this energy loss,' which he showed me, in fact I may have already known it, but certainly he showed it to me, 'which differs just very slightly from your result. So, extend your result and give us the relation between energy of a charged particle and [its] range. That we can use.' And that's what I did and I got the range-energy relation in air and in hydrogen and other substances, and this turned out to be very useful for the Cambridge experimenters then, and for other experimenters later. And it is still useful because nowadays experimenters can measure very easily the change of energy of an electron as it goes through some foil - let's say, a thin aluminum foil - not electron but any particle, and it turned out as it had turned out in the pre-quantum theory that this energy loss depends primarily on the velocity of the particle. Now it's very easy for experimenters to measure the momentum of a particle by deflecting in a magnetic field, and if you have both momentum and velocity, you deduce the mass of the particle, and in this way nowadays experimenters can deduce the masses of all the remarkable particles, mesons of all kinds and excited states of... of the proton and so on. So this Habilitationschrift which was a very interesting thing to do at the time, turned out to be very useful for a long time.

The late German-American physicist Hans Bethe once described himself as the H-bomb's midwife. He left Nazi Germany in 1933, after which he helped develop the first atomic bomb, won the Nobel Prize in Physics in 1967 for his contribution to the theory of nuclear reactions, advocated tighter controls over nuclear weapons and campaigned vigorously for the peaceful use of nuclear energy.

Listeners: Sam Schweber

Silvan Sam Schweber is the Koret Professor of the History of Ideas and Professor of Physics at Brandeis University, and a Faculty Associate in the Department of the History of Science at Harvard University. He is the author of a history of the development of quantum electro mechanics, "QED and the men who made it", and has recently completed a biography of Hans Bethe and the history of nuclear weapons development, "In the Shadow of the Bomb: Oppenheimer, Bethe, and the Moral Responsibility of the Scientist" (Princeton University Press, 2000).

Tags: Cambridge University, UK, Patrick Blackett

Duration: 3 minutes, 1 second

Date story recorded: December 1996

Date story went live: 24 January 2008