Hans Bethe and His Physics

Edited by:
  • Gerald E. Brown &
  • Chang-Hwan Lee
World Scientific Publishing: 2006. 314 pp. $98, £56 (hbk) $38, £22 (pbk) 9812566104 | ISBN: 9-812-56610-4
Hans Bethe won the physics Nobel prize for work on stars, but he has influenced a wide range of fields. Credit: CORNELL UNIV.

It is almost 20 years since the winter's day when I walked out of West Bridge at the California Institute of Technology from my thesis defence, with blood only slightly diluting the adrenaline in my veins, to deliver my newly approved opus to the registrar and record my PhD. As I swaggered across the concrete arch spanning the fountain in the centre of the campus, an elderly, heavy-set man was slowly making his way up the other side, his face a study in concentration as he peered into the water. I wanted to trigger a conversation to somehow mark my newly fledged status, but the words caught in my throat and I stepped aside. Even in my euphoric state I knew better than to interrupt Hans Bethe when he was thinking. Six years of my effort had produced a pedestrian but passable thesis, whereas an afternoon of Bethe's thinking could transform an entire field of physics.

This awe of Bethe and his work is universal among those who knew him, and the contributors to Hans Bethe and His Physics are certainly no exception. Freeman Dyson called him “the supreme problem solver of the twentieth century”, and John Bahcall said it looked as if Bethe's output was the result of a conspiracy by several people all signing their work with the same name. His output must be nearly unmatched in duration, too — he published his first paper (with his father) at the age of 18 in 1924, and his last was submitted by his co-authors to the preprint servers six months after his death in March 2005. Along the way he picked up the 1967 Nobel Prize in Physics for work in the 1930s on the energy-production mechanisms of stars; he introduced the Bethe ansatz, which has found applications throughout physics and mathematics; he wrote three long articles for Reviews of Modern Physics in 1936–7 that were dubbed the 'Bethe Bible' and are said to contain everything that anybody else knew about nuclear physics and quite a bit more besides; and he gave his name to some now-essential formulae for calculating nuclear masses, the energy loss of charged particles passing through matter, and much more as well.

But it was his 1947 calculation of the Lamb shift that perhaps best shows the power of his direct approach to physics. At a conference on Long Island, Willis Lamb announced that accurate measurements of the fine structure of atomic hydrogen were in conflict with Paul Dirac's prevailing theory of electrodynamics. Many of those present, including such towering figures as Niels Bohr and Robert Oppenheimer, saw this as the demonstration of a profound theoretical crisis that would require revolutionary new developments to resolve. On leaving the conference, Bethe took up the problem and had managed to calculate a value of this 'Lamb shift' that matched the observations before his train had travelled the 75 miles to his destination. He did this not with any revolutionary developments, but by performing the simplest calculation that he thought might match the data. This was the Bethe way, or as he put it: “Learn advanced mathematics in case you need it, but use only the minimum necessary for any particular problem,” and “be prepared to make conjectures if it helps”. All real problems are solved with approximations, so genius in physics often consists of knowing which mistakes to make, and Bethe was a master of it. His calculation convinced others that the fledgling field of quantum electrodynamics (QED) was on the right track, and helped spur the developments that led to the great success of QED and hence much of modern theoretical physics. His extremely pragmatic attitude to calculation, which he passed on to the legion of students he taught in his 71-year career at Cornell, still influences the field today (although looking at the current state of theoretical physics, perhaps not enough).

As head of the theory division at Los Alamos during the Manhattan Project, Bethe played a key role in constructing the first nuclear weapons. A refugee from Hitler's Germany, he was well aware of the danger that nuclear weapons could pose in the hands of totalitarian regimes. So, like many who had fled fascism, he devoted himself to ensuring that the democracies got these awful weapons first. After the war, Bethe was sensible enough to know that the development of nuclear weapons would continue in the United States whatever he did. He therefore continued his involvement in the belief that he could be a more effective voice for moderation from the inside — although quotes such as “I sometimes wish I was a more consistent idealist” show his ambivalence about such work. He was also an authoritative and effective advocate for arms control and international cooperation.

To fully describe such a formidable list of accomplishments will keep many a scholar busy for the foreseeable future. Hans Bethe and His Physics is much too short to even attempt such a task. It grew out of a request by Bethe to Gerry Brown to explain his physics to the world; Brown and Chang-Hwan Lee edited this book. It makes no claim to be a comprehensive biography — it is more like a taster menu drawn from half-a-dozen different restaurants. Like Bethe's work, the chapters range from extremely detailed physics to non-technical consideration of some of the biggest issues facing humanity. Much fascinating insight into some of the key figures can be gained without any scientific background, but some chapters require a knowledge of physics to degree level or beyond. A few of the later articles have a faint echo of axes being ground, but a book with no trace of controversy would be a poor reflection on Bethe's life.

I would especially recommend the book to anyone who has been involved in any way with the events described, as it brings alive many of the physicists that some of us knew only slightly, or by reputation alone. Reading about Bethe's pure pleasure as he bulldozed problem after problem using the simplest tools he could get away with was an inspiration — I wanted to grab a piece of paper and have a go myself. He kept that pleasure to the end. A colleague who gave a seminar at Cornell in the week of Bethe's death told me that Bethe closely followed the talk and obviously enjoyed it. His last words to a younger physicist, quoted in one of the best and most touching articles in the book, were simply “Carry on”. We will, Hans.