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Using symmetry to learn about physical laws

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Parity conservation: an inviolable principle?
Murray Gell-Mann Scientist
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The course in mathematical physics was taught by Herman, and one thing I remember about Herman's class, and that is that he assigned one day – and I've written about this also – he assigned one day a problem that consisted of proving by reflection of co-ordinates that parity is conserved. And I spent the weekend on the problem and no matter how I twisted it, I could not prove from mathematics alone that parity was conserved, and so I turned in a little note on Monday that said I don't believe that parity can… parity conservation can be proved by reflection of co-ordinates. It’s a matter of how the Hamiltonian behaves under reflection of co-ordinates and that’s a matter of the laws of physics, and it would be possible, to have… in principle… to have laws of physics that were not symmetrical. The actual electromagnetic theory is symmetrical, but some other theory might not be. So I was not one of those people who thought that parity had to be conserved. And there were others, I discovered. I discovered that Dick Feynman was not particularly impressed with the need to have parity conserved under all conditions, Fermi also, and Dirac. These were all people who felt that parity conservation was a property of certain laws and that you can have others. In fact, if you look back at Fermi's 1934 paper on beta decay you will discover that he adds… that he has a vector interaction for the…  how does it work? A vector interaction for the neutron and proton multiplied… a vector current for the neutron and proton, multiplied by an axial vector current for the electron and neutrino,  something like that.

[Q] I see, so he…

Now, that actually, because of the special properties of the mass-less neutrino and so on, and that actually doesn't have any parity-violating properties because you can transform it into the… into a parity conserving transformation just by defining a new neutrino which is gamma five times the old neutrino. But it indicates that Fermi was paying absolutely no attention to parity, and he never thought that it was a fundamental inviolable principle. So there were a number of people who didn't think it was an inviolable principle. But apart from that, I don't remember learning much from Herman.

New York-born physicist Murray Gell-Mann is known for his creation of the eightfold way, an ordering system for subatomic particles, comparable to the periodic table. His discovery of the omega-minus particle filled a gap in the system, brought the theory wide acceptance and led to Gell-Mann's winning the Nobel Prize in Physics in 1969.

Listeners: Geoffrey West

Geoffrey West is a Staff Member, Fellow, and Program Manager for High Energy Physics at Los Alamos National Laboratory. He is also a member of The Santa Fe Institute. He is a native of England and was educated at Cambridge University (B.A. 1961). He received his Ph.D. from Stanford University in 1966 followed by post-doctoral appointments at Cornell and Harvard Universities. He returned to Stanford as a faculty member in 1970. He left to build and lead the Theoretical High Energy Physics Group at Los Alamos. He has numerous scientific publications including the editing of three books. His primary interest has been in fundamental questions in Physics, especially those concerning the elementary particles and their interactions. His long-term fascination in general scaling phenomena grew out of his work on scaling in quantum chromodynamics and the unification of all forces of nature. In 1996 this evolved into the highly productive collaboration with James Brown and Brian Enquist on the origin of allometric scaling laws in biology and the development of realistic quantitative models that analyse the influence of size on the structural and functional design of organisms.

Tags: Herman Feshbach, Richard Feynman, Enrico Fermi, Paul Dirac

Duration: 2 minutes, 31 seconds

Date story recorded: October 1997

Date story went live: 24 January 2008