Jena 2013 – scientific programme

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GP: Fachverband Geschichte der Physik

GP 3: Session 2

GP 3.4: Talk

Monday, February 25, 2013, 17:45–18:15, HS 7

The density matrix - The story of a failed transfer — •Alexander Blum — MPI für Wissenschaftsgeschichte, Berlin

With the discovery of the positron in 1933, Paul Dirac (along with most other physicists) was forced to really take seriously his earlier suggestion that in the world as we know it all negative energy states are occupied and we are thus surrounded by an infinite sea of electrons. What was needed was a way to treat this large number of electrons in a manageable fashion. Dirac resorted to the use of the density matrix, a technique he had earlier used to describe the large number of electrons in complex atoms. Initially, this transfer from atomic physics to what we would nowadays call particle physics was quite successful, and for a few years the density matrix was the state of the art in describing the Dirac electron sea, but then rapidly fell out of favor. I will investigate the causes of this ultimately failed transfer and how it relates to changes in the physical notion of the vacuum, changes which eventually eliminated the analogy on which the transfer had been based in the first place.