Paul Adrien Maurice Dirac, in a series of four definitive papers, makes a major contribution to physics by devising a form of quantum mechanics, the laws of motion that govern atomic particles. Dirac had the revolutionary idea that the electron could be described by four wave functions, satisfying four simultaneous differential equations. It followed from these equations that the electron must rotate on its axis, an idea that had been developed by other physicists, and also that there must be states of negative energy.

The image above is based on the cloud chamber photographs of Carl David Anderson confirming the existence of positrons i.e., particles equal to the electron in mass but positively charged.
This timeline entry suggestion
courtesy of
Professor David L. Andrews, UEA.




Further info :

Paul Adrien Maurice Dirac - brief biography -
http://www.phy.bg.ac.yu/web_projects/giants/dirac.html

Stanford Linear Accelerator Centre - Quantum Mechanics Theory
http://www2.slac.stanford.edu/vvc/theory/quantum.html

St.Andrews University, Scotland - Quantum Mechanics History
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html

LBNL Image Library - Cloud-chamber photograph -
http://imglib.lbl.gov/ImgLib/COLLECTIONS/BERKELEY-LAB/PARTICLE-DETECTION
/CLOUD-CHAMBERS/index/pg09_cloudchamber.html