how is a penning trap used to determine the upper limit of the electron radius. google hasnt been much help. I found this but I dont understand it at all. http://cerncourier.com/cws/article/cern/29724 What can be learned from the more accurate electron g? The first result beyond g itself is the fine structure constant, α = e2/4πε0hbarc – the fundamental measure of the strength of the electromagnetic interaction, and also a crucial ingredient in our system of fundamental constants. A Dirac point particle has g = 2. QED predicts that vacuum fluctuations and polarization slightly increase this value. The third use of the measured g is in probing the internal structure of the electron – limiting the electron to constituents with a mass m* > m/√(δg/2) = 130 GeV/c2, corresponding to an electron radius R <1 × 10–18 m. If this test was limited only by our experimental uncertainty in g, then we could set a limit m* > 600 GeV. This is not as stringent as the related limit set by LEP, which probes for a contact interaction at 10.3 TeV. However, the limit is obtained quite differently, and is somewhat remarkable for an experiment carried out at 100 mK.