Colloquium Ehrenfestii - Sir Michael Berry (Bristol University): 'Variations on a theme of Aharonov and Bohm'



19:30 hrs


Sitterzaal, Oort building


The Aharonov-Bohm effect (AB) concerns the role in quantum physics of the magnetic vector potential of an impenetrable line of magnetic flux. Its partial anticipation by Ehrenberg and Siday, in terms of interference, was an approximation whose wavefunction was not singlevalued. Its connection with the singlevalued AB wave involves topology: ‘whirling waves’ winding round the flux. AB is a fine illustration of idealization in physics. There are four AB effects, depending on whether the waves and the flux are classical or quantum. In the classical-classical case, many details of the AB wavefunction have been explored experimentally in ripples scattered by a water vortex, where the flow velocity of the water corresponds to the vector potential; this analogy has recently been extended to include superradiance. The AB wave possesses a phase singularity, and there is a similar phenomenon in general interferometers. Connections between the AB wave and the Cornu spiral describing edge diffraction lead to very accurate approximations.

The Colloquium Ehrenfestii takes place on Wednesday evenings starting at 19:30 hours in the main auditorium of the Oort building. Before the Colloquium, there is a common dinner in the canteen located on the ground floor of the  Oort building. The dinner starts at 18:00 hours sharp and is free of charge, under the condition that one attends the colloquium and that one has made a reservation before noon on the Tuesday preceding the colloquium.

The registration form is only accessible from within the University (to block spammers); if you would like to register for the dinner from outside the University, please send an email to   The revolving doors to the Oort building  are accessible until 9 pm after that they are locked and one should instead use the sliding doors directly adjacent to the Huygens building: these can be opened with an electronic key (seek help from locals).