Electrons (blue) passing either side of a current-carrying solenoid shows the Aharonov-Bohm effect in action (Courtesy Physics Today)
By Matin Durrani
The Aharonov-Bohm effect is one of those weird, counter-intuitive consequences of quantum mechanics that makes physics the fascinating subject it is.
Discovered 50 years ago by Yakir Aharonov and the late David Bohm at the University of Bristol in the UK, the AB effect, as it is known to insiders, is being celebrated today at a special conference at Bristol.
In case you weren’t aware, the AB effect describes the fact that an electrically charged particle passing through a region where both the magnetic and electric fields are zero is nevertheless affected by the electromagnetic potential in that region.
It can best be understood by considering a beam of electrons passing through two slits and then around either side of a current-carrying solenoid, as shown by the blue lines in the picture above.
Although there is no magnetic field outside the solenoid, the potential is different on the two sides, which means that the wavefunction of the electrons travelling past one side of the solenoid are phase-shifted by a different amount compared with the electrons travelling past the other.
The AB effect can be verified by allowing the electron beams to interfere: the resulting fringe patterns shift depending on whether the solenoid is on or off.
The conference, which also marks the 25th anniversary of Michael Berry’s discovery of the related “Berry phase”, has attracted a crowd of specialists from around the world, including Aharonov himself.
I went to the conference dinner at the university’s Georgian-period Goldney Hall, where guests were treated to a marvellous menu of roast asparagus with goat’s cheese mousse and Serrano ham crisps, slow-cooked rump of lamb with quince sauce, and confit of raspberries.
Spotted among the guests were Bob Chambers, who confirmed the AB effect experimentally back in 1960, former Brookhaven chairman Michael Hart and independent physicist Julian Barbour, author of The End of Time.
Today’s first lecture session back at the university’s physics department was chaired by Murray Peshkin from Argonne National Laboratory in the US, who introduced Sir Michael by saying “he is a man of few words but many syllables – so listen carefully”.
Berry’s lecture was entitled “Semifluxon degeneracy choreography” and he duly proceded to use a fair few long-syllabled words, including “Gaussian random simulation”, “traceless real symmetric 2×2 matrices” and “rearrangements of nodal domains”.
The talk was a bit over my head, but on such occasions I take comfort in Richard Feynman’s famous phrase that “nobody really understands quantum mechanics”.
The conference ends today.