What Goes Around Is Really Round: "Improved measurement of the shape of the electron"

by Chad Orzel

The big physics story of the week is undoubtedly the new limit on the electric dipole moment (EDM) of the electron from Ed Hinds's group at Imperial College in the UK. As this is something I wrote a long article on for Physics World, I'm pretty psyched to see this getting lots of media attention, and not just from physics outlets.

OK, you said this is about a dipole moment, but the headlines all talk about measuring the shape of an electron. What do these have to do with one another? A "dipole moment" is just a bit of mathematical apparatus used to describe a non-spherical distribution of charge. It turns out to be mathematically convenient to talk about "polar moments" of various fields in electricity and magnetism. The simplest sort of field is a "monopole," made by a point charge, which pushes other like charges directly outward from itself. Slightly more complicated than that is a "dipole" pattern, which is like what you get when you sprinkle iron filings over a magnet-- the field pushes out at one end, and pulls in at the other, and has some sideways component in between. You can make an electric dipole by putting a negative point charge close to but not exactly on top of a positive point charge.

So, an electron is made up of a little positive thing stuck to a bigger negative thing? There doesn't need to be actual positive charge present-- you can just take some of the negative charge from one pole of a spherical ball of charge and move it to the other pole. That creates a little bit of a dipole moment, too, without needing any of the opposite charge.

OK, so an electron is supposed to be like a ball of charge with a bump on one side and a divot on the other? Well, it could effectively look like that, but this measurement shows that it doesn't. Which in some ways isn't surprising, because it shouldn't be anything but round, according to the simplest models of physics.

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http://scienceblogs.com/principles/2011/05/what_goes_around_is_really_rou.php