http://www.youtube.com/watch?v=3wHKBavY_h8

:-)

Jony

I think I'm not quite making my point: there's simply no sense in talking about attributes on the scale of the Plank Length, or the electron missing a cross section. The fact of it is that we simply don't know what's going on in the electron and that's all speculation.

What we do know is that it, electromagnetically, has a magnetic dipole, a charge monopole, and - as best we can tell - no charge dipole. The electromagnetic properties are the shape in the only sense that anyone ever really uses the word shape. So its shape is round.

If you want a picture in your minds eye of what we know about the electron, with the understanding that this is just a picture and the theory is the authoritative description, then a ball of charge is, in my opinion, closer to our current theories than a mysterious point particle. You bring up the classical charge radius above: this is precisely the radius that electron-positron virtual pairs can be created, and in a meaningful sense is something like an electron radius. (Although note that they have set limits on the charge radius by comparing the measured magnetic g-factor with the QED calculation, this is the 10^-20something that wikipedia quotes, but this really measures something different - I think you need to be more specific than just saying size at this length scale).

I understand the complications due to the electrons being in molecules - if you haven't twigged yet, JonyEpsilon == J.J. Hudson == first author of paper under discussion - but it doesn't have any real bearing on what we're talking about.

Hope that makes sense :-)

I don't know why people are so convinced the electron is a point particle. A point particle is an idealisation, one that even theory essentially rejects these days (in as much as the bare electron is a theoretical fiction which can't, even in principle, be considered separately from the vacuum polarisation it induces). I don't think it's particularly misleading to think of a ball of charge either - there's no evidence to suggest this isn't the case! I stand by my comment, and the title of my paper: I don't see what else what terms like "shape" and "spherical" can mean in this context, if one restricts oneself, as one must as a physicist, to things that can be measured.

Oops, yes, exactly right. I shouldn't post before I've had my cornflakes.

Yeah, it really is spherical in any sense that you can mean spherical. It can't have any higher order multipole moments because it only has spin 1/2. (So the Wigner-Eckart theorem tells you that all matrix elements with operators greater than spin 1/2 are necessarily zero.) Jony

We used the compton wavelength of the electron as the size scale for this analogy. You could argue that the 1/e radius of the electron/positron virtual particle cloud would be a better measure, and this is closer to the classical radius (off the top of my head, it's late here), which would give an accuracy measured in mm rather than microns. The 10^-22 number comes from interpreting ion trap measurements of the electron's g-factor, comparing them to QED theory. To some extent, the question of the electron's size depends on what you mean by size and how you might choose to measure it. Jony

It took us ten years to build the experiment. We didn't average for ten years!

Good questions! This is actually one of the central motivations for measuring this is. The standard model of particle physics predicts that the electron will be round. But most physicists think that the standard model isn't the full story. The interesting thing is most of the proposed extensions/replacements to the standard model predict that the electron will be somewhat distorted. To give a concrete example, supersymmetric theories, which are viewed by many as the most promising avenue for extending our theories of physics, usually predict a distorted electron.