Stick with the round balls, for now

So, apparently, electrons are round. Very very round. So when drawing those terribly wrong but evocative pictures of atoms as a lump in the middle (the nucleus) with a collection of round balls in orbit around them (the electrons), go ahead and make them nice and round. Very round. How “very” are we talking about here? According to this report on the recent experimental measurements in the Guardian:

Were the electron scaled up to the size of the solar system, any deviation from its roundness would be smaller than the width of a human hair […]

So you’d have to be using a pretty impressively sharpened pencil to draw it that accurately round. But give it a try.

Ok, what’s the story here? Well, oddly, this seemed to be on a lot of news sources yesterday, and I’m not sure exactly why. Maybe because it mostly seemed to be pitched as a “back to the drawing board for the theorists” story (two major sources I heard had it spun this way), which editors seem to like running with. And the roundness? What’s that about? Well, what they’re taking about is the result of a long careful set of measurements done by Hudson et. al. at Imperial College (my undergraduate alma mater!), reported in Nature, of the electron’s “electric dipole moment” – the simplest deviation from having its electric charge effectively look to be the same amount no matter what direction it is probed from – but I guess they decided that a picture of a soccer ball versus a rugby ball (or American football) was easier to grasp. (This can introduce confusion, since electrons are not to be thought of as composite particles, but as fundamental point particles, so how come the pictures of balls, round or not?* Well, think of doing the probing of it that I referred to above by having it interact with another particle (how else does one ever touch anything anyway?). How it interacts gives an effective measurement of its charge. Now, roughly, one can ask whether its effective charge is the same for all directions the other particle approaches from… Same = spherical or round, not same = not spherical or round.)

It is a rather precise measurement that can be made, and so that is nice in and of itself because it helps provide constraints on ideas about how Nature works. In searching for new theories of unexplained phenomena – and there are many (unexplained phenomena – and candidate theories) – one of the tests is to see whether it fits with what we already know, and so the better we know what we know, the better we are at evaluating the viability of those candidate theories.

Now the article says that supersymmetry (a candidate symmetry of nature that a lot of people like, and that appears naturally in string theories, for example) which is much fancied as a structure that might help us explain various phenomena as yet not understood (look it up – many hope that the Large Hadron Collider (LHC) might discover it, for example, or see hints of it for later experiments – see e.g. here), predicts non-spherical electron, and so this new result potentially has bearing on it. The issue seems to be one of just how non-spherical supersymmetry sets the electron to be, but more urgently for me is the question of how model independent is the answer? I don’t know, not having studied this issue, but I can imagine that there might be as many predictions as there are supersymmetric scenarios, and we don’t yet know which ones are viable candidates. So rather than stating that this result rules out supersymmetry (and gosh, it would be nice to have a definite answer on something!) as seems to have been said or implied in some news articles, might it be that it rules out perhaps a class of supersymmetric models? Not sure. I hope to learn more.

Anyway, slightly flawed presentation or not, it is nice to see a bit of discussion of non-LHC physics experiments in the headline news, since it shows that perhaps the press trust the public to be able to hold more than one particle physics story in their heads in a given period.

And with that, it’s back to the drawing board.


*This is usually the same point at which people get confused about the fact that electrons have spin, since one wants to now invoke a little bit of structure in order to have it turning about an axis. Don’t do that. Spin is more fundamental a property than just the turning about an axis. In a sense, the name’s just a bit of an accident.

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4 Responses to Stick with the round balls, for now

  1. k says:

    Why is the electron called a point particle if you’re saying it can also have some sort of asymmetry in the distribution of its effective charge? a point particle implies that the effective charge should be the same in all directions.
    On the other hand if we speak about the electron having a shape, whether its spherical or not, then it can not be a point.

    I also have a philosophical question. I have frequently heard mentioned that the general linear group and unitary group of order n, arise in particle physics. can you explain the relationship between these groups and particle physics?

  2. Clifford says:

    Hi, on the first point, perhaps you ought to read what I wrote in the post, directly addressing the issue… Charge distributions, etc. [update: oops, maybe you did read about charge distributions… in which case I don’t know how to help you further there. Seems to me now more about semantics, and what one calls a point or not? I don’t have anything further intelligent to say there.] As to the second, I don’t know what it has to do with philosophy, so maybe I am missing what you are asking. Groups like Su(n) arise as both global and local symmetries in particle physics. Sometimes these are real symmetries, analogous to rotations (in fact, Su(2), is closely related to so(3), the group of rotations in three dimemsnsions… The former is a double cover of the latter, and in fact the spin particles have are representations of Su(2), and not just so(3)), and other global symmetries like flavour, sometimes broken a bit, show up in the spectrum of hadrons you can make out of different flavours of quarks…. See the eightfold way and all that… and at other instances they are symmetries of the formulation, but not really the physics itself, like the so-called gauge theories that describe (perturbatively at least) the three non-gravitational fundamental forces… Then groups like su(2), and su(3) show up…. for more, there are very many books on the subject at all levels. I don’t know what level you are at.