The Shape of the Electron

What is round and measures 1 billionth of a millimetre across? Yes, you guessed it, it’s the electron. Theories have predicted that these particles should be spheres, but proving...
31 May 2011

Interview with 

Jony Hudson, Imperial College London


Dr Dhiren Kara adjusting the laser system used to measure the shape of the electron.


Diana -   What is round and measures 1 billionth of a millimetre across?  Yes, you guessed it, it's the electron.  Theories have predicted that these particles should be spheres, but proving this has been tricky.  Now after 10 years of trying, a team at Imperial College London have succeeded, as Jony Hudson explained to Chris.

Jony -   So what we've been working on is we've been trying to measure the shape of the electron.  The conventional theory suggests that the electron is round like a point particle, but we've been wanting to check that and to see whether it really is round.

Chris -   Should that really make a difference?

Jony -   Yeah.  It's important actually and the reason it's important is because although our current theories predict that the electron should be round, some of the advanced theories that go beyond that predict that it won't be round, that it'll have a distortion.  And so by doing this measurement, we look at it as a way to search for physics beyond the physics that we know about at the moment.

Dr Dhiren Kara adjusting the laser system used to measure the shape of the electron.Chris -   And just to give people some grasp as to the scale of the problem that you're grappling with, how big is an electron?

Jony -   The size of the electron, it depends how you measure it.  The classical, what they call the Compton wavelength is one measure of the size of the electron, and that's 10-12 metres, so 1 billionth of a millimetre.

Chris -   Pretty small in other words.

Jony -   Yes.

Chris -   How on Earth do you go about trying to size up something like that?

Jony -   If the electron were perfectly round, when we put it in an electric field, it would just spin around its axis in a perfectly regular way, whereas if it's not round when we put the electron in the electric field, it will develop a kind of wobble, a very distinctive motion where it's wobbling around its axis.  A lot like if you were to put a gyroscope on a stand at an angle, that kind of wobbling motion.  And so we look for that wobbling motion.

Chris -   Is that because the electron itself is an electrically charged particle?  So if you have something which is electrically charged moving, and it's moving in an electric field therefore, the two fields are going to interact and that's going to impart a movement on the particle that you can then pick up?

Jony -   Yeah, that's basically it.  If the electron were round, then it wouldn't matter which way it was oriented with respect to the field because it would just look the same.  If you turn a perfect sphere around a bit, it still looks the same.  Whereas if the electron say if it were egg-shaped, it would have a bit of distortion to it, it matters which direction it's pointing in relative to the field and so, there's a force on it, trying to align it with the field, and it's that force which creates the wobbling motion.

Chris -   So how did you actually do it?

Jony -   So what we do, we don't just use a bare electron because there are some real technical difficulties with working with electrons, namely that they're electrically charged.  So what we did instead is we used the electrons in a particular molecule, a molecule called ytterbium fluoride and this molecule has an unpaired electron orbiting around the outside of it, and that's the electron that we studied.  What we look at is if the electron isn't round, it starts wobbling in its orbit around the molecule.  So actually, what we really look at is we look to see how the molecule spins.

Chris -   And what do you find?

Jony -   What we find is, as best we can tell and we've looked really very carefully, there's no wobble.  The electron shows all signs of being round at our current sensitivity.

Chris -   So that means or that must have quite big implications for other things in the quantum realm then?

Jony -   Yeah.  Like I say, our current theory of physics predicts that the electron should be really almost exactly round.  And so, the first thing to say is our work doesn't contradict our current theory of physics which was - I guess a disappointment for us, it'd be much more fun if it did contradict the theory, but you can't have everything!  What it does do is if you look at some of these theories.  You know, I said people propose these theories that go beyond our current theory of physics.  Some of them predict that the electrons should be really quite distorted and actually, we've shown that it's rounder than some of those theories would predict.  So what's it allowed us to do is it's allowed us to constrain and guide the development of theories that go beyond our current theoryof physics.  It places limits on what theories could possibly be right.

Chris -   And do you think all electrons are made equal?  What I mean by that is, in the context in which you studied the electron, it behaved like that, but what would happen if you took a different molecule as the donor and studied that?  Do you think you might be able to get an electron that was distorted?

Jony -   No, no, no.  There are no forces in the molecule that can distort the shape of the electron.  What we're measuring here is how the electron comes from mother nature.  The molecule itself is not placing any forces on the electron that are distorting it.

Chris -   So you've nailed that one, myth busted, but are there any actual physical applications now that you are armed with this knowledge that you can use to take this forward?

Jony -   Well one thing that physicists are very interested in is there's a big mystery in cosmology, a big mystery in describing the origin of the universe, and the mystery is what they callthe matter-antimatter imbalance.  So our current theory of physics, the best one we have, says that in the Big Bang, matter and antimatter were created in equal measure.  Further, our theories of physics says that the laws that govern antimatter are basically the same as the laws that govern matter.  And so, the logical conclusion would be that we should have an equal amount of antimatter and matter today.  But astronomers have looked and you can search the skies.  You can look wherever you like.  You find only incredibly tiny traces of antimatter and this is a big mystery for people.

Now one potential solution is that there's a slight difference in the behaviour of matter and antimatter, and this would mean over the billions of years that the universe has existed, it could tip the balance and the matter could start to dominate over the antimatter.  If the electron is not round, then you can show that it's not possible for it to behave the same as its antiparticle, the positron.  So if the electron is not round, this would indicate that there has to be a difference between the behaviour of matter and the behaviour of antimatter.  And so, that's one of the motivations for studying this.  People are looking for a difference between the behaviour of matter and antimatter.  They think there must be one that we haven't discovered yet.

Chris -   So could you take your technique and apply it to the antihydrogen that has been successfully made at CERN and ask that very question?

Jony -   That would be a fantastically interesting experiment, but I would say our experiment was so difficult that it took us over a decade of extremely hard work to do.  Their experiment is so difficult that it took them over a decade.  If you sort of multiply them together, I can't even begin to imagine how difficult that would be!

Chris -   Jony Hudson from Imperial College and amazingly the precision with which they say they've made those measurements is equivalent to, were you to scale the electron up to an object the size of our solar system, their measurements are precise to within the thickness of a human hair.  Amazing piece of work.  It's published this week in the journal Nature.


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