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Electron acts like a perfect sphere

Mike Tarbutt

Imperial College London's Dhiren Kara adjusts the laser system used to measure the shape of the electron.

The most precise measurements of the electron ever made suggest that it's perfectly spherical to an accuracy of less than 0.000000000000000000000000001 centimeter — a tiny, tiny number that physicists say can make a big difference in the nature of the cosmos.

In this case, we're actually talking about the "shape" of the electron's interactions with electric fields rather than whether it's a non-spatial point particle or a tiny vibrating string. Those concepts are fine in other contexts, but for the laser experiment conducted by researchers at Imperial College London, size (and shape) matters. The measurements, which were 10 years in the making, are reported in this week's issue of the journal Nature.


If the physicists had seen an irregularity in the electric dipole moment — that is, the orientation of the electron as it spins in an electric field — that would have lent support to some of the non-standard models in particle physics. One example is the idea that an extra supersymmetric particle (a.k.a. "sparticle") exists for every particle we know about in the standard model. Another example is the view that the interactions involving matter are just slightly different from interactions involving antimatter ... which would explain why we see virtually no antimatter in the universe around us.

The fact that the electron seems so perfectly round suggests that the search for "new physics" at Europe's Large Hadron Collider might be harder than some scientists had hoped. That meshes with the LHC's initial findings ... or, should I say, the non-findings of exotic phenomena such as microscopic black holes and supersymmetry. The LHC is still ramping up its data collection rate, however, and physicists say it's way too early to guess at what they'll find or not find.

In any case, the results reported in Nature are considered an observational tour de force.

"We're really pleased that we've been able to improve our knowledge of one of the basic building blocks of matter," research team member Jony Hudson said today in a news release. "It's been a very difficult measurement to make, but this knowledge will let us improve our theories of fundamental physics. People are often surprised to hear that our theories of physics aren't 'finished,' but in truth they get constantly refined and improved by making ever more accurate measurements like this one."

In a Nature commentary, University of Michigan physicist Aaron Leanhardt noted that the previous best attempt to detect the electron's electric dipole moment was reported by other researchers in 2002, by making fine-scale measurements of electrons in a beam of neutral thallium atoms. The Imperial College team pushed the envelope by measuring the motion of electrons in ytterbium monofluoride molecules, using highly precise laser pulses.

If the electrons were not perfectly round, their motion would exhibit a slight wobble, like a spinning top that's running down. As a result, the shape of the ytterbium monofluoride molecules would be distorted. But the physicists saw no sign of such a wobble. Instead, they determined that any variance from perfect roundness, stated in terms of centimeters, would have to be less than 10.5 times 10 to the -28th power times e, where e is the charge of the electron.

"This means that if the electron was magnified to the size of the solar system, it would still appear spherical to within the width of a human hair," Imperial College said in its news release.

Hudson and his colleagues aren't finished yet. They report that they're working on new techniques that could make even finer measurements of the electron, allowing them to "probe for new particle physics at tens of tera-electronvolts." In comparison, the top collision energy achievable at the LHC is a mere 14 tera-electronvolts, or 14 TeV.

It's mind-boggling to think that ultra-fine measurements of electrons can guide physicists' investigations at the highest energies achievable on earth. That perspective is what led Leanhardt to hail the Imperial College team's effort as a significant push into the frontiers of physics.

"Experiments of this genre reach far beyond the realm of atomic, molecular and optical physics: they can be viewed as low-energy windows on the high-energy soul of the cosmos," he wrote.

More about the frontiers of physics:


In addition to Hudson, the authors of "Improved Measurement of the Shape of the Electron" include D.M. Kara, I.J. Smallman, B.E. Sauer, M.R. Tarbutt and E.A. Hinds.

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