By Edwin Cartlidge of Nature magazine
Now that's precision measurement: the electron is a perfect sphere, give or take barely one part in a million billion.
The result comes from the latest in a long line of experiments to probe the shape of the fundamental particle that carries electrical charge. "If you imagine blowing up the electron so that it is the size of the Solar System, then it is spherical to within the width of a human hair," says physicist Edward Hinds at Imperial College London, who led the team responsible for the minuscule measurement.
But this is more than a quest for accuracy. Many physicists are intent on finding out whether the electron is actually slightly squashed, as some theories predict. If the deformity is there, further refinement of the technique that made the latest measurement should pin down the deformity in the coming decade. The discovery would show that time is fundamentally asymmetrical, and could prompt an overhaul of the 'standard model' of particle physics.
Although the electron has traditionally been considered to be an infinitesimally small point of charge, it actually drags a cloud of virtual particles around. These fleeting particles pop in and out of existence, and contribute to the electron's mass and volume. All experiments so far have revealed that this cloud is perfectly spherical, but hypothetical virtual particles predicted by extensions to the standard model would make the cloud bulge slightly along the electron's axis of spin. This bulge would make one side of the electron slightly more negatively charged than the other, creating an electric dipole similar to the north and south poles of a bar magnet.
Physicists argue that we would expect to see this electric dipole in a Universe which consists overwhelmingly of matter. Although equal quantities of matter and antimatter are thought to have been created in the Big Bang, we see almost no antimatter in today's Universe. This asymmetry not only implies a cosmic favoritism for matter, but also suggests that physics does not always work the same way when time is run backwards instead of forwards.
Be kind, rewind
Evidence of this asymmetry could be found by playing a film of a spinning, slightly squashed electron in reverse. Although the direction of the electric dipole would remain unchanged, the magnetic dipole around the electron--which depends on the direction of its spin--would flip to the opposite direction.
The latest study, published today in Nature, looked for the effect of this asymmetry on the spins of electrons exposed to strong electric and magnetic fields--but found nothing. Indeed, the researchers say that any deviations from perfect roundness within electrons must measure less than a billionth of a billionth of a billionth of a centimeter across.
Similar measurements had previously used beams of atoms passing through magnetic and electric fields. But Hinds and colleagues instead used molecules, which can be more sensitive to the fields. Using a pulsed beam of ytterbium fluoride, they were able to improve on the previous best sensitivity--achieved in 2002 by Eugene Commins and colleagues at the University of California, Berkeley, who used thallium atoms--by a factor of about 1.5.
Getting better all the time
Hinds reckons that by increasing the number of molecules per pulse and reducing their speed, his group should be able to raise the sensitivity of measurement by a factor of ten "over the next few years", and, ultimately, by a factor of 100. This would be more than enough to detect the distorting effects of most modifications to the standard model, and would thus provide evidence for the existence of new, very massive particles. A non-discovery, by contrast, would send theorists back to the drawing board.
"We would pretty much rule out all current theories if we went down by a factor of 100 and saw nothing," he says. "But theorists are very creative and would probably come up with models where the electric dipole moment is smaller."
Commins agrees that the latest work opens the door to major discoveries. "In the half-century since such experiments began, this is the first time that the best upper limit on the electric dipole has been achieved using molecules," he says. "Since molecules offer much greater sensitivities than atoms, it is only a question of time before the limit is greatly improved."
David DeMille of Yale University in New Haven, Connecticut, who was a co-author on the 2002 paper with Commins and is carrying out molecular experiments of his own using thorium monoxide, agrees. "On the face of it, the actual improvement in precision in the latest work is rather small," he says. "However, this paper represents the first of what many in the field believe to be a coming wave of potentially much larger improvements, because of new experimental methods that are being developed."
This article is reproduced with permission from the magazine Nature. The article was first published on May 3, 2011.