Not pear-shaped

FUNDAMENTAL building blocks of matter, like quarks (which make up the protons and neutrons in atomic nuclei) and electrons (which orbit those nuclei), are called point particles. This is somewhat misleading. It implies that although they have mass, they are also, like mathematical points, zero-dimensional—in other words, they do not take up any space. In the parlance of quantum mechanics, however, to call a particle pointlike is to say no more than that it is elementary, ie, that it is not known to be made up of smaller bits. Nowhere is it stipulated that it cannot have a shape.

Indeed, shape matters. Take the electron, the most manageable of all elementary particles, and thus the most thoroughly studied. According to the Standard Model, a 40-year-old theory which describes the behaviour of all the known elementary particles and forces of nature apart from gravity, an electron's point mass sits amid a cloud of virtual particles which pop in and out of existence—the sort of thing possible in the weird world of quantum mechanics. Theory suggests that this cloud should be an almost perfect sphere.  The crucial word, though, is "almost".

A departure from Platonic perfection is predicted to be caused by the particle's electric dipole moment. Unfortunately, this has never been measured. That matters for two reasons. First, various versions of the Standard Model make different predictions about the size of the electric dipole moment. Measuring it would help choose between them. Second, many physicists believe the electron's electric dipole moment is a manifestation of the asymmetry that causes the universe to be made of matter.

If the world were completely symmetrical at a fundamental level, equal amounts of matter and antimatter would have been created in the Big Bang and would then have gone on to annihilate each other, with the result that the only thing left in the universe would be radiation. Moreover, this asymmetry implies that the laws of physics would be different if the arrow of time were reversed. This might be an explanation of what is (to a physicist, at least) a strange anomaly in the fabric of the universe, namely that it is possible to travel in any direction in the dimensions of space, but only one direction in the dimension of time. Measuring the electric dipole moment, then, is the sort of thing that really floats physicists' boats. The question is, how to do it?

Besides their putative electric dipole moment electrons have a real and measurable magnetic dipole moment. They act, in other words, like tiny bar magnets with north and south poles, making them rotate in a magnetic field. Any electric moment would arise if the particle's charge were distributed unevenly along the axis around which the particle spins in this way. The consequence would be that the particle's centre of charge and its centre of mass were not the same point, meaning it was not quite round.

A team of physicists at Imperial College, London, led by Edward Hinds, has spent the past ten years trying to see just how round the electron really is. The obvious way to go about this task is to send electrons through an electric field and see whether they twist and turn. Any electric dipole moment would align itself with the the electric field. Since the centre of mass is offset this would make the particles precess like gyroscopes. The stronger the field, and the longer an electron spent floating in it, the more visible any such wobbling would be.

Alas, a free electron carries an electric charge. This means that using a stronger field merely speeds it up, slinging it rapidly into the wall of the apparatus, and reducing the amount of time available for measurement. To make matters worse, an electron moving through an electric field generates its own magnetic field, which couples with its magnetic moment to cause a second, confounding precession. 

So, instead of using electrons, Dr Hinds and his colleagues chose to work with molecules of ytterbium fluoride, a highly ionic substance. An electric field will not accelerate a neutral molecule in the way it would an electron, but it will polarise the strong ionic bond which holds the molecule together, separating the opposite charges and, in effect, isolating some of the electrons within it so that their spins can be studied.

After more than ten years of fiddling with their set-up, Dr Hines's team has succeeded in determining that the electron is round to within one part in a million billion. That will not confoud the theoreticians too much, but Dr Hines hopes to improve the accuracy of his measurements tenfold over the next few years, and eventually to achieve a hundredfold improvement. By then, any anomalies should be obvious. If they are not, then it is the theories of physics themselves that will have gone distinctly pear-shaped.