Perfectly spherical!

The electron is the tiniest of the atomic particles and it acts like a mass and charge located at a point. Finding its actual shape has been neither feasible nor of any consequence, in the normal course.  But crises in physics in explaining all of nature now place importance on whether the electron is even the teeniest bit different from a perfect sphere. Jay Hudson and colleagues at Centre for Cold Matter, Blackett Laboratory, Imperial College, London, report that the best measurements to date still say the electron is round in every way!

The crisis

The nature of very small particles, atoms, nuclei and interactions between them has been understood down to a very great degree of certainty with the help of the quantum theory and the Special Theory of Relativity. Quantum electrodynamics, as this field of study is called, has been verified to the precision of one part in a hundred billion, 10-8, or uncertainty of just 1 mm in a distance or 100km.

At the same time, the General Theory of Relativity, which deals with gravitation and the structure of the cosmos, has also been verified in every experiment and there is no reason whatsoever to question its validity on grounds of facts or principle. And yet, the two theories are limited to the fields of very small dimensions and short range forces, or large masses and distances and long range forces like gravity, respectively. Short range forces, which act between nuclear particles, are irrelevant at the cosmic scale and gravity hardly matters for atomic particles, which have low mass and huge electric charge, for their size.  

An important question, perhaps the only question in physics has been to discover a unified theory, which acts both for short distances and astronomical distances, a set of principles that work for all manifestations of nature.

The most promising in this line is the String Theory, developed by the celebrated Stephen Hawking, which seeks to describe nature with not only the variables of space and time, but introduces many other dimensions, which display themselves in high energy interactions between elementary particles. With these postulated extra dimensions, the theory to seeks to explain, at once, all the phenomena at the atomic scale and also the nature of mass and gravity.

The proof of this complex structure would depend on verification of some of its predictions, apart from the known physics that it must explain. One of these new results is the existence of heavy particles, the ‘supersymmetric’ counterparts of all particles.

An objective of the Large Hadron Collider at CERN is to create reactions of such high energy that these heavy particles could get generated, which would act as verification of supersymmetry. One more consequence is that the electron should show ‘ovality’, so that it may align itself along the direction of electric fields.

The electron is already known to possess a magnetic axis, known as ‘spin’, which can get flipped in magnetic fields. But the current theory does not call for an electric ‘axis’, so that the electron would feel a turning force in an electric field.

At least, the current theory, to the extent verified, does imply that any such ‘ovality’ of the electron is less than one part in 1028. But supersymmetry puts this limit at only one part in 1014 to1019. Measurement of the electric directionality of the electron to that accuracy could thus also act as a check of the validity of the theory.

Electric dipole moment

An objective of research has thus been to detect the force that turns the electron around, when it is placed in an electric field. On the one hand, it is clear that the force is very small, because theory predicts very minute non-spherical distribution of charge in the electron. As the force is much too small, there are two ways of making it larger so that it can be detected.

One is to increase the electric field, the other is to let the field act for a long time, so that the effect of the field adds up. But because the electron has a negative electrical charge, these methods cannot work. In even a moderate electric field, the whole electron, if it is free, is swiftly whisked away by the field and the more the strength of the field, the faster the electron moves out of it.

The way around this difficulty has been to work not on free electrons but electrons that are bound, in atoms.  When an atom is placed in an electric field, the atom, being neutral, is not affected. But as the parts of the atom, the nucleus and the electrons are charged, they get drawn in opposite directions and the atom gets polarised. If the kind of atom and the strength of the field are well chosen, the field on the outer electrons can be quite high, to more readily display any effects of the electron being different from spherical.

Polarisation

It is by refining this idea that the Imperial College group made measurements of great precision. Molecules, which consist of a group of atoms which have lent or borrowed electrons to create forces that hold them together, are even more readily polarised by an electric field than atoms. And the effective field experienced by individual, outer electrons can be made to be even higher. Researchers worked with molecules of ytterbium monoflouride (YbF) and improved on the earlier best results that had come from atoms of thalium.

The results of the experiments were still that no ovality is detected. But the merit of the experiments is that we are able to say with confidence that if there is any ovality, it has to be less than one in 1018, which is how accurate the experiments were.

This limit of ovality is now very near, just a factor of 10, from the limit set by supersymmetry. The group is working on improving the experiment so that it gets more accurate and is able to say – is there ovality at the limit set by supersymmetry, or is there not?  In either case, the result has great value – either it validates supersymmetry and spurs on the effort to discover other high energy manifestations, or it turns the course to work on alternative theories.