The neutron is a constituent of atomic nuclei and is electrically neutral. However, there are small negative and positive charges deep within it. We want to know whether the average position of the positive charge coincides with the average position of the negative charge. Any separation between these average positions would result in the neutron having an electric dipole moment (EDM) - the electrical equivalent of having a bar magnet at its centre.

The search for an EDM in a fundamental particle like the neutron offers one of the most exciting possibilities for progress in particle physics. Why should this be so?

We know that the behaviour of fundamental particles and forces are underpinned by laws of symmetry - a series of mathematical 'mirrors': for example, if all the particles in an interaction are changed for their oppositely charged antiparticle partners (C symmetry), or all of their coordinates in space are reversed (P symmetry), then the resulting situation should become an exact 'reflection' of the original one. The same applies if the process is reversed in time, like a film running backwards (T symmetry). However, we know that these symmetry laws do not work perfectly; if they did, the Universe today would contain exactly the same amount of matter as antimatter, which is clearly not the case. An understanding of these symmetry violations is vital in devising a correct theory of particles and forces.

Searching for these asymmetries is now one of the most important aims of particle physicists, and it turns out that the neutron offers an intriguing route to looking for evidence of time-reversal asymmetry. Neutrons have spin, rather like a spinning top. If you turn a symmetric top upside down, it appears to be running backwards, so flipping over a spinning neutron is somewhat like turning it backwards in time - unless the neutron contains an asymmetry along its axis, such as an EDM, in which case you can tell the difference. The question of just how small the neutron EDM really is then depends upon the mechanism invoked to explain the time-reversal symmetry. This is the motivation for the attempt to measure it.