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Virtual-Particle Magnetism

From The Economist print edition

Particles that exist only fleetingly help make everyday matter magnetic

IN THE world of particle physics, there is no such thing as nothing. Particles of matter, and their anti-matter counterparts, are forever flitting in and out of existence. Theorists have predicted that the presence of such transient visitors has little effect on everyday life. However, a group of experimental physicists has just shown this view to be mistaken.

Atomic nuclei are bundles of protons and neutrons which, along with electrons, are the basis of matter. But protons and neutrons themselves are made of more fundamental particles called quarks. These quarks have fractional electrical charges and combine to give each particle its overall electric charge, whether positive in the case of protons or neutral in the case of neutrons. They also give each particle its magnetic properties.

As well as possessing electric charge, quarks come in one of six “flavours”: up, down, strange, charm, bottom and top. A proton, for example, consists of two up quarks and a down quark, while a neutron consists of two down quarks and an up quark. But besides these permanent quarks, quantum theory predicts that so-called virtual quarks, together with their anti-matter partners, are continuously emerging from the vacuum of space and then disappearing again as a result of Heisenberg's uncertainty principle. So, while a proton has three resident quarks, it also plays host to a lot of short-term visitors. These are mostly up and anti-up, or down and anti-down, but also, occasionally, strange and anti-strange quarks. These guests add their properties to the mix.

A team of physicists at the Thomas Jefferson National Accelerator Facility in Virginia, led by Douglas Beck, decided to investigate just how much of a contribution the visitors make. They did so by probing the interior of protons (in the form of the nuclei of hydrogen atoms) with fast-moving electrons. By analysing the subsequent trajectories of the particles, they could pinpoint the effect of the visitors. The results, reported in this week's Physical Review Letters, showed that this was far greater than had been predicted. In particular, some 5% of a proton's magnetism is contributed not by the host quarks but by visiting strange quarks that have popped out of nowhere.

The magnetism of protons is exploited in the medical technique known as magnetic-resonance imaging (MRI). Patients are placed in a strong magnetic field that aligns the magnetic fields of the protons in their bodies. The protons are then stimulated by radio waves which “unalign” them, and the energy they release as they return to their original alignments gives away their position. Since different tissues have different compositions (and thus different densities of protons), this signal can be turned into a picture of what is going on inside a body. So, next time you have an MRI scan, remember that part of the picture depends on something that isn't really there. Strange, eh?