This image shows a computer simulation of the polarized electron-spin density on a plane that contains the Earth’s rotational axis and Amherst, Massachusetts. The black arrows indicate only the direction of the electron spins, while the coloured shading of the plot indicates the magnitude of the polarized electron-spin density. The violet arrow corresponds to the location of the Amherst apparatus. The vertical axis is along the Earth’s rotational axis, and the axis labels have units of kilometres. (Courtesy: Daniel Ang and Larry Hunter of Amherst College)

Search for ‘unparticles’ focuses on Earth’s crust

Feb 21, 2013

Evidence of a minuscule force that could exist between two particle spins over long distances could be lurking in magnetized iron under the Earth’s surface. That is the conclusion of a new study by physicists in the US, who have used our planet’s vast stores of polarized spin to place exacting limits on the existence of interactions mediated by unusual entities such as “unparticles”.

The intrinsic angular momentum, or “spin”, of a particle gives that particle a magnetic moment, and the interaction between spins generates magnetism. A ferromagnet, such as iron, becomes magnetized when the spins of some of the electrons in its constituent atoms line up, while quantum mechanics tells us that the magnetic force between spins results from the electrons exchanging “virtual” photons.

Some theoretical physicists have suggested that other, as-yet-undiscovered particles might be exchanged virtually and so give rise to new types of spin–spin interaction. In 2007, for example, Howard Georgi of Harvard University proposed the existence of unparticles, which would have the unusual property that their masses would scale with energy or momentum.

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