#dipole

The search for the unicorn’s other horn ….

In the dim and distant past, the days when I was a young scientist, I had a theoretical physicist friend who was trying to predict the behaviour of magnetic monopoles. He likened his project to the quest for the unicorn’s second horn. No one expected to see it any time soon.

With the passage of time it seems that the quest has moved from theoreticians to experimentalists. Last week, a group of scientists reported the first results of their search for magnetic monopoles, which they suggest, should be concentrated in rocks derived from Earth’s deep mantle. Needless to say (sorry, my cynical side is coming out here), they have not yet found one. But they still think they might.

So, why the interest in magnetic monopoles? Well, we know that electric dipoles (a positive and negative charge separated from each other) exist – materials rich in these dipoles are used in your camera flash, in passive infrared burglar detectors, and even in the memory card of your Sony PlayStation. Electric monopoles also exist – as isolated positive or negative electric charge (giving rise to static electricity, for example). But their magnetic equivalents seem different. A magnetic north pole seems always to be accompanied by a magnetic south pole, to form a magnetic dipole (like a bar magnet or a compass needle). Can norths or souths exist in isolation, in the same way that positive or negative electric charges do? Well, experience says no.

In a paper entitled “Search for Magnetic Monopoles in Polar Volcanic Rocks” a team assembled from Sweden, Switzerland, Iceland, Denmark, USA and the UK explain that magnetic monopoles formed in the very early Universe, and predicted by grand-unification theories, may persist by attaching themselves to magnetic nuclei. It has been suggested that they would be present in cosmic rays, so material subjected to cosmic ray bombardment – Moon rocks, rocks from Earth’s crust, and meteorite samples, have been the focus of earlier (fruitless) searches. Bendt and co-workers point out that (heavy) monopoles in Earth’s interior would accumulate towards the core, but would end up trapped at the core-mantle boundary where Earth’s geodynamo forces them to the magnetic axis in the polar regions. Mantle convection then brings them back toward the surface, eventually (over a time scale of around half a billion years) appearing in igneous rocks generated from mantle hot spots.

This thesis has been the reasoning behind the group’s quest for magnetic monopoles in igneous rocks from high latitudes. They have, for example, analysed rocks from Antarctic and Arctic flood basalts and intrusions, such as the Skaergaard intrusion shown here. More than 20 kg of rock have yet to yield any sign of a monopole. While this negative result may seem unremarkable to many, it does set limits on the likely mass of the elusive magnetic monopole.

~SATR

Image: Sødalen scientific camp, near the Sgaergaard intrusion, Eastern Greenland (credit: “submanant”, Flickr).

Links: http://arxiv.org/abs/1301.6530

http://physics.aps.org/articles/v6/34