Predicted by theory but never glimpsed, magnetic monopoles may be so rare that hunting them is futile: the upper limit on the number of these lone magnetic poles that can exist in our patch of the universe has been slashed.

Unlike electric charges, which abound as individual positives and negatives, magnets seem always to have both a north and south end. Yet monopoles pop up in descriptions of the early universe and also help to explain why electric charges come in discrete “quanta”. If monopoles actually exist, will we ever spot one?

Our best hope may be to capture a relic monopole from the early universe, but the likelihood of this depends on how many there are near us. The limit on the number of such high-speed relic monopoles that could inhabit the Milky Way without sapping its magnetic field is called the “Parker bound”.

Now readings from ANITA, a balloon-borne neutrino telescope based in Antarctica that can also detect monopoles, suggest that the maximum is just 1/10,000th of the Parker bound for monopoles moving close to the speed of light, says David Besson at the University of Kansas, Lawrence, and colleagues.


Gift denied

Meanwhile, a second team has produced new, decreased upper limits for slower-moving monopoles, too, using data taken by the AMANDA telescope, an array of sensor-studded strings embedded in the Antarctic ice.

This leaves Thomas Weiler of Vanderbilt University in Nashville, Tennessee, wondering how much longer to hunt monopoles. “It’s an opportunity for nature to give us a gift, and she has decided not to.”

For the time being, however, the search is likely to continue. Analysis is ongoing, for example, on data collected by AMANDA’s successor IceCube, which fills 1 cubic kilometre of ice and is set to be completed later this year.

Journal references: The European Physical Journal C (DOI: 10.1140/epjc/s10052-010-1411-6) and arxiv.org/abs/1008.1282