Monday, May 20, 2013

The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though!

Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very simple.  Construct a loop of wire and then wait for a magnetic monopole to fall through the loop and generate a current.  As described in a review article in Nature[3] the detectors work in the same fashion as the original experiment done by Faraday which showed that when the end of a magnet is pushed through a coil of wire an electric current is induced in the wire. (picture 2)

After recording data for a little more than 120 days, the experiment saw the signal shown in picture 3.  The signal, (shown in the lower half of the picture), is compared to other jiggles in the experimental data caused by known possible sources of error.

No other signals were ever recorded.  In the end, the event was written off as more than likely being caused by trapped magnetic flux in the detector's superconducting loop creeping its way through the detection circuitry.

And of course, there's a gravity probe B[1] connection.  To shield the detector from stray magnetic fields not caused by magnetic monopoles, an apparatus was used that was originally designed to shield the prototypes of the superconducting gyroscopes that would ultimately be used on the spacecraft to detect Earth's gravitomagnetic field.

1.  Fairbank and Gravity Probe B

2.  Cabrera's candidate monopole
Cabrera B. (1982). First Results from a Superconductive Detector for Moving Magnetic Monopoles, Physical Review Letters, 48 (20) 1378-1381. DOI:

3.  Nature review of magnetic monopoles
Frisch H.J. (1990). Quest for magnetic monopoles, Nature, 344 (6268) 706-707. DOI:

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