Magnetic Monopoles And Their Discovery.

This article on magnetic monopoles is a guest article by Ariana Vlad, senior at the International Computers High School of Bucharest, Romania, where she focuses on studying Physics and Mathematics.


Background

Magnetism offers us one of the most indestructible, inseparable pairs: the north and south poles. Even breaking a bar magnet can’t separate one pole from another. This means the “southness” or “northness” of a region in a bar magnet - and neither other magnetic phenomena observable or known until now- is not the result of some specific magnetic net charge. However, modern topics such as superstrings and grand unified theory predict the existence of such charges as magnetic monopoles.

Magnetic Monopoles
When a bar magnet is halved, two additional different poles appear where the division was made. This way, two magnets are created. No matter how many divisions one performs, a dipole is formed and a region with only one “polarity” can’t be separated.

Magnetic Monopoles and maxwell's equations

The standard Maxwell’s equations describe the influences of the electric and magnetic field to each other, taking into consideration only electric charges but no magnetic monopoles. An alternate version of those equations - which includes magnetic charges analogous to the electric monopoles - is completely symmetrical under the interchange of the fields, and can be written if some more physical quantities are included: density of magnetic charges  ρm and "magnetic current density"  Jm.  This new form which implies some analogies between the magnetic and electric field charges has been firstly introduced by Dirac in the 1930s. A moving magnetic monopole creates an electric field just like an electron moving creates a magnetic field. An electric charge in a variable magnetic field starts moving and a changing electric field has the same effect on magnetic charges. 

Related: What do the 4 Maxwell's equations really mean?

Maxwell's equations with magnetic monopoles
The four Maxwell’s equations and the expression of Lorentz force in both standard and alternate form. The usual, standard notations are used. The symmetry between the two cases is clear for Gauss’, Faraday’s and Ampere’s Laws. For the Lorentz force, an additional term has to be included.

Magnetic analogues

Phenomena mathematically similar to magnetic monopoles can be created under special conditions in condensed matter systems. These quasi-particles are called magnetic strings. Thin magnets are stretched on lattices on great distances until the two poles are very distant to each other. When looking at a part of the system, only a pole can be detected, which gives the false illusion of a monopole. In fact, the other pole is also there, but well-separated and isolated from the one observed.

Magnetic string in condensed matter. Although they can trick an observer to believe otherwise, every strip is a magnetic dipole. Magnetic monopoles and quasi-particles are different concepts exhibiting similar properties. 

Experimental search

Such a unique theoretical approach attracted many physicists who tried to design experimental setups in order to detect a magnetic monopole. An apparently simple yet elegant experiment was created by Blas Cabrera almost half a century ago. Knowing that a magnet passing a wire loop would determine a variable signal (let’s say positive for a pole then negative for the other one), Cabrera used an eight loops coil, hoping to detect a signal of a specific polarity (either positive or negative, depending on the initial convention). The magnitude of the signal was also important, as it had to correspond to exactly 8 magnetons (the unit for the hypothetical magnetic monopole). Any other registered value would be an error. 

After some detections of one or two magnetons - deemed irrelevant - the apparatus detected what was thought to be a monopole. On February 14th, 1982 around 2 am the device recorded exactly 8 magnetons. Pleasantly surprised, many scientists built their own similar experiments hoping to detect the magnetic charge. Unfortunately, neither the original nor the latter devices recorded anything relevant. Without further confirmation, the first detection is either an unpredictable glitch in the apparatus or a very fortunate event. 

Other scientists researched this topic in various locations, including CERN, IceCube detector from the South Pole and London Center of Nanotechnology. Theoretical analysis indicates a magnetic charge would have rest energy as high as 10^14 TeV, meaning they had been produced only at the initial stages of Big Bang and very high energy detectors are needed. To this day, no other detection was recorded. However, for such an interesting topic, it's safe to say physicists will not cease to look for magnetic monopoles. 

Related: 5 unsolved problems in particle physics that are way too interesting!

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