The magnetic monopole is a hypothetical particle in physics that is a magnet with only one pole. In more technical terms, it would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unification theory and superstring theory, which predict their existence.  
The magnetic monopole was first hypothesized by Pierre Curie in 1894,  but the quantum theory of magnetic charge started with a 1931 paper by Paul Dirac.  In this paper, Dirac showed that the existence of magnetic monopoles was consistent with Maxwell's equations only if electric charges are quantized, which is observed. Since then, several systematic monopole searches have been performed. Experiments in 1975 (Price et all)  and 1982 (Blas Cabrera)  produced candidate events that were initially interpreted as monopoles, but are now regarded as inconclusive.
Monopole detection is an open problem in experimental physics. Within theoretical physics, some modern approaches assume their existence. Joseph Polchinski, a prominent string-theorist, described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen."  These theories are not necessarily inconsistent with the experimental evidence: in some models magnetic monopoles are unlikely to be observed, because they are too massive to be created in particle accelerators, and too rare in the universe to enter a particle detector.
Some condensed matter systems propose a superficially similar structure, known as a flux tube. The ends of a flux tube form a magnetic dipole, but since they move independently, they can be treated for many purposes as independent magnetic monopole quasiparticles. 
Main properties of magnetic monopolesEdit
Magnetic monopole chargeEdit
Magnetic monopole charge in the CGS units has the same units as the electric charge:
In the SI units magnetic charge has other units:
where is the Planck constant.
Coupling constant for monopoleEdit
It is known that electric charges have a small value of the coupling constat (s.c. fine structure constant). It has the following value in the CGS units:
In the SI units its expression is more complicated:
where is the electric constant.
By analogically, we could define the magnetic coupling constant in the CGS units (Giacomelli et al, 2003): 
In the SI units this coupling constant is (Yakymakha O.L.,1989): 
Magnetic monopole massEdit
Dirac theory doesn't predict the monopole mass. Therefore, there is no real prediction for the mass of the classical Dirac monopole (experiments pointed the lower border limit of mass only). Furthermore, the electron mass is defined from experiments only too, and it isn't predicted by any theory.
Low border limit for monopole massEdit
Low border limit for monopole mass could be obtained using the classical electron radius (SI units):
where is the Compton wave-length for electron, and is the rest mass of electron.
By analogy, we could define a value for the classical monopole radius (SI units):
where is the monopole rest mass. Thus, equating the classical radiuses for electron and monopole, we could obtain the low border limit for the monopole mass:
High border limit for monopole massEdit
In the general case, magnetic monopoles are results of the dynamic interaction of the particles with electric charges, and therefore it shouldn't has the rest mass (or static mass). For example, the gravitational interaction of the two static electron masses will be defined by the Newton law:
It seems that magnetic monopole should to have the "dynamic torsion mass" , since it has the "dynamic magnetic charge" . Then, the interaction of two dynamic torsion masses will be defined as:
where is the dynamic torsion coupling constant, is the gravitomagnetic gravitational constant.
On the other hand, magnetic monopole could have the upper border limit for the rest mass, which gravitational force is:
Equating the static and dynamic forces
we could find out the upper limit for the monopole mass:
It is evident that this value is so great, that it belongs to the cosmological scale. Therefore, any attempts of building such experimental devices to discover even one monopole are out of the scope of any civilization.
It is worth noting, that the supermassive monopoles, predicted by Grand Unified Theory theories, and connected with Early Universe, have the monopole mass values considerably lesser then predicted above.
"Monopoles" in condensed-matter systemsEdit
While a magnetic monopole particle has never been conclusively observed, there are a number of phenomena in condensed-matter physics where a substance, due to the collective behaviour of its electrons and ions, can show emergent phenomena that resemble magnetic monopoles in some respect.   
These should not be confused with actual monopole particles; in particular, the divergence of the microscopic magnetic B-field is zero everywhere in these systems, unlike in the presence of a true magnetic monopole particle. The behaviour of these quasiparticles would only become indistinguishable from true magnetic monopoles — and they would truly deserve the name — if the so-called magnetic flux tubes connecting these quasiparticles became unobservable which also means that these flux tubes would have to be infinitely thin, obey the Dirac quantization rule, and deserve to be called Dirac strings.
In a paper published in Science in September 2009, researchers Jonathan Morris and Alan Tennant from the “Helmholtz-Zentrum Berlin fur Materialien und Energie” (HZB) along with Santiago Grigera from “Instituto de Fisica de Liquidos y Sistemas Biologicos” (IFLYSIB, CONICET) and other colleagues from Dresden University of Technology, University of St. Andrews and Oxford University described the observation of quasiparticles resembling monopoles.  A single crystal of dysprosium titanate in a highly frustrated pyrochlore lattice (F d -3 m) was cooled to 0.6 to 2 K. Using neutron scattering, the magnetic moments were shown to align in the spin ice into interwoven tube-like bundles resembling Dirac strings. The ends of each tube are at the crystallographic defects, where the magnetic field looks like that of a monopole. Using an applied magnetic field to break the symmetry of the system, the researchers were able to control the density and orientation of these strings. A contribution to the heat capacity of the system from an effective gas of these quasiparticles is also described.
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- Particle Data Group summary of magnetic monopole search
- Magnetic Monopole Searches (lecture notes)
- 'Race for the Pole' Dr David Milstead Freeview 'Snapshot' video by the Vega Science Trust and the BBC/OU.