# Experiments looking for monopoles

Background: (skip it if you know it)

In the easiest formulation of classical electromagnetism magnetic monopoles do not exist. In fact, the Maxwell's equation $\nabla \cdot \vec{B}=0$ implies (using Gauss' Theorem) that the surface integral of the flux of $\vec{B}$ over the bounday of any finite surface is zero. Therefore no isolated magnetic charges (i.e monopoles do exist).

However Dirac discovered that even if a single monopole existed in the universe, we could then explain in a rather easy way why the electric charge is quantized. Note that since the charge is not an observable its quantization is completely different than the quantization of energy or momentum in QM, for example.

Furthermore, in the recent developings of QFT, the theoretical model we usually assume implies that every time a gauge symmetry is broken monopoles (and other kind of topological defects such as solitons) arise. Since in the hot big bang model it is usually belived that many gauge symmetries were broken in a primordial of the universe, monopoles of various kinds (not just magnetic monopoles but also Yang-Mills ones) could (at least this is what teorists say) have been produced.

Up to date, not a single monopole has been found by experiments.

With this in mind I ask the following

Question:

Which experiments are currently being carried over to look for magnetic, and other types of, monopoles?

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Here is a link where some experiments are described... spectrum.ieee.org/semiconductors/materials/… –  cryonole Oct 30 '13 at 1:53
Actually something that you would definitely want to say is that the monopoles here - although they have all the theoretical properties of fundamental magnetic monopoles - are indeed quasi-particles that only exist in particular phases of matter. A very interesting article, but I don't believe these would be the particles that give Dirac's quantisation explanation wings. –  WetSavannaAnimal aka Rod Vance Oct 30 '13 at 9:47
Federico, While it's an interesting question I don't think it's an appropriate one for this site. Unless we have a monopole experimentalist to hand the answer would just be a Google search, which anyone could do including you. If there was some aspect of the experimental design/protocol you didn't understand that might be a good question, but as it stands I think this question should be closed as a list question (meta.physics.stackexchange.com/questions/4561/…). –  John Rennie Oct 30 '13 at 9:56
This question appears to be off-topic because it is asking for an open-ended list. See this meta post for a discussion of appropriate list questions. –  tpg2114 Oct 31 '13 at 16:48
@JohnRennie: A Priori how does one know there are no monopole experimentalists? :P –  user7757 Nov 9 '13 at 1:42

I work on an experiment that involves trying to create magnetic monopoles in a type of material called a spin ice (Dy2Ti2O7) so called because its spins obey the same rules as ice water. The interesting part of the structure is a tetrahedra of rare earth atoms where in the ground state you have two spins pointing in and two spins pointing out across its four corners. If you excite the system you can flip one of the spins in the system so you have for example 3 in 1 out spins. This can give a net charge at one tetrahedra. To balance this an opposite spin flip will occur elsewhere in the lattice so you effectively have a dipole. The interesting thing about this is that you will have a chain of connected spin flips (a Dirac string) between the two tetrahedra with net charge but it doesn't cost any energy to flip the spins between these two sites. This means that each part of the dipole can propagate entirely independently of the other meaning that it is a site of charge that is not energetically linked to an opposite site of charge making this a monopole. You can look for these features using neutron diffraction (that being the experiment, to answer your question). If you want to read some more on the topic Castelnovo et al. is a good place to start as is Morris et al.

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The most famous experiment looking for monopoles in condensed matter is the Stanford monopole experiment. –  LlamaD Nov 10 '13 at 16:00