In Basic Physics classes students always learn that a magnet has a north and a south pole and these can be demonstrated using simple experiments, such as the use of iron filings with a bar magnet, i.e.
In more advanced physics courses, however, including university -level classical electrodynamics, e.g.
Introducing Electrodynamics (Part 1)
It is well nigh impossible to disabuse students of their earlier dual pole imagery and conception, i.e. from the earlier bar magnet experiments. It is as if the two-poled nature of magnetic fields is mandated and discussing magnetic monopoles (which may well be one form of dark matter) remains as abstract as talking about one-legged unicorns. This is despite the fact that there is nothing in theoretical physics that says magnetic monopoles can’t exist. After all, they are the hypothetical analogs to electric charges in Maxwell’s equations. However, one of those equations, Ñ · B = 0 does suggest such monopoles can't exist. But that is taking the equations at face value.
In truth, however, adopting such monopoles would render the Maxwell E- M equations more symmetrical: Electric terms could be transformed to magnetic ones and vice versa (See, e.g. the article by Arttu Rajantie, Physics Today, October 2016, page 40). But what one might desire in theory, to enhance the symmetry or beauty of equations or a given theory, is not always easily attained in practice. Look, by way of example, at the difficulties in identifying the Higgs boson. (It was later revealed some of the CERN channels were 'overproducing' the Higgs - a spin 0 boson - while other channels were underproducing it. The upshot? There may not have been a real Higgs discovered but rather a spurious image or mirage abstracted from differing observational channels, never mind the Higgs theorists being awarded the Nobel prize, i.e.
In a somewhat similar vein, the search for magnetic monopoles has failed to identify any so far. But this may be because most experiments have focused on elementary-particle collisions that could produce monopoles that are themselves point-like particles. Further, particle theorists don’t expect those magnetic monopoles to suffer from an exponential suppression of their production cross- section, unlike composite monopoles, e.g.
[2106.07800] Dirac plus Nambu Monopoles in the Standard Model (arxiv.org)
Which have been predicted in various theories that consider physics beyond the standard model. Irrespective of the manifestation, both point-like and composite monopoles are expected to strongly couple to photons. The problem is that such strong coupling has previously prevented researchers from reliably calculating their production cross sections (See: Physics Today, July 2006, page 16).
The break point for discovery may well lie in the collaboration known as MoEDAL—the Monopole and Exotics Detector At the Large Hadron Collider (LHC) which has adopted a different strategy using heavy-ion collisions. To fix ideas, in November 2018 a lead–lead collision experiment at the LHC succeeded in producing a magnetic field with a strength of 1016 T (Tesla). This was the strongest ever field observed in the known universe, since even pulsars (neutron stars) at 1011 T don't possess magnetic fields of comparable magnitude .
Based on a 'New Discovery' column in the February edition of Physics Today, we now know MoEDAL has just published its results. Although no magnetic monopoles were observed, the team did exclude the possibility of monopoles with masses smaller than 75 GeV, which is roughly 80 times the mass of the proton. So whatever monopoles are eventually found will have to be pretty massive critters.
The heavy-ion approach relies on the "Schwinger mechanism", a vacuum-decay effect that produces electron–positron pairs in a decaying electric field. See e.g.
The exceptionally strong magnetic fields thereby created - when heavy ions collide - can then be thought of as the magnetic counterpart to the Schwinger mechanism. Rather than electron–positron pair production, the decaying magnetic fields could create magnetic monopoles and their antiparticles.
The magnetic field produced in the Pb–Pb collision was found to be about 10 000 times as strong as magnetic fields found on the surfaces of neutron stars. To look for possible magnetic monopoles, the collaboration designed detector traps made from aluminum nuclei. The latter's high magnetic moment allowed them to catch particles carrying a magnetic charge. A DC superconducting quantum interference device then scanned the detectors for the presence of magnetic charges.
Although no magnetic monopoles were found, the negative result narrows the range in which future experiments will look for magnetic monopoles. That search will continue this spring with MoEDAL deploying a new detector at the LHC to look for magnetic monopoles with higher mass and magnetic charge. (See e.g.B. Acharya et al., Nature 602, 63, 2022.)
See Also:
And:
[0807.1117] The Schwinger mechanism revisited (arxiv.org)
And:
Electric Charge in Composite Magnetic Monopole Theories | SpringerLink
And:
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