Magnetic monopole: Difference between revisions

In physics, a magnetic monopole is a hypothetical particle that may be loosely described as "a magnet with only one pole" (see electromagnetic theory for more on magnetic poles). In more accurate terms, it would have net "magnetic charge". Interest in the concept stems from particle theories, notably Grand Unified Theories and superstring theories that predict either the existence or the possibility of magnetic monopoles.

Magnets exert forces on one another; similar to electric charges, like poles will repel each other and unlike poles will attract. When a magnet, that is, an object conventionally described as having a north and a south pole, is cut in half across the axis joining those "poles", the resulting pieces are two normal (albeit smaller) magnets each with its own north pole and south pole, rather than two separate north-only and south-only pieces.

The modern understanding of magnetism posits that all magnetic effects are actually due to the motion of charged particles; that is, all magnets are in fact electromagnets. The magnetic force is actually due to the finite speed a disturbance of the electric field, the speed of light, which gives rise to forces that appear to be acting along a line at right angles to the charges. In effect, the magnetic force is the portion of the electric force directed to where the charge used to be.

Even atoms have a tiny field. In the toy model of an atom the electrons orbit the nucleus, and thus have a charge in motion giving rise to a magnetic field. Permanent magnets have a measureable magnetic field because the atoms (and molecules) are arranged in a way that their individual tiny fields are aligned to add up.

In this model, the lack of a single pole makes intuitive sense; cutting a bar magnet in half does nothing to the arrangement of the molecules within, and you end up with two bars with the same arrangement, and thus the the same field. This also explains how heating or simply hitting a magnet made from a soft material will degauss it, as the molecules within are moved about.

Since all known forms of magnetic phenomena involve the motion of electrically charged particles, and since no theory suggests that "pole" is, in that context, a thing rather than a convenient fiction, it may well be that nothing that could be called a magnetic monopole exists or ever did or could.

Maxwell's equations of electromagnetism relate the electric and magnetic fields to the motions of electric charges. The equations are very nearly symmetric under interchange of electric and magnetic field; in fact symmetric equations could be written if one allowed for the possibility of "magnetic charges" exactly analogous to the observed electric charges. When no magnetic charges are present in a region, these symmetric equations reduce to the conventional equations of electromagnetism, that is, ( ∇·B = 0 ).

So, classically, the question is "Why does the magnetic charge always seem to be zero?" Nothing in Maxwell's equations suggests that it has to be, nor has any other development based on these original formulations. This has been a curiosity for a long time, but it has become more of a problem in recent years, when new theories of physics seem to predict the existence of magnetic monopoles.

One of the defining advances in quantum theory was Paul Dirac's work on developing a relativisic quantum electromagnetism. Before his formulation, the presence of electric charge is simply "inserted" into QM, but in 1931 Dirac showed that a discrete charge naturally "falls out" of QM if the product of the electric charge (e) and "magnetic charge" (g) is equal to 2ħc. Given the measured values of ħ, c and e, g must be at least a few thousand times larger than e.

At the time it was not clear if such a thing existed, or even had to. After all, another theory could come along that would explain charge quantization without need for the monopole. The concept remained something of a curiosity. However, in the time since the publication of this seminal work, no other widely accepted explaination of charge quantization has appeared.

In more recent years, a new class of theories has also suggested the presense of a magnetic monopole.

In the early 1970s, the successes of quantum field theory and gauge theory in the development of electroweak theory and the strong nuclear force led many theorists to move on to attempt to combine them in a single theory known as a grand unified theory, or GUT. Several GUT theories were proposed, most of which had the curious feature of suggesting the presense of a real magnetic monopole particle. More accurately, GUTs predicted a range of particles known as dyons, of which the most basic state is a monopole. The charge on magnetic monopoles predicted by GUTs is either 1 or 2gD, depending on the theory.

Many of the particles predicted by these GUTs were beyond the abilities of current experiments to detect. For instance, a wide class of particles known as the X bosons are predicted to mediate the coupling of the electroweak and strong forces, but these particles are extremely heavy and well beyond the capabilities of any reasonable particle accelerator to create. Monopoles, on the other hand, are expected to be stable and fairly low-mass, and therefore should be detectable not only in existing accelerators, but also "in the wild" by looking for them in appropriate detectors. For this reason, monopoles became a major interest in the 1970s and 80s, along with the other "approachable" prediction of GUTs, proton decay.

Monopoles would exist as a side effect of the "freezing out" of the conditions of the early universe, or symmetry breaking. In this model the monopoles arise due to the vacuum configuration in a particular area of the universe. The length scale over which this special vacuum configuration exists is called the correlation length of the system. A correlation length cannot be larger than causality would allow, therefore the correlation length for making magnetic monopoles must be at least as big as the horizon size determined by the metric of the expanding universe. According to that logic, there should be at least one magnetic monopole per horizon volume as it was when the symmetry breaking took place.

This leads to a direct prediction of the amount of monopoles in the universe today, which is about 10¹¹ times the critical density of our universe. The universe appears to be close to critical density, so monopoles should be fairly common.

A number of attempts have been made to detect magnetic monopoles. One of the simplest is to use a loop of superconducting wire that can look for even tiny magnetic sources, a so-called "superconducting quantum interference detector", or SQUID. Given the predicted density, loops the size of a soup would expect to see about one monopole event per year. Although there have been tantalizing events recorded, in particular the event recorded by Blas Cabrera on the night of February 14, 1982, there has never been reproducible evidence for the existence of magnetic monopoles. The lack of such events places a limit on the number monopoles of about 1 monopole per 10²⁹ nucleons.

Other experiments rely on the strong coupling of monopoles with photons, as is the case for any electrically charged particle as well. In experiments involving photon exchange in particle accelerators, monopoles should be produced in reasonable numbers, and detected due to their effect on the scattering of the photons. The probability of a particle being created in such experiments is related to their mass -- heavier particles are less likely to be created -- so by examining such experiments limits on the mass can be calculated. The most recent such experiments suggest that monopoles with masses below 600 GeV/c² do not exist, while upper limits on their mass due to the existance of the universe (which would have collapsed by now if they were too heavy) is about 10²⁶ eV.

Non-inflationary Big Bang cosmology suggests that monopoles should be plentiful, and the failure to find magnetic monopoles is one of the main problems that led to the creation of cosmic inflation theory. In inflation, the visible universe was much smaller in the period before inflation, and despite the very short time before inflation, it would have been small enough for the whole visible universe to have been within the horizon, and thus not requiring many monopoles, perhaps only one. At the moment, versions of inflation seem to be the most likely cosmological theories.

Interestingly, the other major prediction of GUTs, proton decay, has also not been observed. The absence of these two key pieces of evidence has generally led to a decline in work on GUTs, and the introduction of more "radical" proposals, such as superstrings.

• soliton
• dyon
• instanton
• magnetic photon
• meron
• Dirac monopole
• Wu-Yang monopole
• 't Hooft-Polyakov monopole
• Felix Ehrenhaft

• Magnetic Monopole Searches (lecture notes)
• Particle Data Group summary of magnetic monopole search
• "Evidence for Monopoles (?)"
• "A Regular Theory of Monopoles"