Magnetic monopole

In physics, magnetic monopole is a term describing a hypothetical particle that could be quickly clarified to a person familiar with magnets but not electromagnetic theory as "a magnet with only one pole". In more accurate terms, it would have net "magnetic charge". Interest in the concept stems from particle theories like Grand Unified Theories and superstring theories that predict either the existence or the possibility of magnetic monopoles.

Table of contents

1 Background

2 Attempts to find monopoles

3 Theory

4 See also

5 External links

Background

Magnets exert forces on one another; similarly to electric charges, like poles will repel each other and unlike poles will attract. When a magnet, i.e., 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. 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.

A hypothetical isolated magnetic pole is called a magnetic monopole; it has been theorized that such things might exist in the form of tiny particles similar to electrons or protons, forming from topological defects in a similar manner to cosmic strings, but no such particles have ever been found.

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. So, classically, the question is "Why does the magnetic charge always seem to be zero?" 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.

Attempts to find monopoles

A number of attempts have been made to detect magnetic monopoles, ranging from simple experiments with large coils of wire attempting to catch passing monopoles to experiments involving the analysis of collisions in particle accelerators. Although there have been tantalizing events recorded, none of these experiments have produced reproducible evidence for the existence of magnetic monopoles.

There are a number of possible explanations for these results:

monopoles do not exist, or
that they are very rare, or
that they have some property that prevents them from being detected by our current experimental designs, or
that the experiments are flawed.

Theory

In particle theory, a magnetic monopole arises from a topological glitch in the vacuum configuration of gauge fields in a Grand Unified Theory or other gauge unification scenario. 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 creates a problem, because it predicts that the monopole density today should be about 10¹¹ times the critical density of our Universe, according to the Big Bang model. But so far, physicists have been unable to find even one. Also, the Universe appears to be close to its critical density - for all matter combined.

Standard 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. At the moment, versions of inflation seem to be the most likely cosmological theories.

The idea of magnetic monopoles existing is an appealing one, in light of the very natural and elegant way they would fit into a number of theories that physicists find promising. For example, Paul Dirac's conclusion (related to the Aharonov-Bohm effect) that the existence of a magnetic monopole implies that both electric and magnetic charge are quantized is unquestioned.