Magnetic monopole
In physics, a magnetic monopole is the hypothetical construct of a magnet with only one pole. In other words, it would have net magnetic charge. But magnetic monopoles have never been observed or created experimentally. When a magnet with a north and south pole is cut in half, it becomes two magnets, each with its own north and south poles. There doesn't seem to be a way to create a magnet with only one pole. Yet particle theories like Grand Unified Theories and superstring theory predict the existance magnetic monopoles.
Background
Magnets exert forces on one another; similarly to electric charges, like poles will repel each other and unlike poles will attract. Experimentally, magnetic poles have so far been found only in inseparable north-south pairs. (If a magnet is cut in half, the result is not a separate north pole and south pole; it is two smaller magnets, each with its own north and south poles.)
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.
Origin of the 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 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 the 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.
The magnetic monopole problem is one of the main problems that led to the creation of 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.
Attempted proof
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
This is curious, because the existence of magnetic monopoles seems to be very natural and elegant in a number of popular theories. For example, the existence of a magnetic monopole automatically leads to the conclusion that both electric and magnetic charge must be quantized (a result due to Paul Dirac and related to the Aharonov-Bohm effect).
If such monopoles could actually exist, they would cause an unprecedented revolution in electrical engineering. For instance, if one could replace the iron core of a transformer with an identical core that would be made of a substance containing free magnetic monopoles, in other words, of a 'magnetic conductor', this transformer could work as well on direct current as on alternating current, perhaps better on DC, judging from the latest theories claiming that magnetic monopoles are much heavier than electrically charged elementary particles.
Efforts have been made to explain why we don't see them. Standard Big Bang cosmology suggests that monopoles should be plentiful -- this is one of the problems which inflation theory claims to correct.
See also
External links
- A lecture on the search for magentic monopoles
- Some tantalizing events
- A review of monopole search experiments (presented as a table in Adobe pdf format)