Magnetic monopoles

We know a great deal about things that may not exist.

A student recently asked our tutor about magnetic monopoles, and whether they might be used as an energy source.  It seemed unlikely, but particle physics is not his specialty so he asked for time to do some research.

Electric monopoles are familiar, the particles with either a positive or negative charge that carry electricity.  In undergraduate Physics courses we learn the Maxwell equations linking electricity with magnetism.  They have a sort of symmetry that is pleasing to physicists, except that there are no magnetic monopoles: particles with only a north or south magnetism.  Indeed, there are none in the Standard Model of particle physics, a far more elaborate and sophisticated theory.  And none has been detected by any of the searches that have been performed over the years.

So why worry about them at all?  Well, it’s known that the Standard Model, while extremely successful, is incomplete.  Not only is gravity absent from it, there are parts that are put in “by hand.”  It’s sort of like having to add 2 to the end of a long calculation for no apparent reason.  So physicists have long been searching for ways to extend the Model.

How do you go beyond a successful theory?  One way is by analogy: the Standard Model was built out of previous versions using a mathematical technique called Group Theory, so let’s apply it again.  That gives supersymmetry or Grand Unified Theories (GUTs).  There are other ways.  Each of them makes certain predictions about what lies beyond the Standard Model.   Many of them call for magnetic monopoles.

In one popular version, magnetic monopoles can catalyze proton decay.  In the Standard Model protons are absolutely stable, and current measurements put a lower bound on their lifetimes of 10 to the thirty-something years (the universe is roughly 10 to the 10th years old).  But in some GUTs, a magnetic monopole can cause a proton to break down into certain other particles, in principle releasing energy.

So have we found an energy source to replace fossil fuel?  Well, in contrast with this rather remarkably specific prediction, the theories are vague on other points.  For instance, the mass of a monopole is unknown.  It must be above a certain figure, or we would have produced some in the great particle-colliders already.  It may be too high for us to make at all this way.  They might have been produced in the high-energy early universe, but then we have the problem of finding them.  And there can’t be lots of them around, or they’d change the magnetic fields we observe in most of the objects in the universe.

We conclude that magnetic monopoles are unlikely to become a source of energy.  A distinguished scientist we know, however, cautions against saying anything is impossible; nuclear power once seemed completely impractical.  Someone might yet come up with a clever way to find or make monopoles and employ their unusual properties.

If they exist at all.