There are certain coincidences which, we you first hear about them, you sit up all night thinking wild and crazy thoughts. I think my favorite example of this comes from the Kerr-Newman black hole. The Kerr-Newman solution to the Einstein-Maxwell equations describes black holes with charge and angular momentum. What is strange about the Kerr-Newman black holes? Suppose we examine the gyromagnetic ratio for these objects. The gyromagnetic ratio is 2Mm/QJ where M is the mass of the black hole, m is the magnetic moment of the black hole, Q is the charge of the black hole and J is the angular momentum of the black hole. For a Kerr-Newman black hole (and for many other charged solutions in general relativity) the gyromagnetic ratio is exactly 2. Sound familiar? Well it should, because this is the value of the gyromagnetic ratio for a Dirac electron (there are some claims that this value of 2 is a triumph of relativistic quantum field theory, but it must be said that there are nonrelativistic arguments for a value of 2 as well.) Talk about a strange and wonderful coincidence. Or more than a coincidence? So now you can spend the rest of the day worrying about whether the electron is nothing more(!) than a charge spinning black hole!
The Quantum Cardinals