Melvin's "magnetic universe" revisited

Half a century ago Mael Melvin noted that magnetic forcelines do not contract and collapse under their own gravity, no matter how strong the latter may be. He claimed that instead magnetic fields showed an ability to stabilise themselves against gravitational self-contraction. Melvin then raised the question as to whether the magnetic presence could also support against the collapse of an external gravitational field. Here, we look into these issues from a different perspective and reach the same conclusions, namely that magnetic fields do not self-gravitate and also have a generic tendency to support against gravitational collapse. In the process, we also find that the resistance of the field grows stronger as the collapse progresses and that the responsible agent is the field's tension. We argue, in other words, that behind this remarkable magnetic behaviour lies the elasticity of the field lines, that is their tendency to remain "straight" and to react against anything that distorts them from equilibrium. In a sense, it appears as though the elastic properties of the magnetic field have been injected into the fabric of the host space, which seems to act like a spring under tension.