I've written two books about infinity, most recently the fun illustrated title Introducing Infinity , and it's a subject I enjoy writing and thinking about. But for physicists, infinity often means a problem. While we can conceive that the universe might be infinite, because we only ever deal with a part of it, when infinity rears its head in calculations, it usually means trouble. This most famously arises in quantum electrodynamics, the science of the interaction of light and matter on the quantum scale. The solution there has been renormalisation - in effect, putting in the real observed values of some quantities to make the infinities go away. And this works, but it's a bit uncomfortable. Elsewhere, such as at the moment of the big bang or in the heart of a black hole, the infinities are taken to mean that our current theories break down at that point and we need to find new ways to look at what's happening. However, there is another class of infinite entities t...