Some of the many different versions of string theory attempt to unify all four fundamental forces. The problem with any of these theories, indeed the problem with string theory itself, is that they make no predictions that scientists can test. As a consequence, we have no way of knowing if any version of string theory is even remotely correct. As a result, string theory is more a branch of mathematics than a branch of physics, since we have no way of knowing whether it describes a real or entirely fictional universe.