For physicists like James Clerk Maxwell or Albert Einstein, the search for mathematical truths led to new understandings of the ultimate nature of reality. But what are these truths? Taking a tour of nine of the most extraordinary numbers in physics, Antonio Padilla explores Graham's number, which could collapse your head into a singularity; TREE(3), whose finite nature can never be definitively proved; and the number measuring the balance of energy needed to allow the universe to exist for more than just a moment.