"God is a pure mathematician!" declared British astronomer Sir James Jeans. The physical Universe does seem to be organised around elegant mathematical relationships. And one number above all others has exercised an enduring fascination for physicists: 137.03599913. Let me explain. When scientists measure any quantity they must specify the units being used. The speed of light, for example, is either 186,000 or 300,000 depending on whether it is expressed as miles per second or kilometres per second. Likewise your weight might be 150 or 68 according to whether you are measuring in pounds or kilograms. Without knowing the units being...