X P – P X = i ħ That’s it. The letter X indicates the position of a particle, the letter P indicates its speed multiplied by its mass (what we call “momentum”). The letter i is the mathematical symbol of the square root of –1 and, as we have seen, ħ is Planck’s constant divided by 2π. In a sense, Heisenberg and company have added to physics only this simple equation: everything else follows from it—from the quantum computer to the atomic bomb.
