Heisenberg's uncertainty principle
Heisenberg had been looking at changes that happen in an atom when an electron changes its energy level and so gets closer to the center of its atom or gets farther from its center, and, especially, situations in which an electron falls to a lower energy state in two steps. Max Born explained how he took Heisenberg's strange "recipe" for finding the product, C, of some change in an atom from energy level n to energy level n-b, which involved taking the sum of multiplying one change in something called A (which could be, for instance, the frequency of some photon) produced by a change of energy of an electron in the atom between energy state n and energy state n-a) by a succeeding change in something called B (which could be, for instance, the amplitude of a change) produced by another change in energy state from n-a to n-b):