Seeing Further: The Story of Science and the Royal Society

by Bill Bryson

Consider, for example, the Pauli exclusion principle, which states (for example) that in a helium atom with two electrons in the same orbital, the two must have opposite spins. It is not possible for both of them to possess exactly the same state. Each of the two electrons somehow ‘knows’ the direction of the other’s spin and ‘obeys’ the rule that its spin must be different. The Pauli exclusion principle is Leibniz’s identity of indiscernibles principle translated directly into physics. Moreover, the ability of an electron to ‘know’ the state of another electron, without any physical explanation as to how this information is transmitted and stored, is strongly reminiscent of Monadology. Elementary descriptions of quantum mechanics tend to limit themselves to extremely simple systems, such as individual particles or atoms, since beyond there the mathematics becomes intractable. But the same principles apply, albeit in vastly more complex form, in larger systems: the quantum state of each particle is dependent upon the states of all the other particles in the system.