For years I have been accepting quantum entanglement because the experiments show it works. I have never quite understood how it works until just now. I am reading "The Matchbox that ate a Forty-Ton Truck" by Marcus Chown. In it he finally makes a few statements that clarify the reason for it.
The reason that when you detect a quantum state of one particle the quantum state of an entangled particle is immediately determined is because of conservation laws. Take the case of photons, which have been the most studied particles. Photons of course have a property called spin. Photons naturally have a spin of 1. In measuring spin you will measure any given photon as having a spin of 1 or -1. The difference is the direction the photon is pointing, up or down. Spin is a conserved quantity. That is the total spin of a system will always be the same, no matter what happens to the components of the system. So, if an event creates 2 photons then the 2 photons will have a spin of 0, since the 2 photon system had 0 spin before it was created. This zero spin will be the sum of the spins of the 2 photons, -1 + 1 = 0. But the 2 photons exist as wave functions, and the spin of either one of them will be unknown until it is measured, at which time it will randomly take a spin of either 1 or -1. But since spin is conserved the other one will immediately take the opposite spin. And that will happen no matter how far the 2 photons are separate.
Hooray, at last I have a much clearer idea of just what quantum entanglement is all about.
Bill Gill
