This is part 2 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience. Part 1 can be read here.
So, by the mid 1920s, physicists had made significant progress in developing the new quantum theory. It had been shown that light and matter each possess a dual nature as waves and particles, and Schrödinger had derived a mathematical equation that accurately described how the wave part of matter evolves in space and time.
But it was not clear what, exactly, was doing the “waving” in a matter wave. Water waves arise from the oscillation (waving) of water, sound waves arise from the oscillation of molecules in the air, but what is oscillating in a matter wave? Or, to put it another way, what does such a wave represent?
We will try and answer this question by looking at how a matter wave manifests in an actual experiment. It turns out that the best example for demonstrating the wave properties of matter is also the best example for demonstrating the wave properties of light: Young’s classic double slit experiment! However, the double slit experiment was not done with electrons until decades after the foundation of quantum mechanics, so we must briefly step away from our historical discussion to investigate it.