Today we continue on our tour of quantum-mechanical weirdness. After our last installment on probability, many folks started laying down bets that the Heisenberg Uncertainly Principle would be the next stop in the land of weirdness. As the author of the series I, however, was certain they were wrong. In this series I'm trying to go from the most basic assumptions we bring to the world and show how reality, on the quantum level, defies those expectations. So today we stumble across one of the most basic and most weird of quantum conclusions: The Wave Particle Duality.
There is a thing in logic called the Law of Excluded Middle. Aristotle was the first to codify it and it states the obvious fact that a statement about the world should either be true or false.
End of story.
Here we are explicitly focusing on the state of physical or mathematical reality. Statements like "Well, I kind of love her but I am not sure" are not part of the discussion. Think pregnancy. As everyone knows, you can't be "kind of pregnant." The statement "She is pregnant" is either true or it is false.
End of story.
In physics, waves and particles are totally and entirely different kinds of beasts. A particle is a little chunk of mass (mass-energy in fact a la Einstein). Think of a billiard ball. It exists in one place at a time. Waves, on the other hand, exist in many places at once. Think of the ripples spreading out on a pond after a fish breaks the surface. The wave is the entire circular ripple.
Waves and particles behave very differently, as well. When two waves collide they pass through each other. During the moments that they overlap they create either a bigger wave or smaller wave, depending on the positions of their peaks and troughs. This property is called superposition.
Collide two particles, on the other hand, and they either smash apart or stick together. Particles never superpose because there is no way to get them to occupy the same space.
For classical physicists, particles and waves are like "pregnant" and "not pregnant." There is no in-between for these two very different kinds of physical "thing".
And then came quantum mechanics.
I won't run through the history of how we stumbled on this (it's fascinating). But the long and short of the story is this: in quantum mechanics everything is BOTH a particle and wave. Light can act as a wave or it can act as particle. Electrons can act as a wave or a particle. Protons can act as wave or a particle.
If you do an electron experiment that looks for "I am all spread out and superposing like a wave," you will observe that kind of behavior. But if you do an electron experiment that looks for "I am all bound up in one place and time like a particle," you will see that too. And here is the very weird part. You will never see both at the same time. Wave experiments give wave results. Particle experiments give particle results. Try to get clever figure out what the electron really is and nature will always thwart you.
You can forget about trying to imagine a wave-sickle or something like that too. Its been tried and leads to conclusions shot down by experiments. But that is the key conclusion of today's lesson in weirdness. Waves and particles are opposites, like being pregnant and not pregnant, like having a dead cat or having a live cat (more on that in the future). We simply can't imagine, can't draw a picture in our heads, what quantum mechanics is pointing us to in terms of a "picture" of reality. We don't have the mental machinery to create images in our heads that embrace the reality these experiments imply.
Thus we have a duality: both, but neither at the same time. Everything is made of particles and/or/but/also waves.