A big problem with physics is that it’s invented by humans. We see stuff around us and use our observations to build a mental model of the world. Children do this, and of course physicists keep right on doing it. In many cases, this is awesome. It's how we got Newton's laws of motion. But it goes sideways when we try to picture the behavior of tiny, invisible things like subatomic particles.

This is the ‘quantum realm,’ as they call it in Ant-Man. It's not really a place you can visit in a special suit; you're already in it, and it's in you. But quantum phenomena are so alien that this deep layer of reality may as well be an alternate universe. The physicist Richard Feynman once famously said, ‘Nobody really understands quantum mechanics.’ Feynman didn't mean we can't explain quantum behavior. Today we have models that predict quantum outcomes with incredible accuracy. What he meant is that it's impossible to ever wrap our macro-scale arms around it. The stuff I'm about to tell you is true, and it makes no sense whatsoever. You just have to accept the absurdity and proceed from there.

To see this in action, let's revisit one of the most famous experiments in science: the double-slit experiment. Thomas Young first used this setup in 1801 to show that light travels in waves. Then, in 1961, Claus Jönsson built a minuscule version to show that subatomic particles with mass also behave like waves. Think about that. In our world of big things, if you toss a marble into a pond, it causes ripples, yeah? Well, in the quantum realm, it's like physical matter is both a marble and a ripple. And I'm afraid it only gets more preposterous from there.

The Double-Slit Experiment

Let's start in the world of big things. Imagine a wall with two narrow, vertical windows, and we have a machine that shoots tennis balls. Some hit the wall, some pass through on the left, some pass through on the right. In back there's a second wall covered with Velcro, so when a ball hits this back wall, it sticks. What does that look like? As you’d expect, the balls end up in two clumps, corresponding to the two inlets. But what if we run the same experiment with teeny-tiny slits and a machine that shoots electrons? Well, we tend to picture electrons as little balls orbiting an atomic nucleus, so by analogy you might expect a similar result on the screen. Nope. Instead of two clumps of electron hits, we get multiple bands. This is very similar to what you see when light passes through two slits, as Thomas Young discovered. Since light is a wave, it does two things: first, there is diffraction. As the light passes through each opening, it spreads out like an ocean wave passing through a gap in a sea wall. Second, the waves from the two slits overlap and create an interference pattern. Where the waves are in phase, you get a bright spot. Where they're out of phase, you get a dark spot. Here's what that looks like in real life with a red laser as the light source.