The Double-Slit Experiment: The Most Disturbing Result in Science

I want to describe an experiment carefully, because I think it's the kind of thing that sounds like a metaphor until you realize it isn't. It's a real experiment. It's been done thousands of times. The result is always the same, and the result is genuinely strange.

Take a barrier with two narrow slits in it. Fire light at it. On the screen behind, you don't get two bright lines, you get an interference pattern, alternating bands of light and dark. This is wave behavior. The light waves pass through both slits, interfere with each other, and produce the pattern. Fine. Light is a wave. That's been known since the 1800s.

Now do the same experiment with electrons. Electrons are particles, they have mass, they have charge, they leave individual marks on the screen. Fire them one at a time, so there's no possibility of one electron interfering with another. Each electron hits the screen, leaves a dot, and you move on. After thousands of electrons, you look at the pattern of dots. It's an interference pattern. The same alternating bands you got with light.

Each electron, fired alone, somehow "knows" about both slits. It behaves as if it went through both simultaneously, interfered with itself, and then landed according to the resulting probability distribution. This is not a metaphor. This is what happens.

Now add a detector at the slits, any device that tells you which slit the electron went through. The interference pattern disappears. You get two bands, exactly what you'd expect from particles going through one slit or the other. The act of measuring which path the electron took, destroys the interference. The electron, apparently, behaves differently when it's being watched.

Richard Feynman said this experiment contains the only mystery of quantum mechanics. Everything else, he argued, is a consequence of this one result. I think he was right. The experiment doesn't just challenge our intuitions, it challenges the idea that particles have definite properties before we measure them. It suggests that the question 'which slit did it go through?' doesn't have an answer until someone asks.

I've read a lot of attempts to explain this away, none of them fully work. The honest position, I think, is the one Feynman took: accept the result, use the math, and resist the temptation to pretend you understand what's happening at the level of individual events. We don't. Not yet. Maybe not ever.