The double-slit experiment explained

The double-slit experiment explained

Many folks around the world, from all walks of life, have been bewildered and spooked by a famous physics experiment called the double-slit experiment, also known as Young’s interference experiment. In this article, I’m going to attempt to demystify much of the fog surrounding it.

I’m totally going to go into physics mode here, so realize that I am quite capable of normal laughing conversations, but this one will be more of an adventure into the conceptual jungles of quantum mechanics.

We figured out quantum mechanics when we noticed situations where the universe departs from classical predictions — meaning more or less what we see in the macroscopic world, and came up with concepts like “wave-particle duality” to describe what goes on. But this is totally backward: the universe is fundamentally quantum and we’re just too big to see quantum effects normally, so we’ve grafted these big-world concepts (particle, wave) onto something that is much stranger than either. This is why I love physics.

First, I’d like to caution against reading too much into the idioms that people use. When they say “the particle knows we’re watching,” what they mean is that “the act of measurement affects the process”. The words “knows” and “watching” make it seem like consciousness is somehow involved, and it’s not. (New age gurus officially hate me.)

One of the ways we know it’s not, is that it’s possible to cause the wavefunction to collapse even if we COULD have watched but didn’t. There are ways to set up the double-slit experiment to get the “which path” information that can cause it to collapse even if we don’t actually make a measurement!

Now, as far as waves and particles go, here is the really cool part. In quantum mechanics, the waves are not water, or electromagnetic fields, or sound — they are waves of pure probability. Waves of chance!

The next tidbit to know is that when two equal waves have their peaks and troughs lined up, they add to have a bigger wave twice as tall as the first two. And when they are opposing each other, so that one’s peak aligns with the other’s trough, they equal zero — a flat line. And if they are slightly out of phase with each other, funky shapes result from the adding of their values at each point. This is all called superposition.

Now, most explanations would have you believe that a particle is a tiny sphere. This isn’t really true. It turns out that when you add many waves of many frequencies in just the right pattern, you can make a bulge appear in one place in all of space, and the bulge abruptly tapers off towards zero in all directions. It looks like this: ——————

That is the particle. It’s a special case of sums of waves, where there is only one peak in one place. This peak is a peak of probability. It means we are overwhelmingly likely to find the particle there when we measure it… but it never quite dies to zero everywhere else. It is possible to find the particle wayyyy out at the edge of the universe where its tail is still nonzero. But it would take millions of lives of the universe to observe that once — so for practical purposes, it’s zero.

Now here’s the important part. Each particle is made up of many different waves of different frequencies and amplitudes, and each one carries a piece of the total energy. Now each of those waves doesn’t care what the others are doing. What matters to us is how we perceive their sum. When the localized bulge we think of as an electron reaches the double slit, each of its internal pure waves is a plane wave by itself, hitting a double slit.

Now, every kind of plane wave behaves the same when it hits a double slit; it causes a predictable interference pattern that is the same whether it’s water or whatever. So each of these internal waves does that.

But since they are waves of probability, if we happen to place something near one of the slits, the probability of the electron being there is rather high, and the moment that we register a particular location for the electron, all possibility of it being anywhere else goes to zero. This is why when we “watch” one of the slits, we don’t see an interference pattern — we’re catching it when it behaves like a particle, and that act destroys the wave.

But I still haven’t answered why watching makes it a particle! The answer is in the mechanism of watching. “Watching” means that we have a detector, made of many many atoms. These atoms are quantum systems that can only increase their energy by stair-step amounts. Anything below a certain threshold has no effect. So we make one that has its threshold at just the right energy for one electron to cause it to shoot up to an excited level, and then calm back down, emitting a photon or something, and we know we’ve absorbed an electron.

What has really happened is that we’ve encountered an event where the energy of all the different internal waves of the electron happened to coincide at precisely the spot where our detector is, and they all gave up that energy at once to the atom in the detector. The ONLY way for that to happen is if all those internal waves add to form that bulge ——————-, and only if that bulge happens to be right at our detector atom.

Now if we hadn’t put the detector there, that bulge would still have been there — but only for an instant, and then a million other configurations would have happened as the waves traveled towards the screen.

But by having our detector there, we captured the energy from all the waves at once when they happened to lock into their bulge formation (what we call a particle) — and then, since there was no more energy left in the waves, they did not continue. Then, as more electrons pass through, either the same thing happens, or it doesn’t — in which case the waves pass through the other slit. But since no waves are making it past the detector in the first slit, no interference can happen — and that’s why “watching” causes the single particle line on the screen.

So, if you followed all of that, you’re extremely patient. I would like to add that this is very much my own understanding of things, and there are physicists who have different opinions, as well as some who agree with me. I hope some of this was illuminating. I find physics to be an extremely fascinating subject, and I very much enjoy QM literature.