Disclaimer: As always, keep in mind that I’m not a scientist, I’m a layman. The following is what I understand to be the current scientific understanding, but if you are a scientist and you see something wrong, please correct me.

The Dual Slit Experiment is a well known experiment that demonstrates the so-called “wave-particle duality” of light. It’s also cited by new-agers and woo-peddlers as proof that thinking hard enough can change reality. But we’ll get in to that later.

This video starts off by explaining it pretty well, but by the end it sort of goes in to cookoo-land. I’ll explain it a bit more after the video.

The first thing wrong with this video is when it says that particles are like little marbles. They’re not. It’s a little awkwardly phrased to say that electrons sometimes act like particles and sometimes act like waves. That’s sort of like saying lions sometimes act like cats but sometimes they hunt for food.But lions are cats and cats hunt for food. Similarly, electrons are particles, and that’s what all particles do. They don’t sometimes act one way and sometimes act another way, they always act like particles. We are just very bad at describing how they behave in plain English so we say they act like marbles sometimes and like waves other times.

Particles are excitations in a field that, yes, can be described as waves or ripples while they are moving and can be described like a single point when they interact with things, but they don’t change their minds on how to act because someone is watching them, they always act how particles act, very strangely. That’s the other part of the video that bugged me; they made it seem like the act of watching, with our eyes, changes the outcome of the experiment. But that’s not true. An “observation” in the world of physics doesn’t mean someone was watching or videotaping, it means they were taking measurements. But taking a measurement changes the outcome of the experiment because that single electron must be interacted with to be measured. That’s why the results changed when scientists “observed.” It’s not because of magic or woo or whatever Deepak Chopra would have you believe.

If anything, it’s an awesome demonstration of the Heisenberg Uncertainty Principle, which says that, on the quantum mechanical level, you can’t know both the current position, and the current momentum of a particle to an arbitrary degree of certainty. What this means is, the more accurately you know the position of the particle at a moment in time, the less you can know about how fast and in which direction it was traveling. And the more accurately you measure how fast it is traveling and where it is going, the less you can know about where exactly it is right now. A good way I’ve seen it explained is like this: If you take a camera and set the shutter speed to be really fast, and you take a picture of a game of pool in progress, the balls will all be very clear and visible because the shutter was so fast. But, from that picture you can’t tell which balls were moving and how fast because they all look like they’re sitting still. Now if you take another picture with a slower shutter speed, the balls will look like streaks. Now you can better see how fast they were moving and in which direction, but you can’t really say exactly where they were when the picture was taken because they’re blurry. But it gets even more difficult. Pretend the lights are off and the only way to detect where the balls are is to put your hand on the table and wait for a ball to hit you. You know where the ball was at the moment it hit you, it was touching your hand, but by measuring where it was with this method, you’ve changed it’s momentum. That is how “observation” works in this experiment. We can’t sit and watch electrons traveling to the detector, we have to use other detectors to figure our where they are and that obviously changes the way they behave. It has nothing to do with consciousnesses and the particle never decides to act differently. It just acts the way a particle acts, which can be very strange to laypeople like us, but it’s not magic.