How Quantum Entanglement Works – For Dummies

5. December, 2022

So you’ve heard about this “quantum entanglement” stuff and how Einstein was apparently worried it might break the speed of light.

It doesn’t and he wasn’t.

Here is the simple version: Take a piece of paper. Rip it apart once. Check the pieces that you got. Maybe scribble something on it. Mix the two pieces behind your back. Give one of them to a friend without looking. Send the friend to the end of the universe. If he refuses, find a real friend. Wait a few billion years. Open your hand. In that instant, no matter how far away your friend is, you will know what piece he has in his hands.

The complicated version: Quantum is weird but some stuff is actually easy to understand. There are just a few ways to create entangled particles. All of them have something in common: The pieces must add up exactly to what you put in. This isn’t magic or some badly written Star *beep* episode. Imagine you put in a piece of paper (or a photon – a blip of light). You can’t have more paper (or more light) after splitting it. In the case of the photon: If you add energy, you get more light but no entanglement.

So the entangled photon pairs are always half of the original in terms of energy (which roughly translates to “half as bright”). And they always go in exactly opposite directions (see conservation of momentum). Things like that. Which means you know everything about the two particles except one thing: You don’t know which is which unless you look.

If you keep one of them around and sent the other away, and then at some later point look at what you kept, you know exactly and instantly what the other must look like, no matter how far away it is now.

But you can’t change the far particle anymore. Same as you can’t add text to the paper which your friend at the edge of the universe is holding. So you can’t use this to beam information.


Be grateful for having friends like that. A pity that you sent him away.

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