What exactly does it mean to say particles are "entangled"? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Let me give you a (necessarily imperfect, as I’ll explain) classical analogy.

Suppose you travel abroad and you pack your luggage in haste. Upon arrival, you open your suitcase and find half a pair of your favorite brown socks. You know immediately that if someone were to open your sock drawer at home, they’d find the other half. There… the two states are entangled.

Or imagine, instead, if your socks could be either brown or blue, and the color is not determined until you look. So when you check your suitcase in the dark, you find half the pair… and by looking at it in daylight or lamplight, you determine that it’s brown. From this you conclude that back at home, someone not only will find the remaining half in your drawer, but they, too, will observe its color as brown.

Notice that nothing changed in the drawer at home. Your discovery of the mismatched sock in your suitcase, your turning on the light and observing the sock’s color did not magically cause any changes thousands of miles away.

As I said, it is an imperfect analogy. Classical objects, like socks, follow classical statistics. In this case, the color of the sock is never in question; both socks “carry” their color with them, whereas quantum objects carry no such “hidden variables”.

But the principle remains the same. Particles (or socks) are entangled because at some point or another, they interacted (socks being part of a matching pair counts as an “interaction”), and they obey some conservation law (in the case of socks, the law is that socks come in color-matched pairs.)

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