What is this effect good for? · ← Part 1
To send secrets, you both need the same password. But how do you agree on it without anyone listening?
Heads = 0, tails = 1. Your friend sees the opposite and just flips it. Done: the same password on both sides, and it never traveled through any wire.
When the spy looks, the coins lose their magic and turn into ordinary coins "with a little note". Then Bell's game can't get past the wall anymore, and you catch the spy.
With a twin pair and a phone call, you can pass the "state" of one particle to another one far away. It's not like in the movies: no people, and it needs the phone call, so it's never faster than light.
In 2017, the Chinese satellite Micius sent twin pairs to two cities more than 1,200 km apart. The effect was still there.
From Earth: you measured first.
According to Einstein, "at the same time" depends on who is looking. The order flips, and the results still match. So no coin "told" the other: the two are one single thing, in two places.
More than 10,000 times the speed of light (Geneva, 2008) and more than 13,000 (China, 2013). They never found any delay. But remember: no real message ever gets through this way. 📵
Want to see the real numbers? Open the Bell Lab →