#AtoZChallenge – Quantum Entanglement

Time for Q in my Sci-Fi themed A-to-Z Challenge!

As I said in my first A-to-Z post about ansibles, quantum entanglement is often used by sci-fi authors as a way for characters to communicate instantly across space. For Q, I decided I’d take a slightly closer look at quantum entanglement.

Put simply, quantum physics says that unobserved particles exist in all possible states at the same time. As soon as you try to observe and measure a particle, however, you find it in one specific state (its wave function collapses). These states are called “spins”, even though the particles don’t actually rotate. Yeah, I know – quantum physics is weird. I can understand why Einstein wasn’t a fan.

Now, you can have particles interact, like photons fired through crystals and split into pairs of photons. These pairs are entangled – which leads to something very weird indeed. If you now measure one of those paired photons, it will “collapse” into one state, while its partner takes on a state relative to it. So if one particle is observed in a “spin-up” state, the other becomes “spin-down”. This happens instantly, no matter how much distance there is between the entangled pair.

So why can’t you use entanglement for FTL communication?

Short answer: because it’s random. Let’s say I give you one entangled particle and take the other with me to Mars. I measure my particle when I get there, collapsing its wave function. My particle takes on a random state, and yours takes on a relative state, and that happens instantly. But it just looks like some random value to you! All I’ve managed to do is send you random information instantly.

I suppose it could let you know that I arrived on Mars and tried to call you – or that I slipped up and observed my particle too soon. You won’t know until you call me up by conventional means!


More about quantum entanglement: