The Martian Midnight Intercept

Why entanglement can't send faster-than-light signals

Mission 4: Entanglement. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Look sharp, old sport: the Echo Network has launched a rather unsporting cyber-missile at our Ares-7 satellite over Mars, and impact is an uncomfortable thirty seconds away. A classical radio call would crawl through the void for twelve minutes, so Craze Joe is thumping the table swearing we can beat Einstein outright and fire the abort code across space instantly, straight down a shared Bell pair. Bold. Let us put his grand theory on the bench before we wager a satellite on it. Alice holds her half of the pair up on Mars; Bob holds the other back on Earth. Try absolutely everything to make Alice's choice show up in Bob's readout: force her result to a 0 or a 1, switch her measuring basis, and fire off a few hundred pairs. Keep one eye on Bob's own tally the whole time. If entanglement truly carried a message, his 50/50 split would lean whichever way you pushed. It will not budge a hair. The correlation is genuinely there, but it only surfaces once you Phone Bob down an ordinary, light-speed line and compare notes. Nothing you do on Mars sends a single bit to Earth. Prove it to yourself, recruit, then we send the abort code the honest way.