I’m asking this because there is a scifi book I’m reading, and in the book there’s a scene where someone is communicating with a person in a spacecraft moving at lightspeed. I know their ability to communicate would probably not be possible, but let’s just put that aside for a second. Hypothetically, if you could communicate with someone moving lightspeed, would the time dilation make it so that they would appear to be moving and speaking very slowly relative to you?
I want to push back on this - quantum entanglement cannot be used to transmit information faster than the speed of light. The entanglement effect does allow for instantaneous correlations, but you can’t use those correlations to transmit information on their own.
For example, imagine Alice and Bob have a pair of entangled particles. The spins are anti-correlated, so if one is spin up the other is spin down. Alice measures the spin of her particle and sees its spin up. She now knows that Bob’s is spin down. She can learn this even if he is many light years away, without needing to communicate with him.
But what information was sent? If Alice or Bob wanted to communicate a simple “yes or no” to each other, how does knowing the spin of each others particles help?
What could happen is Alice and Bob could agree that spin up is yes, down is no. And Alice could measure her particles spin, then call Bob and say “my answer is the spin of your particle” to communicate “no”. This would be physically secure encryption of her answer which is a big advantage, but it is still communicated at classical speeds.
Actually methods of quantum cryptography are more complicated and involve measuring many entangled particles to prove that no one is intercepting the information.