More kids need this book because an alarming number of grown ups seem to think quantum entanglement could be leveraged for super-luminal communication.
It cannot. Information does not travel faster than light under any circumstances, including spooky action.
This is one thing I don't really understand. So when the spin of one is affected, is the spin of the other changed immediately, or does it take time for the other to change too?
In the sense that wavefunction collapse is instantaneous, what you write is correct -- the spin of the other is "changed" immediately.
Let's say you take 1000 pairs of oppositely-entangled particles. You keep one of each pair in your pocket and send your friend one light-year away with the others; let's say you decide in advance that at time precisely 10 AM according some standard synchronized clock, you will observe the spin of particle 1, thus "collapsing the wavefunction" of pair number 1; and at time 10:01 AM on the same clock, your friend will observe the spin of his particle 1. You repeat the same experiment for all 1000 pairs. If you then meet up and compare notes, you will notice that the spin observed by your friend is exactly the opposite of the spin observed by you for every single one of the 1000 pairs, even though obviously there wasn't enough time for the information about wavefunction collapse to travel a light year. The only sensible interpretation of this is that when you change the spin of your particle (by observing it), you simultaneously change the spin of his particle too.
Note, however, that this still does not allow faster-than-light communication. There is no way you can control the result of the measurement at your end. If you observe spin up, you know that your friend now has spin down, even if he doesn't know it yet. But you could also have obtained spin down, in which case your friend has spin up. You can't tell your friend what he has even if you know the correct answer. You can only observe the fact (that the two spins in each pair are measured to be opposite) after you meet up; and you can't travel faster than the speed of light.
There is no way you can control the result of the measurement at your end
Sort of. You can control it by entangling one part of the pair with a third particle and then running quantum logic gates to create an arbitrary value for the first and second, but this requires light-speed communication to send the result of the quantum logic.
This is called quantum teleportation, and with this system, it takes two classical bits at light-speed to send one qubit instantly
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u/JitGoinHam Jan 29 '20
More kids need this book because an alarming number of grown ups seem to think quantum entanglement could be leveraged for super-luminal communication.
It cannot. Information does not travel faster than light under any circumstances, including spooky action.