What is quantum entanglement?
In: Physics
A particle with conserved quantity Q with a value of q decays into two particles A and B. Quantum mechanics would suggest that measuring Q for either A or B will give a random result over the range of possibilities. For example there are two possibilities q_1 and q_2 and measuring Q for either particle would give us q_1 or q_2 with 50% probability.
Classical physics tells us that a conservation law for Q must be satisfied for example q = q_1 + q_2. This seems impossible given particle A has nothing to do with particle B. As it turns out both predictions are correct and that last assumption is wrong. Particle A has everything to do with particle B to the point where treating the system as two individual particles is pointless, it’s one two-particle system.
So measuring either particle will yield q_1 or q_2 with probabilities given by quantum mechanics but you “affect” the two-particle system with your measurement of Q and so if A ended up on q_1 then B takes q_2 and thus the conservation law remains satisfied. And we call the effect entanglement.
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