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.
Ooh boy. This is one of the most misunderstood concepts in all of physics and it’s very hard to eli5 but I’ll try my best. Quantum entanglement is when particles interact in such a way that their quantum states become linked in such a way that you can’t describe the particles individually. The result is that when you observe the state of one particle, you instantly know the state of the other, because they’re intertwined. For example, if you measure one particle to have spin up, you instantly know the other particle is spin down. This occurs no matter how far away the particles are. The state isn’t determined until you actually measure one of the particles. Once you measure the system, you break the entanglement.
Here’s the part that most people have trouble understanding, which is that you *cannot* use this to communicate faster than light because no information is being transferred. There’s no causal relationship, it’s merely a correlation. Also, it’s not a magical state that forces the two particles to always have opposite states. It only means that the next time you measure both particles, there will be a 100% chance that they are opposite. But if you change one of the particles, nothing happens to the other one. They just aren’t in a correlated state anymore.
Here’s an analogy I really like to sum it all up: Imagine that you know that a friend of yours only has 2 hats, and if he wears one, the other one is on his shelf in his home. You then meet your friend, and see which hat he wears, thus instantly telling you the position of the other hat. Has any FTL communication occurred? No, course not, the information that you gained “traveled” on top of your friends head at whatever speed he was moving at when he left his house to meet you, and then you combine it with a previously established fact (the correlation between the two hats). Entanglement is roughly the same as this, and really not all that much stranger.
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