Entanglement seems to be a hot topic in popular physics. So I thought I’d take a stab at explaining it in a layman friendly manner. Before I do that, though, I’m going to have to give you a brief intro into the nature of quantum mechanics.
-A Crash Course in Quantum-
Okay. Firstly, quantum mechanics tends to deal with very very very small things, like electrons. So from now on, I’ll talk about electrons like they are the only focus of quantum mechanics, however, all subatomic phenomenon must be handled with quantum mechanics. Electrons have a property called spin, for now, you can imagine that it is like the electron… well… spinning. If I say that an electron is “spin up”, then it means if you look at it from the top, you will see it spinning counterclockwise. “Spin down” means it is spinning in the other direction if you are looking at it from the top. I’ll also say things like “spin right” and “spin left”, which mean similar things to spin up and down except looking at the electron from a different direction.
So, how does spin work? Well, let’s say I have a beam of electrons and I want to know which electrons are spin up, and which are spin down. I’d send all these electrons through a machine and some would come out spin up, some spin down. If I took only the spin up ones, and sent them through another machine that asks the same question (are you up or down?) it would tell me that all of them are up. No surprises here. What if I instead took all of those spin up electrons and asked them, are you right or are you left? The machine would tell me that half of them would be right, and the other half would be left. Strange.
Now something even stranger. If I took the left electrons I’d just filtered out, and asked them, “are you up or are you down?”, maybe you’d think that since I was using only up electrons before, that they would all be up. No! Again half of them would be up, and half would be down! It’s like they forgot that they were spin up to begin with!
An important bit of insight is that I didn’t say which of these electrons in the beam were up, down, left or right. I only said what a certain percentage of these electrons would be. In fact, in quantum mechanics, one can NEVER predict whether a specific electron will be up or down, only a probability that it will be up or down. And it’s not that we don’t have enough information. The very nature of subatomic particles is that of probability until you make a measurement.
So what does this have to do with entanglement? When we entangle two electrons we process them in such a way that their spins must cancel out. So, if one is up, the other must be down. If one is right, the other must be left. This is called conservation of angular momentum, but the name is not important, just the idea. But remember, as I said before, you can never know whether one specific electron is up, down, left or right… only the probability that it will be. Here’s the weird part. If two electrons are entangled and one is sent hundreds of miles away from the other, conservation of angular momentum must still be true and their spins must cancel out. If we took one, and asked it, “are you up or down?”, we might find that it happens to be up at the moment. At that very instant we know with absolute certainty that the other must be down! And we didn’t have to measure the other to figure this out.
Now, some people may be quite skeptical at this apparently amazing phenomenon and might reason this way: “If it’s all probability, isn’t it like flipping a coin and discovering that the heads side is pointing up and then being amazed that the tails side of the coin is pointed downwards? Isn’t this obvious?”. Simply put: no. The coin has two distinct sides that will always be heads and tails. Electrons, before they are asked to reveal their spin direction, don’t have a specific spin. By mathematical and physical requirement, their spins must be considered as only a set of probabilities for being up, down, left or right.
Einstein didn’t like this idea at all. It looks like once the first electron’s spin is measured, that information about the first electron’s spin is transfered instantaneously to the second one, even if they are lightyears away! This would violate Einstein’s principal of relativity that says nothing (no information) can travel faster than light. This is known as the EPR paradox (stands for Einstein, Podolsky, and Rosen). One resolution to this “paradox” is that no information is actually transfered. We can’t control whether the first electron is up or down, left or right. Since we can’t control the first electron’s spin, we can’t transfer any information.
So hopefully, I have explained this well enough to make you sufficiently confused about nature and reality. A great physicist, Richard Feynman liked to say, “If you think you understand quantum mechanics, you don’t understand quantum mechanics!“ I’m glad to say, that with my education in physics, I am becoming progressively more and more confused about the strangeness of the very very very small.
