Entanglement is perhaps one of the most confusing aspects of quantum mechanics. On its surface, entanglement allows particles to communicate over vast distances instantly, apparently violating the speed of light. But while entangled particles are connected, they don’t necessarily share information between them.
In quantum mechanics, a particle isn’t really a particle. Instead of being a hard, solid, precise point, a particle is really a cloud of fuzzy probabilities, with those probabilities describing where we might find the particle when we go to actually look for it. But until we actually perform a measurement, we can’t exactly know everything we’d like to know about the particle.
These fuzzy probabilities are known as quantum states. In certain circumstances, we can connect two particles in a quantum way, so that a single mathematical equation describes both sets of probabilities simultaneously. When this happens, we say that the particles are entangled.
When particles share a quantum state, then measuring the properties of one can grant us automatic knowledge of the state of the other. For example, let’s look at the case of quantum spin, a property of subatomic particles. For particles like electrons, the spin can be in one of two states, either up or down. Once we entangle two electrons, their spins are correlated. We can prepare the entanglement in a certain way so that the spins are always opposite of each other.
If we measure the first particle, we might randomly find the spin pointing up. What does this tell us about the second particle? Since we carefully arranged our entangled quantum state, we now know with 100% absolute certainty that the second particle must be pointing down. Its quantum state was entangled with the first particle, and as soon as one revelation is made, both revelations are made.
But what if the second particle was on the other side of the room? Or across the galaxy? According to quantum theory, as soon as one “choice” is made, the partner particle instantly “knows” what spin to be. It appears that communication can be achieved faster than light.
The resolution to this apparent paradox comes from scrutinizing what is happening when – and more importantly, who knows what when.
Let’s say I’m the one making the measurement of particle A, while you are the one responsible for particle B. Once I make my measurement, I know for sure what spin your particle should have. But you don’t! You only get to know once you make your own measurement, or after I tell you. But in either case nothing is transmitted faster than light. Either you make your own local measurement, or you wait for my signal.
While the two particles are connected, nobody gets to know anything in advance. I know what your particle is doing, but I only get to inform you at speed slower than light – or you just figure it out for yourself.
So while the process of entanglement happens instantaneously, the revelation of it does not. We have to use good old-fashioned no-faster-than-light communication methods to piece together the correlations that quantum entanglement demand.