“Then this link endures—even if they are separated across the galaxy…”: Lawrence M. Krauss with a great, high-level explainer of the (odd, fascinating) state of entanglement within quantum physics:
To understand just how spooky entanglement really is, it helps to step back and think about what happens to sensible, “classical” objects when you separate them. (Classical objects are large enough, or interact strongly enough with their environments, for quantum-mechanical effects to wash out.) Imagine that I have a detonator and a bomb. If I separate them across the street from each other and activate the detonator, it can trigger the bomb only by sending a signal at the speed of light or slower. Only after the bomb receives the signal will it detonate.
Quantum theory, however, suggests that objects which have been carefully prepared together and placed into a combined quantum state can, even when separated across galaxy, remain “entangled,” as long as neither has any significant interactions with other objects to break the entanglement. If I perform a measurement on one of two entangled objects, the state of the other object will be instantaneously affected, no matter how far apart the two objects are.
It’s one thing to read this. It’s another to actually *think* about what these means.
There’s a catch, however. Quantum mechanics says that the actual spin direction of either electron is not determined in advance of the measurement; the only thing that’s for sure is that the spins are anti-aligned. Even stranger, until they have been measured, both electrons are actually spinning up and down at the same time. Their measured state is probabilistic: all that can be said is that there’s a fifty-fifty probability that, once one of the electrons is measured, it will be “fixed” in a state of spinning up or down. Because the two electrons are in a single quantum state—because they are entangled—the moment I measure the spin of one electron, I fix the direction of spin of the other electron. It’s as though, by flipping one coin, and coming up “heads,” I force another coin to come up “tails.”
As long as the two electrons remain entangled, then this link endures—even if they are separated across the galaxy. If I measure one electron in my lab, the second electron is affected by the measurement of the first electron with no time delay—instantaneously—even though a signal traveling at the speed of light would take millenia to cross the distance between them.