Quantum Entanglement Connects Particles Across Any Distance
Quantum entanglement is one of the delightfully bizarre phenomena that underpins quantum mechanics. The basic idea behind it is that two particles can be linked to each other—that is, affect each other's quantum states—over any distance, even if that distance is the diameter of the universe. What's more, the effect happens instantaneously, but without going faster than the speed of light. How? Just let us help you wrap your head around it.
Spooky Action At A Distance
What this means in practical terms (if anything about quantum mechanics can be considered practical, that is) is that if we know something about one particle, we know something about its entangled mate. But it's weirder than that. Say you have a pair of gloves. If you know that one glove is right-handed, you automatically know that the other is left-handed, even if it's billions of light years away. But with entangled particles, the act of measuring one particle actually changes the state of the other. It's as if both gloves were in a superposition of right- and left-handedness—that is, both right- and left-handed at the same time—and only when you observed one did it become right-handed, thereby making the other become left-handed that very instant. That is, even if you check both gloves at the same microsecond from opposite sides of the universe, one becomes right-handed and the other becomes left-handed. That's why Albert Einstein called quantum entanglement "spooky action at a distance."
But wouldn't this break the universal speed limit known as the speed of light? No, and here's why: the information isn't "sent" in an instant, the way you might send an email. The relationship between the particles already exists from the time the particles first interacted and became entangled. As Frank Wilczek writes in Quanta Magazine, "in all known cases the correlations between an [entangled] pair must be imprinted when its members are close together, though of course they can survive subsequent separation, as though they had memories." The particles are "sending" information without sending anything at all. If you think that sounds like a fantastic way to create a supercomputer, you're onto something—it's precisely why quantum computing has such a huge potential.
This gets much more complicated, as you might expect. For example, the concept of "complementarity" says, in essence, that particles have certain complementary properties that can't be known at the same time. One quantum physics example of complementary properties are position and momentum, but in our glove example, let's say it's handedness and color. A glove can be either right- or left-handed, and black or brown. Complementarity says that if you know one glove is right-handed, you can't know what color it is; and if you know what color it is, you can't know which hand it goes on.
Albert Einstein, Boris Podolsky, and Nathan Rosen discovered that something strange happens with the complementarity of entangled particles. If you measure both gloves for handedness, you'll find that one is right and one is left. Likewise, if you measure both for color, you may find that both are brown. But if you measure one for color, and the other for handedness, there's no match—a brown glove's mate is equally likely to be right or left handed.
When you put a third particle in the mix, things get even stranger. Let's say Mike likes brown, left-handed gloves and Steve likes black, right-handed gloves. If you take a whole lot of gloves, and measure two entangled gloves for color and one—also entangled—for handedness over and over and over, exactly none or two results are Mike's. But if you measure three entangled gloves for handedness, one or three results are Mike's. So are there an even or odd number of Mike's gloves? The question makes no sense, because in quantum physics, systems like that don't have definite properties—if they did, it wouldn't matter whether or not you measured the gloves. That's the mind-bending essence of quantum mechanics.