![]() Smiling with confident knowledge, I send off a beam of light telling you what you'll find.īut before the signal arrives, you look at your particle, dutifully measuring the quantum-mandated down-spin particle. Eager as always, I look at my particle - performing the all-important measurement - and find an up-spin particle. Let's say I keep one of the entangled pair of particles and send the other off to you. The resolution to Einstein's question comes via an excruciatingly careful examination of who knows what and when. Haven't we noticed this apparent contradiction? Living in a quantum world It does so even when we've sent the entangled particles as far apart as we possibly can.īut physicists still go around talking about how important the speed of light is and how nothing can violate that sacred limit. ![]() These tests have also shown that this "spooky action at a distance" (as Einstein called this mysterious quantum backchannel communication system that entangled particles seem to employ) does indeed happen in an instant. He argued that quantum mechanics fell short of being a full description of the subatomic world and that particles carried with them so-called hidden variables that enabled them to coordinate their states prior to being measured.īut sophisticated tests in the intervening decades have conclusively shown, time and again, that no such hidden variables exist. ![]() In a snappy 1935 paper written with Boris Podolsky and Nathan Rosen, he used a similar line of thinking to point out that the newfangled quantum theory wasn't consistent with itself - the ultimate slap in the face to any self-respecting theory of nature. To Einstein, the fault was obviously with quantum mechanics. We prepare our entangled quantum state, send our particles off on their merry ways and begin to make our measurement. In the second possible outcome, the spins are flipped. The other particle has spin pointing down. In the first outcome, one particle has a spin pointing up (if you don't know what " spin" means here, don't worry, that's the subject of another article and doesn't really matter for this example). We'll prepare our superspecial entangled quantum state so that there are two and only two possible outcomes, each with a perfect 50/50 chance of appearing when we make the measurement. Let's look at an extremely simple, but surprisingly realistic, example. Stumping AlbertĪt first, that sounds innocent enough and probably like something only academics would care about, but something funny pops up with this so-called "entangled" state of two particles. In certain circumstances, we can connect two particles in a quantum way, so that a single mathematical equation describes both sets of probabilities simultaneously. That's all well and good - if horrendously mind-bending - but the real fun begins when we get two particles to share a quantum state. They're neat mathematical equations that summarize all the probabilities of the particle property we want to probe. ![]() These fuzzy probabilities are known as quantum states. If it's something we're interested in measuring, chances are we don't know what we're going to get in advance. A lot of fuzziness also attaches to a particle's speed, angular momentum, spin and so on. And those fuzzy probabilities apply to more than just the position of a particle. ![]()
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