 She's fast enough for you old man. What's the cargo? Only particles, myself, de Broglie, are momentum, and no questions asked. What is it, some sort of local realism? Let's just say that we'd rather avoid any quantum entanglements. What I want to do here is tell you about this beautiful experiment and why it's so cool. But to do that, we're going to have to go through quantum mechanics. If you want to know more, please check out the links that I show here and in the description. So quantum mechanics, a branch of physics that covers very, very small stuff like electrons. It has a deserved reputation for being vastly unintuitive and weird. But quantum physicists will just do some math in their office on a whiteboard without seeing any data whatsoever, say the answers 5.19266 out to 20 decimal places. Then years later, when someone finally builds an apparatus to test that value, they find out that that number is exactly right every single time. So as much as any other science, we know that this stuff is true, even when it gets pretty bizarre. For example, there are special ways to generate pairs of quantum particles, like electrons, where the math says that the state of one can't be separated from the state of the other one. This phenomenon, called entanglement, is real and demonstrable. If you generate a pair of entangled particles and then measure certain properties about them, like their spin or momentum, you'll find that they're invariably opposites. If one is up, the other one is always, always down. You can create entangled light particles, or photons, pretty easily by shining laser light through a barium borate crystal, splitting each high-energy photon into two entangled lower-energy photons. Whatever polarization you measure one of them to have, which is just which direction its wave is oscillating in, the other one will be polarized at a right angle to it. So if photon A is going up and down, photon B will be going left and right. If photon A is at 30 degrees, photon B will be at 120 degrees. But how exactly are you measuring these things? When you're talking about bigger stuff, things like chairs or planets, you can get a fair amount of information with a tape measure or a telescope. But when you're talking about something the size of a photon, any information you want to get is going to cost you something. Like if you want to know where a chair is, you can flick on a lamp, let some light bounce off of it into your eyes and say, oh, it's over there. If you want to know where a specific photon is, you can turn on a lamp, let some light composed of other photons bounce off of it, like a bunch of soccer balls hitting another soccer ball, and then, hey, where did it go? This is an example of Heisenberg's uncertainty principle, another law of quantum mechanics that was discovered just by doing math and confirmed perfectly by experiment. The harder you measure certain things about particles, like where your photon is, the more you sort of shoot yourself in the foot to know other things about them, like how fast it's going after you punched it with light to find it. And weirdly enough, that principle of uncertainty manifests in a very physical way. If you don't measure quantum particles at all, just let them do their thing undisturbed, then you'll find that they behave as though they had a whole bunch of different values at the same time, like they were 50% 0.5, 25% 0.25, 25% 0.75. But if you do measure them in the middle of an experiment to try and figure out what value they actually are, all of a sudden, they stop behaving that way and instead become boring little billiard balls instead. Ugh, fine, I'm a 0.6. Can I go now? And this happens on a continuum, just as the uncertainty principle promises. The harder you measure them, the more they act like they were billiard balls. The more you leave them alone, the more they act like they had every single possible value they might have had if you had looked. That implies an important question. When we're not measuring them, when they're doing their thing, are these particles just switching between many discrete values, according to some hidden variable that we don't know about yet? So they just appear to hold all of these values simultaneously, like they're looking at a very tiny watch and saying, okay, I'm a 0.6, 3, 2, 1, now I'm a 0.27, and now I'm a 0.7. Or do they actually exist as probability functions, holding all those different values at the same time to varying degrees and only collapsing into a discrete number like 0.27 or 0.6 when we measure them? In a way, that's a question about the fundamental nature of the universe. Everything else in physics is deterministic. If you plug in the same values for the same variables, then you get the same answer every single time. But if the values of quantum particles aren't strictly determined by some variable, if they really are just a roll of the dice, that's a much different picture. That's a big deal, so much so that three physicists, Podolski, Rosen, and Einstein weighed in on it in 1935. They pose something now called the EPR paradox to prove that it must be the hidden variable thing, basically saying either relativity or probability functions. Pick one. According to Einstein's theory of relativity, the speed limit on information in the universe is the speed of light. If something happens on the moon, if someone waves high or the moon explodes suddenly for no reason, people on earth can't know anything about those events, can't see them or feel any effects whatsoever for 1.8 seconds, which is how long it takes light to get here from there. But let's say that you got an entangled pair of photons, and then moved them far enough apart so that, even at the speed of light, photon A can't know what B is doing for a half second. If you measured them both at more or less the same time, you'll always find that A is polarized at a right angle to B. But how do they know? There wasn't enough time for A to say, hey, I just got measured and I'm going up, down, you go left, right, go left, right! Einstein and his crew said that this proved that A and B must be switching between discrete values according to some hidden variable and just synchronize their watches ahead of time so they don't need to talk to each other. After all, if they were probability functions that only became real numbers when we measured them, then they'd have to be talking to each other faster than the speed of light, which relativity forbids. But there is another option here. Suppose, just suppose, that they can talk to each other faster than the speed of light. What if Einstein's theory of relativity applies to everything in the universe except entangled particles? Physicist John Stuart Bell published a brilliant empirical way to test this in the 60s and tests were first performed in the 70s. The test uses a pair of polarized photons and two detectors with polarizing filters in front of them. Filters which will allow through photons that are polarized in a specific direction will block all photons that are polarized at a right angle to that, with a linearly decreasing number of photons allowed through as you move from 0 to 90 degrees. If you happen to set the detectors up so they were both perfectly aligned, say both were up, down, then shine a pair of entangled photons through them, if one photon was going up, down, the other one would have to be going left, right, so you'd never see both photons at the same time. If instead you set one filter to be up, down, and the other one to be left, right, then you'd always see both photons at the same time. Easy. At the angles in between, Bell proved mathematically that if the particles were switching between discrete values in sync, according to some hidden variable, then you would have to see a straight line of correlation as you moved from filters that were perfectly aligned to filters that were 90 degrees offset from each other. If you actually run the experiment and plot the data, you don't get that line. You get this curve, which just happens to be exactly what the probability function theory predicts. If that's true, that would mean that the particles are only choosing a polarity when they get to the filter, which means that they are communicating faster than the speed of light. There are a couple of loopholes which would still allow us to see the probability function curve even if the photons are actually doing the value synchronization thing, and not, you know, violating relativity. First, maybe the detectors are somehow talking to each other and the photons. If everything gets synced up before we even start the test, then maybe we could get this result even with a hidden variable. So put the detectors far enough apart that they can't talk to each other. In this experiment, different buildings. Simple answer. Same result. Okay, um, maybe the detectors aren't being set randomly in the first place. It's possible that the random number generator that we're using to set arbitrary angles for the detectors is tapped into the same hidden variable that the photons are, and is only spitting out angles that will give us this curve instead of a nice straight line. That's quite a stretch, but we are talking about breaking one of the most robust and well respected laws of physics, so these physicists wanted to be damn sure. They ditched the random number generator, and instead they used the variations in light from stars 600 light years away to set the angles of the filters. No matter what bizarre quantum crap is happening here on Earth, this light is arriving from outside that system at the speed of light. So it's clean. We've closed all the loopholes. If any information is being transmitted here between photons or detectors or the stars, it has to be going faster than the speed of light. Sure enough, even controlling for a secret conspiracy of random number generators, the experiment still produces this curve. It seems that these photons exist at all of these different values at the same time, and can tell each other what value to be instantaneously when they get measured, in violation of relativity. Well, technically, there's still one more loophole. If the universe is super deterministic, that is, every single quantum interaction in its entire lifespan has been dictated since the moment of the Big Bang, just everything predestined since the beginning of time, then it's possible that the light from the stars knows that it's going to be used in this experiment and blah blah blah. But unfortunately, no physics department has the budget to buy another universe to use as a control. It's just not something that we can test. As far as physicists are concerned, the relativity-breaking communication between entangled particles is true. Even if it's really, really weird, is using the light from distant stars to control for a quantum entanglement experiment the coolest physics you've ever heard of? Please leave a comment below and let me know what you think. Thank you very much for watching. Don't forget to blah blah subscribe, blah share, and don't stop thunking.