How to make sense of quantum entanglement

00:00:02
Speaker 1: Hey everyone, it's Daniel with a quick note about my new book, which is out now, Do Aliens Speak Physics. It's all about how easy or hard it might be to talk to arriving aliens about physics. I think it's right up your alley, but you don't have to trust me. Here's what Shamus Blackly, creator of the Xbox, said about it quote. You should buy this book if you don't buy it for the charming, smart Vundy prose. Buy it for the lovely and poignant illustrations. Buy it for the wonderful, imaginative daydreams about how we might meet aliens and what they'd be like. Buy it because they will gently accidentally educate you by the history of language, translation, decryption, and the cultural impact of the writing and transmitting ideas. Not enough, buy it for the thoughtful, clear, and genuinely entertaining grand tour of the fundamentals of math, physics, and the universality of consciousness. You won't go wrong, So please consider picking up a copy of Do Aliens Speak Physics. The goal of science is to make sense of the world, to unravel the laws that controls it and translate them into something that makes sense to us. But there's no guarantee that the universe runs on rules that we can understand. What if our intuition, built in a slow and large environment doesn't equip us with ways of understanding that can be mapped to the quantum world. One of the trickiest elements of quantum mechanics is its strange randomness. We like to think that the universe follows rules that determine what happens, that at any moment there is a true story of reality. But quantum mechanics says no, there are only probabilities until you look and that a random one is selected. And if you have two quantum objects whose fates are intertwined, who have to coordinate their outcomes to follow some rule, then those outcomes are somehow determined together, even across back distances of space instantaneously. How does that make sense? Today, we're going to do a deep dive into quantum entanglement and try to untangle some misconceptions. Welcome to Daniel and Kelly's Extraordinary quantum Universe.

00:02:28
Speaker 2: Hello, I'm Kelly leader Smith. I study parasites and space, and I'm excited that I'm going to have all of my confusion about entangled particles cleared up today, I will be an expert.

00:02:39
Speaker 1: Hi. I'm Daniel. I'm a particle physicist, and often I feel like a superposition of many Daniels.

00:02:46
Speaker 2: Okay, all right, psychologist Kelly, I'm putting on my psychology hat. What does that mean, Daniel?

00:02:53
Speaker 1: Well, you know, the simplest way to think about it is, we have so many roles in our lives, right. You know, you're a spouse, you're a parent, you're a scientist, you're an author, you're a citizen. Sometimes we feel like such different roles with conflicting needs. It almost feels like you're a different person in each context. Didn't you get that way?

00:03:11
Speaker 3: Yeah?

00:03:11
Speaker 2: Yeah, you forgot goat herder, but goose lord and goose lord, Lady of the goose, the geese, lady of the geese. Yeah, we just named our geese Jacques and Francine Gusto. We're very excited about that.

00:03:27
Speaker 1: And some of this is reflected in my CV, and so I have to put you on the spot, Kelly, did you do your homework?

00:03:32
Speaker 4: Did?

00:03:32
Speaker 2: I didn't look at your CV? No?

00:03:35
Speaker 1: Oh my god, you didn't look at my CV? Well, the blank stare I just got fakes? You would be amazed.

00:03:40
Speaker 2: I can't believe you're.

00:03:41
Speaker 1: Asking me minute.

00:03:44
Speaker 2: I still have dreams about forgetting to do my homework.

00:03:47
Speaker 1: Oh well, here you go. Yeah, and giving you a zero on this one, but you know to turn it in late. Maybe we can give you some some partial credit.

00:03:54
Speaker 2: I didn't think you actually expected me to go find your CV. How about you send it to me in an email?

00:04:00
Speaker 1: Okay, okay, it's not that hard. Just google me.

00:04:03
Speaker 2: All right, all right, I'll hear. I'm gonna make myself a note after I heard the goats, I'll find Daniel CV.

00:04:14
Speaker 1: I'm second in importance to the goats. I feel so honored.

00:04:18
Speaker 2: Well, I might also take the geese for a swim first.

00:04:24
Speaker 1: All right, Well, many of you out there also have multiple roles in your lives. You are podcast listeners. You are curious about science, but you are also nurses and teachers and firefighters and bankers and all sorts of stuff. We love hearing from you, so right in, tell us who you are, what you do, and how this podcast superimposes on your life.

00:04:42
Speaker 2: Oh man, I never get tired of physics puns.

00:04:46
Speaker 1: Maybe, and today we're going to hear about something I hear about a lot from listeners who really want to understand one of the weirdest, most confounding, yet most revealing things we've learned about the universe, which is quantum entanglement and how it all works.

00:05:05
Speaker 2: This is a super confusing topic, I think, and it's one of those topics where I have to admit and I think I've said this on the show before I hear about it, and I think physicists are missing something. Can't this can't be true. But I'm sure by the end of the episode you're going to convince me this is definitely true and we should give more money to science to figure it out better.

00:05:27
Speaker 1: Well, you're right, actually, because the story we're going to tell the end is about physicists misinterpreting their own results and misunderstanding it and propagating that misunderstanding for decades until we understood really what these experiments were telling us. So even physicists at the top level we're talking about von Neumann and his colleagues misunderstood or misrepresented what these experiments mean. So it's not easy.

00:05:53
Speaker 2: I'm thrilled to report that that has never happened in biology.

00:06:00
Speaker 5: Right.

00:06:00
Speaker 1: Well, before we dig into quantum entanglement, I wanted to know what people out there thought about it and what they found confusing about it to make sure that all of the quantum itches out there got scratched. So I reached out to our group of volunteers, which you are very welcome to join. Write to us to questions at Danielankelly dot org and we'll add you to the Volunteer Question Answering Corps. Here is what people had to say when I asked them, what's the most confusing thing about quantum? Entangled particles?

00:06:28
Speaker 2: And I'm going to add my voice to the chorus. All of it.

00:06:35
Speaker 6: Particles are entangled because they're a part of a system and that entangles them. And then they go off and become part of another system, and so they're entangled in another system. It's just how they would move from being entangled to one to another. It just seems arbitrary how they would do that.

00:06:53
Speaker 7: Speed, which deemed for life from troubles.

00:06:55
Speaker 3: Like, what's so special about two particles being formed at the same place at the same time, so that like they're entangled.

00:07:03
Speaker 1: But probably the thing that is most confused is that it is a method for faster than light communication.

00:07:12
Speaker 8: I think it's confusing that if there's a collapse of quantum state on one particle, there's an instantaneous corresponding collapse on the other particle. But regardless of distance, this isn't a transmission of information faster than the speed of light.

00:07:31
Speaker 7: The most confusing part of quantum intangled particles is how we keep their states from collapsing to figure out they were entangled in the first place.

00:07:39
Speaker 1: I heard something recently that said entanglement could be non traversible wormholes between particles.

00:07:46
Speaker 8: I'd say, that's that's really confusing. How does the universe get the information to the other particle when one of the two is measured, it's just mind boggling.

00:07:57
Speaker 5: Well, for me, it's that there's no information transferred. So if you got an up and a down particle and you separate them, one turns out to be up, the other one has to be down. But it's got to be more complicated than that.

00:08:10
Speaker 7: I can't think of anything about them that is not confusing.

00:08:13
Speaker 1: Well, this is just about what I expect that you know, good deep questions about how do we know the universe really is random? How do we know it wasn't actually determined in advance? Somehow how does this whole thing happen across great distances? Can you use this to communicate faster than light? It is all very confusing.

00:08:30
Speaker 2: And we did chat on a previous episode about whether or not you can use it to communicate faster than light, and I remember that the answer is no, that's right, that's right. But you know, let's remind every one of all of the details, and like all truly exciting explanations, we're going to start with definition.

00:08:48
Speaker 1: So hey, if you want to be crisp and really explain stuff, you got to use these words that mean something. And so we have to agree on what words mean, which is why, yes, every important philisold of conversation starts with like, what do you mean by science anyway? Because words are fuzzy and slippery, right.

00:09:07
Speaker 2: Yeah, no, I do. I do absolutely agree it is important. So let's start by defining random, which I know is a term that can get many statistics people very angry. If you say I randomly picked blah and you weren't actually random. You can see that they're like the blood is boiling, and they'll like steam is about to come out of their ears, And so why are statisticians mad at me? When I say I pick something randomly and it wasn't really random.

00:09:33
Speaker 1: Yeah, And teenagers have their own definition of random, you know, like some random on the internet. Well, it's not really random, is it? But the teenagers don't want to hear that.

00:09:42
Speaker 2: What teenagers don't want to hear what their parents have to say when we want to correct them and make sure they're accurate. Impossible?

00:09:48
Speaker 1: Actually answer right. So let's distinguish true randomness from our typical experience of things we call random. So, for example, we use like dice or coin when we need a random number. You're playing a game, you need a number between one and six. You don't want to chosen in advance. You roll a die, you have to decide who gets the last couple of ice cream. You flip a coin. Right, But these processes are not truly random. They're actually chaotic and they're not random because they're deterministic, which means that the initial conditions exactly how you roll the die or exactly how you flip the coin determines the outcome. If you did it exactly the same way twice, you would get exactly the same answer. Because it's following laws of physics.

00:10:33
Speaker 9: Classical physics which are deterministic, but you know you're never you really doing it the same way twice, so why why are you being this way?

00:10:46
Speaker 1: And you're right, that's almost impossible to do, maybe literally impossible to do, which is why it's a useful stand in for random processes. When you need something unpredictable, then you use a chaotic process, and that's what these are and dice. They are chaotic. They are very very difficult to predict because they're very sensitive to the initial conditions. We have these points and these edges and the die so that if you toss it slightly differently, it has a chance to go left or a chance to go right. And the coin is very delicately balanced on its edge, so that it's super sensitive to exactly how you flipped it, and the wind conditions and if your partner is glaring at you or whatever. All these tiny details which make it effectively impossible to predict. Lets the coin and the dice do its job, and we use chaos because we don't have in our normal, everyday life access to true randomness, by which I mean something which if you ran it exactly the same way multiple times, would give you different answers a spectrum of answers determined by some probability distribution.

00:11:48
Speaker 2: But the good news is when I use the random number generator on my computer, I do get random numbers, and the statistician should.

00:11:55
Speaker 3: Leave me alone.

00:11:57
Speaker 1: This is rage base wrong, unfortunately, because how does your computer work? Right? Your computer is not a quantum computer. It's a classical computer. It's deterministic. That's the best thing about the computer is that if you run the same program twice you get the same answer. What else could it do? Right? It's literally just following the rules of digital logic, which are crisp and deterministic. And so you run a random number generator on your computer, what is it doing. It generates a string of digits and it picks one for you. This is not a random string. You set the seed to the same value, you get the same series of digits. They are roughly distributed in a uniform manner between two different numbers, and so in that sense they're useful, but they're not actually random because again, the same initial conditions lead to the same string of numbers.

00:12:48
Speaker 2: So if you were to use a random process to create ten numbers and a chaotic process to create ten numbers, like, how meaningfully different would your result be? Are the statisticians being silly?

00:13:03
Speaker 1: Statisticians are very silly because they also have this concept they call a random variable, which isn't random in the sense that we're talking about from a physics point of view. It's like a mapping between outcomes and numbers on the number line, which really is not random at all, And so it's like totally misnamed statisticians. Feel free to email me hate messages about this, but I will stand by this position. So your question is like does it matter? And mostly it doesn't, which is why it takes really clever, very subtle experiments to distinguish between a universe where things really are random with the microscopic level, and things are deterministic and chaotic. And we'll get into those experiments in a minute. But philosophically it makes a big difference. Right. It tells you that you live in a very different universe if the laws are deterministic and if the laws are probabilistic. Right, let's start with deterministic laws. If the universe is fully deterministic, meaning that the currency set up of the universe, every particle where it is its velocity determines the future completely. That says something really powerful about the universe. It says the future is determined right What's going to happen tomorrow may be hard to calculate, may require a supercomputer, and may depend on a butterfly's wings, But in principle, there is only one possible future, and we might not be able to extract it from our limited knowledge of the current situation and our limited ability to computationally apply the laws of physics. But in principle it is determined by the current state of the universe. So that deterministic universe, like a clockwork universe, is amazing and fascinating, but also kind of scary because like, hmmm, how do I fit into that? Am I just a robot following the conditions of the universe?

00:14:46
Speaker 2: Okay, So just to summarize real quick, if it's chaotic, then if you rewind a situation and play it forward again, you'll get the exact same results. And if it's random, then you'll rewind the situation and even if everything about the situation say the same, you're going to get a different result exactly.

00:15:03
Speaker 1: And there's something really subtle there, which I think is often overlooked about what it means to be random, because random is not arbitrary. Right, We're not saying, look, whatever happens just happens. By happ the universe is lawless. Right, we still have laws of physics. Quantum mechanics is actually deterministic, and that's a very confusing thing to say, but hold on for a moment. Is deterministic in a different way than classical physics. Classical physics says you hit the cue ball the same way, you're always going to get the same outcome. You flip the coin the same way, you're always going to get the same outcome. Quantum mechanics says you run the same experiment twice, you don't get the same outcome, but you get exactly the same probability of outcomes. So physics doesn't give up, It just retreats one step. It says, I can't tell you exactly what's going to happen for any individual experiment, but I do absolutely determine the probabilities of various things happening. So some thing's impossible to happen, zero probability, they will never happen. Other things very likely. Some thing's very improbable. So for example, when we smash particles together at the large Hadron collider, we don't know and we can't know what's going to happen for any individual collision, but we can and we do calculate the probabilities of X happening or why happening or Z happening, and then we go off and we measure the rates of which those things happen. We compare them to our calculations, and they agree. So quantum mechanics is not arbitrary. It's just deterministic in a different way. It's like generalizing determinism, not at the individual experiment level, but at the possibilities of the outcomes.

00:16:36
Speaker 2: Okay, all right, I totally followed that. And so if you want to get true random numbers, do you have to be doing quantum mechanics or are there other ways to get random numbers?

00:16:48
Speaker 1: Quantum mechanics is, as far as I know, the only source of true randomness in the universe. All classical physics is deterministic, right, Every classical theory depends on the initial conditions, and the outcomes are totally determined by those initial conditions. And so classical physics, yeah, totally deterministic and philosophically, this was like mind blowing for people once they understood it before we had quantum mechanics, they were like, oh my gosh, wow, seems like the universe is deterministic. We are all effectively philosophical clocks, right, we're robots determined by the early universe. And you know, us being on this podcast was set in stone once we had the universe at a certain stage billions of years ago, and then quantum mechanics says, actually, no, there's an important layer at which the universe is not deterministic. There's a probabilistic nature there. There's some randomness there.

00:17:41
Speaker 2: So the world is lucky that we ended up doing this podcast.

00:17:45
Speaker 1: Well, it opened up a whole rabbits hole of philosophical questions like does that actually allow for free will? And you know my answer is, I'm not sure it does, because the universe is not arbitrary. It doesn't open the door for like mind body duality, where you can have like some non physical mind now affecting the physical universe. It just says that there's some randomness, right, and free will is not randomness. Right. When you go to choose ice cream at the store, you're making a choice for chunky monkey. Right, it's not randomly decided. And so anyway, that's a whole philosolphal rabbit hole. We're not going to go down to this hole.

00:18:22
Speaker 2: It's an important it's.

00:18:24
Speaker 1: A pretty chunky rabbit hole, yes, for sure. But let's take one more step towards entanglement. So so far we've talked about randomness and what that means. And so let's imagine, for example, a classical coin. Right, you flip it, it's hard to predict, but it is determined. Imagine now you had some quantum version, a coin which you could flip and was really random, right, It was not determined by the initial conditions. The other amazing thing about this quantum coin is that it preserves both possibilities until you look. So the classical coin, you flip it, it lands in your hand and you cover it up. But under your hand it is heads or it is tails, right, you just don't know it yet. The quantum coin you flip it is under your hand until you look, it has the superposition of all the possibilities. Maybe it's heads, maybe it's tails. Right, So, not only is quantum mechanics random, but it's also undetermined until it's measured, which is going to be an important factor in our later conversations. So that quantum coin is undetermined, and that's weird, and you can ask the same question you asked a minute ago, like, well, how different is it because you look, it's got an answer. How do you really know it's undetermined? How do you really know it's random? And how do you really know that it's undetermined until you look?

00:19:39
Speaker 7: Yeah?

00:19:40
Speaker 2: Right? And this is where I help the whole field of physics by letting you all know you've just got to be wrong about that. That doesn't feel like it makes any sense to me, and so go back to the drawing table and try again. Guys and gals. All right, So we are about five percent of the way through our outline and about a third of the way through our episode. I got really excited about randomness and chaos. Let's take a break, get ourselves back on track, and when we come back, Daniel will convince me that I'm wrong about all the physics.

00:20:17
Speaker 1: Wait, so your homework is check Daniel CV. My homework is convinced Kelly that all the physics is correct. Wow, this doesn't feel like an equitable distribution of tasks.

00:20:26
Speaker 2: Well, you know, if physics got things right in the first place, you wouldn't have to worry about this. This is why you gotta be in biology. You know, we got it all figured out. It depends.

00:20:36
Speaker 1: Nothing's right or wrong anyway.

00:20:38
Speaker 4: So yeah, there you go, there you go. All right, let's take that break.

00:21:01
Speaker 2: Okay, So at the end of our last session, I dropped the bombshell that all of physics is wrong, and Daniel is going to let me know why he thinks I'm wrong. So, all right, Daniel, quantum entanglement. How can that coin that you flipped and it's in your hand but you've covered it. How can it be both heads and tails superimposed at the same time but doesn't actually end up as one until you move your hand away.

00:21:26
Speaker 1: Yeah, it's bizarre. And to really probe this, we're gonna have to make this setup one step more complicated and even more counterintuitive. And so to explain this, we're gonna have to make this setup a little bit more complicated. We're gonna need two objects, and we're gonna need them to be connected in an important way. We go need their fates to be connected. And so instead of having a coin, let's imagine that we have two bags. One has a red ball in it and one has a blue ball in it. Okay, and these are just classical, normal balls, it's no big deal. And I have one and Kelly has the other, and we pick bags and we go back to our homes. I'm in California, you're Virginia. And I look in my bag and I see that I have the blue ball in my bag. Now, I instantly know that you have the red ball, right, because we knew there was one blue and one red. And I know you have the red ball because I'm applying this constraint, this condition, this requirement that there was only one blue and one red, and therefore, if I had the blue, you have to have the red. No magic at all, right, And also no instantaneous communication of information, right, Like I know instantly that you have the red ball. You don't know that, right. I know that I know something about a ball that's really far away, and I know that instantly. But again, there's been no instantaneous communication of information, right.

00:22:39
Speaker 2: I Mean, I'll be honest, Daniel, I probably peaked.

00:22:42
Speaker 1: You are a cheater, aren't you. I knew that about you.

00:22:47
Speaker 2: Okay, But I totally understand the scenario you've laid out.

00:22:50
Speaker 1: All right, Now, let's imagine the quantum version and the quantum version things are different. Okay, So number one, it's not determined who has the blue and the red. Like in the classical version, I had the blue one the whole time, I just didn't know it, right. In the quantum version, it's not determined I could have the blue or the red. And so the ball is in this undetermined state. It has a probability of being red and a probability of being blue, and your ball is also has a probability of being red and a probability of being blue. But because we know there's only one blue ball, if I look inside my bag and I see the blue ball, now I know you have a red ball, right, And so then something amazing happens. My ball goes from both possibilities to being blue, and at the same time, your ball goes from both possibilities to only one possibility of being red.

00:23:40
Speaker 2: Okay, So is the ball both blue and red or it's just neither? Until you look, what is the right way to be thinking about this?

00:23:51
Speaker 1: Yeah, so people like to say that it's both things at the same time. Electrons going to be in multiple places at the same time. I think that's confusing in a way that doesn't educate and isn't accurate, because it's not true that it's in both locations at the same time, or that it has both colors. It just has the probability to have both colors, and it's not yet determined, right, So it doesn't make any sense to say the ball is blue and it's red, or it has both colors. It's just that it has both possibilities and we don't know yet, and the universe has not yet decided, right. And the weird thing about this is like, how can you tell? Right, all I'm doing is I'm picking a bag, I'm going to California and opening up, I'm seeing it's blue. How do I know that the universe didn't actually just decide when I picked the ball which one I had in which one you had? How can I tell that it was really uncertain and that it really is random? And that's the crux of the question. That's what we want to know, like what's really going on inside these bags? And of course when we do these experiments, there are no quantum versions of balls and bags, and so we do it with particles, and we do with particles that are constrained by laws of physics to have some opposite characteristic. So, for example, you create two electrons in such a way that one has to be spin up and the other one has to be spinned down to conserve angular momentum. And so when you measure one, you learn that it's spin up. You know, the other one has to be spinned down. And that's just because we need a quantum property. You have a quantum object. And we've been talking about bags and balls, but there aren't actually quantum bags and balls, and so in the real world we do this with spin and with particles.

00:25:24
Speaker 2: So I think when we were talking about quantum internet, you explain some of this there. And I asked how this differs than the Schrodinger's cat example, is like, is the cat both alive and dead? And I think you told me that that's not even almost about what we're talking about, but I've forgotten because my memory is great. So how does this is this the same thing?

00:25:50
Speaker 1: This is the same concept? Yes, okay, The schirtingers Koint experiment says you have a cat in a box and it's going to be killed based on some quantum process which is un predictable, right, It's truly random, and it does this thing. We're trying to connect that quantum process to something classical and intuitive, which is a cat. And so you know, before you open the box, the cat has a probability being alive and a probability of being dead. And people like to say it's both dead and alive, which I don't think makes any sense. I think it has a probability of being dead and a probability of being alive. And the quantum version of the story is that before you open the box, it has both probabilities and the universe has not yet decided. And the classical view of that it says, no, no, no, it's determined. You just don't know until you open the box. And the question, the deep question, is can we tell the difference? Can we really know if the universe is playing this undetermined random game, which would be really really strange, or is there some way in which the universe decides all this stuff in advance and it's all predetermined. We just don't know how it does it. It's just some hidden detail that we're missing that tells one particle to go up and particles go down. And so those are the two questions. Is it actually random and quantum mechanical and undetermined or is there some hidden variable, some detail which is controlling this that we just are missing or not understanding.

00:27:13
Speaker 2: All right, so how could you possibly tell the difference? What experiments do we need to do?

00:27:18
Speaker 1: Well, the best thing to do is the most obvious. It's like, well, just repeat the experiment multiple times, start it exactly the same way, set it up exactly the same way, or the balls or the electrons or whatever, and see do you get different outcomes? Because if you do it the same way, starting with exactly the same initial conditions multiple times, and you always get the same outcome, then you know it's determined. And if you use these same initial conditions and you get different outcomes, then you know that it's not. That sounds great, right, what a clean experiment, Just test it. The problem is how do you do the same experiment exactly the same way twice? Right? Like you know you can never step in the same river twice. If you repeat an experiment the next day, the Earth is in a different place around the sun and the temperature are slightly different, and you had a different breakfast, and there's like a zillion things that you could never control for, and the quantum philosophy nerds are like really nerdy about all these loopholes, and so that's just impossible. Even at the particle accelerator, Like we smashed protons together millions of times a second, but it's never exactly the same collision. The angles are slightly different, the energies are slightly different, and so that's essentially impossible. So we need something more clever. You can't just run the same experiment twice, which is a bummer, because man, that would be awesome.

00:28:32
Speaker 2: That would be awesome. Okay, but I'm hoping physicists haven't just thrown in the towel. But maybe all did because you were like, oh, shoot, we're probably wrong.

00:28:40
Speaker 1: No, okay, no, we did not throw in the towel. There was a very clever guy named John Bell who came up with an experiment that could tell us the difference between these two hypotheses. One that the universe has somehow figured this out in advance and we just are missing the information, and two that know it's actually random and undetermined until you look that these particles, even when they're separated by great distance, somehow decide together at the same moment which one is up and which one is down. And it sounds like impossible to tell the difference, but he came up with this really clever way. And unfortunately there's no like smoking gun individual experiment where you can say I'm looking at the outcome and it proves a versus b right. This is not like do unicorns exist or I found one, therefore we know there are unicorns. The results are a subtle statistical correlation across many experiments.

00:29:34
Speaker 2: So is it a quantum result. There's a constant probability that you're wrong.

00:29:40
Speaker 1: And briefly, you take measurements of these two distant, entangled particles and you look at how often you get the same result, and if they are hidden variables, you can't get the same result more than two thirds of the time. But quantum mechanics allows you to get the same result on these two particles more than two thirds of the time. It breaks that restriction by not determining the result in advance. But it's a correlation, right, It's not like any individual experiment proves it. It's like a pattern among many, many, many runs of the experiment. So it's a little frustratingly indirect, but it's also mathematically very crisp, and I want to try to walk you through as you can get an intuition for what's going on here. Okay, So how does this Bell's experiment work? And why is two thirds an important threshold? So imagine I have an electron here in California and there's an electron in Virginia, and we've entangled them, so we know if one is up, the other one is down. So what are the possibilities If I measure up, then Virginia is down. If California measures down, Virginia is up. Okay, So we get opposite spins one hundred percent of the time. So far. This is not evidence of anything. This just says our particles are constrained. They're entangled, right, And it's important for people to understand really what this means, because a lot of people don't get the simplicity of what entanglement means. The entanglement just removes some possible outcomes, like without entanglement, I could get up and you could get it up. I could get down and you could get down. Entanglement just says no, those possibilities are zero. So entanglement just removes possibilities and only leaves the ones that satisfy in this case, like angler momentum. All right, so we know that they have to be opposite. So far we haven't learned anything. But remember that also I don't have to measure spin in the same direction as you do. We have three dimensions of space X, y, and Z right, and I could measure spin along like a Z axis, and you can measure it along a Y axis that's perpendicular. Or for example, I can measure it in one direction and you could flip your machine upside down. What happens if you flip your machine upside down? Then we expect that we always get the same answer. If I read up, then you're also going to read up. I would have read down on your particle, but your machine is upside down. So I get up, you get up. If I get down, you get down. So in that scenario we get the same results one hundred percent of the time. Right where our machines are flipped, but also the particles are entangled, so they have opposite spins yep R with me still uh huh okay. Bell's experiment says, let's get even weirder folks, Let's pick three axes in advance. So like, I'm gonna pick three directions in space, maybe up and then left and then also some weird angle in between. Okay, so we pick that in advance. We have our electrons, one in California and one in Virginia. Now I randomly choose which of those three axes I'm gonna measure my electron on? Is it the Z? Is it? The Y? Is it the in between? You're also gonna do that. You're gonna randomly choose an axis?

00:32:35
Speaker 2: Okay, how do I randomly choose an axis?

00:32:37
Speaker 7: If?

00:32:38
Speaker 2: I if randomness is so hard to uh, I can't use the random number generator on my computer. That's chaotic.

00:32:46
Speaker 1: No, you're you're being persnickety about it, but in a really actually fascinating way that quantum theorists get really nerdy about it. We're going to talk about that later on. People use like lava lamps and cosmic rays to try to be like really truly random to make sure they're not being like you're influenced by something, because that randomness is absolutely essential for this whole argument. So we're going to come back to that point. Okay, and it's going to involve like scripts of Gilligan's Island. It's really weird.

00:33:10
Speaker 2: That's great. I was hoping that's where this episode would end up.

00:33:14
Speaker 1: All Right, So we each have an electron, and we each have three axes, and we randomly pick which axis we're going to measure this electron on. Right, So imagine that these things actually are determined, that some hidden variable on this electron makes mine be up and yours be down along some axis. Right, it's not random, it's not quantum mechanical. Let's imagine that hidden variables are really at work here. Well, then what would happen? Well, it's all determined in advance, right, I have my three axes, I have my particle, You have your particle. But my particle is actually pointing in some direction. Your particle is pointing in some direction. And so it's all determined in advance, and you can actually enumerate all the possibilities. Right, And we have three axes, and so one third of the time we're going to be choosing the same axis, right, Like if I choose Z, you're gonna choose Z. Because we have three axes and we're both choosing randomly, So a third of the time we choose the same axis, which means they will get the opposite results, Like we'll choose the same axis, I'll get up and you'll get down. Right, So at least a third of the time we get the opposite results, which means that we get the same result less than two thirds of the time, right, Okay, So that's what we expect for hidden variables, and that just comes out of having three axes and choosing them randomly. Okay, So what if there aren't hidden variables? So what if there's quantum mechanics going on?

00:34:35
Speaker 6: Gas?

00:34:36
Speaker 1: Okay, So what happens if this quantum mechanics going on? So if there's no hidden variables, if quantum mechanics is at play, then there's something sneaky going on here, which is that then Heisenberg uncertainty principles sneaks in the door. Heisenberg says that there's some things you can't know simultaneously about the universe, like you can't know the speed and the location of a particle at the same time. Right, Well, it also applies to spins of a particle in different directions. So, for example, if I measure the spin of a particle in one axis, I can't know it in the other ones, or if I measure it in some access, I can't know it on my first one. So there is no true spin direction of these particles in the quantum mechanical view. Right, It's not like there is a true spin and we're measuring along some axis, so we get up or down. It's like scrambled in this weird way. And in the quantum mechanical view, you represent the probability of these particles being spin up or spin down using these complex numbers. It comes out of the shortening your equation, and it's all determined by these complex amplitudes. And like in many quantum effects, these complex amplitudes can interfere with each other, and so quantum mechanics allows these particles to interfere with each other. It's not like all set up in advance. They dynamically respond to the situation, and quantum mechanics predicts that you should get the same spin around three reforths of the time. Now remember the hidden variables prediction says you cannot get the same spin more than two thirds at the time. Absolutely not totally impossible. That would break logic quantum mechanics says no, no, no, at these weird angles, then three fourths of the time you can get the same spin. If you arrange things right, depending on the angles between your axes, you can get the same spin more than two thirds of the time, up to three fourths of the time.

00:36:25
Speaker 2: Okay, so I was totally following you, but adding the Heisenberg uncertainty principle feels like cheating because I don't really understand why that works. And it's like, okay, but also now we're playing by a totally different set of rules that you don't understand, and it makes sense. So could we take a quick break and talk about Heisenberg uncertainty principle just for a second?

00:36:45
Speaker 1: Yeah? Sure. So you know, Heisenberg un certainty principle is just a way of thinking about the spread of possible outcomes and what's allowed, and the fact that measurements are connected, that you can't measure one thing independently, put that in the box, know it, and then move on to measure something else. It's just a way of thinking about how like the truth isn't totally determined, and so in that way, it's kind of a shorthand. We can actually do without introducing the Heisenberg un certainty principle entirely, if we can just think about the nature of quantum measurements, and so really all you need to know is that the quantum mechanical prediction for whether my California particle is spin up or spin down along some axis is probabilistic, right, that's the quantum mechanical nature of it. It predicts some probability of this and some probability of that, and yours it predicts the opposite. And that's very simple. But if you rotate your axis so you're not measuring along the same axis as I am, there's a relationship between those probabilities. But that relationship is different in the quantum version than in the hidden variables version, because we have these amplitudes, because we have these complex numbers that are interfering with each other. And Bell realized that as that rotates, the number changes differently for the quantum version than it does for the true everything is determined hidden variables version. And it's because of those complex amplitudes and the way that calculation happens. And it's really fascinating because normally these complex amplitudes aren't things that you can see, but when there's interference, then those results are apparent. It's sort of like in the double slit experiment. You can't see the probabilities, but you can see them interfering with each other. And so that's roughly what's happening here, is that the probability of measuring along one axis is interfering with the probability of measuring along a different axis in a way that gives us a different dependence as the angle changes, and so that comes up with a different prediction. And that's why it's not an individual experiment. You're like, I ran it, I got up and down, and therefore kronme mechanics is correct and hidden variables is wrong. It's like I ran it a thousand times and I got the same direction for both particles seventy two percent of the time, which is impossible in the hidden variables theory. So it's an average over many experiments.

00:38:54
Speaker 2: Okay, so at the end of this experiment we can say that what's happening is definitely random, not chaotic. Yeah, and so there are no hidden variables.

00:39:04
Speaker 1: That's right, and it's incredibly powerful and broad result. It's saying it cannot be a hidden variable. Right, you don't even have to know what the hidden variable is. You can imagine some like additional dimension of space, and these particles have some features in that space, and that's what's determining it. No, we don't even have to discover those dimensions. This proves that that cannot be happening. No hidden variable theory satisfies these experiments. It's really incredible. The consequences are huge. But there's a very important caveat right. We've been talking about hidden variables, and Bell's experiment actually only rules out local hidden variables. Information that's like connected to the particle that like a little detailed that like the electron has tucked to do its pocket. That's determining whether it's going to be plus or minus. Right, local hidden variables. And this was actually misunderstood for decades.

00:39:56
Speaker 2: And when we get back from the break, we'll find out why the answer was a scured for so long. All Right, we're back, and Bell's experiments were misunderstood for decades, and Daniel's going to explain to us why.

00:40:29
Speaker 1: So. This very smart guy, John von Neumann, he's like widely considered one of the smartest dudes in history and he's the guy who showed that Heisenberg's matrix quantum mechanics was the same thing mathematically as Schrodinger's wave quantum mechanics, even though those two guys hated each other really like a towering figure. And he did this proof, this conceptual proof, before Bell's experiments that showed that quantum mechanics couldn't have any hidden variables at all. But it turns out there was a mistake in it, and people actually argue about like was it von Neuman's mistake or did people misinterpret what von Neuman was saying and he really understood it. They don't like pointing out mistakes in genius's work. But for a long time, the lore was that quantum mechanics was inconsistent with any kind of hidden variable. And it was Bell who came up with these experiments that showed actually what they do is they show no local hidden variables. His experiments can't disprove a different kind of hidden variables, non local right like global hidden variables. And as a result, they're like interpretations of quantum mechanics like bomy and mechanics that have these like global guiding functions. This pilot wave that like tells this particle to go positive in that particle to go negative. So they are deterministic, but they required this like weird non local coordination between all the particles and the universe in a strange way. That's very counterintuitive. But the big picture result for Bell's experiment is not that it shows no hidden variables, but no local hidden variables, which means that quantum mechanics is weirdly non local. Right, Like, what's happening here is my particle is collapsing and your particle is collapsing at the same time. It's instantaneous across time and space. Quantum mechanics is non local.

00:42:15
Speaker 2: Okay, So Bell proved that there was no local hidden variable. Yeah, but did we ever convince ourselves that there's no Bomian global guiding function or could That's still an open question.

00:42:30
Speaker 1: That's still an open question that these Bell's experiments can't tell us whether Bomian mechanics is correct and there are hidden variables but they're global, or there are no hidden variables at all. Right, So people often interpret Bell's experiments too broadly. They say, well, there's no hidden variables, but actually they just show no local hidden variables. So if your theory has like weird global hidden variables, yeah, that's still cool. And that's what Bomian mechanics is. And Bell actually was a strong proponent of hidden variable right. He thought the global hidden variables were the way the universe worked. So it's sort of weird because he's like famous for devolishing hidden variables, but he actually believed in them. But he believed in the global version of it. And people have actually done these experiments. These are not just thought experiments. The first ones were done in the seventies, and then they do them in fancier and fancier ways, and they keep the particles further and further apart to test this question of like is this really happening instantaneously across time and space. And the way they do this is they entangle the particles and they really do separate them in vast macroscopic distances and then make their measurements at the same time. So there's not enough time for light to go from like California to Virginia to inform my California particle what happened to your Virginia particle, So we know that this really does have to happen at the same time.

00:43:50
Speaker 2: Well, wouldn't that have to mean that the information is traveling faster than light?

00:43:55
Speaker 1: Yeah? And so this was a question from a listener who wrote in and asked something similar. Here's Mohammad asking his question.

00:44:01
Speaker 7: Hi, Kelly and Daniel, I would like to understand that if information travels have the speed of light, then how do we know that quantum entanglement is instantaneous? How do we know that the particles and suence each the faster than the speed of light when the measurement itself is captive the speed of flight? How can we measure them precisely at the same time When the particles have moved far apart after they untangled, they are subjected to change in gravitational field. And that seems to me like it is enough to throw it forthing out of whack. So what kind of witchcraft allows us to technolo gr all right?

00:44:37
Speaker 1: And so yeah, Muhammad is asking, how do we know as faster than light? And Kelly is asking, doesn't that violate everything I thought I know about physics? And so to answer Mohammed's question, what they do is they bring these things really far apart and they make them measurement as simultaneously as they can. So if they know they make the measurement within a microsecond, then as long as there further a part than a light microsecond, further part than light can go in a microsecond, then they know that the collapse is faster than light. You can't prove it's literally instantaneous, but you can prove that it's faster than light because the particles are separated by more distance than light could go in the intervening time. And so how does that not violate relativity? Well, relativity tells us no information can be transmitted from California to Virginia faster than light, but no information is being transmitted. Like if I measure my particle and it goes from undetermined to up, then Kelly's particle goes from undetermined to down. But she doesn't know that there's no information she's gathered there. She just has her particle she hasn't measured yet. When she goes to measure it, she sees, oh, it's down. She doesn't know whether it was collapsed or not. People often write in and they're like, what if Daniel collapses his particle, then Kelly can see that it's collapsed, and that's a way to communicate information faster than light. But Kelly has no way to know whether her particle is collapsed or not. All she can do is measure and say, oh, I got a minus. She doesn't know she's collapsing it to get that minus, or if it was already collapsed. So, yes, the quantum wave function is non local. It extends from California to Virginia and collapses simultaneously across space, which is really weird. But you can't use it to transmit any information from California to Virginia faster than light, and so it doesn't break special relativity, which feels like a really loyally loophole, but it's true.

00:46:29
Speaker 2: All right. Alright, let's clear up Kelly's misconception here. So my first thought was, well, the information between those two electrons has already been transmitted when they were entangled, and so no information is being transmitted when one is actually observed because they already were tied. But they don't know if they're up or down yet until they're observed, and so there is Okay, I get it.

00:46:54
Speaker 1: That's really insightful. Yeah, So what's happening when you're entangling them is you're removing some possibilities, right, You're removing the up up and the down down possibilities. You're only leaving the up down and the down up. However, we're leaving those possibilities undetermined. And so when I make my measurement, it crosses one more off your list, leaving you with only one possibility. So that is happening across space and time. But you're right, the entanglement itself is local, and it was made when the particles were created in these initial states.

00:47:28
Speaker 3: Hey, Daniel and Kelly, this is Matt. You may remember me from editing audio on your show. I was in the middle of editing this episode when some of this stuff that Daniel was saying started to hurt my brain much in the way seemingly that it hurt Kelly's brain. And I have a question for you. One thing in particular that I'm having an especially hard time understanding is the observational element of quantum physics. How and why does it matter if the state of a particle is or isn't being observed. Do quantum particles have awareness? Are they shy? What is it about observation that determines or sets a quantum object in place, so to speak. Doesn't this mean that consciousness is somehow related to the state of things, that consciousness somehow dictates or influences reality, And if so, doesn't this indirectly mean that before consciousness evolved in the universe, the universe didn't fully exist. If you could shed some light on this matter, I'd really appreciate it, But I'm fully prepared to be confused about this for the rest of my life.

00:48:41
Speaker 1: Thanks. All right, great question, Matt. So First, remember that observation is not passive. It's active. It requires some kind of interaction with the thing you're studying. You want to see a particle, you have to bounce a photon a probe off of it in order to see where it is. You want to measure the spin of your electron, you have to put it through a magnetic field that acts like a probe and interacts with it. So observation requires interaction. Now, if that interaction yields information, that means that your probe is now entangled with the system. That's because if the way your probe particle behaves afterwards depends on the electron spin, then your probe is entangled with the quantum system because the spin of the electron determines what the probe does. So now your probe is part of the system. But if, for example, your probe was badly designed so it doesn't depend on the electron spin, it doesn't extract information, then it's decoupled and it's not entangled. So it's not about consciousness or shyness. It's about interacting in a way that yields information about the state of the system. Now, whether there's like wave function collapse or not is another question of philosophy. Copenhagen interpretation says interaction collapses the wave function if one of the object involved is classical, like if the probe is classical, But it also doesn't define what classical means and why quantum objects, when they come together, somehow become classical. The major alternative many world hypothesis says the interaction entangles you and you become part of the system. So now you only see one outcome. This no collapse, You're just along one of the branches. But philosophically, the whole thing is kind of a mess. All right, great question, But Kelly, let's go back to your other fun loophole, which is this experiment requires people to choose axes at random and then measure how often something happens over a bunch of random trials. How do we know that those are random? Or testing randomness? It relies on randomness? Is there some circularity? There are we on firm ground? And there's a really fun kind of paranoid theory of quantum mechanics called super determinism, and it says, look, the outcome of these experiments relies on those things being random. But what if they weren't. What if Kelly and Dana were manipulated somehow into choosing axes that give this result, right, Because if they're not random, then the result can't be relied on. And you know what if it all is actually determined by something that happened a billion years ago and set this series in motion. And so to try to get around this loophole, they've done really hilarious things, like they've made the choice super duper chaotic, Like they randomly sample scripts from television shows and you know, like if the letter is greater than K, then they choose this axis and if it's less than G, you know, on the third line of page two, this kind of stuff, and they combine it with lava lamps and cosmic rays to try to get like as random as possible. But in the end, superdeterminism is something you can never really totally knock down, because there's always some crazy paranoid theory you could have that these things are just being like orchestrated by folks in the simulation or super intelligent aliens or something.

00:52:00
Speaker 2: All of our best efforts at getting random numbers say the same thing that Bell's experiment works.

00:52:06
Speaker 1: Yeah, okay, yeah, And so broadly the consensus in the community is, yes, the universe really is random at the microscopic level, really is undetermined, and these extraordinarily subtle but very clever experiments reveal that that's how the universe works at the lowest level. And it relies not just on the universe being random and the results being undetermined, but having these tests with particles that are distant from each other yet quantum mechanically having their fates connected to each other.

00:52:34
Speaker 2: Okay, and so now nobody should ever be confused again about entangled particles.

00:52:40
Speaker 1: This stuff entangles your brain for.

00:52:42
Speaker 2: Sure, Yeah it does. Yeah, follow up questions welcome.

00:52:46
Speaker 1: But it's a moment where you should be skeptical, where you should ask, hmm, how do we know that's really true? I hear this all the time. People are telling me quantum mechanics is random? What experiment really proves it? And you should demand an answer, and you should demand that intuitive explanation that satisfies you, because this is something that's true about the whole universe, and the philosophical implications are huge and far reaching. And so before you like update your priors and change the way you look at the universe, like, make sure it makes sense to you.

00:53:14
Speaker 2: I think my new favorite conspiracy theory is superdeterminism.

00:53:20
Speaker 1: Because it includes all the other conspiracy theory.

00:53:23
Speaker 2: It's right, that's right, all right.

00:53:26
Speaker 1: Well, thanks very much everyone who wrote in asking for an explanation of quanam entanglement. I hope that helped.

00:53:32
Speaker 2: It Sure helped me. Have a good day everyone. Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

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