Daniel and Jorge Explain the UniverseDaniel and Jorge step through the details of a mind-boggling version of the double slit experiment. Check out their new book: universefaq.com
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Hey, it's Horehan and Daniel here, and we want to tell you about our new book.
It's called Frequently Asked Questions about the universe.
Because you have questions about the universe, and so we decided to write a book all about them.
We talk about your questions, we give some answers, we make a bunch of silly jokes.
As usual, and we tackle all kinds of questions, including what happens if I fall into a black hole? Or is there another version of you out there that's right?
Like usual, we tackle the deepest, darkest, biggest, craziest questions about this incredible cosmos.
If you want to support the podcast, please get the book and get a copy of not just for yourself, but you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters, and.
For the aliens. So get your copy of Frequently Asked Questions about the Universe. It's available for pre order now, coming out November two. You can find more details at the book's website, universe faq dot com. Thanks for your support, and.
If you have a hamster that can read, please let us know. We'd love to have them on the podcast. Hey Daniel, do you ever feel like you really understand quantum mechanics?
No, you know, I think it's probably just too alien for us to really ever feel comfortable with.
I guess it's too bad there aren't any macroscopic big quantum optics we can really like poke and play with.
I know, like a big fat electron. But actually I'm working on a theory that children are governed by the rules of quantum mechanics.
Oh really, like there's uncertainty about where they are. Have you lost your kids?
No, but I've noticed that you can't like observe your children without sort of perturbing the system, right.
I think I know what you mean. Like, they won't do their homework unless you're there watching.
Exactly, And when I walk into the room, somehow all their conversations collapse suddenly into silence.
Yeah, it's like short angers children. They're both excited and sad to see it.
Hi.
I am horhand made cartoonist and the creator of PhD comics.
Hi I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm always in many quantum states at once.
Really, so that does that mean you're not real?
It means I don't even know if I'm real. Man.
Well, I hope you are, because that would mean that I'm talking to myself right now, and that would be a little concerning.
Maybe you are the only brain in the universe and the rest of the universe is just part of your mind.
That would make a lot of sense why I'm so successful and good looking. But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we do try to explode our minds out to capture the entire universe. We want to take this vast, glittering, crazy, violent, wild and white cosmos and wrap it all up inside our brains. It's not enough for us to just live in this universe, to experience it and to see it. We want to understand it. We want to download the whole thing into our minds, and that means understanding the basic rules about how it works, what's really going on with tiny little particles or whatever is happening at the smallest scale. On this podcast, we dive the deepest you can into the hardest and trickiest of questions and we try to explain all of them to you.
Yeah, because it is a pretty tricky universe. It's full of interesting rules and interesting phenomena that happens at the smallest of levels and at the largest of scales, and a lot of it is understandable, even if it's not very intuitive to think about.
That's right, And I look at the universe like a big puzzle. It's like a detective novel or a murder mystery, and I want to figure out who did it. I want to understand how it works. It's amazing to me, sort of philosophically, that the universe is presented to us that way, like this big puzzle that isn't obvious to figure out, but yet can somehow be understood if you push hard enough.
Do you think the universe is understandable? Daniel? That's a big question in physics, isn't it.
It's a big question in the philosophy of physics. And my answer is it doesn't make sense for it to be understandable, Like how could it be possible that the complexities of this maybe infinite universe could be stored in the minds of a human. On the other hand, we have all these theories that work really well, like surprisingly well, and so I don't know how to hold those two ideas in my mind. They're in quantum conflict.
You're both confused and the feeling smart at the same time. That's what this podcast is all about. I'm the confused quantum state and you are the feeling smart state.
That's right. And all scientists sort of hold those two feelings in their mind at once, like, look at all we have understood, and yet look at all that we do not, And that's both exciting and terrifying.
Yeah, and we have a lot of questions about the universe. And by the way, speaking of questions, we have a new book coming out pretty soon on November two.
That's right, Orhean and I celebrate asking questions about the universe, and we love thinking about these questions. I love hearing about your questions about the universe. So we wrote a book that wraps up the most frequently asked questions that we get about the universe.
Yes, if you like this podcast and you want to support us, please check it out. It's called Frequently Ask Questions about the Universe and it's on a pre order right now. I can order it now and get it as soon as it comes out. And I think you know what's important is that we kind of didn't quite write it for our listeners, right, Daniel. We kind of wrote it for the people that know our listeners you know, if you ever have like a nephew or a cousin or an uncle who you want to share this amazing information about the universe, I think this is the book for you or for.
Them, that's right. So every one of you out there, you should buy five copies and give them to all your friends and family so that they can understand the answers to questions like where did the universe come from? Or how can we travel to the stars.
Yeah, we tackle all kinds of pretty cool questions in it, and we try to answer it for people like your relatives and friends with all kinds of interesting and clear answers. And also cartoons, which is I think something you don't see in every day and physics books.
That's right, all these awesome fun drawings that help clarify the topic and amuse you along the way that Jorge added to this book. So if you like this blend of physics and silly jokes, then I think you'll enjoy this book. So go out and get your copy. You can find it at universe faq dot com.
All right, well, speaking of questions, we are tackling a question today and it has something to do with quantum mechanics, which is I guess, for lack of a better technical term, bonkers.
It's my favorite kind of bonkers. It's the kind of bonkers that doesn't make sense to your mind. But the math works perfectly and it keeps predicting absurd experimental conclusions that experimentalists keep verifying.
Yeah, because I guess you started with a little nugget of an experiment, and then you worked out some math, and then you find out that the crazy math that it suggests is actually also true.
Yeah, we had to change the basic concept of what we thought was going on at the very heart of the universe or of everything that's around.
Me and you.
Even though it seems intuitive and like it follows rules that we're familiar with from growing up, it turns out that the tiniest little parts inside are following totally different rules, which mean that the nature of reality is quite different from the one that we thought it was. And people worked that out and they thought that's crazy, and if it's true, it would mean you could do this Bonker's experiment which would have this nonsense result, so obviously it can't be true. And then business went out and did the experiment and got the nonsense result, which turns out to be the truth of our reality.
Yeah, because I guess at the core of it, it's kind of weird for us humans to think of things that are like two things at the same time, right, I mean not just in a conceptual lede, but like actually in reality, in quantum objects, things can be multiple things at the same time.
That's right. And that phrase in reality is the key there, because we imagine at the very basic level that there is a reality out there, that there is a truth that even if we're not looking, the universe is there and it's operating and it's following some rule and it doesn't really matter if we are looking or not. But the reality suggests that the universe is quite different from that, that it does matter if you interact with it, and that what is happening in the universe is not exactly well determined until you interact with it.
Yeah, you might say that in reality, the way the world works and the universe works, it's kind of fuzzy, kind of not quite as solid as we might think it is from our everyday lives.
That's right. We have a weird and particular view of this quantum universe. We are only used to interacting with enormous quantum objects like baseballs and rocks and trees, which are quantum objects, but they contain like ten to the twenty six quantum objects, and when you have that many, they do things differently than when you have one or two of them isolated to reveal their sort of true fundamental nature. So today we're going to be talking about some really crazy experiments that try to reveal exactly what the rules are of how these particles work at the smallest scale when they're left alone.
Yeah, and so it turns out that also reality it's not just a little bit fuzzy, but it may not even be as permanent as we think it is. Quantum information and quantum things we think they're there for real, but it turns out that maybe things can be taken away from the universe.
That's right, This fuzzy question of what things are doing when you're not looking at them, and if you look at them and then look away, and it doesn't matter who's looking at them, and how they look at them, and whether they store the information and look at it later. All these fun thought experiments can help us try to understand what's really going on at the smallest scales.
So to the end the podcast, we'll be talking about what is a quantum eraser now, Daniel, is this a rubber eraser or what is it made out of?
It's something which will erase your mind if you think about it too much.
It's kind to stretch it out like a piece.
Of rubber exactly, and push it too hard and it might just snap.
No.
Quantum eraser refers to the concept of quantum information and what happens if you create information and then erase that information from the universe. So it's an extension of some really fun experiments that listeners on this podcast have heard us talk about, the double slit experiment, which reveals how particles can interfere with themselves and have the chance to be in multiple places at once. We dug into that experiment with a fun conversation with Adam Becker, the author of What Is Real, and today we're gonna go double down on those experiments and think about even crazier versions.
Yeah, so, what is a quantum eraser now, Daniel? Is this a thing or like a concept?
Yeah, it's both. It was first a concept and then people made it a thing. It's like that in quantum mechanics. A lot people think, well, if the universe really is that way, here's a ridiculous scenario that should lead to a silly result. And then physicists go out and they do the experiment. They make it real, They figure out a way to like build it in their lab to test that crazy property in the universe, and they get these ridiculous results, which in the end you have to accept because that's what the experiment says. They say the universe really works that m.
All right, Well we'll dig into it. We'll rub that quantum eraser all over our brains and see there's anything left at the end. But first we were wondering if how many people out there had thought about this question or even heard the term quantum eraser. So Daniel went out there and asked people on the internet what they thought a quantum eraser is.
That's right, So if you'd like to be a participant in our virtual person on the street interviews and love to speculate about physics without looking anything up, then please write to us two questions at Danielandthorge dot com.
All right, so think about it for a second. If a random physicist came up to you on the street, and you didn't run away first, And I actually listened to them and they asked you, what is a quantum eraser? What would you say? Your people's answers.
Something an angry physics grad student uses, I have no idea. Maybe something that erases things at random and like gets rid of things, because that's what quantum generally is, is the randomness.
Quantum eraser could be something we invent in the future to erase quantum mechanics and quantum physics, just because it's pretty hard to understand. If we don't have it around anymore, we don't have to deal with it. We can stick with general relativity and regular gravity, So yeah, just get rid of that stuff.
Really, I don't know either if I were to compare a quantum eraser to a regular eraser, which essentially just kind of distorts the little graphite particles and absorbs.
Them and in a way that you know, it gets rid of the material on a piece of paper or something, so that you know you can reuse that material to un on something. Maybe a quantum eraser is some sort of force or phenomenon that causes sometimes particles to break up from a given area or concentration, so that the information is very very difficult to understand or to receive or to observe.
Maybe well, I'm a teacher, and I know that my students use a raise as to high the mistakes. So I think a quantum eraser is something that quantum physicists use to hide their errors from everyone else.
All I can come up with for that is, sometimes you make a mistake and it's for the better, it's a better idea than what you originally planned. And sometimes it's a big catastrophe. But since you don't know ahead of time, you use your quantum eraser to either undo your mistake or make it permanent, and you just roll the dice and let fate decide.
No idea.
Never heard of a quantum eraser, but the image of a really, really tiny eraser pops into my mind. So let's say a tool that allows you to.
Change just supotomic structure of stuff.
I would guess that a quantum eraser is having to do with erasing particles that have certain quantum states, thereby leaving behind particles that you are in the state you want that, or it's the weapon that they used in the movie Eraser with Arnold Schwarzenegger.
All right, a lot of fun answers here. Everyone's a comedian on the internet.
Well, especially if I have no idea what we're talking about, then they got to go to that joking place. I like the joke about the really tiny eraser.
Hmmm right, Yeah, Like if you have a quantum pencil, I guess it would have a quantum eraser on the one end.
Of it, yeah, or erases quantum particles or something like that. I'm imagining a super tiny little vacuum cleaner that like slurps up.
Electrons, yeah, and then does one with them, passes.
Them through a wormhole into another universe.
I guess, tantalizing. All right, well, let's get into this concept of a quantum eraser. Daniel. You're saying it has something to do with the double slit experiment. Now, this is going to be kind of hard because I feel like this is a podcast and it's audio only, and this is a very kind of visual experiment. But I guess we can try our best to describe what it is.
Yeah, maybe we can add a dance element to it. You think that'll help, Yeah, Or I could drop our tuons I'm doing watercolor painting at the same time as we do our podcast.
By the way, Oh really, you're in a quantum artistic.
State, I am. This quantum eraser is an experiment that's like a permutation or an add on to the basic double slit experiment. So to understand why the quantum e racer is so weird and crazy, you definitely have to understand what's going on in the double slit experiment. And so I think we're gonna have to use our words to describe the wiggly crazy nature of that experiment, and then we can build on it to get to the quantumer racer.
All right, So I guess the double slid experiment starts with a single slit first. I guess we'll explain that one, and then we'll multiply by two. So the basic experiment is like you have a wall, like a barrier, like a plate of metal, and you cut a little slit on it, like a little opening that's long in one direction, and then you shoot like a laser or like just a regular beam of light through it and then onto a wall behind the first wall.
Yeah, exactly, So imagine in your mind some source of light. A laser is good because then you have photons of all the same wavelength in the same direct action, and then a screen on the other side where the laser hits. What do you get. You get a laser spot. Now, as you say, put something in between, like a barrier that has a very thin slit in it. Then what do you see on the screen Instead of the full laser spot. Now you see like a slice of that spot. You get a smooth pattern on the screen, but it's sort of cut by the slit in the barrier, and it has smooth edges, not a sharp edge, because that's what happens when light goes through a slit. It tends to like spread out a little bit and smooth out. So the thing you start with is this single slit where the light goes through and hits the barrier on the other.
Side, right, and you get a smooth light pattern on the other side. Now, the weird thing is then what happens if you put a second slit next to the first slit.
Right, that's right, So now you put two slits really close together so that the beam could pass through one slit or the other slit, and once you get on the other side, instead of having like two smooth patterns, or you know, the simple addition of two patterns like you saw before. Now you get an interference pattern, which means you get the patterns of light and dark and light and dark and light and dark and what's happening there is interference. Just like if you have waves when you add them, if one wave is going up while the other wave is going down, then they cancel each other out. Whereas if one wave is going up and the other one is going up at the same time, then they add up on top of each other. They get twice as strong. So the interference pattern has these slices that are twice as bright as the previous pattern, and these dark slices as well. And that's because you have two sources of light. Now, each of the slits is giving you photons and they can either constructively or destructively interfere on the screen.
Right, because I guess you're shooting a laser at both slits at the same time, or you're like shooting a laser and the beam of the laser kind of goes through both slits at the same time. Right.
Yeah, the slits are very narrow and very very close together. This only works if the scale we're talking about here is sort of related to the wavelength of light that we're shooting at it. So this needs to be very microscopic.
Right, So if you have one slit, you get a fuzzy, like a plain fuzzy image on the other side. But if you have two, then suddenly it's not a smooth fuzzy image. It's like a weird Rippley kind of image, which means that somehow light is interacting with itself.
Yeah, in this case, we don't know if the light is interacting with itself. You could say, hey, look, light is a wave, and waves interfere. This happens with waves in the bathtub, it happens with waves in the air. Like noise canceling headphones, right, they generate a second pattern of noise to cancel out the noise that's coming into your ear. So interference in waves is not necessarily a quantum mechanical thing. It's just a wave thing. So in this version of the experiment so far, you could just say, look, light's a wave, it's interfering. No big whoop. It gets quantum when you remember that the beam is actually made not of waves but of photons, little individual packets and so you can take it to the next step by slowing down the experiment and dimming the laser so that it's shooting like one photon through the experiment at a time.
Yeah, you shoot one photon at a time, and then you would think that just throwing like one footon at a time, photon would pick like the right or the left slit and then end up on the other side of the wall, and you would get the same fuzzy smooth pattern. But the weird part I guess is that you're shooting one photon at a time, but you still get the ripley kind of interference pattern on the other side.
Exactly. You expect that if you shoot one photon at a time, then it can't interfere because you were thinking, well, the interference comes from two photons going through both slits at the same time. Now you have just one photon in the experiment, So what's it interfering with because you still see the interference pattern on the other side of the screen when you shoot one photon through at a time. It's just that it takes longer to build up if you watch it for an hour or so, as those photons go through one lands here one lance, there, one lens, this other spot. It gradually builds up that interference pattern. So what's it interfering with. It's interfering with itself. It has the probability to go through both slits, and that wave function, which controls where a quantum particle goes, interferes with itself and creates this probability distribution on the screen for where it might land, and that probability distribution has the interference effects inside of it, and that's why you get this interference pattern. Every photon that goes through like randomly pulls a number from the probability distribution on the screen which has the interference pattern built in, and lands there, and gradually it builds up that distribution.
It's almost like, you know, if you were to shoot a photon as a little ball, it would go through one of the slids, but because it's quantum, it's almost like it's going through both slits at the same time, right, that's kind of the quantum thing. It's going through both slits at the same time, and then it's sort of going through both slits and then interacting with itself in a quantum way, so that when it gets to the screen. It's not a smooth pattern.
Yeah, it's tempting to say that it's in two places at once, or that goes through both slits at the same time, and that's our tendency to try to like tell a story for what happens. But I'm not sure that's the right way to think about it. The way I think about it is that it has a probability to go through both at once. What it actually does is not determined, you know, until it gets to the other side. So happened when it went through the slits? We don't know, We might never know. There isn't necessarily a story there. So it's a small change in wording, but an important change in meaning for me to say that it had probability to go through both slits rather than it actually went through both.
Right, it's like saying, it's not that the cat is alive and dead, as you said, it has the same probability, or it has a certain probability of being alive and a certain probability of being dead. All right, Well, then now the weird part here. Now it is going to come when we try observing this photon, and that's when we get into this idea of quantum erasors. So let's talk about that. But first let's take a quick break.
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All right, we were asking the question what is a quantum eraser? And now, Daniel, I feel like half of my brain is already erased trying to talk about quantum objects. And now we were explaining the double slit experiment, and so I think we were done with that. Like if you shoot a laser at a two small slits on a screen, then on the other side you're going to get an interference parent because of the way the quantum probabilities kind of affect each other. And so now the weird thing happens when you try to like add a detector right where and you try to see which slid it actually went through.
That's right, because our tendency is to want to know, like, well, what happened, right, did it go through one slit or did it go through the other. We feel like it must have gone through one or the other, right, because you know, if it was over here and then it is over there, so it must go from here to there is our sense. And so to try to get in like a more accurate understanding of what happened, you could add a little detector one that gives you a signal if a photon goes through slit A, for example, instead of slip B. So that way you can know, hey, did it go through slid A or slip B. Because you know, photons are observable, right, You can interact with them, They make splashes of light, you can measure them. They are quantum objects, but they are also physical. And so what happens when you do that, when you ask, when you insist on knowing which slid it went through, is that the interference pattern disappears. When you add that detector that just tries to understand which slid it went through, then the interference pattern is gone.
So if you try to like measure and the photon as it goes through slit, how would you even do that? Don't you have to stop the photon to do that.
That's the crux of the matter right there. To measure a photon, you have to interact with it somehow. You can't just like observe a photon without interacting with it. You know, a photon that like passes in front of you, you can't see it. It's a piece of light, but unless it hits your eyes, you can't see it unless you put something in front of it to stop it, measure it, and then re emit it. Right, So, for example, the reason you see something in front of you as red is because a photon has hit that object and then emitted red photons. So you can't see things without interacting with them, even light, right, You need to interact with it somehow. So you can make up lots of different physical systems that could do this, but the simplest one is, you know, just like a simple photon detector, a photo multiplier tube for example, or a scintillator screen that indicates when a photon went through and then re emits it on the other side.
Oh, I see what you're saying, Like if you catch it and then release it back on the other side, that's one way you could measure the photon.
And you'd like to think, Oh, can't I just take a peek? Can I just look and see where it went without touching it, without interfering with it, without messing with it in any way, But you can't do that. Quantum mechanics tells us that the only way to get information about an object is to interact with it. You can like bounce a photon off of it, or you can bounce an electron off of it, or you can put another little screen, but somehow you have to interact with it. You can't get information from that particle without somehow interfering with its path.
Yeah, because I guess in our everyday lies, we're used to this idea of being able to like see things but not touch them, and so we think we can tell where something is without actually like influencing it. But when you get down to the smallest of levels, like all seeing is interacting in a way.
That's right, And it's also true with the macroscopic scale. It's just that you don't notice it. Like if you are walking outside at night and you want to know, hey, is there a rock in my path? You turn on your flashlight. You are shooting photons at that rock. Those photons are hitting the rock, they're warming up the rock. Then the rock is reflecting photons back at you. So yeah, you're not touching the rock, but you're definitely interacting with the rock and you're changing its quantum state. It's just like it doesn't like really heat up the rock or push the rock far away. When these are quantum particles, they can have significant effects if you interact with them by shooting beams of light at them or other particles.
All right, So then in the double slit experimentment, if you try to measure these photons before they hit the wall in the back, then what you're doing is you're collapsing the quantum wave, right, You're messing up the quantum information.
Yeah. So here's where the different interpretations of quantum mechanics all tell you different stories for what happens. The experiments say if you measure the photon before or after the screen, that the interference pattern goes away. The classical interpretation of quantum mechanics. The Copenhagen interpretation is what you just described. It says that the probability to go through both slits only exists if you haven't made a measurement. But then if you interact with it, it collapses the wave function. Now it can only go through one slit or the other. So there's no interference because the interference came from the ambiguity came from the probabilities to go through both. The Many World's interpretation tells a different story. It says collapse is nonsense, that doesn't happen, it's ridiculous. It says that the universe splits into two, one where the photon went through one slit and one where the photon went through the other, and you're in one of those universes and not the other.
Right, So those are the two ways in which you can interpret what happens. So how does that relate to the information and the quantum information?
So the idea here is that you have the information about whether it went through slit A or slit B, and that's what destroys the interference. Because you've made this measurement. Somehow it changes the experiment. And you know, this is not something that we understand very well, this whole concept of measurement in quantum mechanics, and that was the topic of our episode with Adam Becker, and then recently we did an episode with Carlo Ravelli and he's got a whole new theory for how to understand measurement in quantum mechanics. It's not something that physics understands very well. How interacting with something changes its wave function? Does it collapse it, does it split the universe? All of this kind of stuff. But the key idea here is that you extract information about which way the photon went, then there's no interference.
Right, You somehow get rid of the quantumness of it. Like when I poke something, it's no longer quantum.
If you poke something with a classical object, like a big detector, right, that big detector can't be in two stays at once. It can't be like, well, yes I saw it and no I didn't. Has to make a decision, and so it decoheres and you get this weird thing where a quantum object is interacting with the classical objects and so now it has to like follow the classical rules so the quantum eraser is an attempt to get around that, is to say, what if instead of poking it with a big finger or a big classical detector, what if we've got this information but we somehow kept it quantum at the same time.
I see. So it's almost like the cat and Strodenger's box, Like, before you open the box, it's both alive and dead. This is a probability of it being one or the other. And if you open the box, that's the classical way of checking it out, Like you open it and it's either alive or dead, and then you get rid of the quantum probabilities. You're saying, can I like somehow, you know, poke my quantum finger into the box and measure, but not kind of destroy that superposition of being both alive and dead exactly.
The quantum eraser experiment tries to do that. It says, well, let's try to get this information out, but not look at it directly, not use like our classical objects, our eyeballs, our brain, it's even our computers to access that information, so it can stay in a quantum superposition, so we can make a decision later about whether we want that information and here's where the mind bending stuff comes in. If you can like extract that information about which way the photon went, keep it in a quantum state by storing it in some other entangled particles, then you can decide after the photons have hit the screen whether or not you want to know which way it went.
Wait, see it again.
So the idea is, you want to know which way the photons went, right, do they go through split air slip b. You know, if you add a classical object like a big detector, you're going to collapse the wave function. So instead you add a quantum detector one that can record this information, but maybe without collapsing the state of the wave function. Somehow it gets this information, but because it's a quantum object, it doesn't trigger the collapse, right, So it can be like entangled with the photon without like forcing the photon to decohere completely. And so it's different from interacting with like your big body or something. You interact with it like with a single particle, and that's it stores the information about which way the photon went, but you haven't looked at it, you haven't collapsed that wave function yet. You let the photon then go hit the screen, and then after the photon has already hit the screen and decide where it's going to land, then you access that quantum information. It's called the delayed choice version, where you decide after the photon has hit the screen whether or not you want to know the information about which way it went.
You're saying that the photon did go through one of the two slits, like once it hits the screen in the back, then it sort of chooses a history of having gone through the left or the right slit.
That's right. This is trying to like force the photon to make its decision about whether or not to make an interference pattern before you decide whether you want to know which slit it went through. So it's sort of like, you know, trying to play quantum bluff with the photon. And that's when we get into these really funny questions of like, how does the photon know whether it's going to be measured? How does the photon know whether you're going to have information about it?
Mmmm.
It's almost like you want to, you know, peek inside of the Schrodinger's box but not look at the answer, so that it's still alive and dead inside the box, but you sort of have the answer in your pocket, but you don't. You haven't looked at the answer yet exactly, And so why is that? Call it a quantum eraser?
Right, So this is not yet a quantum eraser. This is the delayed choice version.
The delayed measurement.
Yes, exactly, the delayed measurement choosing whether or not you have the information. That's the delay. So you might wonder, like, well, what happens on the screen, What does the screen look like? What does the experiment look like? If you do this, if you capture this information in a quantum state, but you don't look at it yet, Well, what happens is you don't see interference on the screen because by doing this, by slurping this information out of the photons, you have destroyed the interference. But people think, well, that's interesting. But I haven't yet looked at that information, right, So what happens if I then erase that information? This is where the quantum eraser comes in. If I take that quantum information which is stored in these quantum objects, but I haven't looked at it yet, If I erase that information somehow, can I then recover the interference? Can I make the interference pattern reappear on the screen by adding this quantum eraser, which like deletes that information from the universe because I never peaked at it.
Wait, are you saying that this is an actual experiment, Like we've sort of intercepted the photon before it goes into the slits, and we've stored that information, and we see that it now it doesn't generate an interference pattern, a weekly pattern on the screen, even though nobody really knows which slid it went through.
That's right, nobody knows what slid went through, though it is in principles stored in this quantum object. Although that quantum object can be in a superposition, it doesn't have to be in a definite state.
Right.
You can say there's a probability of one and a probability of the other, and we don't see that interference pattern. So then people thought, well, what if we delete that information? What would the universe do if we measure the photon but don't look at it, keep it a quantum state, and then erase that information. Can it somehow go back and recover the interference pattern.
I see you're saying that maybe it's not the fact that it interactive with your little secret finger poking quantum poking. That destroyed the interference pattern. Maybe if I poke it with my quantum finger and then I destroy my finger, will it go back to being a quantum object? Is that what you mean? Like, we know that if I poke it, even with a quantum finger and not look at the answer, it destroys the quantum information. But now what happens if I poke it with the quonda finger and then destroy the finger, will go back to being a quantum object? Is that kind of the idea?
That's the quantum eraser is destroying that quantum information you've extracted from the experiment, but having yet looked at so it's still quantum. So that's the crazy experiment. And I can hear you react and say, what is that a real experiment? Did we really do that? And yes, we have really done this experiment, and we have done it with photons, and you can go up and google and learn all about the details of this experiment. I think there's a slightly simpler version that's easier to talk about, where you use electrons, But the principles are all the same. And so how can you destroy quantum information? Well, for example, if when the photon is passing through the slit, you have some detector, and that detector takes an electron and puts it in like a spin upstate if the photon went through one slit, and a spin downstate if the photon went through another slit. This is just a way to like store that information about which way the photon went and keep it quantum.
Right.
We don't want to like mark it on a piece of paper or put it in a computer some big classical object. We want to keep it as quantum information. So here the electron is just like a single cube bit. It contains some quantum information, but it can be in a superposition. It can be a little spin up and a little spin down.
We don't know yet, right, but I feel like you bumped the photon right, Like the photon was going through the slip, but you made it bump into this electron. And now it feels like it's now an impure experiment because you bumped it right, not just in a quantum way, but you did sort of. It's not maybe the same photon, or it's not the same path as a photon who didn't bump into an electron.
That's right, and that's why you no longer see the interference. Right, you add this experiment where you're bumping it into the electron, it destroys the interference pattern because that information is now stored in the electron, so it can be extracted. The knowledge of which way the photon went can be measured in the universe, and so that destroys this interference pattern on the screen. So you're right, it's a different experiment.
Right, Like the quantumness went from being on the w behind the screen to now being in this electron who you coked it with? And now I guess the question is if I destroyed the information in that electron, do I get back my weekly pattern on the screen.
That's the idea. It's totally mind bending and crazy what actually happens. I love it. And so you take this electron and you might wonder like, well, how do you destroy the information? How do you erase the information? Well, it's actually not that hard to erase quantum information. It happens all the time. Like if you, for example, measure a particle's momentum, then that scrambles your knowledge of the particle's position because the Heisberg uncertainty principle says you can't know both very very precisely. If you have a particle, for example, you measure its position really precisely, and then you measure its momentum, then you've erased the quantum information about its position because you can't have both simultaneously. So you can do sometin things sort of similar to this electron. You can't know an electron spin in one direction and in another direction at the same time. So if you want to erase this spin up and down information, all you need to do is measure the spin of the electron sort of left right, and that will scramble the information about the electron spin up down.
I poked the box with the cat with my quantum finger, and now I'm sort of running my finger through a filter that then kind of scrambles or filters out the information from the cat.
That's right. So now it's no longer possible to know which state it was in. Was it spin up or was it s been down? We don't know anymore, and it's scrambled. It's not like the information existed and we've overwritten it. It was in a quantum superposition, it was undetermined, and now the information about those probabilities is lost. So that's the quantum eraser. It says, destroy the information that you've extracted from this experiment.
All right, So we did the experiment, actually we poked it with something and then we erased the information. And did they actually somebody actually built this.
Somebody actually built this, and they did it. They did this experiment. And so there's a lot of discussion of this kind of experiment online and I found a lot of these to be sort of misleading because they suggest that what happens when you apply the quantum eraser, when you erase this experiment, is that the interference pattern like reappears on the screen, which is impossible because you could do this like quantum eraser experiment like years later, after you've already done the original experiment. You know, you could like store these electrons somehow, and then five years later decide to erase the information. You can't go back in time and then change the interference pattern on the screen. So that's not what happens. That would be crazy, in bonkers and awesome. But instead, what happens is that if you do this, if you erase the quantum information, then you are making a measurement of those electrons, you're measuring them like left right instead of up down. If you take those results and you look at only the ones that have like electron that turned out to be left, or only the ones electron that turned out to be right, then you see the interference pattern. So the photons that had like a right spinning electron, you see an interference pattern in those photons, and there's an interference pattern in the photons that had a left spinning electron. If you put them together, they add up to the same smooth shape. So sort of like the interference pattern was hiding inside that smooth shape. And if you scramble the information that you knew about which photon went where, you can recover that interference pattern from within the smooth shape that you saw on the screen.
Oh right, But then that still requires an observation, right, because you're using some of that information you thought you destroyed, but you didn't really destroy it in a way or like, do you destroy the information in one direction and so the quantum objects sort of adjusted into the other direction.
Yeah, you destroyed the original information. You can't know which way the photon went right, and so that allows you to have interference, and you can recover that interference if you look at, like some of the photons. And the reason is that you know some of these photons are entangled with some of these electrons in this way, and you need to like know how to pull out the subset of photons that have the interference pattern you're looking for. You can only get that if you have erased the quantum information you were looking for. If you access the quantum information directly, if you measure spin up or down so you know which photon went through which slit, then that collapses the wave functions essentially and means so you see no interference. You cannot access any interference only if you erase the information in those electrons by measuring left right instead of up down. Can you then go back and split the photons into two categories, each of which shows interference.
Interesting? All right, let's get into what this all means and what it can mean about how we see reality. But first, let's take another in quick break.
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Hi, I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I'm a neuroscientists at Stanford, and I've spent my career exploring the three pound universe.
In our heads.
We're looking at a whole new series of episodes this season to understand why and how our lives looked the way they do. Why does your memory drift so much? Why is it so hard to keep a secret, When should you not trust your intuition? Why do so easily fall for magic tricks? And why do they love conspiracy theories. I'm hitting these questions and hundreds more because the more we know about what's running under the hood, the better we can steer our lives. Join me weekly to explore the relationship between your brain and your life by digging into unexpected questions. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple.
Podcasts or wherever you get your podcasts.
Guess what, Mango?
What's that?
Well, so iHeart is giving us a whole minute to promote our podcast, Part Time Genius.
I know.
That's why I spent my whole week composing a haikup for the occasion. It's about my emotional journey in podcasting over the last seven years, and it's called Earthquake.
House, Mega Mango, I'm going to cut you off right there. Why don't we just tell people about.
Our show instead?
Yeah, that's a better idea. So every week on Fartime Genius, we feed our curiosity by answering the world's most important questions, things like when did America start dialing nine to one? One? Is William Shatner's best acting work in Esperanto?
Also?
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All right, Daniel, I'm still sort of stuck in the cat in the box experiment because I feel like it's a little bit easier to grasp. So we had a cat in the box. We talked up with a quantum finger, and then we measured my finger in one direction, and then we sort of destroyed that information by measuring the finger in a different direction. And then we see that the cat is still sort of alive and dead. But only if we use the information we got from the finger.
Right, Yeah, exactly. I think if you want to talk about cats and boxes, then you'll need like one hundred and one hundred boxes because in the end, this is a probabilistic effect. Just like with the double slit experiment, this interference patterns are only obvious if you do a lot of photons, so you can see the patterns. Because a single photon could hit the screen wherever, you can't tell if you're seeing an interference pattern or not from one photon. So let's say you have you know, you're a cat person. Your house is swarming with cats. Each one you put in a box, right, and then you make this quantum measurement of each one, but you don't look at the result, right, you poke it with a quantum finger, you don't look at the results.
All right, So then you figured out I guess with this experiment that the objects sort of goes back to being quantum, but not really because maybe you're in a way, you're sort of cheating, right, You're using some of the information you got from poking it to make it look quantum again in a way. Right, It's almost like the photon went back to being quantum, but only half quantum because you were able to measure some of it.
Yeah, exactly. It's a bit mind bending because we like to think about what happens to these particles, like what do they really doing? And we like to think that things can't go back in time and change their decision. And you know, the core fuzziness of this experiment is that if you think about these things in terms of particles and waves, you like to think that it's a wave and then it gets collapsed into a particle after it goes through the slit, if there's a detector there, and then needs to decide like am I a particle or am I a wave before it hits the screen, so that it can either make an interference pattern or not. Right, it doesn't seem like it would make sense for that to depend on what you do later on, because you can make this like quantum information decision a year later, or ten years later, or a thousand years later. So some people are at tempted to say that this means that there's retro causality that based on what you do later, it can go back in time and change the results of the experiment. I think that's kind of nonsense. Really, what you're doing here is just interpreting the experiment in a different way using additional information you've extracted from the experiment. As you said, you're sort of cheating, but it's backwards. Right now, you're making a measurement to know how to separate those photons to see the interference pattern. You're not making a measurement about which way the photon went about which slid it went through. Instead, you're just separating the photons into the ones that were entangled with electrons in one way versus photons that were entangled with the electrons in the other way. And those subsets do have the interference going on, it's just it was masked because when you add up the two kinds of interference, they add up to the smooth pattern.
It's like it's still the same sort of regular quantum that was going on before. Like the photon looked like it had lost this quantum information, but really, like if you take that information that you got from the poking with the finger, then you can sort of find its quantumness in the direction that you didn't poke it in.
Yeah, exactly. And so it's a really fun experiment to try to think about this nature of decoherence. Like you were saying before, if you poke something with a big classical object, it decoheres. It gets entangled with the whole environment millions and millions of particles, and so all of its quantum properties are essentially lost.
Here.
What we're doing is we're like kind of cheating, we're decohering it, only it tiny little bit by interacting with it with a quantum object. So we can play quantum games with that decoherence later and in the end recover some of that interference by erasing that information. And so it's really sort of a great mental exercise to think about whether you understand decoherence. And we had a whole podcast episode about what quantum decoherence is. It's closely connected to this question of what is a quantum measurement and what happens when you measure something, but it's not quite the same thing. It's more about whether quantum properties can be observed because the different quantum states are still coherent, whether they add up and cancel out in just the right ways to make something have a quantum effect.
I think the main point is that you know everything's quantum, but quantumness of something can exist kind of in different directions in a way, or in like different aspects that are part of the whole, but it's still you can sort of take out half of the quantumness of an object and still preserve sort of the other half of the quantumness that it has in the other direction.
Yeah, exactly. And so you know, the trickiness here relies on the fact that, like by becoming entangled with a single electron rather than the whole environment, these photons hit the screen only become kind of decohered, right, and so it's just a single particle to worry about. We're sort of able to think about measuring it in different ways, and that's really fun, and it's easier to think about what this experiment means in some interpretations of quantum mechanics than in others. Like in many worlds, it's not that big a deal because the whole universe has a wave function and now we're just talking about the quantum wave function of the photon and the electron and they're kind of entangled and that's no big deal, whereas in like a strict theory where you have collapse, then you have to wonder like, well, did the photon collapse or not, because if I don't destroy the information and I measure it, then the photon has to collapse because I knew which way it went. But if I do destroy the information, then how am I getting an interference pattern later on? Because for that to happen, it has to stay a wave. And so this is sort of troublesome for the collapse theories of quantum mechanics, not so much trouble for other theories like many worlds and relational quantum mechanics.
Doesn't that just mean that maybe like the wave collapse in one direction but not the other direction, Like couldn't you still you know, use the coping dating interpretation and just say that it collapse in like one direction and not the other.
Well, the electron is the one that has these multiple directions of information that spin up down versus spin left right. The photon is either interfering or it's not, you know, And it either collapses and it's just like a single source which gives you the smooth pattern, or it doesn't collapse and you have the wave function which does interfere. There aren't multiple directions there. And so it's hard to understand how a collapse theory can really work because that does kind of require going back in time and like uncollapsing the wave function to me collapsing the wave function makes no sense at all. It's not even consistent with quantum mechanics because it destroys quantum information in a way that we know violates basic principles and it violates the time continuity of quantum mechanics. You should be able to run experiments forward and backwards. So the collapse theory never made any sense to me, really, and I think this experiment really highlights how it's sort of nonsense.
But then the only other interpretation that we have is the multi world's theory, right, which this multiverse theory that every time a quantum object makes a decision, that the two universes are created.
Yeah, that's another interpretation, and that one is pretty happy with this experiment. There are other interpretations that you can use that are consistent with this experiment, Like relational quantum mechanics works well with this because it says that like, hey, everything in the universe has its own measurement of these things, and so it doesn't matter what you measure, there is no reality anyway. And then there are also like other variants of collapse theories that are not as strict. You know, let's say, well, collapse happens in this way or in that way. So there's a whole spectrum of them. But this is troublesome for like the most hardcore collapse theories.
All right, well, then I guess to answer the question what is a quantum eraser? I feel like the answer to that is sort of straightforward, but it's sort of the implications of what quantum eraser can do. That's really sort of what we spend an hour, and that's really hard to sort of get your head around. So a quantum eraser is just taking quantum information from something and eraising it in a way. Right, Like, if I have quantum information stored in one direction of an electron spin, by measuring it in the other direction, I can destroy that quantum information. Right, that's the idea of a quantum eraser.
And if that electron happens to be entangled with photons which may or may not be interfering, then whether or not you erase that information or not can determine whether or not you can see interference in those photons.
Well, it doesn't determine whether or not you can see it in a way, it tells you how to look for that interference.
Well, if you measure the which way of the photons using those electrons, then you cannot see interference. The only way to see interference is to destroy that information and then use the results of destroying that information to pick out the interference patterns from the photons. You can't do that if you measure which way the photon went.
Right, Right, But you're sort of still measuring the electron, and then that's telling you how to look for the interference in the photon.
Right. Yes, you're measuring the electron, but you're not measuring which way the original photon went. You're measuring something else about the electron, which destroys that information.
All right, it sounds like we erase people's brain and hopefully not their time for the last hour.
Thanks very much for going on this journey into the weird quantum world. I love these thought experiments, the ones people think of and say, whoa, what would actually happen? Because that's the fun thing about experimental physics is confronting the universe and saying, all right, universe, show us what you got. We set up a situation that forces you to reveal what's happening, and the quantum universe always responds with something crazy.
And that's why we're here to talk about the craziness and to hopefully get you to wrap my mind around all the different and interesting implications about what it means about the things around you that you see in touch or maybe don't see your touch.
And so, if you're a person who likes questions and maybe even answers, check out our book Frequently ask Questions about the Universe, available now and coming out in just a couple of weeks.
You can find the links at Universe faq dot com. All right, well, thanks for joining us. We hope you enjoyed that. See you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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Guess what will?
What's that mago?
I've been trying to write a promo for our podcast, Part Time Genius, but even though we've done over two hundred and fifty episodes, we don't really talk about murders or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We all also solve mysteries like how Chinese is your Chinese food? And how do dollar stores make money? And then of course can you game a dog show? So what you're saying is everyone should be listening. Listen to part Time Genius on the iHeartRadio app or wherever you get your podcasts.
