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Speaker 1: thinking about the environmental impact. But the people in the

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Speaker 2: Hi.

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Speaker 3: I'm David Ego from the podcast Inner Cosmos, which recently

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Speaker 3: hit the number one science podcast in America. I mean

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Speaker 3: neuroscientists at Stanford, and I've spent my career exploring the

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Speaker 3: three pound universe in our heads.

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Speaker 2: Join me weekly to explore the relationship.

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Speaker 3: Between your brain and your life, because the more we

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Speaker 3: know about what's running under the hood, be or we

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Speaker 3: can steer our lives. Listen to Inner Cosmos with David

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Speaker 3: Eagleman on the iHeartRadio app, Apple Podcasts or wherever you

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Speaker 3: get your podcasts.

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Speaker 2: Hi.

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Speaker 4: Everyone, it's me Katie Couric. You know, lately, I've been

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Speaker 4: overwhelmed by the whole wellness industry, so much information out

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Speaker 4: there about flaxed pelvic floor serums and anti aging. So

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Speaker 4: I launched a newsletter it's called Body and Soul to

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Speaker 4: share expert approved advice for your physical and mental health.

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Speaker 4: And guess what's free. Just sign up at katiecurreic dot

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Speaker 4: com slash Body and Soul. That's k A T I

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Speaker 4: E c O U ric dot com slash Body and Soul.

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Speaker 4: I promise it will make you happier and healthier.

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Speaker 2: Hey, it's Horehan and Daniel here, and we want to

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Speaker 2: tell you about our new book.

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Speaker 1: It's called Frequently Asked Questions about the universe.

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Speaker 2: Because you have questions about the universe, and so we

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Speaker 2: decided to write a book all about them.

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Speaker 1: We talk about your questions, we give some answers, we

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Speaker 1: make a bunch of silly jokes.

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Speaker 2: As usual, and we tackle all kinds of questions, including

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Speaker 2: what happens if I fall into a black hole? Or

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Speaker 2: is there another version of you out there that's right?

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Speaker 1: Like usual, we tackle the deepest, darkest, biggest, craziest questions

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Speaker 1: about this incredible cosmos.

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Speaker 2: If you want to support the podcast, please get the

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Speaker 2: book and get a copy of not just for yourself,

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Speaker 2: but you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters, and.

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Speaker 1: For the aliens. So get your copy of Frequently Asked

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Speaker 1: Questions about the Universe. It's available for pre order now,

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Speaker 1: coming out November two. You can find more details at

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Speaker 1: the book's website, universe faq dot com. Thanks for your support, and.

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Speaker 2: If you have a hamster that can read, please let

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Speaker 2: us know. We'd love to have them on the podcast.

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Speaker 2: Hey Daniel, do you ever feel like you really understand

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Speaker 2: quantum mechanics?

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Speaker 1: No, you know, I think it's probably just too alien

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Speaker 1: for us to really ever feel comfortable with.

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Speaker 2: I guess it's too bad there aren't any macroscopic big

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Speaker 2: quantum optics we can really like poke and play with.

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Speaker 1: I know, like a big fat electron. But actually I'm

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Speaker 1: working on a theory that children are governed by the

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Speaker 1: rules of quantum mechanics.

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Speaker 2: Oh really, like there's uncertainty about where they are. Have

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Speaker 2: you lost your kids?

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Speaker 1: No, but I've noticed that you can't like observe your

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Speaker 1: children without sort of perturbing the system, right.

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Speaker 2: I think I know what you mean. Like, they won't

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Speaker 2: do their homework unless you're there watching.

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Speaker 1: Exactly, And when I walk into the room, somehow all

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Speaker 1: their conversations collapse suddenly into silence.

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Speaker 2: Yeah, it's like short angers children. They're both excited and

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Speaker 2: sad to see it.

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Speaker 1: Hi.

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Speaker 2: I am horhand made cartoonist and the creator of PhD comics.

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Speaker 1: Hi I'm Daniel. I'm a particle physicist and a professor

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Speaker 1: at UC Irvine, and I'm always in many quantum states

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Speaker 1: at once.

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Speaker 2: Really, so that does that mean you're not real?

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Speaker 1: It means I don't even know if I'm real. Man.

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Speaker 2: Well, I hope you are, because that would mean that

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Speaker 2: I'm talking to myself right now, and that would be

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Speaker 2: a little concerning.

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Speaker 1: Maybe you are the only brain in the universe and

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Speaker 1: the rest of the universe is just part of your mind.

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Speaker 2: That would make a lot of sense why I'm so

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Speaker 2: successful and good looking. But anyways, welcome to our podcast

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Speaker 2: Daniel and Jorge Explain the Universe, a production of iHeartRadio.

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Speaker 1: In which we do try to explode our minds out

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Speaker 1: to capture the entire universe. We want to take this vast, glittering, crazy, violent,

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Speaker 1: wild and white cosmos and wrap it all up inside

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Speaker 1: our brains. It's not enough for us to just live

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Speaker 1: in this universe, to experience it and to see it.

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Speaker 1: We want to understand it. We want to download the

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Speaker 1: whole thing into our minds, and that means understanding the

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Speaker 1: basic rules about how it works, what's really going on

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Speaker 1: with tiny little particles or whatever is happening at the

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Speaker 1: smallest scale. On this podcast, we dive the deepest you

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Speaker 1: can into the hardest and trickiest of questions and we

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Speaker 1: try to explain all of them to you.

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Speaker 2: Yeah, because it is a pretty tricky universe. It's full

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Speaker 2: of interesting rules and interesting phenomena that happens at the

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Speaker 2: smallest of levels and at the largest of scales, and

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Speaker 2: a lot of it is understandable, even if it's not

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Speaker 2: very intuitive to think about.

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Speaker 1: That's right, And I look at the universe like a

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Speaker 1: big puzzle. It's like a detective novel or a murder mystery,

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Speaker 1: and I want to figure out who did it. I

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Speaker 1: want to understand how it works. It's amazing to me,

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Speaker 1: sort of philosophically, that the universe is presented to us

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Speaker 1: that way, like this big puzzle that isn't obvious to

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Speaker 1: figure out, but yet can somehow be understood if you

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Speaker 1: push hard enough.

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Speaker 2: Do you think the universe is understandable? Daniel? That's a

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Speaker 2: big question in physics, isn't it.

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Speaker 1: It's a big question in the philosophy of physics. And

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Speaker 1: my answer is it doesn't make sense for it to

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Speaker 1: be understandable, Like how could it be possible that the

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Speaker 1: complexities of this maybe infinite universe could be stored in

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Speaker 1: the minds of a human. On the other hand, we

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Speaker 1: have all these theories that work really well, like surprisingly well,

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Speaker 1: and so I don't know how to hold those two

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Speaker 1: ideas in my mind. They're in quantum conflict.

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Speaker 2: You're both confused and the feeling smart at the same time.

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Speaker 2: That's what this podcast is all about. I'm the confused

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Speaker 2: quantum state and you are the feeling smart state.

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Speaker 1: That's right. And all scientists sort of hold those two

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Speaker 1: feelings in their mind at once, like, look at all

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Speaker 1: we have understood, and yet look at all that we

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Speaker 1: do not, And that's both exciting and terrifying.

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Speaker 2: Yeah, and we have a lot of questions about the universe.

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Speaker 2: And by the way, speaking of questions, we have a

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Speaker 2: new book coming out pretty soon on November two.

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Speaker 1: That's right, Orhean and I celebrate asking questions about the universe,

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Speaker 1: and we love thinking about these questions. I love hearing

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Speaker 1: about your questions about the universe. So we wrote a

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Speaker 1: book that wraps up the most frequently asked questions that

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Speaker 1: we get about the universe.

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Speaker 2: Yes, if you like this podcast and you want to

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Speaker 2: support us, please check it out. It's called Frequently Ask

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Speaker 2: Questions about the Universe and it's on a pre order

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Speaker 2: right now. I can order it now and get it

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Speaker 2: as soon as it comes out. And I think you

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Speaker 2: know what's important is that we kind of didn't quite

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Speaker 2: write it for our listeners, right, Daniel. We kind of

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Speaker 2: wrote it for the people that know our listeners you know,

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Speaker 2: if you ever have like a nephew or a cousin

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Speaker 2: or an uncle who you want to share this amazing

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Speaker 2: information about the universe, I think this is the book

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Speaker 2: for you or for.

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Speaker 1: Them, that's right. So every one of you out there,

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Speaker 1: you should buy five copies and give them to all

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Speaker 1: your friends and family so that they can understand the

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Speaker 1: answers to questions like where did the universe come from?

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Speaker 1: Or how can we travel to the stars.

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Speaker 2: Yeah, we tackle all kinds of pretty cool questions in it,

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Speaker 2: and we try to answer it for people like your

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Speaker 2: relatives and friends with all kinds of interesting and clear answers.

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Speaker 2: And also cartoons, which is I think something you don't

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Speaker 2: see in every day and physics books.

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Speaker 1: That's right, all these awesome fun drawings that help clarify

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Speaker 1: the topic and amuse you along the way that Jorge

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Speaker 1: added to this book. So if you like this blend

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Speaker 1: of physics and silly jokes, then I think you'll enjoy

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Speaker 1: this book. So go out and get your copy. You

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Speaker 1: can find it at universe faq dot com.

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Speaker 2: All right, well, speaking of questions, we are tackling a

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Speaker 2: question today and it has something to do with quantum mechanics,

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Speaker 2: which is I guess, for lack of a better technical term, bonkers.

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Speaker 1: It's my favorite kind of bonkers. It's the kind of

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Speaker 1: bonkers that doesn't make sense to your mind. But the

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Speaker 1: math works perfectly and it keeps predicting absurd experimental conclusions

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Speaker 1: that experimentalists keep verifying.

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Speaker 2: Yeah, because I guess you started with a little nugget

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Speaker 2: of an experiment, and then you worked out some math,

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Speaker 2: and then you find out that the crazy math that

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Speaker 2: it suggests is actually also true.

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Speaker 1: Yeah, we had to change the basic concept of what

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Speaker 1: we thought was going on at the very heart of

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Speaker 1: the universe or of everything that's around.

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Speaker 5: Me and you.

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Speaker 1: Even though it seems intuitive and like it follows rules

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Speaker 1: that we're familiar with from growing up, it turns out

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Speaker 1: that the tiniest little parts inside are following totally different rules,

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Speaker 1: which mean that the nature of reality is quite different

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Speaker 1: from the one that we thought it was. And people

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Speaker 1: worked that out and they thought that's crazy, and if

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Speaker 1: it's true, it would mean you could do this Bonker's

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Speaker 1: experiment which would have this nonsense result, so obviously it

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Speaker 1: can't be true. And then business went out and did

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Speaker 1: the experiment and got the nonsense result, which turns out

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Speaker 1: to be the truth of our reality.

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Speaker 2: Yeah, because I guess at the core of it, it's

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Speaker 2: kind of weird for us humans to think of things

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Speaker 2: that are like two things at the same time, right,

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Speaker 2: I mean not just in a conceptual lede, but like

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Speaker 2: actually in reality, in quantum objects, things can be multiple

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Speaker 2: things at the same time.

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Speaker 1: That's right. And that phrase in reality is the key there,

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Speaker 1: because we imagine at the very basic level that there

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Speaker 1: is a reality out there, that there is a truth

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Speaker 1: that even if we're not looking, the universe is there

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Speaker 1: and it's operating and it's following some rule and it

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Speaker 1: doesn't really matter if we are looking or not. But

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Speaker 1: the reality suggests that the universe is quite different from that,

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Speaker 1: that it does matter if you interact with it, and

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Speaker 1: that what is happening in the universe is not exactly

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Speaker 1: well determined until you interact with it.

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Speaker 2: Yeah, you might say that in reality, the way the

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Speaker 2: world works and the universe works, it's kind of fuzzy,

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Speaker 2: kind of not quite as solid as we might think

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Speaker 2: it is from our everyday lives.

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Speaker 1: That's right. We have a weird and particular view of

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Speaker 1: this quantum universe. We are only used to interacting with

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Speaker 1: enormous quantum objects like baseballs and rocks and trees, which

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Speaker 1: are quantum objects, but they contain like ten to the

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Speaker 1: twenty six quantum objects, and when you have that many,

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Speaker 1: they do things differently than when you have one or

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Speaker 1: two of them isolated to reveal their sort of true

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Speaker 1: fundamental nature. So today we're going to be talking about

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Speaker 1: some really crazy experiments that try to reveal exactly what

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Speaker 1: the rules are of how these particles work at the

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Speaker 1: smallest scale when they're left alone.

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Speaker 2: Yeah, and so it turns out that also reality it's

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Speaker 2: not just a little bit fuzzy, but it may not

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Speaker 2: even be as permanent as we think it is. Quantum

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Speaker 2: information and quantum things we think they're there for real,

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Speaker 2: but it turns out that maybe things can be taken

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Speaker 2: away from the universe.

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Speaker 1: That's right, This fuzzy question of what things are doing

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Speaker 1: when you're not looking at them, and if you look

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Speaker 1: at them and then look away, and it doesn't matter

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Speaker 1: who's looking at them, and how they look at them,

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Speaker 1: and whether they store the information and look at it later.

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Speaker 1: All these fun thought experiments can help us try to

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Speaker 1: understand what's really going on at the smallest scales.

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Speaker 2: So to the end the podcast, we'll be talking about

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Speaker 2: what is a quantum eraser now, Daniel, is this a

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Speaker 2: rubber eraser or what is it made out of?

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Speaker 1: It's something which will erase your mind if you think

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Speaker 1: about it too much.

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Speaker 2: It's kind to stretch it out like a piece.

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Speaker 1: Of rubber exactly, and push it too hard and it

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Speaker 1: might just snap.

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Speaker 6: No.

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Speaker 1: Quantum eraser refers to the concept of quantum information and

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Speaker 1: what happens if you create information and then erase that

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Speaker 1: information from the universe. So it's an extension of some

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Speaker 1: really fun experiments that listeners on this podcast have heard

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Speaker 1: us talk about, the double slit experiment, which reveals how

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Speaker 1: particles can interfere with themselves and have the chance to

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Speaker 1: be in multiple places at once. We dug into that

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Speaker 1: experiment with a fun conversation with Adam Becker, the author

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Speaker 1: of What Is Real, and today we're gonna go double

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Speaker 1: down on those experiments and think about even crazier versions.

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Speaker 2: Yeah, so, what is a quantum eraser now, Daniel? Is

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Speaker 2: this a thing or like a concept?

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Speaker 1: Yeah, it's both. It was first a concept and then

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Speaker 1: people made it a thing. It's like that in quantum mechanics.

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Speaker 1: A lot people think, well, if the universe really is

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Speaker 1: that way, here's a ridiculous scenario that should lead to

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Speaker 1: a silly result. And then physicists go out and they

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Speaker 1: do the experiment. They make it real, They figure out

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Speaker 1: a way to like build it in their lab to

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Speaker 1: test that crazy property in the universe, and they get

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Speaker 1: these ridiculous results, which in the end you have to

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Speaker 1: accept because that's what the experiment says. They say the

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Speaker 1: universe really works that m.

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Speaker 2: All right, Well we'll dig into it. We'll rub that

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Speaker 2: quantum eraser all over our brains and see there's anything

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Speaker 2: left at the end. But first we were wondering if

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Speaker 2: how many people out there had thought about this question

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Speaker 2: or even heard the term quantum eraser. So Daniel went

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Speaker 2: out there and asked people on the internet what they

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Speaker 2: thought a quantum eraser is.

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Speaker 1: That's right, So if you'd like to be a participant

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Speaker 1: in our virtual person on the street interviews and love

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Speaker 1: to speculate about physics without looking anything up, then please

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Speaker 1: write to us two questions at Danielandthorge dot com.

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Speaker 2: All right, so think about it for a second. If

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Speaker 2: a random physicist came up to you on the street,

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Speaker 2: and you didn't run away first, And I actually listened

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Speaker 2: to them and they asked you, what is a quantum eraser?

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Speaker 2: What would you say? Your people's answers.

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Speaker 7: Something an angry physics grad student uses, I have no idea.

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Speaker 7: Maybe something that erases things at random and like gets

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Speaker 7: rid of things, because that's what quantum generally is, is

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Speaker 7: the randomness.

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Speaker 8: Quantum eraser could be something we invent in the future

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Speaker 8: to erase quantum mechanics and quantum physics, just because it's

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Speaker 8: pretty hard to understand. If we don't have it around anymore,

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Speaker 8: we don't have to deal with it. We can stick

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Speaker 8: with general relativity and regular gravity, So yeah, just get

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Speaker 8: rid of that stuff.

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Speaker 9: Really, I don't know either if I were to compare

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Speaker 9: a quantum eraser to a regular eraser, which essentially just

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Speaker 9: kind of distorts the little graphite particles and absorbs.

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Speaker 6: Them and in a way that you know, it gets

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Speaker 6: rid of the material on a piece of paper or something,

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Speaker 6: so that you know you can reuse that material to

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Speaker 6: un on something. Maybe a quantum eraser is some sort

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Speaker 6: of force or phenomenon that causes sometimes particles to break

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Speaker 6: up from a given area or concentration, so that the

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Speaker 6: information is very very difficult to understand or to receive

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Speaker 6: or to observe.

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Speaker 10: Maybe well, I'm a teacher, and I know that my

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Speaker 10: students use a raise as to high the mistakes. So

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Speaker 10: I think a quantum eraser is something that quantum physicists

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Speaker 10: use to hide their errors from everyone else.

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Speaker 11: All I can come up with for that is, sometimes

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Speaker 11: you make a mistake and it's for the better, it's

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Speaker 11: a better idea than what you originally planned. And sometimes

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Speaker 11: it's a big catastrophe. But since you don't know ahead

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Speaker 11: of time, you use your quantum eraser to either undo

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Speaker 11: your mistake or make it permanent, and you just roll

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Speaker 11: the dice and let fate decide.

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Speaker 1: No idea.

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Speaker 8: Never heard of a quantum eraser, but the image of

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Speaker 8: a really, really tiny eraser pops into my mind. So

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Speaker 8: let's say a tool that allows you to.

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Speaker 2: Change just supotomic structure of stuff.

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Speaker 5: I would guess that a quantum eraser is having to

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Speaker 5: do with erasing particles that have certain quantum states, thereby

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Speaker 5: leaving behind particles that you are in the state you

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Speaker 5: want that, or it's the weapon that they used in

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Speaker 5: the movie Eraser with Arnold Schwarzenegger.

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Speaker 2: All right, a lot of fun answers here. Everyone's a

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Speaker 2: comedian on the internet.

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Speaker 1: Well, especially if I have no idea what we're talking about,

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Speaker 1: then they got to go to that joking place. I

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Speaker 1: like the joke about the really tiny eraser.

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Speaker 2: Hmmm right, Yeah, Like if you have a quantum pencil,

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Speaker 2: I guess it would have a quantum eraser on the

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Speaker 2: one end.

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Speaker 1: Of it, yeah, or erases quantum particles or something like that.

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Speaker 1: I'm imagining a super tiny little vacuum cleaner that like

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Speaker 1: slurps up.

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Speaker 2: Electrons, yeah, and then does one with them, passes.

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Speaker 1: Them through a wormhole into another universe.

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Speaker 2: I guess, tantalizing. All right, well, let's get into this

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Speaker 2: concept of a quantum eraser. Daniel. You're saying it has

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Speaker 2: something to do with the double slit experiment. Now, this

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Speaker 2: is going to be kind of hard because I feel

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Speaker 2: like this is a podcast and it's audio only, and

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Speaker 2: this is a very kind of visual experiment. But I

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Speaker 2: guess we can try our best to describe what it is.

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Speaker 1: Yeah, maybe we can add a dance element to it.

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Speaker 1: You think that'll help, Yeah, Or I could drop our

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Speaker 1: tuons I'm doing watercolor painting at the same time as

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Speaker 1: we do our podcast.

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Speaker 2: By the way, Oh really, you're in a quantum artistic.

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Speaker 1: State, I am. This quantum eraser is an experiment that's

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Speaker 1: like a permutation or an add on to the basic

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Speaker 1: double slit experiment. So to understand why the quantum e

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Speaker 1: racer is so weird and crazy, you definitely have to

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Speaker 1: understand what's going on in the double slit experiment. And

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Speaker 1: so I think we're gonna have to use our words

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Speaker 1: to describe the wiggly crazy nature of that experiment, and

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Speaker 1: then we can build on it to get to the

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Speaker 1: quantumer racer.

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Speaker 2: All right, So I guess the double slid experiment starts

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Speaker 2: with a single slit first. I guess we'll explain that one,

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Speaker 2: and then we'll multiply by two. So the basic experiment

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Speaker 2: is like you have a wall, like a barrier, like

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Speaker 2: a plate of metal, and you cut a little slit

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Speaker 2: on it, like a little opening that's long in one direction,

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Speaker 2: and then you shoot like a laser or like just

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Speaker 2: a regular beam of light through it and then onto

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Speaker 2: a wall behind the first wall.

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Speaker 1: Yeah, exactly, So imagine in your mind some source of light.

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Speaker 1: A laser is good because then you have photons of

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Speaker 1: all the same wavelength in the same direct action, and

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Speaker 1: then a screen on the other side where the laser hits.

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Speaker 1: What do you get. You get a laser spot. Now,

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Speaker 1: as you say, put something in between, like a barrier

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Speaker 1: that has a very thin slit in it. Then what

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Speaker 1: do you see on the screen Instead of the full

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Speaker 1: laser spot. Now you see like a slice of that spot.

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Speaker 1: You get a smooth pattern on the screen, but it's

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Speaker 1: sort of cut by the slit in the barrier, and

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Speaker 1: it has smooth edges, not a sharp edge, because that's

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Speaker 1: what happens when light goes through a slit. It tends

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Speaker 1: to like spread out a little bit and smooth out.

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Speaker 1: So the thing you start with is this single slit

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Speaker 1: where the light goes through and hits the barrier on

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Speaker 1: the other.

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Speaker 2: Side, right, and you get a smooth light pattern on

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Speaker 2: the other side. Now, the weird thing is then what

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Speaker 2: happens if you put a second slit next to the

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Speaker 2: first slit.

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Speaker 1: Right, that's right, So now you put two slits really

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Speaker 1: close together so that the beam could pass through one

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Speaker 1: slit or the other slit, and once you get on

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Speaker 1: the other side, instead of having like two smooth patterns,

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Speaker 1: or you know, the simple addition of two patterns like

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Speaker 1: you saw before. Now you get an interference pattern, which

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Speaker 1: means you get the patterns of light and dark and

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Speaker 1: light and dark and light and dark and what's happening

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Speaker 1: there is interference. Just like if you have waves when

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Speaker 1: you add them, if one wave is going up while

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Speaker 1: the other wave is going down, then they cancel each

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Speaker 1: other out. Whereas if one wave is going up and

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Speaker 1: the other one is going up at the same time,

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Speaker 1: then they add up on top of each other. They

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Speaker 1: get twice as strong. So the interference pattern has these

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Speaker 1: slices that are twice as bright as the previous pattern,

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Speaker 1: and these dark slices as well. And that's because you

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Speaker 1: have two sources of light. Now, each of the slits

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Speaker 1: is giving you photons and they can either constructively or

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Speaker 1: destructively interfere on the screen.

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Speaker 2: Right, because I guess you're shooting a laser at both

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Speaker 2: slits at the same time, or you're like shooting a

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Speaker 2: laser and the beam of the laser kind of goes

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Speaker 2: through both slits at the same time. Right.

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Speaker 1: Yeah, the slits are very narrow and very very close together.

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Speaker 1: This only works if the scale we're talking about here

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Speaker 1: is sort of related to the wavelength of light that

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Speaker 1: we're shooting at it. So this needs to be very microscopic.

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Speaker 2: Right, So if you have one slit, you get a fuzzy,

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Speaker 2: like a plain fuzzy image on the other side. But

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Speaker 2: if you have two, then suddenly it's not a smooth

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Speaker 2: fuzzy image. It's like a weird Rippley kind of image,

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Speaker 2: which means that somehow light is interacting with itself.

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Speaker 1: Yeah, in this case, we don't know if the light

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Speaker 1: is interacting with itself. You could say, hey, look, light

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Speaker 1: is a wave, and waves interfere. This happens with waves

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Speaker 1: in the bathtub, it happens with waves in the air.

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Speaker 1: Like noise canceling headphones, right, they generate a second pattern

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Speaker 1: of noise to cancel out the noise that's coming into

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Speaker 1: your ear. So interference in waves is not necessarily a

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Speaker 1: quantum mechanical thing. It's just a wave thing. So in

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Speaker 1: this version of the experiment so far, you could just say, look,

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Speaker 1: light's a wave, it's interfering. No big whoop. It gets

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Speaker 1: quantum when you remember that the beam is actually made

441
00:20:40,840 --> 00:20:45,080
Speaker 1: not of waves but of photons, little individual packets and

442
00:20:45,119 --> 00:20:47,120
Speaker 1: so you can take it to the next step by

443
00:20:47,200 --> 00:20:50,439
Speaker 1: slowing down the experiment and dimming the laser so that

444
00:20:50,440 --> 00:20:53,560
Speaker 1: it's shooting like one photon through the experiment at a time.

445
00:20:53,760 --> 00:20:56,680
Speaker 2: Yeah, you shoot one photon at a time, and then

446
00:20:56,760 --> 00:20:59,040
Speaker 2: you would think that just throwing like one footon at

447
00:20:59,040 --> 00:21:01,920
Speaker 2: a time, photon would pick like the right or the

448
00:21:02,000 --> 00:21:03,720
Speaker 2: left slit and then end up on the other side

449
00:21:03,720 --> 00:21:05,639
Speaker 2: of the wall, and you would get the same fuzzy

450
00:21:05,720 --> 00:21:08,320
Speaker 2: smooth pattern. But the weird part I guess is that

451
00:21:08,359 --> 00:21:10,400
Speaker 2: you're shooting one photon at a time, but you still

452
00:21:10,440 --> 00:21:13,320
Speaker 2: get the ripley kind of interference pattern on the other side.

453
00:21:13,359 --> 00:21:16,239
Speaker 1: Exactly. You expect that if you shoot one photon at

454
00:21:16,240 --> 00:21:18,600
Speaker 1: a time, then it can't interfere because you were thinking, well,

455
00:21:18,600 --> 00:21:22,280
Speaker 1: the interference comes from two photons going through both slits

456
00:21:22,280 --> 00:21:24,800
Speaker 1: at the same time. Now you have just one photon

457
00:21:24,840 --> 00:21:27,679
Speaker 1: in the experiment, So what's it interfering with because you

458
00:21:27,720 --> 00:21:30,639
Speaker 1: still see the interference pattern on the other side of

459
00:21:30,680 --> 00:21:32,720
Speaker 1: the screen when you shoot one photon through at a time.

460
00:21:32,960 --> 00:21:35,160
Speaker 1: It's just that it takes longer to build up if

461
00:21:35,160 --> 00:21:36,960
Speaker 1: you watch it for an hour or so, as those

462
00:21:36,960 --> 00:21:40,520
Speaker 1: photons go through one lands here one lance, there, one lens,

463
00:21:40,560 --> 00:21:43,840
Speaker 1: this other spot. It gradually builds up that interference pattern.

464
00:21:43,960 --> 00:21:47,760
Speaker 1: So what's it interfering with. It's interfering with itself. It

465
00:21:47,760 --> 00:21:50,520
Speaker 1: has the probability to go through both slits, and that

466
00:21:50,640 --> 00:21:54,960
Speaker 1: wave function, which controls where a quantum particle goes, interferes

467
00:21:55,000 --> 00:21:58,520
Speaker 1: with itself and creates this probability distribution on the screen

468
00:21:58,600 --> 00:22:02,400
Speaker 1: for where it might land, and that probability distribution has

469
00:22:02,480 --> 00:22:05,439
Speaker 1: the interference effects inside of it, and that's why you

470
00:22:05,440 --> 00:22:08,600
Speaker 1: get this interference pattern. Every photon that goes through like

471
00:22:08,760 --> 00:22:11,960
Speaker 1: randomly pulls a number from the probability distribution on the

472
00:22:11,960 --> 00:22:15,320
Speaker 1: screen which has the interference pattern built in, and lands there,

473
00:22:15,359 --> 00:22:17,600
Speaker 1: and gradually it builds up that distribution.

474
00:22:17,800 --> 00:22:20,119
Speaker 2: It's almost like, you know, if you were to shoot

475
00:22:20,160 --> 00:22:21,840
Speaker 2: a photon as a little ball, it would go through

476
00:22:21,840 --> 00:22:24,080
Speaker 2: one of the slids, but because it's quantum, it's almost

477
00:22:24,119 --> 00:22:27,000
Speaker 2: like it's going through both slits at the same time, right,

478
00:22:27,080 --> 00:22:29,000
Speaker 2: that's kind of the quantum thing. It's going through both

479
00:22:29,040 --> 00:22:31,800
Speaker 2: slits at the same time, and then it's sort of

480
00:22:32,160 --> 00:22:34,760
Speaker 2: going through both slits and then interacting with itself in

481
00:22:34,760 --> 00:22:37,280
Speaker 2: a quantum way, so that when it gets to the screen.

482
00:22:37,520 --> 00:22:38,600
Speaker 2: It's not a smooth pattern.

483
00:22:38,720 --> 00:22:40,960
Speaker 1: Yeah, it's tempting to say that it's in two places

484
00:22:41,000 --> 00:22:43,919
Speaker 1: at once, or that goes through both slits at the

485
00:22:43,960 --> 00:22:46,439
Speaker 1: same time, and that's our tendency to try to like

486
00:22:46,520 --> 00:22:48,960
Speaker 1: tell a story for what happens. But I'm not sure

487
00:22:48,960 --> 00:22:50,880
Speaker 1: that's the right way to think about it. The way

488
00:22:50,880 --> 00:22:53,200
Speaker 1: I think about it is that it has a probability

489
00:22:53,200 --> 00:22:55,960
Speaker 1: to go through both at once. What it actually does

490
00:22:56,119 --> 00:22:58,800
Speaker 1: is not determined, you know, until it gets to the

491
00:22:58,840 --> 00:23:02,040
Speaker 1: other side. So happened when it went through the slits?

492
00:23:02,240 --> 00:23:04,680
Speaker 1: We don't know, We might never know. There isn't necessarily

493
00:23:04,720 --> 00:23:07,679
Speaker 1: a story there. So it's a small change in wording,

494
00:23:07,680 --> 00:23:09,720
Speaker 1: but an important change in meaning for me to say

495
00:23:10,160 --> 00:23:12,359
Speaker 1: that it had probability to go through both slits rather

496
00:23:12,400 --> 00:23:14,000
Speaker 1: than it actually went through both.

497
00:23:14,200 --> 00:23:15,920
Speaker 2: Right, it's like saying, it's not that the cat is

498
00:23:15,960 --> 00:23:18,840
Speaker 2: alive and dead, as you said, it has the same probability,

499
00:23:19,200 --> 00:23:21,320
Speaker 2: or it has a certain probability of being alive and

500
00:23:21,320 --> 00:23:23,800
Speaker 2: a certain probability of being dead. All right, Well, then

501
00:23:23,920 --> 00:23:25,840
Speaker 2: now the weird part here. Now it is going to

502
00:23:25,880 --> 00:23:29,520
Speaker 2: come when we try observing this photon, and that's when

503
00:23:29,520 --> 00:23:32,200
Speaker 2: we get into this idea of quantum erasors. So let's

504
00:23:32,200 --> 00:23:34,560
Speaker 2: talk about that. But first let's take a quick break.

505
00:23:38,800 --> 00:23:41,760
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Speaker 2: All right, we were asking the question what is a

574
00:27:18,080 --> 00:27:20,800
Speaker 2: quantum eraser? And now, Daniel, I feel like half of

575
00:27:20,800 --> 00:27:25,280
Speaker 2: my brain is already erased trying to talk about quantum objects.

576
00:27:25,359 --> 00:27:28,320
Speaker 2: And now we were explaining the double slit experiment, and

577
00:27:28,359 --> 00:27:30,199
Speaker 2: so I think we were done with that. Like if

578
00:27:30,200 --> 00:27:32,520
Speaker 2: you shoot a laser at a two small slits on

579
00:27:32,600 --> 00:27:34,760
Speaker 2: a screen, then on the other side you're going to

580
00:27:34,800 --> 00:27:37,199
Speaker 2: get an interference parent because of the way the quantum

581
00:27:37,200 --> 00:27:40,320
Speaker 2: probabilities kind of affect each other. And so now the

582
00:27:40,359 --> 00:27:42,639
Speaker 2: weird thing happens when you try to like add a

583
00:27:42,680 --> 00:27:44,800
Speaker 2: detector right where and you try to see which slid

584
00:27:44,840 --> 00:27:45,680
Speaker 2: it actually went through.

585
00:27:45,800 --> 00:27:48,440
Speaker 1: That's right, because our tendency is to want to know, like, well,

586
00:27:48,600 --> 00:27:50,960
Speaker 1: what happened, right, did it go through one slit or

587
00:27:51,000 --> 00:27:52,919
Speaker 1: did it go through the other. We feel like it

588
00:27:53,000 --> 00:27:55,480
Speaker 1: must have gone through one or the other, right, because

589
00:27:55,800 --> 00:27:57,159
Speaker 1: you know, if it was over here and then it

590
00:27:57,200 --> 00:27:59,480
Speaker 1: is over there, so it must go from here to

591
00:27:59,720 --> 00:28:02,200
Speaker 1: there is our sense. And so to try to get

592
00:28:02,200 --> 00:28:05,199
Speaker 1: in like a more accurate understanding of what happened, you

593
00:28:05,200 --> 00:28:07,240
Speaker 1: could add a little detector one that gives you a

594
00:28:07,280 --> 00:28:10,560
Speaker 1: signal if a photon goes through slit A, for example,

595
00:28:10,560 --> 00:28:13,199
Speaker 1: instead of slip B. So that way you can know, hey,

596
00:28:13,280 --> 00:28:15,000
Speaker 1: did it go through slid A or slip B. Because

597
00:28:15,040 --> 00:28:18,040
Speaker 1: you know, photons are observable, right, You can interact with them,

598
00:28:18,119 --> 00:28:20,280
Speaker 1: They make splashes of light, you can measure them. They

599
00:28:20,280 --> 00:28:22,800
Speaker 1: are quantum objects, but they are also physical. And so

600
00:28:22,840 --> 00:28:24,720
Speaker 1: what happens when you do that, when you ask, when

601
00:28:24,720 --> 00:28:27,680
Speaker 1: you insist on knowing which slid it went through, is

602
00:28:27,760 --> 00:28:31,400
Speaker 1: that the interference pattern disappears. When you add that detector

603
00:28:31,480 --> 00:28:33,520
Speaker 1: that just tries to understand which slid it went through,

604
00:28:33,880 --> 00:28:35,600
Speaker 1: then the interference pattern is gone.

605
00:28:35,680 --> 00:28:38,120
Speaker 2: So if you try to like measure and the photon

606
00:28:38,160 --> 00:28:40,120
Speaker 2: as it goes through slit, how would you even do that?

607
00:28:40,560 --> 00:28:42,440
Speaker 2: Don't you have to stop the photon to do that.

608
00:28:42,440 --> 00:28:44,880
Speaker 1: That's the crux of the matter right there. To measure

609
00:28:44,920 --> 00:28:48,400
Speaker 1: a photon, you have to interact with it somehow. You

610
00:28:48,440 --> 00:28:52,000
Speaker 1: can't just like observe a photon without interacting with it.

611
00:28:52,040 --> 00:28:54,360
Speaker 1: You know, a photon that like passes in front of you,

612
00:28:54,360 --> 00:28:57,120
Speaker 1: you can't see it. It's a piece of light, but

613
00:28:57,200 --> 00:29:00,400
Speaker 1: unless it hits your eyes, you can't see it unless

614
00:29:00,520 --> 00:29:02,760
Speaker 1: you put something in front of it to stop it,

615
00:29:03,120 --> 00:29:05,960
Speaker 1: measure it, and then re emit it. Right, So, for example,

616
00:29:06,000 --> 00:29:07,760
Speaker 1: the reason you see something in front of you as

617
00:29:07,880 --> 00:29:10,440
Speaker 1: red is because a photon has hit that object and

618
00:29:10,440 --> 00:29:14,680
Speaker 1: then emitted red photons. So you can't see things without

619
00:29:14,800 --> 00:29:17,600
Speaker 1: interacting with them, even light, right, You need to interact

620
00:29:17,640 --> 00:29:19,760
Speaker 1: with it somehow. So you can make up lots of

621
00:29:19,800 --> 00:29:22,840
Speaker 1: different physical systems that could do this, but the simplest

622
00:29:22,840 --> 00:29:25,040
Speaker 1: one is, you know, just like a simple photon detector,

623
00:29:25,120 --> 00:29:28,479
Speaker 1: a photo multiplier tube for example, or a scintillator screen

624
00:29:28,720 --> 00:29:30,960
Speaker 1: that indicates when a photon went through and then re

625
00:29:31,040 --> 00:29:32,320
Speaker 1: emits it on the other side.

626
00:29:32,600 --> 00:29:34,960
Speaker 2: Oh, I see what you're saying, Like if you catch

627
00:29:35,000 --> 00:29:37,080
Speaker 2: it and then release it back on the other side,

628
00:29:37,160 --> 00:29:39,360
Speaker 2: that's one way you could measure the photon.

629
00:29:39,960 --> 00:29:41,760
Speaker 1: And you'd like to think, Oh, can't I just take

630
00:29:41,800 --> 00:29:44,200
Speaker 1: a peek? Can I just look and see where it

631
00:29:44,280 --> 00:29:47,560
Speaker 1: went without touching it, without interfering with it, without messing

632
00:29:47,640 --> 00:29:49,680
Speaker 1: with it in any way, But you can't do that.

633
00:29:49,760 --> 00:29:52,200
Speaker 1: Quantum mechanics tells us that the only way to get

634
00:29:52,240 --> 00:29:55,320
Speaker 1: information about an object is to interact with it. You

635
00:29:55,320 --> 00:29:57,440
Speaker 1: can like bounce a photon off of it, or you

636
00:29:57,480 --> 00:29:59,240
Speaker 1: can bounce an electron off of it, or you can

637
00:29:59,240 --> 00:30:02,160
Speaker 1: put another little screen, but somehow you have to interact

638
00:30:02,200 --> 00:30:05,440
Speaker 1: with it. You can't get information from that particle without

639
00:30:05,600 --> 00:30:07,640
Speaker 1: somehow interfering with its path.

640
00:30:07,840 --> 00:30:10,200
Speaker 2: Yeah, because I guess in our everyday lies, we're used

641
00:30:10,240 --> 00:30:12,640
Speaker 2: to this idea of being able to like see things

642
00:30:12,640 --> 00:30:15,120
Speaker 2: but not touch them, and so we think we can

643
00:30:15,200 --> 00:30:18,960
Speaker 2: tell where something is without actually like influencing it. But

644
00:30:19,040 --> 00:30:21,320
Speaker 2: when you get down to the smallest of levels, like

645
00:30:21,600 --> 00:30:23,800
Speaker 2: all seeing is interacting in a way.

646
00:30:23,680 --> 00:30:25,880
Speaker 1: That's right, And it's also true with the macroscopic scale.

647
00:30:25,880 --> 00:30:27,840
Speaker 1: It's just that you don't notice it. Like if you

648
00:30:27,880 --> 00:30:30,320
Speaker 1: are walking outside at night and you want to know, hey,

649
00:30:30,320 --> 00:30:31,840
Speaker 1: is there a rock in my path? You turn on

650
00:30:31,880 --> 00:30:35,360
Speaker 1: your flashlight. You are shooting photons at that rock. Those

651
00:30:35,400 --> 00:30:37,880
Speaker 1: photons are hitting the rock, they're warming up the rock.

652
00:30:38,160 --> 00:30:41,160
Speaker 1: Then the rock is reflecting photons back at you. So yeah,

653
00:30:41,200 --> 00:30:43,880
Speaker 1: you're not touching the rock, but you're definitely interacting with

654
00:30:43,920 --> 00:30:46,200
Speaker 1: the rock and you're changing its quantum state. It's just

655
00:30:46,240 --> 00:30:48,480
Speaker 1: like it doesn't like really heat up the rock or

656
00:30:48,520 --> 00:30:51,320
Speaker 1: push the rock far away. When these are quantum particles,

657
00:30:51,400 --> 00:30:54,080
Speaker 1: they can have significant effects if you interact with them

658
00:30:54,120 --> 00:30:56,440
Speaker 1: by shooting beams of light at them or other particles.

659
00:30:56,520 --> 00:30:59,040
Speaker 2: All right, So then in the double slit experimentment, if

660
00:30:59,080 --> 00:31:01,720
Speaker 2: you try to measure these photons before they hit the

661
00:31:01,720 --> 00:31:04,320
Speaker 2: wall in the back, then what you're doing is you're

662
00:31:04,360 --> 00:31:09,320
Speaker 2: collapsing the quantum wave, right, You're messing up the quantum information.

663
00:31:09,480 --> 00:31:12,280
Speaker 1: Yeah. So here's where the different interpretations of quantum mechanics

664
00:31:12,360 --> 00:31:15,320
Speaker 1: all tell you different stories for what happens. The experiments

665
00:31:15,320 --> 00:31:18,320
Speaker 1: say if you measure the photon before or after the screen,

666
00:31:18,720 --> 00:31:22,040
Speaker 1: that the interference pattern goes away. The classical interpretation of

667
00:31:22,120 --> 00:31:25,040
Speaker 1: quantum mechanics. The Copenhagen interpretation is what you just described.

668
00:31:25,040 --> 00:31:28,280
Speaker 1: It says that the probability to go through both slits

669
00:31:28,560 --> 00:31:31,080
Speaker 1: only exists if you haven't made a measurement. But then

670
00:31:31,080 --> 00:31:33,440
Speaker 1: if you interact with it, it collapses the wave function.

671
00:31:33,800 --> 00:31:36,080
Speaker 1: Now it can only go through one slit or the other.

672
00:31:36,240 --> 00:31:39,160
Speaker 1: So there's no interference because the interference came from the

673
00:31:39,240 --> 00:31:42,360
Speaker 1: ambiguity came from the probabilities to go through both. The

674
00:31:42,360 --> 00:31:45,600
Speaker 1: Many World's interpretation tells a different story. It says collapse

675
00:31:45,640 --> 00:31:48,640
Speaker 1: is nonsense, that doesn't happen, it's ridiculous. It says that

676
00:31:48,680 --> 00:31:51,320
Speaker 1: the universe splits into two, one where the photon went

677
00:31:51,360 --> 00:31:53,160
Speaker 1: through one slit and one where the photon went through

678
00:31:53,160 --> 00:31:55,080
Speaker 1: the other, and you're in one of those universes and

679
00:31:55,120 --> 00:31:55,640
Speaker 1: not the other.

680
00:31:55,920 --> 00:31:57,920
Speaker 2: Right, So those are the two ways in which you

681
00:31:57,960 --> 00:32:01,360
Speaker 2: can interpret what happens. So how does that relate to

682
00:32:01,520 --> 00:32:03,160
Speaker 2: the information and the quantum information?

683
00:32:03,280 --> 00:32:06,040
Speaker 1: So the idea here is that you have the information

684
00:32:06,240 --> 00:32:09,000
Speaker 1: about whether it went through slit A or slit B,

685
00:32:09,440 --> 00:32:13,280
Speaker 1: and that's what destroys the interference. Because you've made this measurement.

686
00:32:13,360 --> 00:32:16,040
Speaker 1: Somehow it changes the experiment. And you know, this is

687
00:32:16,080 --> 00:32:18,920
Speaker 1: not something that we understand very well, this whole concept

688
00:32:18,960 --> 00:32:21,720
Speaker 1: of measurement in quantum mechanics, and that was the topic

689
00:32:21,760 --> 00:32:24,120
Speaker 1: of our episode with Adam Becker, and then recently we

690
00:32:24,120 --> 00:32:26,240
Speaker 1: did an episode with Carlo Ravelli and he's got a

691
00:32:26,240 --> 00:32:29,440
Speaker 1: whole new theory for how to understand measurement in quantum mechanics.

692
00:32:29,560 --> 00:32:33,040
Speaker 1: It's not something that physics understands very well. How interacting

693
00:32:33,040 --> 00:32:35,800
Speaker 1: with something changes its wave function? Does it collapse it,

694
00:32:35,840 --> 00:32:38,200
Speaker 1: does it split the universe? All of this kind of stuff.

695
00:32:38,240 --> 00:32:41,360
Speaker 1: But the key idea here is that you extract information

696
00:32:41,440 --> 00:32:45,080
Speaker 1: about which way the photon went, then there's no interference.

697
00:32:45,440 --> 00:32:47,880
Speaker 2: Right, You somehow get rid of the quantumness of it.

698
00:32:47,960 --> 00:32:51,200
Speaker 2: Like when I poke something, it's no longer quantum.

699
00:32:50,760 --> 00:32:53,360
Speaker 1: If you poke something with a classical object, like a

700
00:32:53,360 --> 00:32:56,400
Speaker 1: big detector, right, that big detector can't be in two

701
00:32:56,440 --> 00:32:58,560
Speaker 1: stays at once. It can't be like, well, yes I

702
00:32:58,560 --> 00:33:01,040
Speaker 1: saw it and no I didn't. Has to make a decision,

703
00:33:01,480 --> 00:33:04,520
Speaker 1: and so it decoheres and you get this weird thing

704
00:33:04,560 --> 00:33:07,160
Speaker 1: where a quantum object is interacting with the classical objects

705
00:33:07,160 --> 00:33:09,320
Speaker 1: and so now it has to like follow the classical

706
00:33:09,400 --> 00:33:12,320
Speaker 1: rules so the quantum eraser is an attempt to get

707
00:33:12,360 --> 00:33:14,959
Speaker 1: around that, is to say, what if instead of poking

708
00:33:15,000 --> 00:33:17,840
Speaker 1: it with a big finger or a big classical detector,

709
00:33:17,960 --> 00:33:20,720
Speaker 1: what if we've got this information but we somehow kept

710
00:33:20,760 --> 00:33:22,360
Speaker 1: it quantum at the same time.

711
00:33:22,560 --> 00:33:25,240
Speaker 2: I see. So it's almost like the cat and Strodenger's box, Like,

712
00:33:25,280 --> 00:33:27,280
Speaker 2: before you open the box, it's both alive and dead.

713
00:33:27,320 --> 00:33:29,920
Speaker 2: This is a probability of it being one or the other.

714
00:33:30,120 --> 00:33:32,000
Speaker 2: And if you open the box, that's the classical way

715
00:33:32,040 --> 00:33:33,840
Speaker 2: of checking it out, Like you open it and it's

716
00:33:33,880 --> 00:33:36,160
Speaker 2: either alive or dead, and then you get rid of

717
00:33:36,160 --> 00:33:40,760
Speaker 2: the quantum probabilities. You're saying, can I like somehow, you know,

718
00:33:40,960 --> 00:33:45,520
Speaker 2: poke my quantum finger into the box and measure, but

719
00:33:45,600 --> 00:33:49,240
Speaker 2: not kind of destroy that superposition of being both alive

720
00:33:49,240 --> 00:33:49,960
Speaker 2: and dead exactly.

721
00:33:50,040 --> 00:33:52,440
Speaker 1: The quantum eraser experiment tries to do that. It says, well,

722
00:33:52,680 --> 00:33:55,680
Speaker 1: let's try to get this information out, but not look

723
00:33:55,720 --> 00:33:59,360
Speaker 1: at it directly, not use like our classical objects, our eyeballs,

724
00:33:59,400 --> 00:34:03,200
Speaker 1: our brain, it's even our computers to access that information,

725
00:34:03,320 --> 00:34:06,400
Speaker 1: so it can stay in a quantum superposition, so we

726
00:34:06,440 --> 00:34:09,480
Speaker 1: can make a decision later about whether we want that

727
00:34:09,560 --> 00:34:12,239
Speaker 1: information and here's where the mind bending stuff comes in.

728
00:34:12,280 --> 00:34:14,920
Speaker 1: If you can like extract that information about which way

729
00:34:14,920 --> 00:34:17,480
Speaker 1: the photon went, keep it in a quantum state by

730
00:34:17,480 --> 00:34:20,759
Speaker 1: storing it in some other entangled particles, then you can

731
00:34:20,800 --> 00:34:24,080
Speaker 1: decide after the photons have hit the screen whether or

732
00:34:24,080 --> 00:34:25,759
Speaker 1: not you want to know which way it went.

733
00:34:26,080 --> 00:34:26,840
Speaker 2: Wait, see it again.

734
00:34:26,920 --> 00:34:28,920
Speaker 1: So the idea is, you want to know which way

735
00:34:28,920 --> 00:34:30,960
Speaker 1: the photons went, right, do they go through split air

736
00:34:31,040 --> 00:34:34,040
Speaker 1: slip b. You know, if you add a classical object

737
00:34:34,080 --> 00:34:36,440
Speaker 1: like a big detector, you're going to collapse the wave function.

738
00:34:36,760 --> 00:34:39,719
Speaker 1: So instead you add a quantum detector one that can

739
00:34:39,760 --> 00:34:43,640
Speaker 1: record this information, but maybe without collapsing the state of

740
00:34:43,680 --> 00:34:46,600
Speaker 1: the wave function. Somehow it gets this information, but because

741
00:34:46,640 --> 00:34:49,799
Speaker 1: it's a quantum object, it doesn't trigger the collapse, right,

742
00:34:49,840 --> 00:34:52,480
Speaker 1: So it can be like entangled with the photon without

743
00:34:52,520 --> 00:34:55,399
Speaker 1: like forcing the photon to decohere completely. And so it's

744
00:34:55,400 --> 00:34:57,680
Speaker 1: different from interacting with like your big body or something.

745
00:34:57,719 --> 00:34:59,759
Speaker 1: You interact with it like with a single particle, and

746
00:34:59,760 --> 00:35:02,560
Speaker 1: that's it stores the information about which way the photon went,

747
00:35:02,760 --> 00:35:04,759
Speaker 1: but you haven't looked at it, you haven't collapsed that

748
00:35:04,800 --> 00:35:07,520
Speaker 1: wave function yet. You let the photon then go hit

749
00:35:07,520 --> 00:35:10,560
Speaker 1: the screen, and then after the photon has already hit

750
00:35:10,560 --> 00:35:12,879
Speaker 1: the screen and decide where it's going to land, then

751
00:35:12,960 --> 00:35:16,960
Speaker 1: you access that quantum information. It's called the delayed choice version,

752
00:35:17,000 --> 00:35:19,200
Speaker 1: where you decide after the photon has hit the screen

753
00:35:19,520 --> 00:35:21,439
Speaker 1: whether or not you want to know the information about

754
00:35:21,440 --> 00:35:22,120
Speaker 1: which way it went.

755
00:35:22,239 --> 00:35:24,080
Speaker 2: You're saying that the photon did go through one of

756
00:35:24,080 --> 00:35:26,480
Speaker 2: the two slits, like once it hits the screen in

757
00:35:26,560 --> 00:35:28,799
Speaker 2: the back, then it sort of chooses a history of

758
00:35:28,800 --> 00:35:30,439
Speaker 2: having gone through the left or the right slit.

759
00:35:30,560 --> 00:35:32,880
Speaker 1: That's right. This is trying to like force the photon

760
00:35:33,000 --> 00:35:35,440
Speaker 1: to make its decision about whether or not to make

761
00:35:35,440 --> 00:35:39,640
Speaker 1: an interference pattern before you decide whether you want to

762
00:35:39,719 --> 00:35:42,080
Speaker 1: know which slit it went through. So it's sort of like,

763
00:35:42,120 --> 00:35:44,719
Speaker 1: you know, trying to play quantum bluff with the photon.

764
00:35:45,280 --> 00:35:47,480
Speaker 1: And that's when we get into these really funny questions

765
00:35:47,480 --> 00:35:50,359
Speaker 1: of like, how does the photon know whether it's going

766
00:35:50,440 --> 00:35:52,919
Speaker 1: to be measured? How does the photon know whether you're

767
00:35:52,920 --> 00:35:54,680
Speaker 1: going to have information about it?

768
00:35:54,880 --> 00:35:55,320
Speaker 5: Mmmm.

769
00:35:55,440 --> 00:35:57,640
Speaker 2: It's almost like you want to, you know, peek inside

770
00:35:57,680 --> 00:36:00,640
Speaker 2: of the Schrodinger's box but not look at the answer,

771
00:36:00,880 --> 00:36:03,120
Speaker 2: so that it's still alive and dead inside the box,

772
00:36:03,120 --> 00:36:04,960
Speaker 2: but you sort of have the answer in your pocket,

773
00:36:05,040 --> 00:36:07,240
Speaker 2: but you don't. You haven't looked at the answer yet exactly,

774
00:36:07,560 --> 00:36:09,800
Speaker 2: And so why is that? Call it a quantum eraser?

775
00:36:09,920 --> 00:36:12,239
Speaker 1: Right, So this is not yet a quantum eraser. This

776
00:36:12,320 --> 00:36:14,160
Speaker 1: is the delayed choice version.

777
00:36:13,960 --> 00:36:15,400
Speaker 2: The delayed measurement.

778
00:36:15,080 --> 00:36:17,759
Speaker 1: Yes, exactly, the delayed measurement choosing whether or not you

779
00:36:17,800 --> 00:36:20,480
Speaker 1: have the information. That's the delay. So you might wonder, like, well,

780
00:36:20,480 --> 00:36:22,759
Speaker 1: what happens on the screen, What does the screen look like?

781
00:36:22,800 --> 00:36:24,440
Speaker 1: What does the experiment look like? If you do this,

782
00:36:24,520 --> 00:36:27,120
Speaker 1: if you capture this information in a quantum state, but

783
00:36:27,160 --> 00:36:29,319
Speaker 1: you don't look at it yet, Well, what happens is

784
00:36:29,400 --> 00:36:32,799
Speaker 1: you don't see interference on the screen because by doing this,

785
00:36:32,880 --> 00:36:35,799
Speaker 1: by slurping this information out of the photons, you have

786
00:36:35,920 --> 00:36:38,600
Speaker 1: destroyed the interference. But people think, well, that's interesting. But

787
00:36:38,640 --> 00:36:41,319
Speaker 1: I haven't yet looked at that information, right, So what

788
00:36:41,480 --> 00:36:44,960
Speaker 1: happens if I then erase that information? This is where

789
00:36:44,960 --> 00:36:48,320
Speaker 1: the quantum eraser comes in. If I take that quantum

790
00:36:48,400 --> 00:36:50,680
Speaker 1: information which is stored in these quantum objects, but I

791
00:36:50,719 --> 00:36:53,560
Speaker 1: haven't looked at it yet, If I erase that information somehow,

792
00:36:53,719 --> 00:36:56,560
Speaker 1: can I then recover the interference? Can I make the

793
00:36:56,600 --> 00:37:01,000
Speaker 1: interference pattern reappear on the screen by adding this quantum eraser,

794
00:37:01,040 --> 00:37:03,680
Speaker 1: which like deletes that information from the universe because I

795
00:37:03,719 --> 00:37:04,400
Speaker 1: never peaked at it.

796
00:37:04,440 --> 00:37:06,640
Speaker 2: Wait, are you saying that this is an actual experiment,

797
00:37:06,760 --> 00:37:10,040
Speaker 2: Like we've sort of intercepted the photon before it goes

798
00:37:10,080 --> 00:37:13,200
Speaker 2: into the slits, and we've stored that information, and we

799
00:37:13,280 --> 00:37:17,120
Speaker 2: see that it now it doesn't generate an interference pattern,

800
00:37:17,160 --> 00:37:20,319
Speaker 2: a weekly pattern on the screen, even though nobody really

801
00:37:20,360 --> 00:37:21,560
Speaker 2: knows which slid it went through.

802
00:37:21,680 --> 00:37:24,000
Speaker 1: That's right, nobody knows what slid went through, though it

803
00:37:24,040 --> 00:37:27,120
Speaker 1: is in principles stored in this quantum object. Although that

804
00:37:27,200 --> 00:37:29,400
Speaker 1: quantum object can be in a superposition, it doesn't have

805
00:37:29,480 --> 00:37:30,959
Speaker 1: to be in a definite state.

806
00:37:31,040 --> 00:37:31,160
Speaker 2: Right.

807
00:37:31,200 --> 00:37:33,040
Speaker 1: You can say there's a probability of one and a

808
00:37:33,040 --> 00:37:35,880
Speaker 1: probability of the other, and we don't see that interference pattern.

809
00:37:35,920 --> 00:37:38,440
Speaker 1: So then people thought, well, what if we delete that information?

810
00:37:38,719 --> 00:37:41,600
Speaker 1: What would the universe do if we measure the photon

811
00:37:41,640 --> 00:37:43,479
Speaker 1: but don't look at it, keep it a quantum state,

812
00:37:43,560 --> 00:37:46,560
Speaker 1: and then erase that information. Can it somehow go back

813
00:37:46,560 --> 00:37:48,400
Speaker 1: and recover the interference pattern.

814
00:37:48,520 --> 00:37:50,960
Speaker 2: I see you're saying that maybe it's not the fact

815
00:37:51,000 --> 00:37:55,080
Speaker 2: that it interactive with your little secret finger poking quantum poking.

816
00:37:55,280 --> 00:37:58,400
Speaker 2: That destroyed the interference pattern. Maybe if I poke it

817
00:37:58,440 --> 00:38:01,760
Speaker 2: with my quantum finger and then I destroy my finger,

818
00:38:02,040 --> 00:38:04,920
Speaker 2: will it go back to being a quantum object? Is

819
00:38:04,920 --> 00:38:06,160
Speaker 2: that what you mean? Like, we know that if I

820
00:38:06,200 --> 00:38:08,719
Speaker 2: poke it, even with a quantum finger and not look

821
00:38:08,760 --> 00:38:11,480
Speaker 2: at the answer, it destroys the quantum information. But now

822
00:38:11,480 --> 00:38:13,080
Speaker 2: what happens if I poke it with the quonda finger

823
00:38:13,080 --> 00:38:15,120
Speaker 2: and then destroy the finger, will go back to being

824
00:38:15,520 --> 00:38:17,240
Speaker 2: a quantum object? Is that kind of the idea?

825
00:38:17,360 --> 00:38:20,440
Speaker 1: That's the quantum eraser is destroying that quantum information you've

826
00:38:20,480 --> 00:38:23,120
Speaker 1: extracted from the experiment, but having yet looked at so

827
00:38:23,200 --> 00:38:25,799
Speaker 1: it's still quantum. So that's the crazy experiment. And I

828
00:38:25,800 --> 00:38:27,440
Speaker 1: can hear you react and say, what is that a

829
00:38:27,440 --> 00:38:30,160
Speaker 1: real experiment? Did we really do that? And yes, we

830
00:38:30,200 --> 00:38:32,480
Speaker 1: have really done this experiment, and we have done it

831
00:38:32,520 --> 00:38:34,640
Speaker 1: with photons, and you can go up and google and

832
00:38:34,719 --> 00:38:37,080
Speaker 1: learn all about the details of this experiment. I think

833
00:38:37,080 --> 00:38:39,600
Speaker 1: there's a slightly simpler version that's easier to talk about,

834
00:38:39,600 --> 00:38:43,000
Speaker 1: where you use electrons, But the principles are all the same.

835
00:38:43,120 --> 00:38:46,759
Speaker 1: And so how can you destroy quantum information? Well, for example,

836
00:38:46,960 --> 00:38:49,319
Speaker 1: if when the photon is passing through the slit, you

837
00:38:49,400 --> 00:38:52,640
Speaker 1: have some detector, and that detector takes an electron and

838
00:38:52,680 --> 00:38:55,240
Speaker 1: puts it in like a spin upstate if the photon

839
00:38:55,280 --> 00:38:57,640
Speaker 1: went through one slit, and a spin downstate if the

840
00:38:57,680 --> 00:38:59,719
Speaker 1: photon went through another slit. This is just a way

841
00:38:59,760 --> 00:39:03,360
Speaker 1: to like store that information about which way the photon

842
00:39:03,440 --> 00:39:05,000
Speaker 1: went and keep it quantum.

843
00:39:05,080 --> 00:39:05,239
Speaker 7: Right.

844
00:39:05,239 --> 00:39:06,560
Speaker 1: We don't want to like mark it on a piece

845
00:39:06,600 --> 00:39:08,640
Speaker 1: of paper or put it in a computer some big

846
00:39:08,680 --> 00:39:11,280
Speaker 1: classical object. We want to keep it as quantum information.

847
00:39:11,600 --> 00:39:13,719
Speaker 1: So here the electron is just like a single cube bit.

848
00:39:14,200 --> 00:39:16,480
Speaker 1: It contains some quantum information, but it can be in

849
00:39:16,760 --> 00:39:18,600
Speaker 1: a superposition. It can be a little spin up and

850
00:39:18,640 --> 00:39:19,480
Speaker 1: a little spin down.

851
00:39:19,520 --> 00:39:21,200
Speaker 2: We don't know yet, right, but I feel like you

852
00:39:21,239 --> 00:39:23,839
Speaker 2: bumped the photon right, Like the photon was going through

853
00:39:23,840 --> 00:39:25,880
Speaker 2: the slip, but you made it bump into this electron.

854
00:39:26,000 --> 00:39:29,880
Speaker 2: And now it feels like it's now an impure experiment

855
00:39:29,880 --> 00:39:32,359
Speaker 2: because you bumped it right, not just in a quantum way,

856
00:39:32,400 --> 00:39:34,760
Speaker 2: but you did sort of. It's not maybe the same photon,

857
00:39:34,840 --> 00:39:36,520
Speaker 2: or it's not the same path as a photon who

858
00:39:36,520 --> 00:39:37,920
Speaker 2: didn't bump into an electron.

859
00:39:37,719 --> 00:39:40,480
Speaker 1: That's right, and that's why you no longer see the interference. Right,

860
00:39:40,520 --> 00:39:43,640
Speaker 1: you add this experiment where you're bumping it into the electron,

861
00:39:43,719 --> 00:39:46,399
Speaker 1: it destroys the interference pattern because that information is now

862
00:39:46,480 --> 00:39:49,600
Speaker 1: stored in the electron, so it can be extracted. The

863
00:39:49,680 --> 00:39:52,200
Speaker 1: knowledge of which way the photon went can be measured

864
00:39:52,239 --> 00:39:55,320
Speaker 1: in the universe, and so that destroys this interference pattern

865
00:39:55,360 --> 00:39:58,080
Speaker 1: on the screen. So you're right, it's a different experiment.

866
00:39:57,680 --> 00:39:59,840
Speaker 2: Right, Like the quantumness went from being on the w

867
00:40:00,360 --> 00:40:02,920
Speaker 2: behind the screen to now being in this electron who

868
00:40:03,080 --> 00:40:05,640
Speaker 2: you coked it with? And now I guess the question

869
00:40:05,719 --> 00:40:08,400
Speaker 2: is if I destroyed the information in that electron, do

870
00:40:08,520 --> 00:40:11,280
Speaker 2: I get back my weekly pattern on the screen.

871
00:40:11,400 --> 00:40:14,719
Speaker 1: That's the idea. It's totally mind bending and crazy what

872
00:40:14,840 --> 00:40:17,440
Speaker 1: actually happens. I love it. And so you take this

873
00:40:17,520 --> 00:40:19,799
Speaker 1: electron and you might wonder like, well, how do you

874
00:40:19,880 --> 00:40:23,319
Speaker 1: destroy the information? How do you erase the information? Well,

875
00:40:23,320 --> 00:40:26,520
Speaker 1: it's actually not that hard to erase quantum information. It

876
00:40:26,560 --> 00:40:29,720
Speaker 1: happens all the time. Like if you, for example, measure

877
00:40:29,760 --> 00:40:33,560
Speaker 1: a particle's momentum, then that scrambles your knowledge of the

878
00:40:33,600 --> 00:40:36,840
Speaker 1: particle's position because the Heisberg uncertainty principle says you can't

879
00:40:36,840 --> 00:40:39,120
Speaker 1: know both very very precisely. If you have a particle,

880
00:40:39,120 --> 00:40:42,200
Speaker 1: for example, you measure its position really precisely, and then

881
00:40:42,280 --> 00:40:45,680
Speaker 1: you measure its momentum, then you've erased the quantum information

882
00:40:45,719 --> 00:40:49,160
Speaker 1: about its position because you can't have both simultaneously. So

883
00:40:49,200 --> 00:40:51,440
Speaker 1: you can do sometin things sort of similar to this electron.

884
00:40:51,760 --> 00:40:54,880
Speaker 1: You can't know an electron spin in one direction and

885
00:40:55,040 --> 00:40:57,799
Speaker 1: in another direction at the same time. So if you

886
00:40:57,840 --> 00:41:00,640
Speaker 1: want to erase this spin up and down information, all

887
00:41:00,640 --> 00:41:02,560
Speaker 1: you need to do is measure the spin of the

888
00:41:02,560 --> 00:41:06,080
Speaker 1: electron sort of left right, and that will scramble the

889
00:41:06,160 --> 00:41:08,720
Speaker 1: information about the electron spin up down.

890
00:41:09,000 --> 00:41:11,440
Speaker 2: I poked the box with the cat with my quantum finger,

891
00:41:11,600 --> 00:41:14,359
Speaker 2: and now I'm sort of running my finger through a

892
00:41:14,400 --> 00:41:18,319
Speaker 2: filter that then kind of scrambles or filters out the

893
00:41:18,360 --> 00:41:19,359
Speaker 2: information from the cat.

894
00:41:19,480 --> 00:41:21,960
Speaker 1: That's right. So now it's no longer possible to know

895
00:41:23,080 --> 00:41:24,960
Speaker 1: which state it was in. Was it spin up or

896
00:41:25,040 --> 00:41:26,680
Speaker 1: was it s been down? We don't know anymore, and

897
00:41:26,719 --> 00:41:29,640
Speaker 1: it's scrambled. It's not like the information existed and we've

898
00:41:29,680 --> 00:41:33,480
Speaker 1: overwritten it. It was in a quantum superposition, it was undetermined,

899
00:41:33,960 --> 00:41:37,279
Speaker 1: and now the information about those probabilities is lost. So

900
00:41:37,320 --> 00:41:40,560
Speaker 1: that's the quantum eraser. It says, destroy the information that

901
00:41:40,600 --> 00:41:42,160
Speaker 1: you've extracted from this experiment.

902
00:41:42,280 --> 00:41:44,920
Speaker 2: All right, So we did the experiment, actually we poked

903
00:41:44,920 --> 00:41:47,960
Speaker 2: it with something and then we erased the information. And

904
00:41:48,280 --> 00:41:49,759
Speaker 2: did they actually somebody actually built this.

905
00:41:49,920 --> 00:41:52,239
Speaker 1: Somebody actually built this, and they did it. They did

906
00:41:52,239 --> 00:41:55,239
Speaker 1: this experiment. And so there's a lot of discussion of

907
00:41:55,280 --> 00:41:57,239
Speaker 1: this kind of experiment online and I found a lot

908
00:41:57,239 --> 00:41:59,600
Speaker 1: of these to be sort of misleading because they suggest

909
00:42:00,040 --> 00:42:02,480
Speaker 1: that what happens when you apply the quantum eraser, when

910
00:42:02,480 --> 00:42:05,000
Speaker 1: you erase this experiment, is that the interference pattern like

911
00:42:05,080 --> 00:42:08,560
Speaker 1: reappears on the screen, which is impossible because you could

912
00:42:08,560 --> 00:42:12,359
Speaker 1: do this like quantum eraser experiment like years later, after

913
00:42:12,360 --> 00:42:14,560
Speaker 1: you've already done the original experiment. You know, you could

914
00:42:14,600 --> 00:42:17,479
Speaker 1: like store these electrons somehow, and then five years later

915
00:42:17,520 --> 00:42:20,000
Speaker 1: decide to erase the information. You can't go back in

916
00:42:20,080 --> 00:42:22,800
Speaker 1: time and then change the interference pattern on the screen.

917
00:42:23,160 --> 00:42:25,160
Speaker 1: So that's not what happens. That would be crazy, in

918
00:42:25,200 --> 00:42:28,439
Speaker 1: bonkers and awesome. But instead, what happens is that if

919
00:42:28,480 --> 00:42:31,359
Speaker 1: you do this, if you erase the quantum information, then

920
00:42:31,360 --> 00:42:34,000
Speaker 1: you are making a measurement of those electrons, you're measuring

921
00:42:34,000 --> 00:42:36,319
Speaker 1: them like left right instead of up down. If you

922
00:42:36,400 --> 00:42:38,960
Speaker 1: take those results and you look at only the ones

923
00:42:39,000 --> 00:42:40,880
Speaker 1: that have like electron that turned out to be left,

924
00:42:41,200 --> 00:42:43,279
Speaker 1: or only the ones electron that turned out to be right,

925
00:42:43,480 --> 00:42:46,400
Speaker 1: then you see the interference pattern. So the photons that

926
00:42:46,480 --> 00:42:49,840
Speaker 1: had like a right spinning electron, you see an interference

927
00:42:49,880 --> 00:42:53,480
Speaker 1: pattern in those photons, and there's an interference pattern in

928
00:42:53,520 --> 00:42:55,840
Speaker 1: the photons that had a left spinning electron. If you

929
00:42:55,880 --> 00:42:58,920
Speaker 1: put them together, they add up to the same smooth shape.

930
00:42:59,040 --> 00:43:02,480
Speaker 1: So sort of like the interference pattern was hiding inside

931
00:43:02,520 --> 00:43:05,600
Speaker 1: that smooth shape. And if you scramble the information that

932
00:43:05,640 --> 00:43:08,320
Speaker 1: you knew about which photon went where, you can recover

933
00:43:08,640 --> 00:43:11,920
Speaker 1: that interference pattern from within the smooth shape that you

934
00:43:11,920 --> 00:43:12,720
Speaker 1: saw on the screen.

935
00:43:13,040 --> 00:43:16,719
Speaker 2: Oh right, But then that still requires an observation, right,

936
00:43:16,800 --> 00:43:19,560
Speaker 2: because you're using some of that information you thought you destroyed,

937
00:43:19,600 --> 00:43:21,440
Speaker 2: but you didn't really destroy it in a way or like,

938
00:43:21,480 --> 00:43:24,319
Speaker 2: do you destroy the information in one direction and so

939
00:43:24,400 --> 00:43:27,440
Speaker 2: the quantum objects sort of adjusted into the other direction.

940
00:43:27,560 --> 00:43:31,040
Speaker 1: Yeah, you destroyed the original information. You can't know which

941
00:43:31,080 --> 00:43:34,080
Speaker 1: way the photon went right, and so that allows you

942
00:43:34,160 --> 00:43:37,480
Speaker 1: to have interference, and you can recover that interference if

943
00:43:37,520 --> 00:43:40,440
Speaker 1: you look at, like some of the photons. And the

944
00:43:40,480 --> 00:43:42,799
Speaker 1: reason is that you know some of these photons are

945
00:43:42,920 --> 00:43:45,680
Speaker 1: entangled with some of these electrons in this way, and

946
00:43:45,719 --> 00:43:48,040
Speaker 1: you need to like know how to pull out the

947
00:43:48,080 --> 00:43:51,279
Speaker 1: subset of photons that have the interference pattern you're looking for.

948
00:43:51,719 --> 00:43:54,200
Speaker 1: You can only get that if you have erased the

949
00:43:54,280 --> 00:43:57,160
Speaker 1: quantum information you were looking for. If you access the

950
00:43:57,239 --> 00:43:59,560
Speaker 1: quantum information directly, if you measure spin up or down

951
00:43:59,640 --> 00:44:02,440
Speaker 1: so you know which photon went through which slit, then

952
00:44:02,440 --> 00:44:05,719
Speaker 1: that collapses the wave functions essentially and means so you

953
00:44:05,760 --> 00:44:09,040
Speaker 1: see no interference. You cannot access any interference only if

954
00:44:09,040 --> 00:44:12,000
Speaker 1: you erase the information in those electrons by measuring left

955
00:44:12,080 --> 00:44:14,480
Speaker 1: right instead of up down. Can you then go back

956
00:44:14,680 --> 00:44:17,520
Speaker 1: and split the photons into two categories, each of which

957
00:44:17,640 --> 00:44:18,759
Speaker 1: shows interference.

958
00:44:18,960 --> 00:44:21,840
Speaker 2: Interesting? All right, let's get into what this all means

959
00:44:22,160 --> 00:44:24,280
Speaker 2: and what it can mean about how we see reality.

960
00:44:24,680 --> 00:44:26,319
Speaker 2: But first, let's take another in quick break.

961
00:44:30,520 --> 00:44:32,319
Speaker 1: When you pop a piece of cheese into your mouth

962
00:44:32,440 --> 00:44:35,560
Speaker 1: or enjoy a rich spoonful of Greek yogurt, you're probably

963
00:44:35,600 --> 00:44:39,640
Speaker 1: not thinking about the environmental impact of each and every bite,

964
00:44:39,680 --> 00:44:42,320
Speaker 1: but the people in the dairy industry are US Dairy

965
00:44:42,360 --> 00:44:46,640
Speaker 1: has set themselves some ambitious sustainability goals, including being greenhouse

966
00:44:46,680 --> 00:44:49,239
Speaker 1: gas neutral by twenty to fifty. That's why they're working

967
00:44:49,280 --> 00:44:51,600
Speaker 1: hard every day to find new ways to reduce waste,

968
00:44:51,680 --> 00:44:55,919
Speaker 1: conserve natural resources, and drive down greenhouse gas emissions. Take water,

969
00:44:55,960 --> 00:44:59,040
Speaker 1: for example, most dairy farms reuse water up to four

970
00:44:59,120 --> 00:45:02,560
Speaker 1: times the same water cools the milk, cleans equipment, washes

971
00:45:02,600 --> 00:45:05,400
Speaker 1: the barn, and irrigates the crops. How is US Dairy

972
00:45:05,440 --> 00:45:09,160
Speaker 1: tackling greenhouse gases? Many farms use anaerobic digestors that turn

973
00:45:09,239 --> 00:45:13,120
Speaker 1: the methane from maneuver into renewable energy that can power farms, towns,

974
00:45:13,160 --> 00:45:15,239
Speaker 1: and electric cars. So the next time you grab a

975
00:45:15,280 --> 00:45:17,279
Speaker 1: slice of pizza or lick an ice cream cone, know

976
00:45:17,360 --> 00:45:20,040
Speaker 1: that dairy farmers and processors around the country are using

977
00:45:20,080 --> 00:45:23,560
Speaker 1: the latest practices and innovations to provide the nutrient dense

978
00:45:23,680 --> 00:45:26,399
Speaker 1: dairy products we love with less of an impact. Visit

979
00:45:26,520 --> 00:45:29,279
Speaker 1: usdairy dot com slash sustainability to learn more.

980
00:45:29,960 --> 00:45:33,520
Speaker 3: Hi, I'm David Eagleman from the podcast Inner Cosmos, which

981
00:45:33,520 --> 00:45:36,919
Speaker 3: recently hit the number one science podcast in America. I'm

982
00:45:36,920 --> 00:45:40,680
Speaker 3: a neuroscientists at Stanford, and I've spent my career exploring

983
00:45:40,760 --> 00:45:42,440
Speaker 3: the three pound universe.

984
00:45:42,160 --> 00:45:42,880
Speaker 2: In our heads.

985
00:45:43,200 --> 00:45:45,640
Speaker 3: We're looking at a whole new series of episodes this

986
00:45:45,719 --> 00:45:49,560
Speaker 3: season to understand why and how our lives looked the

987
00:45:49,560 --> 00:45:52,759
Speaker 3: way they do. Why does your memory drift so much?

988
00:45:53,320 --> 00:45:56,600
Speaker 3: Why is it so hard to keep a secret, When

989
00:45:56,640 --> 00:46:01,160
Speaker 3: should you not trust your intuition? Why do so easily

990
00:46:01,239 --> 00:46:04,880
Speaker 3: fall for magic tricks? And why do they love conspiracy theories.

991
00:46:05,560 --> 00:46:09,200
Speaker 3: I'm hitting these questions and hundreds more because the more

992
00:46:09,239 --> 00:46:12,279
Speaker 3: we know about what's running under the hood, the better

993
00:46:12,320 --> 00:46:16,200
Speaker 3: we can steer our lives. Join me weekly to explore

994
00:46:16,239 --> 00:46:19,960
Speaker 3: the relationship between your brain and your life by digging

995
00:46:20,000 --> 00:46:24,360
Speaker 3: into unexpected questions. Listen to Inner Cosmos with David Eagleman

996
00:46:24,480 --> 00:46:26,359
Speaker 3: on the iHeartRadio app, Apple.

997
00:46:26,120 --> 00:46:28,640
Speaker 2: Podcasts or wherever you get your podcasts.

998
00:46:30,680 --> 00:46:31,440
Speaker 13: Guess what, Mango?

999
00:46:31,680 --> 00:46:32,000
Speaker 2: What's that?

1000
00:46:32,120 --> 00:46:34,600
Speaker 13: Well, so iHeart is giving us a whole minute to

1001
00:46:34,640 --> 00:46:36,800
Speaker 13: promote our podcast, Part Time Genius.

1002
00:46:37,120 --> 00:46:37,400
Speaker 5: I know.

1003
00:46:37,520 --> 00:46:39,719
Speaker 14: That's why I spent my whole week composing a haikup

1004
00:46:39,719 --> 00:46:42,800
Speaker 14: for the occasion. It's about my emotional journey in podcasting

1005
00:46:42,880 --> 00:46:45,719
Speaker 14: over the last seven years, and it's called Earthquake.

1006
00:46:45,160 --> 00:46:47,799
Speaker 13: House, Mega Mango, I'm going to cut you off right there.

1007
00:46:47,880 --> 00:46:49,320
Speaker 13: Why don't we just tell people about.

1008
00:46:49,080 --> 00:46:49,880
Speaker 2: Our show instead?

1009
00:46:50,040 --> 00:46:53,360
Speaker 14: Yeah, that's a better idea. So every week on Fartime Genius,

1010
00:46:53,360 --> 00:46:57,120
Speaker 14: we feed our curiosity by answering the world's most important questions,

1011
00:46:57,320 --> 00:47:00,280
Speaker 14: things like when did America start dialing nine to one? One?

1012
00:47:00,560 --> 00:47:03,719
Speaker 14: Is William Shatner's best acting work in Esperanto?

1013
00:47:04,160 --> 00:47:04,480
Speaker 7: Also?

1014
00:47:04,920 --> 00:47:08,120
Speaker 13: What happened to Esperanto? Plus we cover questions like how

1015
00:47:08,239 --> 00:47:11,319
Speaker 13: Chinese is your Chinese food? How do dollar stores stay

1016
00:47:11,320 --> 00:47:14,880
Speaker 13: in business? And of course is there an Illuminati of cheese?

1017
00:47:14,960 --> 00:47:18,200
Speaker 14: There absolutely is, and we are risking our lives by

1018
00:47:18,200 --> 00:47:20,880
Speaker 14: talking about it. But if you love mind blowing facts,

1019
00:47:20,920 --> 00:47:24,400
Speaker 14: incredible history, and really bad jokes, make your brands happy

1020
00:47:24,440 --> 00:47:26,040
Speaker 14: and tune into Part Time Genius.

1021
00:47:26,400 --> 00:47:28,880
Speaker 13: Listen to Part Time Genius on the iHeartRadio app or

1022
00:47:28,880 --> 00:47:30,319
Speaker 13: wherever you get your podcasts.

1023
00:47:31,360 --> 00:47:34,520
Speaker 15: Parents, are you looking for a screen free, engaging way

1024
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1025
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1026
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1027
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1028
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1029
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1030
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Speaker 15: hundreds and hundreds of beautiful episodes that bring the Bible

1031
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1032
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1033
00:48:05,080 --> 00:48:08,640
Speaker 15: your kids. We want kids to see how incredible God's

1034
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1035
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Speaker 15: Bible Stories Podcast. Listen to Kant's Bible Stories podcast on

1036
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Speaker 15: the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.

1037
00:48:29,080 --> 00:48:31,080
Speaker 2: All right, Daniel, I'm still sort of stuck in the

1038
00:48:31,120 --> 00:48:33,160
Speaker 2: cat in the box experiment because I feel like it's

1039
00:48:33,160 --> 00:48:35,319
Speaker 2: a little bit easier to grasp. So we had a

1040
00:48:35,320 --> 00:48:37,320
Speaker 2: cat in the box. We talked up with a quantum finger,

1041
00:48:37,440 --> 00:48:41,719
Speaker 2: and then we measured my finger in one direction, and

1042
00:48:41,760 --> 00:48:45,680
Speaker 2: then we sort of destroyed that information by measuring the

1043
00:48:45,760 --> 00:48:48,440
Speaker 2: finger in a different direction. And then we see that

1044
00:48:48,440 --> 00:48:50,440
Speaker 2: the cat is still sort of alive and dead. But

1045
00:48:50,719 --> 00:48:53,400
Speaker 2: only if we use the information we got from the finger.

1046
00:48:53,200 --> 00:48:56,680
Speaker 1: Right, Yeah, exactly. I think if you want to talk

1047
00:48:56,719 --> 00:48:59,840
Speaker 1: about cats and boxes, then you'll need like one hundred

1048
00:49:00,239 --> 00:49:02,719
Speaker 1: and one hundred boxes because in the end, this is

1049
00:49:02,760 --> 00:49:05,960
Speaker 1: a probabilistic effect. Just like with the double slit experiment,

1050
00:49:06,200 --> 00:49:08,600
Speaker 1: this interference patterns are only obvious if you do a

1051
00:49:08,600 --> 00:49:10,480
Speaker 1: lot of photons, so you can see the patterns. Because

1052
00:49:10,520 --> 00:49:12,960
Speaker 1: a single photon could hit the screen wherever, you can't

1053
00:49:13,000 --> 00:49:14,960
Speaker 1: tell if you're seeing an interference pattern or not from

1054
00:49:15,000 --> 00:49:17,560
Speaker 1: one photon. So let's say you have you know, you're

1055
00:49:17,560 --> 00:49:20,480
Speaker 1: a cat person. Your house is swarming with cats. Each

1056
00:49:20,520 --> 00:49:23,319
Speaker 1: one you put in a box, right, and then you

1057
00:49:23,360 --> 00:49:26,239
Speaker 1: make this quantum measurement of each one, but you don't

1058
00:49:26,280 --> 00:49:27,880
Speaker 1: look at the result, right, you poke it with a

1059
00:49:27,920 --> 00:49:30,920
Speaker 1: quantum finger, you don't look at the results.

1060
00:49:30,520 --> 00:49:32,680
Speaker 2: All right, So then you figured out I guess with

1061
00:49:32,760 --> 00:49:36,279
Speaker 2: this experiment that the objects sort of goes back to

1062
00:49:36,320 --> 00:49:40,000
Speaker 2: being quantum, but not really because maybe you're in a way,

1063
00:49:40,000 --> 00:49:41,480
Speaker 2: you're sort of cheating, right, You're using some of the

1064
00:49:41,480 --> 00:49:45,040
Speaker 2: information you got from poking it to make it look

1065
00:49:45,280 --> 00:49:47,520
Speaker 2: quantum again in a way. Right, It's almost like the

1066
00:49:47,520 --> 00:49:50,200
Speaker 2: photon went back to being quantum, but only half quantum

1067
00:49:50,239 --> 00:49:53,080
Speaker 2: because you were able to measure some of it.

1068
00:49:53,320 --> 00:49:56,080
Speaker 1: Yeah, exactly. It's a bit mind bending because we like

1069
00:49:56,160 --> 00:49:59,120
Speaker 1: to think about what happens to these particles, like what

1070
00:49:59,160 --> 00:50:01,120
Speaker 1: do they really doing? And we like to think that

1071
00:50:01,160 --> 00:50:03,520
Speaker 1: things can't go back in time and change their decision.

1072
00:50:03,920 --> 00:50:07,319
Speaker 1: And you know, the core fuzziness of this experiment is

1073
00:50:07,440 --> 00:50:09,319
Speaker 1: that if you think about these things in terms of

1074
00:50:09,320 --> 00:50:11,640
Speaker 1: particles and waves, you like to think that it's a

1075
00:50:11,680 --> 00:50:14,600
Speaker 1: wave and then it gets collapsed into a particle after

1076
00:50:14,600 --> 00:50:16,839
Speaker 1: it goes through the slit, if there's a detector there,

1077
00:50:17,280 --> 00:50:19,280
Speaker 1: and then needs to decide like am I a particle

1078
00:50:19,360 --> 00:50:21,280
Speaker 1: or am I a wave before it hits the screen,

1079
00:50:21,400 --> 00:50:24,160
Speaker 1: so that it can either make an interference pattern or not. Right,

1080
00:50:24,160 --> 00:50:26,520
Speaker 1: it doesn't seem like it would make sense for that

1081
00:50:26,560 --> 00:50:29,040
Speaker 1: to depend on what you do later on, because you

1082
00:50:29,080 --> 00:50:32,839
Speaker 1: can make this like quantum information decision a year later,

1083
00:50:33,000 --> 00:50:35,759
Speaker 1: or ten years later, or a thousand years later. So

1084
00:50:35,800 --> 00:50:37,720
Speaker 1: some people are at tempted to say that this means

1085
00:50:37,719 --> 00:50:41,920
Speaker 1: that there's retro causality that based on what you do later,

1086
00:50:42,280 --> 00:50:45,440
Speaker 1: it can go back in time and change the results

1087
00:50:45,440 --> 00:50:48,600
Speaker 1: of the experiment. I think that's kind of nonsense. Really,

1088
00:50:48,600 --> 00:50:51,640
Speaker 1: what you're doing here is just interpreting the experiment in

1089
00:50:51,680 --> 00:50:56,440
Speaker 1: a different way using additional information you've extracted from the experiment.

1090
00:50:56,440 --> 00:50:58,360
Speaker 1: As you said, you're sort of cheating, but it's backwards.

1091
00:50:58,400 --> 00:51:00,920
Speaker 1: Right now, you're making a measurement to know how to

1092
00:51:00,960 --> 00:51:04,160
Speaker 1: separate those photons to see the interference pattern. You're not

1093
00:51:04,200 --> 00:51:06,839
Speaker 1: making a measurement about which way the photon went about

1094
00:51:06,840 --> 00:51:09,480
Speaker 1: which slid it went through. Instead, you're just separating the

1095
00:51:09,480 --> 00:51:12,440
Speaker 1: photons into the ones that were entangled with electrons in

1096
00:51:12,480 --> 00:51:15,280
Speaker 1: one way versus photons that were entangled with the electrons

1097
00:51:15,320 --> 00:51:17,880
Speaker 1: in the other way. And those subsets do have the

1098
00:51:17,920 --> 00:51:20,880
Speaker 1: interference going on, it's just it was masked because when

1099
00:51:20,920 --> 00:51:23,200
Speaker 1: you add up the two kinds of interference, they add

1100
00:51:23,239 --> 00:51:24,400
Speaker 1: up to the smooth pattern.

1101
00:51:24,560 --> 00:51:27,279
Speaker 2: It's like it's still the same sort of regular quantum

1102
00:51:27,520 --> 00:51:30,560
Speaker 2: that was going on before. Like the photon looked like

1103
00:51:30,600 --> 00:51:33,399
Speaker 2: it had lost this quantum information, but really, like if

1104
00:51:33,400 --> 00:51:36,040
Speaker 2: you take that information that you got from the poking

1105
00:51:36,120 --> 00:51:39,120
Speaker 2: with the finger, then you can sort of find its

1106
00:51:39,200 --> 00:51:41,839
Speaker 2: quantumness in the direction that you didn't poke it in.

1107
00:51:42,320 --> 00:51:44,920
Speaker 1: Yeah, exactly. And so it's a really fun experiment to

1108
00:51:44,920 --> 00:51:48,120
Speaker 1: try to think about this nature of decoherence. Like you

1109
00:51:48,160 --> 00:51:49,960
Speaker 1: were saying before, if you poke something with a big

1110
00:51:50,000 --> 00:51:53,560
Speaker 1: classical object, it decoheres. It gets entangled with the whole

1111
00:51:53,640 --> 00:51:56,360
Speaker 1: environment millions and millions of particles, and so all of

1112
00:51:56,400 --> 00:51:58,360
Speaker 1: its quantum properties are essentially lost.

1113
00:51:58,719 --> 00:51:58,919
Speaker 7: Here.

1114
00:51:59,000 --> 00:52:01,239
Speaker 1: What we're doing is we're like kind of cheating, we're

1115
00:52:01,239 --> 00:52:04,680
Speaker 1: decohering it, only it tiny little bit by interacting with

1116
00:52:04,719 --> 00:52:07,600
Speaker 1: it with a quantum object. So we can play quantum

1117
00:52:07,640 --> 00:52:11,160
Speaker 1: games with that decoherence later and in the end recover

1118
00:52:11,320 --> 00:52:14,960
Speaker 1: some of that interference by erasing that information. And so

1119
00:52:15,080 --> 00:52:17,920
Speaker 1: it's really sort of a great mental exercise to think

1120
00:52:17,960 --> 00:52:21,160
Speaker 1: about whether you understand decoherence. And we had a whole

1121
00:52:21,160 --> 00:52:24,880
Speaker 1: podcast episode about what quantum decoherence is. It's closely connected

1122
00:52:25,160 --> 00:52:27,839
Speaker 1: to this question of what is a quantum measurement and

1123
00:52:28,000 --> 00:52:30,400
Speaker 1: what happens when you measure something, but it's not quite

1124
00:52:30,440 --> 00:52:33,359
Speaker 1: the same thing. It's more about whether quantum properties can

1125
00:52:33,360 --> 00:52:36,560
Speaker 1: be observed because the different quantum states are still coherent,

1126
00:52:36,640 --> 00:52:38,880
Speaker 1: whether they add up and cancel out in just the

1127
00:52:38,960 --> 00:52:41,600
Speaker 1: right ways to make something have a quantum effect.

1128
00:52:41,719 --> 00:52:44,320
Speaker 2: I think the main point is that you know everything's quantum,

1129
00:52:44,480 --> 00:52:47,400
Speaker 2: but quantumness of something can exist kind of in different

1130
00:52:47,440 --> 00:52:50,360
Speaker 2: directions in a way, or in like different aspects that

1131
00:52:50,400 --> 00:52:52,200
Speaker 2: are part of the whole, but it's still you can

1132
00:52:52,239 --> 00:52:54,160
Speaker 2: sort of take out half of the quantumness of an

1133
00:52:54,160 --> 00:52:56,920
Speaker 2: object and still preserve sort of the other half of

1134
00:52:56,960 --> 00:52:59,520
Speaker 2: the quantumness that it has in the other direction.

1135
00:52:59,719 --> 00:53:03,200
Speaker 1: Yeah, exactly. And so you know, the trickiness here relies

1136
00:53:03,200 --> 00:53:06,319
Speaker 1: on the fact that, like by becoming entangled with a

1137
00:53:06,360 --> 00:53:09,640
Speaker 1: single electron rather than the whole environment, these photons hit

1138
00:53:09,680 --> 00:53:12,960
Speaker 1: the screen only become kind of decohered, right, and so

1139
00:53:13,000 --> 00:53:15,919
Speaker 1: it's just a single particle to worry about. We're sort

1140
00:53:15,920 --> 00:53:18,640
Speaker 1: of able to think about measuring it in different ways,

1141
00:53:18,719 --> 00:53:20,959
Speaker 1: and that's really fun, and it's easier to think about

1142
00:53:20,960 --> 00:53:24,200
Speaker 1: what this experiment means in some interpretations of quantum mechanics

1143
00:53:24,280 --> 00:53:26,880
Speaker 1: than in others. Like in many worlds, it's not that

1144
00:53:26,920 --> 00:53:29,439
Speaker 1: big a deal because the whole universe has a wave

1145
00:53:29,440 --> 00:53:31,960
Speaker 1: function and now we're just talking about the quantum wave

1146
00:53:31,960 --> 00:53:34,239
Speaker 1: function of the photon and the electron and they're kind

1147
00:53:34,239 --> 00:53:36,799
Speaker 1: of entangled and that's no big deal, whereas in like

1148
00:53:36,880 --> 00:53:39,799
Speaker 1: a strict theory where you have collapse, then you have

1149
00:53:39,840 --> 00:53:43,239
Speaker 1: to wonder like, well, did the photon collapse or not,

1150
00:53:43,719 --> 00:53:46,799
Speaker 1: because if I don't destroy the information and I measure it,

1151
00:53:46,840 --> 00:53:48,960
Speaker 1: then the photon has to collapse because I knew which

1152
00:53:49,000 --> 00:53:51,520
Speaker 1: way it went. But if I do destroy the information,

1153
00:53:51,880 --> 00:53:54,399
Speaker 1: then how am I getting an interference pattern later on?

1154
00:53:54,600 --> 00:53:56,760
Speaker 1: Because for that to happen, it has to stay a wave.

1155
00:53:57,239 --> 00:54:00,000
Speaker 1: And so this is sort of troublesome for the collapse

1156
00:54:00,160 --> 00:54:03,080
Speaker 1: theories of quantum mechanics, not so much trouble for other

1157
00:54:03,120 --> 00:54:05,800
Speaker 1: theories like many worlds and relational quantum mechanics.

1158
00:54:05,880 --> 00:54:08,280
Speaker 2: Doesn't that just mean that maybe like the wave collapse

1159
00:54:08,320 --> 00:54:11,160
Speaker 2: in one direction but not the other direction, Like couldn't

1160
00:54:11,200 --> 00:54:14,759
Speaker 2: you still you know, use the coping dating interpretation and

1161
00:54:14,840 --> 00:54:17,160
Speaker 2: just say that it collapse in like one direction and

1162
00:54:17,200 --> 00:54:17,640
Speaker 2: not the other.

1163
00:54:17,800 --> 00:54:19,880
Speaker 1: Well, the electron is the one that has these multiple

1164
00:54:19,880 --> 00:54:23,279
Speaker 1: directions of information that spin up down versus spin left right.

1165
00:54:23,560 --> 00:54:26,920
Speaker 1: The photon is either interfering or it's not, you know,

1166
00:54:27,040 --> 00:54:29,680
Speaker 1: And it either collapses and it's just like a single

1167
00:54:29,719 --> 00:54:32,520
Speaker 1: source which gives you the smooth pattern, or it doesn't

1168
00:54:32,520 --> 00:54:35,760
Speaker 1: collapse and you have the wave function which does interfere.

1169
00:54:35,800 --> 00:54:39,319
Speaker 1: There aren't multiple directions there. And so it's hard to

1170
00:54:39,400 --> 00:54:42,439
Speaker 1: understand how a collapse theory can really work because that

1171
00:54:42,480 --> 00:54:45,040
Speaker 1: does kind of require going back in time and like

1172
00:54:45,280 --> 00:54:48,760
Speaker 1: uncollapsing the wave function to me collapsing the wave function

1173
00:54:48,800 --> 00:54:51,160
Speaker 1: makes no sense at all. It's not even consistent with

1174
00:54:51,239 --> 00:54:54,160
Speaker 1: quantum mechanics because it destroys quantum information in a way

1175
00:54:54,200 --> 00:54:57,680
Speaker 1: that we know violates basic principles and it violates the

1176
00:54:57,960 --> 00:55:00,560
Speaker 1: time continuity of quantum mechanics. You should be able to

1177
00:55:00,600 --> 00:55:03,399
Speaker 1: run experiments forward and backwards. So the collapse theory never

1178
00:55:03,440 --> 00:55:05,560
Speaker 1: made any sense to me, really, and I think this

1179
00:55:05,640 --> 00:55:08,400
Speaker 1: experiment really highlights how it's sort of nonsense.

1180
00:55:08,520 --> 00:55:10,920
Speaker 2: But then the only other interpretation that we have is

1181
00:55:10,960 --> 00:55:14,640
Speaker 2: the multi world's theory, right, which this multiverse theory that

1182
00:55:14,960 --> 00:55:17,879
Speaker 2: every time a quantum object makes a decision, that the

1183
00:55:17,920 --> 00:55:19,280
Speaker 2: two universes are created.

1184
00:55:19,400 --> 00:55:22,560
Speaker 1: Yeah, that's another interpretation, and that one is pretty happy

1185
00:55:22,600 --> 00:55:25,640
Speaker 1: with this experiment. There are other interpretations that you can

1186
00:55:25,760 --> 00:55:28,480
Speaker 1: use that are consistent with this experiment, Like relational quantum

1187
00:55:28,520 --> 00:55:31,279
Speaker 1: mechanics works well with this because it says that like, hey,

1188
00:55:31,320 --> 00:55:33,879
Speaker 1: everything in the universe has its own measurement of these things,

1189
00:55:33,880 --> 00:55:35,799
Speaker 1: and so it doesn't matter what you measure, there is

1190
00:55:35,840 --> 00:55:38,719
Speaker 1: no reality anyway. And then there are also like other

1191
00:55:38,800 --> 00:55:41,759
Speaker 1: variants of collapse theories that are not as strict. You know,

1192
00:55:41,840 --> 00:55:43,920
Speaker 1: let's say, well, collapse happens in this way or in

1193
00:55:43,960 --> 00:55:46,000
Speaker 1: that way. So there's a whole spectrum of them. But

1194
00:55:46,160 --> 00:55:49,360
Speaker 1: this is troublesome for like the most hardcore collapse theories.

1195
00:55:49,480 --> 00:55:51,480
Speaker 2: All right, well, then I guess to answer the question

1196
00:55:51,520 --> 00:55:53,840
Speaker 2: what is a quantum eraser? I feel like the answer

1197
00:55:53,840 --> 00:55:56,480
Speaker 2: to that is sort of straightforward, but it's sort of

1198
00:55:56,480 --> 00:56:00,000
Speaker 2: the implications of what quantum eraser can do. That's really

1199
00:56:00,120 --> 00:56:03,640
Speaker 2: sort of what we spend an hour, and that's really

1200
00:56:03,680 --> 00:56:05,680
Speaker 2: hard to sort of get your head around. So a

1201
00:56:05,760 --> 00:56:08,960
Speaker 2: quantum eraser is just taking quantum information from something and

1202
00:56:09,640 --> 00:56:11,200
Speaker 2: eraising it in a way. Right, Like, if I have

1203
00:56:11,280 --> 00:56:14,319
Speaker 2: quantum information stored in one direction of an electron spin,

1204
00:56:14,520 --> 00:56:17,720
Speaker 2: by measuring it in the other direction, I can destroy

1205
00:56:17,800 --> 00:56:21,080
Speaker 2: that quantum information. Right, that's the idea of a quantum eraser.

1206
00:56:21,200 --> 00:56:23,960
Speaker 1: And if that electron happens to be entangled with photons

1207
00:56:24,000 --> 00:56:27,000
Speaker 1: which may or may not be interfering, then whether or

1208
00:56:27,000 --> 00:56:29,879
Speaker 1: not you erase that information or not can determine whether

1209
00:56:29,960 --> 00:56:32,640
Speaker 1: or not you can see interference in those photons.

1210
00:56:32,719 --> 00:56:34,480
Speaker 2: Well, it doesn't determine whether or not you can see

1211
00:56:34,480 --> 00:56:35,719
Speaker 2: it in a way, it tells you how to look

1212
00:56:35,760 --> 00:56:36,640
Speaker 2: for that interference.

1213
00:56:36,840 --> 00:56:39,560
Speaker 1: Well, if you measure the which way of the photons

1214
00:56:39,680 --> 00:56:42,839
Speaker 1: using those electrons, then you cannot see interference. The only

1215
00:56:42,880 --> 00:56:45,600
Speaker 1: way to see interference is to destroy that information and

1216
00:56:45,640 --> 00:56:48,799
Speaker 1: then use the results of destroying that information to pick

1217
00:56:48,840 --> 00:56:51,520
Speaker 1: out the interference patterns from the photons. You can't do

1218
00:56:51,600 --> 00:56:54,440
Speaker 1: that if you measure which way the photon went.

1219
00:56:54,440 --> 00:56:56,840
Speaker 2: Right, Right, But you're sort of still measuring the electron,

1220
00:56:57,000 --> 00:56:58,680
Speaker 2: and then that's telling you how to look for the

1221
00:56:58,719 --> 00:56:59,960
Speaker 2: interference in the photon.

1222
00:57:00,440 --> 00:57:03,120
Speaker 1: Right. Yes, you're measuring the electron, but you're not measuring

1223
00:57:03,239 --> 00:57:05,959
Speaker 1: which way the original photon went. You're measuring something else

1224
00:57:06,000 --> 00:57:08,240
Speaker 1: about the electron, which destroys that information.

1225
00:57:08,480 --> 00:57:13,279
Speaker 2: All right, it sounds like we erase people's brain and

1226
00:57:13,560 --> 00:57:15,440
Speaker 2: hopefully not their time for the last hour.

1227
00:57:16,240 --> 00:57:18,440
Speaker 1: Thanks very much for going on this journey into the

1228
00:57:18,480 --> 00:57:21,680
Speaker 1: weird quantum world. I love these thought experiments, the ones

1229
00:57:21,680 --> 00:57:24,160
Speaker 1: people think of and say, whoa, what would actually happen?

1230
00:57:24,600 --> 00:57:27,800
Speaker 1: Because that's the fun thing about experimental physics is confronting

1231
00:57:27,840 --> 00:57:30,360
Speaker 1: the universe and saying, all right, universe, show us what

1232
00:57:30,400 --> 00:57:32,720
Speaker 1: you got. We set up a situation that forces you

1233
00:57:32,800 --> 00:57:36,480
Speaker 1: to reveal what's happening, and the quantum universe always responds

1234
00:57:36,520 --> 00:57:37,520
Speaker 1: with something crazy.

1235
00:57:37,720 --> 00:57:40,160
Speaker 2: And that's why we're here to talk about the craziness

1236
00:57:40,240 --> 00:57:42,720
Speaker 2: and to hopefully get you to wrap my mind around

1237
00:57:42,880 --> 00:57:45,640
Speaker 2: all the different and interesting implications about what it means

1238
00:57:45,640 --> 00:57:48,160
Speaker 2: about the things around you that you see in touch

1239
00:57:48,280 --> 00:57:49,479
Speaker 2: or maybe don't see your touch.

1240
00:57:49,640 --> 00:57:51,440
Speaker 1: And so, if you're a person who likes questions and

1241
00:57:51,560 --> 00:57:54,720
Speaker 1: maybe even answers, check out our book Frequently ask Questions

1242
00:57:54,800 --> 00:57:58,120
Speaker 1: about the Universe, available now and coming out in just

1243
00:57:58,160 --> 00:57:58,920
Speaker 1: a couple of weeks.

1244
00:57:59,040 --> 00:58:02,440
Speaker 2: You can find the links at Universe faq dot com.

1245
00:58:02,480 --> 00:58:04,760
Speaker 2: All right, well, thanks for joining us. We hope you

1246
00:58:04,840 --> 00:58:06,720
Speaker 2: enjoyed that. See you next time.

1247
00:58:14,720 --> 00:58:17,520
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain

1248
00:58:17,600 --> 00:58:21,600
Speaker 1: the Universe is a production of iHeartRadio. For more podcasts

1249
00:58:21,600 --> 00:58:26,240
Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

1250
00:58:26,320 --> 00:58:39,600
Speaker 1: you listen to your favorite shows. When you pop a

1251
00:58:39,600 --> 00:58:41,920
Speaker 1: piece of cheese into your mouth, you're probably not thinking

1252
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Speaker 1: about the environmental impact. But the people in the dairy

1253
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Speaker 1: industry are. That's why they're working hard every day to

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Speaker 1: drive down greenhouse gas emissions. How is us dairy tackling

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Speaker 1: greenhouse gases? Many farms use anaerobic digestors to turn the

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Speaker 1: and electric cars. Visit you as dairy dot COM's Last

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Speaker 1: Sustainability to learn more.

1260
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Speaker 2: Guess what will?

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