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Speaker 1: Hey, Jorhan, Daniel here, and we want to tell you

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Speaker 1: about our new book. It's called Frequently Asked Questions about

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Speaker 1: the Universe because you have questions about the universe, and

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Speaker 1: so we 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 as usual, and we

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Speaker 1: tackle all kinds of questions, including what happens if I

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Speaker 1: fall into a black hole? Or is there another version

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Speaker 1: of you out there that's right? Like usual, we tackle

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Speaker 1: the deepest, darkest, biggest, craziest questions about this incredible cosmos.

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Speaker 1: If you want to support the podcast, please get the

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Speaker 1: book and get a copy, not just for yourself, but

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Speaker 1: you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,

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Speaker 1: and for the aliens. So get your copy of Frequently

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Speaker 1: Asked Questions about the Universe is 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 f a Q dot com. Thanks

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Speaker 1: for your support, and if you have a hamster that

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Speaker 1: can read, please let us know. We'd love to have

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Speaker 1: them on the podcast. Daniel, do you ever feel like

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Speaker 1: you really understand quantum mechanics. No, you know, I think

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Speaker 1: it's probably just too alien for us to really ever

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Speaker 1: feel comfortable with. I guess it's too bad there aren't

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Speaker 1: any macroscopic big quantum optics we can really like poke

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Speaker 1: and play with, I know, like a big fat electron.

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Speaker 1: But actually I'm working on a theory that children are

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Speaker 1: governed by the rules of quantum mechanics. Oh really, like

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Speaker 1: there's uncertainty about where they are. Have you lost your kids? No,

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Speaker 1: but I've noticed that you can't like observe your children

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Speaker 1: without sort of perturbing the system, right, I think I

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Speaker 1: know what you mean. Like they won't do their homework

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Speaker 1: unless you're there watching exactly. And when I walk into

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Speaker 1: the room somehow all their conversations collapse suddenly into silence. Yeah,

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Speaker 1: it's like sure the anger as children, they're both excited

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Speaker 1: and sad to see you. Hi am for Handmad cartoonists

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Speaker 1: and the creator of PhD comics. Hi. I'm Daniel. I'm

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Speaker 1: a particle physicist and a professor UC Irvine, and I'm

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Speaker 1: always in many quantum states at once. Really, so that

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Speaker 1: does that mean you're not real? It means I don't

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Speaker 1: even know if I'm real man. Well, I hope you are,

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Speaker 1: because that would mean that I'm talking to myself right now,

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Speaker 1: and that would be a little concerning. Maybe you are

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Speaker 1: the only brain in the universe and the rest of

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

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Speaker 1: make a lot of sense why I'm so successful and

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Speaker 1: good looking. But anyways, welcome to our podcast. Daniel and

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Speaker 1: Jorge explain the university production of I Heart Radio, in

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

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Speaker 1: 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 download the whole

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

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

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Speaker 1: tiny little particles or whatever is happening at the smallest scale.

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Speaker 1: On this podcast, we dive the deepest you can into

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Speaker 1: the hardest and trickiest of questions and we try to

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Speaker 1: explain all of them to you. Yeah, because it is

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Speaker 1: a pretty tricky universe. It's full of interesting rules and

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Speaker 1: interesting phenomena that happens at the smallest of levels and

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Speaker 1: at the largest of scales, and a lot of it

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Speaker 1: is understandable, even if it's not very um intuitive to

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Speaker 1: think about. That's right. And I look at the universe

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

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Speaker 1: a murder mystery, and I want to figure out who

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Speaker 1: did it. I want to understand how it works. It's

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Speaker 1: amazing to me, sort of philosophically, that the universe is

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Speaker 1: presented to us that way, like this big puzzle that

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Speaker 1: isn't obvious to figure out, but yet can somehow be

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Speaker 1: understood if you push hard enough. Do you think the

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Speaker 1: universe is understandable? Daniel? That's a big question in physics,

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Speaker 1: isn't it. It's a big question in the philosophy of

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Speaker 1: physics x and And my answer is it doesn't make

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Speaker 1: sense for it to be understandable, Like how could it

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Speaker 1: be possible that the complexities of this maybe infinite universe

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Speaker 1: could be stored in the minds of a human. On

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Speaker 1: the other hand, we have all these theories that work

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Speaker 1: really well, like surprisingly well, and so I don't know

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Speaker 1: how to hold those two ideas. In my mind, they're

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Speaker 1: in quantum conflict. You're both confused and feeling smart at

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Speaker 1: the same time. That's what this podcast is all about.

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Speaker 1: I'm the confused quantum state and you are the feeling smart.

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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. Yeah, and

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Speaker 1: we have a lot of questions about the universe. And

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Speaker 1: by the way, speaking of questions, we have a new

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Speaker 1: book coming out pretty soon in on November two. That's right.

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Speaker 1: Jorge and I celebrate asking questions about the universe, and

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Speaker 1: we love thinking about these questions. We love hearing about

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

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

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Speaker 1: get about the universe. Yeah. So, if you like this

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Speaker 1: podcast and you want to support us, please check it out.

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Speaker 1: It's called Frequently Ask Questions about the Universe and it's

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Speaker 1: on a pre order right now. You can order it

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Speaker 1: now and get it as soon as it comes out.

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Speaker 1: And I think you know what's important is that we

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Speaker 1: kind of didn't quite write it for our listeners, right, Daniel.

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Speaker 1: We kind of wrote it for the people that know

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Speaker 1: our listeners. You know, if you ever have like a

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Speaker 1: nephew or a cousin or an uncle who you want

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Speaker 1: to share this amazing information about the universe, I think

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Speaker 1: this is the book for you or for them, that's right.

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Speaker 1: So every one of you out there, you should buy

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Speaker 1: five copies and give them to all your friends and

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Speaker 1: family so that they can understand the answers to questions

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Speaker 1: like where did the universe come from? Or how can

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Speaker 1: we travel to the stars. Yeah, we tackle all kinds

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Speaker 1: of pretty cool questions in it, and we try to

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Speaker 1: answer it for people like your relatives and friends with

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Speaker 1: all kinds of interesting and clear answers. And also cartoons,

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Speaker 1: which is I think something you don't see in every

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Speaker 1: day in physics books. That's right. All these awesome fun

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Speaker 1: drawings that help clarify the topic and amuse you along

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Speaker 1: the way. That or added to this book. So if

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Speaker 1: you like this blend of physics and silly jokes, then

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Speaker 1: I think you'll enjoy this book. So go out and

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Speaker 1: get your copy. You can find it at universe f

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Speaker 1: a Q dot com all right, Well, speaking of questions,

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Speaker 1: we are tackling a question today and it has something

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Speaker 1: to do with quantum mechanics, which is, I guess, for

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Speaker 1: lack of a better technical term, bonkers. It is my

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

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Speaker 1: doesn't make sense to your mind. But the math works

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Speaker 1: perfectly and it keeps predicting absurd experimental conclusions that experimentalists

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Speaker 1: keep verifying. Yeah, because I guess you started with a

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Speaker 1: little nugget of an experiment, and then you worked out

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Speaker 1: some math, and then you find out that the crazy

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Speaker 1: math that it suggests is actually also true. Yeah, we

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

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Speaker 1: was going on at the very heart of the universe,

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Speaker 1: at the core of everything that's around 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 turn turns

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Speaker 1: out that the tiniest little parts inside are following totally

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

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Speaker 1: quite different from the one that we thought it was.

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Speaker 1: And people work that out and they thought that's crazy.

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Speaker 1: And if it's true, it would mean you could do

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

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Speaker 1: obviously it can't be true. And then business went out

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Speaker 1: and did the experiment and got the nonsense results, which

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Speaker 1: turns out to be the truth of our reality. Yeah,

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Speaker 1: because I guess at the core of it, it's kind

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Speaker 1: of weird for us humans to think of things that

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Speaker 1: are like two things at the same time, right, I mean,

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Speaker 1: not just in a conceptual ot but like actually in reality,

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Speaker 1: in quantum objects, things can be multiple things at the

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Speaker 1: same time. That's right, And that phrase in reality is

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Speaker 1: the key there, because we imagine at the very basic

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Speaker 1: level that there is a reality out there, that there's

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Speaker 1: a truth that even if we're not looking, the universe

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Speaker 1: is there and it's operating and it's following some rules

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

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Speaker 1: But the reality suggests that the universe is quite different

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

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

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Speaker 1: exactly well determined until you interact with it. Yeah, you

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Speaker 1: might say that in reality, the way the world works

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Speaker 1: and the universe works, it's kind of fuzzy, kind of

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Speaker 1: not quite as solid as we might think it is.

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Speaker 1: From our everyday lives. That's right. We have a weird

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Speaker 1: and particular view of this quantum universe. We are only

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Speaker 1: used to interacting with enormous quantum objects like baseballs and

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Speaker 1: rocks and trees, which are quantum objects. But they contain

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Speaker 1: like ten to the twenty six quantum objects. And when

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Speaker 1: you have that many, they do things differently than when

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Speaker 1: you have one or two of them isolated to reveal

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Speaker 1: their sort of true fundamental nature. So today we're gonna

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Speaker 1: be talking about some really crazy experiments that try to

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Speaker 1: reveal exactly what the rules are of how these particles

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Speaker 1: work at the smallest scale when they're left alone. Yeah,

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Speaker 1: and so it turns out that also reality is not

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Speaker 1: just a little bit fuzzy, but it may not even

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Speaker 1: be as permanent as we think it is. Quantum in

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Speaker 1: for meation and quantum things. We think they're there for real,

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Speaker 1: but it turns out that maybe things can be taken

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Speaker 1: away from the universe. That's right. This fuzzy question of

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Speaker 1: what things are doing when you're not looking at them,

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Speaker 1: and if you look at them and then look away,

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Speaker 1: and it doesn't matter who's looking at them and how

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Speaker 1: they look at them, and whether they store the information

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Speaker 1: and look at it later. All these fun thought experiments

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Speaker 1: can help us try to understand what's really going on

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Speaker 1: at the smallest scales. So to be on the podcast,

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Speaker 1: we'll be talking about what is a quantum eraser? Now, Daniel,

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Speaker 1: is this a rubber eraser or what is it made

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Speaker 1: out of? It's something which will erase your mind if

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Speaker 1: you think about it too much. It's kind of stretch

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Speaker 1: it out like a piece of rubber exactly, and push

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Speaker 1: it too hard and it might just snap. No. Quantum

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

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

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

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Speaker 1: on experiments that listeners on this podcast have heard us

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

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

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Speaker 1: in multiple places at once. We dog into that experiment

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

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

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Speaker 1: on those experiments and think about even crazier versions. Yeah, so,

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Speaker 1: what is a quantum erasor now, Daniels this a thing

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Speaker 1: or like a concept. Yeah, it's both. It was first

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Speaker 1: a concept and then people made it a thing. It's

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Speaker 1: like that in quantum mechanics, a lot of people think, well,

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Speaker 1: if the universe really is that way, here's a ridiculous

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Speaker 1: scenario that should lead to a silly result. And then

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Speaker 1: physicists go out and they do the experiment. They make

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Speaker 1: it real. They figure out a way to like build

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Speaker 1: it in their lab to test that crazy property universe,

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Speaker 1: and they get these ridiculous results, which in the end

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Speaker 1: you have to accept because that's what the experiment says.

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Speaker 1: They say, the universe really works that way. All right,

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Speaker 1: Well we'll dig into it, we'll rub that quantum erazor

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Speaker 1: all of our brains and see there is anything left

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Speaker 1: at the end. But first we were wondering if how

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Speaker 1: many people out there had thought about this question or

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Speaker 1: even heard the term quantum eraser. So Daniel went out

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Speaker 1: there and ask people on the internet what they thought

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Speaker 1: a quantum eraser is. That's right. So if you'd like

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Speaker 1: to be a participant in our virtual person on the

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Speaker 1: street interviews and love to speculate about physics without looking

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Speaker 1: anything up, then please write to us two questions at

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Speaker 1: Daniel and Jorge dot com. All right, so think about

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Speaker 1: it for a second. If a random physicist came up

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Speaker 1: to you on the street and you didn't run away first,

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Speaker 1: and I actually listened to them, and they asked you,

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Speaker 1: what is a quantum eraser? What would you say? You

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Speaker 1: are people's answers something an angry physics grad student uses.

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Speaker 1: I have no idea. Maybe something that erases things at

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Speaker 1: random and like gets rid of things, because that's what

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Speaker 1: quantum generally is, is the randomness. Quantum araser could be

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Speaker 1: something that we invent in the future to erase quantum

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Speaker 1: mechanics and quantum physics just because pretty hard to understand.

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Speaker 1: If we don't have it around anymore, we don't have

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Speaker 1: to deal with it. We can stick with general relativity

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Speaker 1: and regular gravity. So yeah, just get rid of that stuff. Really,

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Speaker 1: I don't know either if I were to compare a

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Speaker 1: quantum or race or to a regular racer, which essentially

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Speaker 1: just kind of distorts the little graphite particles and absorbs

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Speaker 1: them in in a way that you know, it gets

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Speaker 1: rid of the material on a piece of paper or something,

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Speaker 1: so that you know you can reuse that material to

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Speaker 1: write on something. Maybe a quantum race, or some sort

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Speaker 1: of force or phenomenon that causes sometime of particles to

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Speaker 1: break up from a given area or concentration, so that

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Speaker 1: the information is very very difficult to understand, or to

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Speaker 1: receive or to observe. Maybe well, I'm a teacher, and

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Speaker 1: I know that my students use a razors to hide

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Speaker 1: their mistakes, So I think a quantum e razor is

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Speaker 1: something that quantum physicists used to hide their errors from

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Speaker 1: everyone else. All I can come up with for that is,

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Speaker 1: sometimes you make a mistake and it's for the better,

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Speaker 1: it's a better idea than what you originally planned. And

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Speaker 1: sometimes it's a big catastrophe. But since you don't know

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Speaker 1: ahead of time, you use your quantum eraser two either

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Speaker 1: undo your mistake or make it permanent, and you just

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Speaker 1: roll the dice and let fate decide. No idea. Never

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Speaker 1: heard of a quantum eracer, but the image of a really,

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Speaker 1: really kindy eraser pops into my mind. So let's say

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Speaker 1: a tool that allows you to change the sub potomic

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Speaker 1: structure of stuff. I would guess that a quantum eracer

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Speaker 1: is having to do with erasing particles that have certain

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Speaker 1: quantum states. Thereby leaving behind particles that are in the

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Speaker 1: state you want, uh that, or it's the weapon that

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Speaker 1: they used in the movie Racer with Arnold Schwarzenegger. All right,

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Speaker 1: a lot of fun answers here. Everyone's a comedian on

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Speaker 1: the internet. Well, especially if I have no idea what

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Speaker 1: we're talking about, then they got to go to that

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Speaker 1: joking place. I like the joke about the really tiny eraser, right, Yeah,

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Speaker 1: like if you have a quantum pencil, I guess it

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Speaker 1: would have a quantum eraser on the one end of it. Yeah,

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Speaker 1: or e raises quantum particles or something like that. I'm

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Speaker 1: imagining a super tiny little vacuum cleaner that like slurps

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Speaker 1: up electrons. Yeah, and and those one with them passes

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Speaker 1: them through a wormhole into another universe. I guess, tantalizing.

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Speaker 1: All right, well, let's get into this concept of a

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Speaker 1: quantum erasor Daniel, you're saying it has something to do

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Speaker 1: with the double slit experiment. Now, this is going to

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Speaker 1: be kind of hard because I feel like this is

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Speaker 1: a podcast and it's an audio only and this is

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Speaker 1: a very kind of visual experiment. But I guess we

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Speaker 1: can try our best to describe what it is. Yeah,

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

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Speaker 1: you think that will help? Yeah? Or I could draw

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Speaker 1: our tunes. I'm doing watercolor painting at the same time

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Speaker 1: as we do our podcast. By the way, Oh really,

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Speaker 1: you're in a quantum artistic s am. Know. This quantum

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Speaker 1: er racer is an experiment that's like a permutation or

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Speaker 1: an add on to the basic double slit experiment. So

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Speaker 1: to understand why the quantum racer is so weird and crazy,

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Speaker 1: you definitely have to understand what's going on in the

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Speaker 1: double slit experiment. And so I think we're gonna have

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Speaker 1: to use our words to describe the wiggly crazy nature

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Speaker 1: of that experiment, and then we can build on it

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Speaker 1: to get to the quantumer racer. All right, So I

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Speaker 1: guess the double slicks experiment starts with a single slit first.

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Speaker 1: I guess we'll explain that one, and then we'll multiply

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Speaker 1: by two. So the basic experiment is like you have

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Speaker 1: a wall, like a barrier, like a plate of metal,

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Speaker 1: and you cut a little slit on it, like a

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Speaker 1: little opening that's long in one direction, and then you

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Speaker 1: shoot like a laser or like just a regular beam

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Speaker 1: of light through it and then onto a wall behind

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Speaker 1: the first wall. Yeah, exactly. So imagine your mind some

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Speaker 1: source of light. A laser is good because then you

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Speaker 1: have photons of all the same wavelength in the same direction,

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Speaker 1: and then a screen on the other side with the

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Speaker 1: laser hits. 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, intends to

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

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

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

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Speaker 1: other side, right, and you get a smooth light pattern

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Speaker 1: on the other side. Now, the weird thing is in

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Speaker 1: what happens if you put a second slit next to

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Speaker 1: the first slip, right, that's right, So now you put

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Speaker 1: two slits really close together so that the beam could

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Speaker 1: pass through one slit or the other slit, and once

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Speaker 1: you get on the other side, instead of having like

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

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Speaker 1: two patterns like you saw before, now you get an

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Speaker 1: interference pattern, which means that you get these patterns of

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Speaker 1: light and dark and light and dark and light and dark.

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Speaker 1: And what's happening there is interference. Just like if you

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Speaker 1: have waves when you add them, if one wave is

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Speaker 1: going up while the other wave is going down, then

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Speaker 1: they cancel each other out. Whereas if one wave is

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Speaker 1: going up and the other one is going up at

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Speaker 1: the same time, then they add up on top of

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Speaker 1: each other. They get twice as strong. So the interference

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Speaker 1: pattern has these slices that are twice as bright as

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Speaker 1: the previous pattern, and these dark slices as well. And

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Speaker 1: that's because you have two sources of light. Now each

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Speaker 1: of the slits is giving you photons and they can

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Speaker 1: either constructively or destructively interfere on the screen. Right because

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Speaker 1: I guess you're shooting a laser at both slits at

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Speaker 1: the same time, or you're like shooting a laser and

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Speaker 1: it and the beam of the laser kind of goes

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Speaker 1: through both slits at the same time, right, Yeah, the

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Speaker 1: slits are very narrow and very very close together. This

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

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Speaker 1: sort of related to the wavelength of light that we're

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Speaker 1: shooting at it. So this needs to be very microscopic. Right,

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Speaker 1: So if you have one slid, you get a fuzzy,

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Speaker 1: like a plain fuzzy image on the other side. But

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Speaker 1: if you have two slids, then suddenly it's not a smooth,

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Speaker 1: fuzzy image. It's like a weird Rippley kind of image,

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Speaker 1: which means at somehow light is interacting with itself. Yeah,

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

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

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

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

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

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Speaker 1: noise to cancel out the noise that's coming into your ear.

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Speaker 1: So interference in waves is not necessarily a quantum mechanical thing.

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Speaker 1: It's just a wave thing. So in this version of

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Speaker 1: the experiment so far, you could just say, look, light's

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

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Speaker 1: when you remember that the beam is actually made not

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Speaker 1: of waves but of photons, little individual packets, and so

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Speaker 1: you can take it to the next step by slowing

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Speaker 1: down the experiment and dimming the laser. So that's shooting

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Speaker 1: like one photon through the experiment at a time. Yeah,

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Speaker 1: you shoot one photon at a time, and then you

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Speaker 1: would think that just throwing like one ft doon at

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Speaker 1: a time, this photon would pick like the right or

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Speaker 1: the left slid and then end up on the other

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Speaker 1: side of the wall and you would get the same

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Speaker 1: fuzzy smooth pattern. But the weird part I guess is

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Speaker 1: that you're shooting one ft at a time, but you

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Speaker 1: still get the ripley kind of interference pattern on the

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Speaker 1: other side. Exactly. You expect that if you shoot one

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Speaker 1: photon at a time that it can't interfere because you

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Speaker 1: were thinking, well, the interference comes from two photons going

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Speaker 1: through both slits at the same time. Now you have

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Speaker 1: just one photon in the experiment, so what's it interfering with?

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Speaker 1: Because you still see the interference pattern on the other

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Speaker 1: side of the screen and you shoot one photon through

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Speaker 1: at a time. It's just that it takes longer to

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Speaker 1: build up if you watch it for an hour or so.

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Speaker 1: As those photons go through, one lands here, one lands there,

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Speaker 1: one lands this other spot, it gradually builds up that

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Speaker 1: interference pattern. So what's it interfering with. It's interfering with itself.

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Speaker 1: It has the probability to go through both slits, and

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Speaker 1: that wave function, which controls where a quantum particle goes,

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Speaker 1: interferes with itself and creates this probability distribution on the

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Speaker 1: screen for where it might land, and that probability distribution

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Speaker 1: has the interference effects inside of it, and that's why

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Speaker 1: you get this interference pattern. Every photon that goes through

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Speaker 1: like randomly pulls a number from the probability distribution on

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Speaker 1: the screen which has the interference pattern built in, and

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Speaker 1: lands there, and gradually it builds up that distribution. It's

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Speaker 1: almost like, you know, if you were to shoot a

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Speaker 1: photon as a little ball, it would go through one

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Speaker 1: of the slids, but because it's quantum, it's almost like

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Speaker 1: it's going through both slits at the same time. Right.

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Speaker 1: That's that's kind of the quantum thing. It's going through

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Speaker 1: both slits. At the same time, and then it's sort

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Speaker 1: of going through both slits and then interacting with itself

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Speaker 1: in a quantum way so that when it gets to

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Speaker 1: the screen it's not a smooth pattern. Yeah, it's tempting

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Speaker 1: to say that it's in two places at once, or

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Speaker 1: that goes through both slits at the same time, and

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Speaker 1: that's our tendency to try to like tell a story

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Speaker 1: for what happens. But I'm not sure that's the right

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Speaker 1: way to think about it. The way I think about

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Speaker 1: it is that it has a probability to go through

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Speaker 1: both at once. What it actually does is not determined,

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Speaker 1: you know, until it gets to the other side. So

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Speaker 1: what happened when it went through the slits we don't know.

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Speaker 1: We might never know. There isn't necessarily a story there.

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Speaker 1: So it's a all change in wording, but an important

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Speaker 1: change in meaning for me to say that it had

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Speaker 1: probability to go through both slits rather than it actually

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Speaker 1: went through both. Right, it's like saying that it's not

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Speaker 1: that the cat is alive and dead. It's just said

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Speaker 1: it has the same probability, where it has a certain

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Speaker 1: probability of being alive and a certain probability of being dead.

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Speaker 1: All right, Well, then now the weird part here. Now

431
00:21:18,520 --> 00:21:21,240
Speaker 1: it's going to come when we try observing this photon,

432
00:21:21,760 --> 00:21:24,920
Speaker 1: and that's when we get into this idea of quantum razors.

433
00:21:25,000 --> 00:21:27,080
Speaker 1: So let's talk about that. But first let's take a

434
00:21:27,119 --> 00:21:42,000
Speaker 1: quick break. Alright, we're asking the question what is a

435
00:21:42,119 --> 00:21:44,919
Speaker 1: quantum erasor? And now Daniel feel like half of my

436
00:21:44,960 --> 00:21:49,320
Speaker 1: brain is already erased trying to talk about quantum objects.

437
00:21:49,400 --> 00:21:52,359
Speaker 1: And now we were explaining the double slit experiment, and

438
00:21:52,400 --> 00:21:54,199
Speaker 1: so I think we were done with that. Like if

439
00:21:54,240 --> 00:21:56,520
Speaker 1: you shoot a laser at a two small slits on

440
00:21:56,640 --> 00:21:58,919
Speaker 1: a screen, then on the other side you're gonna get

441
00:21:58,960 --> 00:22:01,920
Speaker 1: an interference parent because of the way the quantum probabilities

442
00:22:02,240 --> 00:22:04,640
Speaker 1: kind of affect each other. And so now the weird

443
00:22:04,680 --> 00:22:07,320
Speaker 1: thing happens when you try to like add a detector, right,

444
00:22:07,359 --> 00:22:09,760
Speaker 1: when you try to see which slid it actually went through.

445
00:22:09,800 --> 00:22:12,480
Speaker 1: That's right, because our tendency is to want to know, like, well,

446
00:22:12,600 --> 00:22:15,000
Speaker 1: what happened, Right, did it go through one slit or

447
00:22:15,000 --> 00:22:16,919
Speaker 1: did it go through the other. We feel like it

448
00:22:17,040 --> 00:22:19,479
Speaker 1: must have gone through one or the other, right, because

449
00:22:19,840 --> 00:22:21,680
Speaker 1: you know it was over here and then it's over there.

450
00:22:21,720 --> 00:22:25,200
Speaker 1: So it must go from here to there is our sense,

451
00:22:25,480 --> 00:22:27,320
Speaker 1: and so trying to get it like a more accurate

452
00:22:27,400 --> 00:22:30,480
Speaker 1: understanding of what happened. You can add a little detector

453
00:22:30,520 --> 00:22:33,240
Speaker 1: when that gives you a signal if a photon goes

454
00:22:33,280 --> 00:22:36,000
Speaker 1: through slit A, for example, instead of slit B. So

455
00:22:36,040 --> 00:22:37,800
Speaker 1: that way you can know, hey, did it go through

456
00:22:37,840 --> 00:22:41,040
Speaker 1: slit A or slip B. Because you know, photons are observable, right,

457
00:22:41,080 --> 00:22:43,320
Speaker 1: you can interact with them. They make splashes of light,

458
00:22:43,359 --> 00:22:45,439
Speaker 1: you can measure them. They are quantum objects, but they

459
00:22:45,440 --> 00:22:47,919
Speaker 1: are also physical. And so what happens when you do that,

460
00:22:47,960 --> 00:22:50,720
Speaker 1: when you ask, when you insist on knowing which slid

461
00:22:50,760 --> 00:22:54,320
Speaker 1: it went through, is that the interference pattern disappears. When

462
00:22:54,400 --> 00:22:56,720
Speaker 1: you add that detector that just tries to understand which

463
00:22:56,720 --> 00:22:59,600
Speaker 1: slid it went through, then the interference pattern is gone.

464
00:22:59,640 --> 00:23:02,160
Speaker 1: So if you try to like measure at the photon

465
00:23:02,200 --> 00:23:04,160
Speaker 1: as it goes through slid how would you even do that?

466
00:23:04,600 --> 00:23:06,440
Speaker 1: Don't you have to stop the photon to do that.

467
00:23:06,440 --> 00:23:08,920
Speaker 1: That's the crux of the matter right there. To measure

468
00:23:08,960 --> 00:23:12,399
Speaker 1: a photon, you have to interact with it somehow. You

469
00:23:12,440 --> 00:23:16,000
Speaker 1: can't just like observe a photon without interacting with it.

470
00:23:16,040 --> 00:23:18,400
Speaker 1: You know, a photon that like passes in front of you,

471
00:23:18,400 --> 00:23:21,119
Speaker 1: you can't see it. It's a piece of light, but

472
00:23:21,200 --> 00:23:23,960
Speaker 1: unless it hits your eyes, you can't see it, or

473
00:23:24,000 --> 00:23:26,840
Speaker 1: unless you put something in front of it to stop it,

474
00:23:27,119 --> 00:23:30,000
Speaker 1: measure it and then re emit it. Right, So, for example,

475
00:23:30,040 --> 00:23:31,800
Speaker 1: the reason you see something in front of you as

476
00:23:31,840 --> 00:23:34,640
Speaker 1: red is because the photons hit that object and then

477
00:23:34,680 --> 00:23:39,399
Speaker 1: emitted red photons. So you can't see things without interacting

478
00:23:39,400 --> 00:23:41,720
Speaker 1: with them, even light, right, you need to interact with

479
00:23:41,760 --> 00:23:44,199
Speaker 1: it somehow. So you can make up lots of different

480
00:23:44,200 --> 00:23:47,240
Speaker 1: physical systems that could do this, but the simplest one is,

481
00:23:47,280 --> 00:23:49,480
Speaker 1: you know, just like a simple photon detector, a photo

482
00:23:49,560 --> 00:23:53,360
Speaker 1: multiplier tube for example, or a scintilator screen that indicates

483
00:23:53,359 --> 00:23:55,480
Speaker 1: when a photon went through and then re emits it

484
00:23:55,520 --> 00:23:57,840
Speaker 1: on the other side. Oh, I see what you're saying,

485
00:23:57,920 --> 00:24:00,440
Speaker 1: Like if you catch it and then really sit back

486
00:24:00,440 --> 00:24:02,800
Speaker 1: on the other side, that's one way you can measure

487
00:24:02,840 --> 00:24:05,399
Speaker 1: the photon. And you'd like to think, oh, can't I

488
00:24:05,440 --> 00:24:07,800
Speaker 1: just take a peek? Can I just look and see

489
00:24:07,880 --> 00:24:10,959
Speaker 1: where it went without touching it, without interfering with it,

490
00:24:11,000 --> 00:24:13,440
Speaker 1: without messing with it in any way. But you can't

491
00:24:13,440 --> 00:24:15,880
Speaker 1: do that. Quantum mechanics tells us that the only way

492
00:24:15,880 --> 00:24:19,000
Speaker 1: to get information about an object is to interact with it.

493
00:24:19,280 --> 00:24:21,359
Speaker 1: You can like bounce a photon off of it, or

494
00:24:21,400 --> 00:24:23,119
Speaker 1: you can bounce an electron off of it, or you

495
00:24:23,119 --> 00:24:25,679
Speaker 1: can put another little screen, but somehow you have to

496
00:24:25,760 --> 00:24:28,840
Speaker 1: interact with it. You can't get information from that particle

497
00:24:29,119 --> 00:24:32,600
Speaker 1: without somehow interfering with its path. Yeah, because I guess

498
00:24:32,640 --> 00:24:35,000
Speaker 1: in our everyday lives were used to this idea of

499
00:24:35,000 --> 00:24:37,680
Speaker 1: being able to like see things but not touch them,

500
00:24:37,920 --> 00:24:40,359
Speaker 1: and so we think we can tell where something is

501
00:24:40,480 --> 00:24:43,639
Speaker 1: without actually like influencing it. But when you get down

502
00:24:43,680 --> 00:24:47,159
Speaker 1: to the smallest of levels, like all seeing is interacting

503
00:24:47,240 --> 00:24:48,879
Speaker 1: in a way, that's right, and it's also true with

504
00:24:48,960 --> 00:24:51,479
Speaker 1: the macroscopic scales, just that you don't notice it. Like

505
00:24:51,600 --> 00:24:53,920
Speaker 1: if you are walking outside at night and you want

506
00:24:53,920 --> 00:24:55,360
Speaker 1: to know, hey, is there a rock in my path?

507
00:24:55,480 --> 00:24:58,560
Speaker 1: You turn on your flashlight. You are shooting photons at

508
00:24:58,600 --> 00:25:01,320
Speaker 1: that rock. Those photons are hitting the rock, they're warming

509
00:25:01,400 --> 00:25:04,040
Speaker 1: up the rock. Then the rock is reflecting photons back

510
00:25:04,080 --> 00:25:06,600
Speaker 1: at you. So yeah, you're not touching the rock, but

511
00:25:06,640 --> 00:25:09,160
Speaker 1: you're definitely interacting with the rock and you're changing its

512
00:25:09,200 --> 00:25:11,800
Speaker 1: quantum state. It's just like it doesn't like really heat

513
00:25:11,880 --> 00:25:14,160
Speaker 1: up the rock or push the rock far away. When

514
00:25:14,280 --> 00:25:16,920
Speaker 1: these are quantum particles that they can have significant effects

515
00:25:17,160 --> 00:25:19,240
Speaker 1: if you interact with them by shooting beams of light

516
00:25:19,280 --> 00:25:21,280
Speaker 1: at them or other particles. All right, So then in

517
00:25:21,320 --> 00:25:24,439
Speaker 1: the double slit experiment, meant if you try to measure

518
00:25:24,440 --> 00:25:26,400
Speaker 1: these photons before they hit the wall in the bag,

519
00:25:26,880 --> 00:25:31,560
Speaker 1: then what you're doing is you're collapsing the quantum wave, right,

520
00:25:31,600 --> 00:25:34,520
Speaker 1: You're messing up the quantum information. Yeah, so here's where

521
00:25:34,520 --> 00:25:37,200
Speaker 1: the different interpretations of quantum mechanics. I'll tell you different

522
00:25:37,240 --> 00:25:40,119
Speaker 1: stories for what happens the experiments. Save you measure the

523
00:25:40,160 --> 00:25:43,919
Speaker 1: photon before or after the screen, that the interference pattern

524
00:25:43,960 --> 00:25:48,040
Speaker 1: goes away. The classical interpretation of quantum mechanics, the Copenhagen interpretation,

525
00:25:48,119 --> 00:25:50,840
Speaker 1: is what you just described. It says that the probability

526
00:25:50,880 --> 00:25:53,920
Speaker 1: to go through both slits only exists if you haven't

527
00:25:53,920 --> 00:25:56,040
Speaker 1: made a measurement. But then if you interact with it,

528
00:25:56,040 --> 00:25:58,600
Speaker 1: it collapses the wave function. Now it can only go

529
00:25:58,680 --> 00:26:01,200
Speaker 1: through one slit or the there so there's no interference

530
00:26:01,240 --> 00:26:04,639
Speaker 1: because the interference came from the ambiguity came from the

531
00:26:04,680 --> 00:26:07,760
Speaker 1: probabilities to go through both. The many worlds interpretation tells

532
00:26:07,760 --> 00:26:10,960
Speaker 1: a different story. It says collapse is nonsense, that doesn't happen,

533
00:26:11,000 --> 00:26:14,320
Speaker 1: it's ridiculous. It says that the universe splits into two,

534
00:26:14,400 --> 00:26:16,200
Speaker 1: one where the photon went through one slit and one

535
00:26:16,200 --> 00:26:17,919
Speaker 1: where the photon went through the other, and you're in

536
00:26:17,960 --> 00:26:20,239
Speaker 1: one of those universes and not the other. Right, So

537
00:26:20,280 --> 00:26:22,560
Speaker 1: that's those are the two ways in which you can

538
00:26:22,720 --> 00:26:25,480
Speaker 1: interpret what happens. So how does that relate to, like

539
00:26:25,520 --> 00:26:28,240
Speaker 1: the information and the quantum information. So the idea here

540
00:26:28,359 --> 00:26:31,160
Speaker 1: is that you have the information about whether it went

541
00:26:31,200 --> 00:26:34,560
Speaker 1: through slit A or slit B, and that's what destroys

542
00:26:34,600 --> 00:26:38,359
Speaker 1: the interference because you've made this measurement. Somehow it changes

543
00:26:38,400 --> 00:26:40,760
Speaker 1: the experiment. And you know, this is not something that

544
00:26:40,800 --> 00:26:43,800
Speaker 1: we understand very well, this whole concept of measurement and

545
00:26:43,880 --> 00:26:46,400
Speaker 1: quantum mechanics. And that was the topic of our episode

546
00:26:46,400 --> 00:26:48,840
Speaker 1: with Adam Becker, and then recently we did an episode

547
00:26:48,840 --> 00:26:51,000
Speaker 1: with Carlo Rovelli and he's got a whole new theory

548
00:26:51,080 --> 00:26:53,959
Speaker 1: for how to understand measurement and quantum mechanics. It's not

549
00:26:54,080 --> 00:26:57,440
Speaker 1: something that physics understands very well, how interacting with something

550
00:26:57,560 --> 00:27:00,159
Speaker 1: changes its way? Function, does it collapse it as it

551
00:27:00,200 --> 00:27:02,320
Speaker 1: split the universe? All of this kind of stuff. But

552
00:27:02,359 --> 00:27:05,720
Speaker 1: the key idea here is if you extract information about

553
00:27:05,760 --> 00:27:09,600
Speaker 1: which way the photon went, then there's no interference. Right,

554
00:27:09,680 --> 00:27:12,080
Speaker 1: you somehow get rid of the quantumness of it, Like

555
00:27:12,119 --> 00:27:14,920
Speaker 1: when when I poke something, it's no longer quantum if

556
00:27:14,920 --> 00:27:18,600
Speaker 1: you poke something with a classical object like a big detector, right,

557
00:27:18,920 --> 00:27:21,120
Speaker 1: that big detector can't be in two stays at once.

558
00:27:21,160 --> 00:27:23,000
Speaker 1: It can't be like well, yes I saw it and

559
00:27:23,080 --> 00:27:25,639
Speaker 1: no I didn't. It has to make a decision, and

560
00:27:25,720 --> 00:27:28,760
Speaker 1: so it decoheres, and you get this weird thing where

561
00:27:28,880 --> 00:27:31,359
Speaker 1: quantum object is interacting with the classical objects and so

562
00:27:31,400 --> 00:27:34,159
Speaker 1: now it has to like follow the classical rules. So

563
00:27:34,240 --> 00:27:36,879
Speaker 1: the quantum or racer is an attempt to get around that,

564
00:27:37,040 --> 00:27:39,199
Speaker 1: is to say, what if instead of poking it with

565
00:27:39,240 --> 00:27:42,280
Speaker 1: a big finger or a big classical detector, what if

566
00:27:42,320 --> 00:27:45,360
Speaker 1: we got this information but we somehow kept it quantum

567
00:27:45,400 --> 00:27:47,760
Speaker 1: at the same time. I see. So it's almost like

568
00:27:47,760 --> 00:27:49,880
Speaker 1: the cat in show Dingerous Box, Like before you open

569
00:27:49,920 --> 00:27:52,399
Speaker 1: the box, it's both alive and dead. The probability of

570
00:27:52,480 --> 00:27:54,680
Speaker 1: it being one or the other. And if you open

571
00:27:54,720 --> 00:27:56,840
Speaker 1: the box, that's the classical way of checking it out,

572
00:27:56,880 --> 00:27:58,800
Speaker 1: Like you open it and it's either alive or dead,

573
00:27:58,880 --> 00:28:01,720
Speaker 1: and and then you get it of the quantum probabilities.

574
00:28:01,960 --> 00:28:05,399
Speaker 1: You're saying, can I like somehow, you know, poke my

575
00:28:05,840 --> 00:28:10,280
Speaker 1: quantum finger into the box and measure but not kind

576
00:28:10,280 --> 00:28:14,000
Speaker 1: of destroy that superpocisition of being both alive and dead exactly?

577
00:28:14,040 --> 00:28:16,520
Speaker 1: The quantum or racer experiment tries to do that as well.

578
00:28:16,720 --> 00:28:19,720
Speaker 1: Let's try to get this information out, but not look

579
00:28:19,760 --> 00:28:23,359
Speaker 1: at it directly, not use like our classical objects, our eyeballs,

580
00:28:23,400 --> 00:28:27,439
Speaker 1: our brains, even our computers to access that information, so

581
00:28:27,480 --> 00:28:30,600
Speaker 1: we can stay in a quantum superposition, so we can

582
00:28:30,640 --> 00:28:34,159
Speaker 1: make a decision later about whether we want that information.

583
00:28:34,480 --> 00:28:36,439
Speaker 1: And here's where the mind bending stuff comes in. If

584
00:28:36,440 --> 00:28:39,000
Speaker 1: you can like extract that information about which way the

585
00:28:39,040 --> 00:28:41,880
Speaker 1: photon went, keep it in a quantum state by storing

586
00:28:41,920 --> 00:28:45,280
Speaker 1: it in some other entangled particles, then you can decide

587
00:28:45,680 --> 00:28:48,239
Speaker 1: after the photons I hit the screen whether or not

588
00:28:48,320 --> 00:28:50,280
Speaker 1: you want to know which way it went wait to

589
00:28:50,320 --> 00:28:52,239
Speaker 1: see it again. So the idea is you want to

590
00:28:52,280 --> 00:28:54,320
Speaker 1: know which way the photons went, right? Did they go

591
00:28:54,360 --> 00:28:56,800
Speaker 1: through slit air slip B. You know, if you add

592
00:28:56,840 --> 00:28:59,400
Speaker 1: a classical object like a big detector, you're going to

593
00:28:59,480 --> 00:29:02,760
Speaker 1: collapse the a function. So instead you add a quantum

594
00:29:02,760 --> 00:29:06,480
Speaker 1: detector one that can record this information, but maybe without

595
00:29:06,520 --> 00:29:09,280
Speaker 1: collapsing the state of the wave function. Somehow it gets

596
00:29:09,320 --> 00:29:12,120
Speaker 1: this information. But because it's a quantum object, it doesn't

597
00:29:12,200 --> 00:29:15,200
Speaker 1: trigger the collapse, right, So it can be like entangled

598
00:29:15,240 --> 00:29:18,040
Speaker 1: with the photon without like forcing the photon to deco

599
00:29:18,120 --> 00:29:20,640
Speaker 1: here completely. And so it's different from interacting with like

600
00:29:20,680 --> 00:29:22,520
Speaker 1: your big body or something. You interact with it like

601
00:29:22,560 --> 00:29:25,400
Speaker 1: with a single particle, and that stores the information about

602
00:29:25,440 --> 00:29:27,800
Speaker 1: which way the photon went, but you haven't looked at it.

603
00:29:27,840 --> 00:29:30,560
Speaker 1: You haven't collapsed that wave function yet. You let the

604
00:29:30,560 --> 00:29:33,440
Speaker 1: photon then go hit the screen, and then after the

605
00:29:33,440 --> 00:29:35,840
Speaker 1: photon has already hit the screen and decide where it's

606
00:29:35,840 --> 00:29:39,120
Speaker 1: going to land, then you access that quantum information. It's

607
00:29:39,120 --> 00:29:42,120
Speaker 1: called the delayed choice version, where you decide after the

608
00:29:42,120 --> 00:29:44,360
Speaker 1: photon has hit the screen whether or not you want

609
00:29:44,400 --> 00:29:46,440
Speaker 1: to know the information about which way it went. You're

610
00:29:46,440 --> 00:29:48,160
Speaker 1: saying that the photon did go through one of the

611
00:29:48,200 --> 00:29:50,640
Speaker 1: two slits, like once it hits the screen in the

612
00:29:50,640 --> 00:29:53,040
Speaker 1: bag then it sort of chooses a history of having

613
00:29:53,040 --> 00:29:54,960
Speaker 1: gone through the left or the right slit. That's right.

614
00:29:55,000 --> 00:29:57,520
Speaker 1: This is trying to like force the photon to make

615
00:29:57,600 --> 00:30:00,000
Speaker 1: its decision about whether or not to make an interfere

616
00:30:00,000 --> 00:30:03,800
Speaker 1: it's pattern before you decide whether you want to know

617
00:30:03,880 --> 00:30:06,120
Speaker 1: which that it went through. So it's sort of like,

618
00:30:06,160 --> 00:30:08,720
Speaker 1: you know, trying to play quantum bluff with the photon.

619
00:30:09,280 --> 00:30:11,479
Speaker 1: And that's when we get into these really funny questions

620
00:30:11,480 --> 00:30:14,320
Speaker 1: of like, how does the photon know whether it's going

621
00:30:14,360 --> 00:30:16,920
Speaker 1: to be measured? How does the photon know whether you're

622
00:30:16,920 --> 00:30:20,400
Speaker 1: going to have information about it. It's almost like you wanna,

623
00:30:20,680 --> 00:30:23,120
Speaker 1: you know, peek inside of the short Inger's box, but

624
00:30:23,280 --> 00:30:26,000
Speaker 1: not look at the answer, so that it's still alive

625
00:30:26,040 --> 00:30:27,800
Speaker 1: and dead inside the box. But you sort of have

626
00:30:27,920 --> 00:30:29,800
Speaker 1: the answer in your pocket, but you don't. You haven't

627
00:30:29,800 --> 00:30:32,360
Speaker 1: looked at the answer yet exactly. And so why is

628
00:30:32,360 --> 00:30:34,480
Speaker 1: that call it a quantum eraser? Right, So this is

629
00:30:34,520 --> 00:30:37,880
Speaker 1: not yet a quantum eraser. This is the delayed choice version,

630
00:30:38,000 --> 00:30:41,560
Speaker 1: the delayed measurement, Yes, exactly, delayed measurement choosing whether or

631
00:30:41,560 --> 00:30:43,640
Speaker 1: not you have the information, that's the delay. So you

632
00:30:43,720 --> 00:30:45,800
Speaker 1: might wonder, like, well, what happens on the screen. What

633
00:30:45,840 --> 00:30:47,920
Speaker 1: does the screen look like? What does the experiment look like?

634
00:30:47,960 --> 00:30:50,240
Speaker 1: If you do this, If you capture this information in

635
00:30:50,280 --> 00:30:52,720
Speaker 1: a quantum state, but you don't look at it yet, well,

636
00:30:52,760 --> 00:30:55,320
Speaker 1: what happens is you don't see interference on the screen

637
00:30:55,840 --> 00:30:58,800
Speaker 1: because by doing this, by slurping this information out of

638
00:30:58,840 --> 00:31:01,960
Speaker 1: the photons, you have stored the interference. But people think, well,

639
00:31:01,960 --> 00:31:04,760
Speaker 1: that's interesting. But I haven't yet looked at that information, right,

640
00:31:05,040 --> 00:31:08,640
Speaker 1: So what happens if I then erase that information? This

641
00:31:08,720 --> 00:31:11,880
Speaker 1: is where the quantum eracer comes in. If I take

642
00:31:11,920 --> 00:31:14,560
Speaker 1: that quantum information which is stored in these quantum objects,

643
00:31:14,560 --> 00:31:16,240
Speaker 1: but I haven't looked at it yet, if I erase

644
00:31:16,360 --> 00:31:20,080
Speaker 1: that information somehow, can I then recover the interference? Can

645
00:31:20,120 --> 00:31:23,520
Speaker 1: I make the interference pattern reappear on the screen by

646
00:31:23,640 --> 00:31:26,800
Speaker 1: adding this quantum eracer, which like deletes that information from

647
00:31:26,800 --> 00:31:28,760
Speaker 1: the universe because I never peeked at it. What are

648
00:31:28,760 --> 00:31:31,280
Speaker 1: you saying that this is an actual experiment, Like we've

649
00:31:31,320 --> 00:31:34,840
Speaker 1: sort of intercepted the photon before it goes into the slids,

650
00:31:34,920 --> 00:31:37,800
Speaker 1: and we've stored that information and we see that it

651
00:31:38,040 --> 00:31:42,040
Speaker 1: now it doesn't generate an interference pattern. A weekly pattern

652
00:31:42,160 --> 00:31:45,120
Speaker 1: on the screen, even though nobody really knows which slid

653
00:31:45,120 --> 00:31:47,080
Speaker 1: it went through. That's right, nobody knows which slid it

654
00:31:47,120 --> 00:31:49,719
Speaker 1: went through, though it is in principle stored in this

655
00:31:49,800 --> 00:31:52,840
Speaker 1: quantum object. Although that quantum object can be in a superposition,

656
00:31:52,840 --> 00:31:55,200
Speaker 1: it doesn't have to be in a definite state. Right.

657
00:31:55,240 --> 00:31:57,040
Speaker 1: You can say there's a probability of one and a

658
00:31:57,080 --> 00:32:00,000
Speaker 1: probability of the other, and we don't see that interference patterns.

659
00:32:00,000 --> 00:32:02,560
Speaker 1: And then people thought, well, what if we delete that information?

660
00:32:02,720 --> 00:32:05,560
Speaker 1: What would the universe do if we measure the photon

661
00:32:05,680 --> 00:32:07,520
Speaker 1: but don't look at it, keep in the quantum state,

662
00:32:07,600 --> 00:32:10,560
Speaker 1: and then erase that information. Can it somehow go back

663
00:32:10,600 --> 00:32:13,800
Speaker 1: and recover the interference pattern? I see you're saying that

664
00:32:13,920 --> 00:32:16,280
Speaker 1: maybe it's not the fact that it's interactive with your

665
00:32:16,320 --> 00:32:21,240
Speaker 1: little secret finger poking quantum poking that destroyed the interference pattern.

666
00:32:21,320 --> 00:32:23,800
Speaker 1: Maybe if I poke it with my quantum finger and

667
00:32:23,840 --> 00:32:27,000
Speaker 1: then I destroy my finger, will it go back to

668
00:32:27,040 --> 00:32:29,520
Speaker 1: being a quantum object? Does that what you mean? Like,

669
00:32:29,560 --> 00:32:31,000
Speaker 1: we know that if I poke it, even with a

670
00:32:31,040 --> 00:32:34,040
Speaker 1: quantum finger and not look at the answer, it destroys

671
00:32:34,040 --> 00:32:36,320
Speaker 1: the quantum information. But now what happens if I poke

672
00:32:36,360 --> 00:32:37,600
Speaker 1: it with a band of finger and then the story

673
00:32:37,640 --> 00:32:40,320
Speaker 1: to finger will go back to being a quantum object.

674
00:32:40,400 --> 00:32:42,520
Speaker 1: Is that kind of the idea, that's the quantum eraser

675
00:32:42,680 --> 00:32:45,760
Speaker 1: is destroying that quantum information you've extracted from the experiment

676
00:32:45,920 --> 00:32:48,440
Speaker 1: but haven't yet looked at, so it's still quantum. So

677
00:32:48,560 --> 00:32:50,560
Speaker 1: that's the crazy experiment. And I can hear you react

678
00:32:50,600 --> 00:32:52,760
Speaker 1: and say, what is that real experiment? Did we really

679
00:32:52,800 --> 00:32:55,720
Speaker 1: do that? And yes, we have really done this experiment.

680
00:32:55,880 --> 00:32:57,920
Speaker 1: We have done it with photons, and you can go

681
00:32:58,000 --> 00:33:00,040
Speaker 1: up and google and learn all about the details of

682
00:33:00,120 --> 00:33:02,760
Speaker 1: this experiment. I think there's a slightly simpler version that's

683
00:33:02,760 --> 00:33:05,880
Speaker 1: easier to talk about, where you use electrons. But the

684
00:33:05,920 --> 00:33:08,000
Speaker 1: principles are all the same. And so how can you

685
00:33:08,080 --> 00:33:11,840
Speaker 1: destroy quantum information? Well, for example, if when the photon

686
00:33:11,920 --> 00:33:14,600
Speaker 1: is passing through the slit, you have some detector, and

687
00:33:14,640 --> 00:33:17,360
Speaker 1: that detector takes an electron and puts it in like

688
00:33:17,400 --> 00:33:19,720
Speaker 1: a spin up state. If the photon went through one

689
00:33:19,760 --> 00:33:22,360
Speaker 1: slit and it's been downstate, if the photon went through

690
00:33:22,400 --> 00:33:24,800
Speaker 1: another slit. This is just a way to like store

691
00:33:24,880 --> 00:33:28,320
Speaker 1: that information about which way the photon went and keep

692
00:33:28,320 --> 00:33:30,240
Speaker 1: it quantum. Right, we don't want to mark it on

693
00:33:30,280 --> 00:33:32,160
Speaker 1: a piece of paper, or put it in a computer

694
00:33:32,280 --> 00:33:34,160
Speaker 1: or some big classical object. We want to keep it

695
00:33:34,160 --> 00:33:36,760
Speaker 1: as quantum information. So here the electron is just like

696
00:33:36,760 --> 00:33:39,920
Speaker 1: a single cube bit. It contains some quantum information. But

697
00:33:40,000 --> 00:33:41,880
Speaker 1: it can be in a super position. It can be

698
00:33:41,880 --> 00:33:43,600
Speaker 1: a little spin up and a little spin down. We

699
00:33:43,600 --> 00:33:45,560
Speaker 1: don't know yet, right, but I feel like you bumped

700
00:33:45,600 --> 00:33:48,200
Speaker 1: the photon right, Like the photon was going through the slip,

701
00:33:48,240 --> 00:33:50,440
Speaker 1: but you made a bump into this electron. And now

702
00:33:51,080 --> 00:33:54,120
Speaker 1: it feels like it's in now an impure experiment because

703
00:33:54,160 --> 00:33:56,400
Speaker 1: you bumped it right, not just in a quantum way,

704
00:33:56,440 --> 00:33:58,760
Speaker 1: but you did sort of. It's not maybe the same photon,

705
00:33:58,880 --> 00:34:00,560
Speaker 1: or it's not the same path as a photon who

706
00:34:00,560 --> 00:34:02,680
Speaker 1: didn't bump into an electron. That's right, And that's why

707
00:34:02,720 --> 00:34:05,120
Speaker 1: you no longer see the interference. Right, you add this

708
00:34:05,440 --> 00:34:08,279
Speaker 1: experiment where you're bumping it into the electron, it destroys

709
00:34:08,280 --> 00:34:11,040
Speaker 1: the interference pattern because that information is now stored in

710
00:34:11,120 --> 00:34:14,160
Speaker 1: the electron, so it can be extracted. The knowledge of

711
00:34:14,200 --> 00:34:16,840
Speaker 1: which way the photon went can be measured in the universe,

712
00:34:16,880 --> 00:34:19,840
Speaker 1: and so that destroys this interference pattern on the screen.

713
00:34:19,880 --> 00:34:22,000
Speaker 1: So you're right, it's a different experiment, right, Like the

714
00:34:22,360 --> 00:34:25,120
Speaker 1: quantumness went from being on the wall behind the screen

715
00:34:25,200 --> 00:34:28,279
Speaker 1: to now being in this electron you poked it with,

716
00:34:28,760 --> 00:34:30,520
Speaker 1: And now I guess the question is if I destroy

717
00:34:30,560 --> 00:34:33,439
Speaker 1: the information in that electron, do I get back my

718
00:34:34,040 --> 00:34:37,040
Speaker 1: weekly pattern on the screen. That's the idea. It's totally

719
00:34:37,040 --> 00:34:40,160
Speaker 1: mind bending and crazy what actually happens. I love it.

720
00:34:40,560 --> 00:34:43,320
Speaker 1: And so you take this electron and you might wonder like, well,

721
00:34:43,360 --> 00:34:45,760
Speaker 1: how do you destroy the information? How do you erase

722
00:34:45,880 --> 00:34:49,080
Speaker 1: the information? Well, it's actually not that hard to erase

723
00:34:49,160 --> 00:34:52,319
Speaker 1: quantum information. It happens all the time. Like if you,

724
00:34:52,400 --> 00:34:57,440
Speaker 1: for example, measure particles momentum, then that scrambles your knowledge

725
00:34:57,440 --> 00:35:00,160
Speaker 1: of the particle's position because the Heisberg uncertainty prints will

726
00:35:00,160 --> 00:35:02,600
Speaker 1: says you can't know both very very precisely. If you

727
00:35:02,600 --> 00:35:05,400
Speaker 1: have a particle, for example, you measure its position really precisely,

728
00:35:05,760 --> 00:35:08,759
Speaker 1: and then you measure its momentum, then you've erased the

729
00:35:08,840 --> 00:35:12,480
Speaker 1: quantum information about its position because you can't have both simultaneously.

730
00:35:13,080 --> 00:35:14,759
Speaker 1: So you can do certain things sort of similar to

731
00:35:14,800 --> 00:35:18,160
Speaker 1: this electron. You can't know an electron spin in one

732
00:35:18,200 --> 00:35:21,560
Speaker 1: direction and in another direction at the same time. So

733
00:35:21,600 --> 00:35:24,480
Speaker 1: if you want to erase the spin up and down information,

734
00:35:24,520 --> 00:35:26,480
Speaker 1: all you need to do is measure the spin of

735
00:35:26,480 --> 00:35:29,680
Speaker 1: the electron sort of left right, and that will scramble

736
00:35:30,040 --> 00:35:33,400
Speaker 1: the information about the electron spin up down. I plucked

737
00:35:33,440 --> 00:35:35,719
Speaker 1: the box with the cat with my quantum finger, and

738
00:35:35,760 --> 00:35:39,000
Speaker 1: now instead of running my finger through a filter, that

739
00:35:39,200 --> 00:35:43,040
Speaker 1: then kind of scrambles or filters out the information from

740
00:35:43,040 --> 00:35:45,600
Speaker 1: the cat. That's right. So now it's no longer possible

741
00:35:45,640 --> 00:35:48,680
Speaker 1: to know which state it was in. Was it's been

742
00:35:48,719 --> 00:35:50,560
Speaker 1: up or was it's been down. We don't know anymore.

743
00:35:50,600 --> 00:35:53,400
Speaker 1: And it's scrambled. It's not like the information existed and

744
00:35:53,440 --> 00:35:56,360
Speaker 1: we've overridden it. It was in a quantum superposition. It

745
00:35:56,400 --> 00:36:00,840
Speaker 1: was undetermined, and now the information about those probability is lost.

746
00:36:01,200 --> 00:36:04,520
Speaker 1: So that's the quantum eraser. It says, destroy the information

747
00:36:04,520 --> 00:36:07,000
Speaker 1: that you've extracted from this experiment. All right, So we

748
00:36:07,080 --> 00:36:09,719
Speaker 1: did the experiment, actually we poked it with something and

749
00:36:09,760 --> 00:36:13,160
Speaker 1: then we erased the information. And did they actually somebody

750
00:36:13,200 --> 00:36:15,680
Speaker 1: actually built this. Somebody actually built this, and they did it.

751
00:36:15,880 --> 00:36:18,680
Speaker 1: They did this experiment. And so there's a lot of

752
00:36:18,719 --> 00:36:21,040
Speaker 1: discussion of this kind of experiment online, and I find

753
00:36:21,040 --> 00:36:23,040
Speaker 1: a lot of these to be sort of misleading because

754
00:36:23,040 --> 00:36:26,319
Speaker 1: they suggest that what happens when you apply the quantum racer,

755
00:36:26,360 --> 00:36:28,840
Speaker 1: when you erase this experiment, is that the interference pattern

756
00:36:28,920 --> 00:36:32,360
Speaker 1: like reappears on the screen, which is impossible because you

757
00:36:32,360 --> 00:36:35,719
Speaker 1: could do this like quantum eraser experiment like years later,

758
00:36:36,120 --> 00:36:38,360
Speaker 1: after you've already done the original experiment. You know, you

759
00:36:38,360 --> 00:36:41,239
Speaker 1: could like store these electrons somehow and then five years

760
00:36:41,280 --> 00:36:43,879
Speaker 1: later decided to erase the information. You can't go back

761
00:36:43,920 --> 00:36:46,840
Speaker 1: in time and then change the interference pattern on the screen.

762
00:36:47,200 --> 00:36:49,160
Speaker 1: So that's not what happens. That would be crazy in

763
00:36:49,200 --> 00:36:52,439
Speaker 1: bonkers and awesome. But instead, what happens is that if

764
00:36:52,520 --> 00:36:55,359
Speaker 1: you do this, if you erase the quantum information, then

765
00:36:55,400 --> 00:36:58,000
Speaker 1: you are making a measurement of those electrons. You're measuring

766
00:36:58,040 --> 00:37:00,360
Speaker 1: them like left right instead of up down. If you

767
00:37:00,440 --> 00:37:03,000
Speaker 1: take those results and you look at only the ones

768
00:37:03,040 --> 00:37:04,920
Speaker 1: that have like electron that turned out to be left,

769
00:37:05,239 --> 00:37:06,920
Speaker 1: or only the ones that electron that turned out to

770
00:37:06,960 --> 00:37:09,480
Speaker 1: be right, then you see the interference pattern. So the

771
00:37:09,520 --> 00:37:13,200
Speaker 1: photons that had like a right spinning electron, you see

772
00:37:13,200 --> 00:37:17,000
Speaker 1: an interference pattern in those photons, And there's an interference

773
00:37:17,000 --> 00:37:19,480
Speaker 1: pattern in the photons that had a left spinning electron.

774
00:37:19,640 --> 00:37:21,440
Speaker 1: If you put them together, they add up to the

775
00:37:21,480 --> 00:37:24,759
Speaker 1: same smooth shape. So it's sort of like the interference

776
00:37:24,800 --> 00:37:27,960
Speaker 1: pattern was hiding inside that smooth shape. And if you

777
00:37:28,239 --> 00:37:31,480
Speaker 1: scramble the information that you knew about which photon went where,

778
00:37:31,640 --> 00:37:35,440
Speaker 1: you can recover the interference pattern from within the smooth

779
00:37:35,440 --> 00:37:37,719
Speaker 1: shape that you saw on the screen. All right, But

780
00:37:37,800 --> 00:37:42,040
Speaker 1: then that still requires an observation, right, because you're seeing

781
00:37:42,040 --> 00:37:43,719
Speaker 1: some of that information you thought you destroyed, but you

782
00:37:43,760 --> 00:37:45,600
Speaker 1: didn't really destroy it in a way, or like, did

783
00:37:45,640 --> 00:37:48,480
Speaker 1: you destroy the information in one direction and so the

784
00:37:48,560 --> 00:37:51,799
Speaker 1: quantum objects sort of adjusted into the other direction. Yeah,

785
00:37:51,840 --> 00:37:55,319
Speaker 1: you destroyed the original information, you can't know which way

786
00:37:55,360 --> 00:37:58,239
Speaker 1: the photon went right, and so that allows you to

787
00:37:58,400 --> 00:38:01,600
Speaker 1: have interference, and you can recover that interference if you

788
00:38:01,640 --> 00:38:04,840
Speaker 1: look at like some of the photons. And the reason

789
00:38:04,960 --> 00:38:07,360
Speaker 1: is that you know some of these photons are entangled

790
00:38:07,400 --> 00:38:09,840
Speaker 1: with some of these electrons in this way, and you

791
00:38:09,880 --> 00:38:12,520
Speaker 1: need to like know how to pull out the subset

792
00:38:12,560 --> 00:38:15,319
Speaker 1: of photons that have the interference pattern you're looking for.

793
00:38:15,760 --> 00:38:18,200
Speaker 1: You can only get that if you have erased the

794
00:38:18,280 --> 00:38:21,520
Speaker 1: quantum information you're looking for. If you access the quantum

795
00:38:21,560 --> 00:38:23,799
Speaker 1: information directly. If you measure spin up or down so

796
00:38:23,840 --> 00:38:26,640
Speaker 1: you know which photon went through with slit, then that

797
00:38:26,760 --> 00:38:30,000
Speaker 1: collapses the way functions essentially and means that you see

798
00:38:30,000 --> 00:38:33,120
Speaker 1: no interference. You cannot access any interference only if you

799
00:38:33,160 --> 00:38:36,239
Speaker 1: erase the information in those electrons by measuring left right

800
00:38:36,280 --> 00:38:38,839
Speaker 1: instead of up down. Can you then go back and

801
00:38:38,880 --> 00:38:43,400
Speaker 1: split the photons into two categories, each of which shows interference. Interesting?

802
00:38:43,400 --> 00:38:46,279
Speaker 1: All right, let's get into what this all means and

803
00:38:46,320 --> 00:38:48,759
Speaker 1: what it can mean about how we see reality. But

804
00:38:48,840 --> 00:39:03,640
Speaker 1: first let's take another quick break. All right, Daniel, I'm

805
00:39:03,680 --> 00:39:05,439
Speaker 1: still sort of stuck in the cat in the box

806
00:39:05,440 --> 00:39:08,040
Speaker 1: experiment because I feel like that's a little easier to grasp.

807
00:39:08,200 --> 00:39:10,040
Speaker 1: So we had a cat in the box, we puked

808
00:39:10,080 --> 00:39:12,319
Speaker 1: up with a plantum finger, and then we measured my

809
00:39:12,400 --> 00:39:16,600
Speaker 1: finger in one direction, and then we sort of destroyed

810
00:39:16,640 --> 00:39:21,000
Speaker 1: that information by measuring the finger in a different direction,

811
00:39:21,360 --> 00:39:22,960
Speaker 1: and then we see that the cat is still sort

812
00:39:23,000 --> 00:39:25,120
Speaker 1: of alive and that but only if we use the

813
00:39:25,160 --> 00:39:29,440
Speaker 1: information we got from the finger, right, Yeah, exactly. I

814
00:39:29,480 --> 00:39:31,600
Speaker 1: think if you want to talk about cats and boxes,

815
00:39:31,840 --> 00:39:34,880
Speaker 1: then you'll need like a hundred cats and a hundred boxes,

816
00:39:35,239 --> 00:39:37,879
Speaker 1: because in the end, This is a probabilistic effect. Just

817
00:39:37,960 --> 00:39:41,000
Speaker 1: like with the double slid experiment, these interference patterns are

818
00:39:41,040 --> 00:39:43,000
Speaker 1: only obvious if you do a lot of photons, so

819
00:39:43,040 --> 00:39:45,080
Speaker 1: you can see the patterns. Because a single photon could

820
00:39:45,120 --> 00:39:47,279
Speaker 1: hit the screen wherever, you can't tell if you're seeing

821
00:39:47,280 --> 00:39:49,960
Speaker 1: an interference pattern or not from one photon. So let's

822
00:39:49,960 --> 00:39:52,359
Speaker 1: say you have you know you're a cat person. Your

823
00:39:52,360 --> 00:39:54,759
Speaker 1: house is swarming with cats. Each one you put in

824
00:39:54,760 --> 00:39:58,560
Speaker 1: a box, right, and then you make this quantum measurement

825
00:39:58,640 --> 00:40:00,720
Speaker 1: of each one, but you don't look at the result.

826
00:40:00,960 --> 00:40:03,359
Speaker 1: You poke it with a quantum finger, you don't look

827
00:40:03,400 --> 00:40:05,839
Speaker 1: at the result, all right, So then you've figured out

828
00:40:05,880 --> 00:40:09,359
Speaker 1: I guess with this experiment that the objects sort of

829
00:40:09,440 --> 00:40:12,640
Speaker 1: goes back to being quantum, but not really because maybe

830
00:40:12,719 --> 00:40:14,600
Speaker 1: you're in a way, you're sort of cheating, right, You're

831
00:40:14,640 --> 00:40:17,000
Speaker 1: using some of the information you got from poking it

832
00:40:17,120 --> 00:40:20,440
Speaker 1: to make it look quantum again in a way. Right,

833
00:40:20,480 --> 00:40:22,720
Speaker 1: It's almost like the photon went back to being quantum,

834
00:40:22,760 --> 00:40:26,000
Speaker 1: but only half quantum because you were able to measure

835
00:40:26,320 --> 00:40:28,840
Speaker 1: some of it. Yeah, exactly. It's a bit mind bending

836
00:40:28,960 --> 00:40:31,879
Speaker 1: because we like to think about what happens to these

837
00:40:31,920 --> 00:40:34,319
Speaker 1: particles like what are they really doing? And we like

838
00:40:34,360 --> 00:40:36,160
Speaker 1: to think that things can't go back in time and

839
00:40:36,280 --> 00:40:39,600
Speaker 1: change their decision. And you know, the core fuzziness of

840
00:40:39,600 --> 00:40:42,480
Speaker 1: this experiment is that if you think about these things

841
00:40:42,520 --> 00:40:44,839
Speaker 1: in terms of particles and waves, you like to think

842
00:40:44,880 --> 00:40:46,960
Speaker 1: that it's a wave and then it gets collapsed into

843
00:40:47,040 --> 00:40:49,640
Speaker 1: a particle after it goes through the slit if there's

844
00:40:49,640 --> 00:40:52,359
Speaker 1: a detector there, and then needs to decide like am

845
00:40:52,360 --> 00:40:54,279
Speaker 1: I a particle or am I a wave before it

846
00:40:54,320 --> 00:40:56,040
Speaker 1: hits the screen, so that it can either make an

847
00:40:56,040 --> 00:40:58,640
Speaker 1: interference pattern or not. Right, it doesn't seem like it

848
00:40:58,680 --> 00:41:01,000
Speaker 1: would make sense for or that to depend on what

849
00:41:01,080 --> 00:41:03,560
Speaker 1: you do later on, because you can make this like

850
00:41:03,840 --> 00:41:07,520
Speaker 1: quantum information decision a year later, or ten years later,

851
00:41:07,600 --> 00:41:10,440
Speaker 1: or a thousand years later. So some people are attempted

852
00:41:10,480 --> 00:41:13,920
Speaker 1: to say that this means that there's retro causality that

853
00:41:14,200 --> 00:41:17,040
Speaker 1: based on what you do later, you can go back

854
00:41:17,080 --> 00:41:20,400
Speaker 1: in time and change the results of the experiment. I

855
00:41:20,440 --> 00:41:23,120
Speaker 1: think that's kind of nonsense. Really, what you're doing here

856
00:41:23,280 --> 00:41:26,800
Speaker 1: is just interpreting the experiment in a different way using

857
00:41:26,920 --> 00:41:30,600
Speaker 1: additional information you've extracted from the experiment. As you said,

858
00:41:30,600 --> 00:41:32,640
Speaker 1: you're sort of cheating but it's backwards right now. You're

859
00:41:32,719 --> 00:41:35,800
Speaker 1: making a measurement to know how to separate those photons

860
00:41:35,960 --> 00:41:38,600
Speaker 1: to see the interference pattern. You're not making a measurement

861
00:41:38,600 --> 00:41:41,120
Speaker 1: about which way the photon went about which slid it

862
00:41:41,160 --> 00:41:44,200
Speaker 1: went through. Instead, you're just separating the photons into the

863
00:41:44,200 --> 00:41:47,160
Speaker 1: ones that were entangled with electrons in one way versus

864
00:41:47,200 --> 00:41:49,799
Speaker 1: photons that were entangled with the electrons in the other way.

865
00:41:49,960 --> 00:41:52,880
Speaker 1: And those subsets do have the interference going on, it's

866
00:41:52,960 --> 00:41:55,080
Speaker 1: just it was masked because when you add up the

867
00:41:55,080 --> 00:41:58,120
Speaker 1: two kinds of interference, they add up to the smooth pattern.

868
00:41:58,239 --> 00:42:00,200
Speaker 1: It's like it's still the same sort of get to

869
00:42:00,239 --> 00:42:03,840
Speaker 1: a quantum that was going on before. Like the photon

870
00:42:03,920 --> 00:42:06,400
Speaker 1: looked like it had lost this quantum information, but really

871
00:42:06,560 --> 00:42:09,000
Speaker 1: like if you take that information that you got from

872
00:42:09,040 --> 00:42:11,640
Speaker 1: the poking with the finger, then you can sort of

873
00:42:11,840 --> 00:42:15,200
Speaker 1: find its quantumness in the direction that you didn't poke

874
00:42:15,239 --> 00:42:18,040
Speaker 1: it in. Yeah, exactly. And so it's a really fun

875
00:42:18,040 --> 00:42:21,240
Speaker 1: experiment to try to think about this nature of decoherence.

876
00:42:21,560 --> 00:42:23,320
Speaker 1: Like you were saying before, if you poke something with

877
00:42:23,360 --> 00:42:26,759
Speaker 1: a big classical object, it decoheres. It gets entangled with

878
00:42:26,840 --> 00:42:29,560
Speaker 1: the whole environment. Millions and millions of particles, and so

879
00:42:29,760 --> 00:42:32,799
Speaker 1: all of its quantum properties are essentially lost. Here. What

880
00:42:32,800 --> 00:42:35,200
Speaker 1: we're doing is we're like kind of cheating, we're deco

881
00:42:35,280 --> 00:42:38,359
Speaker 1: hearing it only Italian a little bit by interacting with

882
00:42:38,400 --> 00:42:41,280
Speaker 1: it with a quantum object. So we can play quantum

883
00:42:41,320 --> 00:42:44,840
Speaker 1: games with that decoherence later and in the end recover

884
00:42:45,000 --> 00:42:48,680
Speaker 1: some of that interference by erasing that information. And so

885
00:42:48,760 --> 00:42:51,600
Speaker 1: it's really sort of a great mental exercise to think

886
00:42:51,600 --> 00:42:54,800
Speaker 1: about whether you understand decoherence. And we had a whole

887
00:42:54,840 --> 00:42:58,560
Speaker 1: podcast episode about what quantum decoherence is. It's closely connected

888
00:42:58,840 --> 00:43:01,480
Speaker 1: to this question of what is a quantum measurement and

889
00:43:01,680 --> 00:43:04,080
Speaker 1: what happens when you measure something, but it's not quite

890
00:43:04,080 --> 00:43:07,040
Speaker 1: the same thing. It's more about whether quantum properties can

891
00:43:07,040 --> 00:43:10,239
Speaker 1: be observed because the different quantum states are still coherent,

892
00:43:10,280 --> 00:43:12,560
Speaker 1: whether they add up and cancel out in just the

893
00:43:12,640 --> 00:43:15,480
Speaker 1: right ways to make something have a quantum effect. I

894
00:43:15,480 --> 00:43:18,000
Speaker 1: think the main point is that you know everything is quantum,

895
00:43:18,120 --> 00:43:21,080
Speaker 1: but quantumness of something can exist kind of in different

896
00:43:21,120 --> 00:43:24,000
Speaker 1: directions in a way, or in like different aspects that

897
00:43:24,040 --> 00:43:25,839
Speaker 1: are part of the whole, but it's still you can

898
00:43:25,880 --> 00:43:27,839
Speaker 1: sort of take out half of the quantumness of an

899
00:43:27,840 --> 00:43:30,600
Speaker 1: object and still for serve sort the other half of

900
00:43:30,600 --> 00:43:34,360
Speaker 1: the quantumness that it has in the other direction. Yeah, exactly,

901
00:43:34,880 --> 00:43:37,040
Speaker 1: And so you know, the trickiness here relies in the

902
00:43:37,080 --> 00:43:41,000
Speaker 1: fact that, like by becoming entangled with a single electron

903
00:43:41,120 --> 00:43:43,720
Speaker 1: rather than the whole environment, these photons hit the screen

904
00:43:43,760 --> 00:43:46,960
Speaker 1: only become kind of decohered, right, and so it's just

905
00:43:47,000 --> 00:43:50,320
Speaker 1: a single particle to worry about. We're sort of able

906
00:43:50,360 --> 00:43:53,280
Speaker 1: to think about measuring in different ways, and that's really fun,

907
00:43:53,400 --> 00:43:55,760
Speaker 1: and it's easier to think about what this experiment means

908
00:43:55,800 --> 00:43:59,239
Speaker 1: in some interpretations of quantum mechanics than in others. Like

909
00:43:59,320 --> 00:44:01,560
Speaker 1: in many world it's not that big a deal because

910
00:44:01,640 --> 00:44:03,919
Speaker 1: the whole universe has a wave function and now we're

911
00:44:03,920 --> 00:44:06,640
Speaker 1: just talking about the quantum wave function of the photon

912
00:44:06,760 --> 00:44:08,839
Speaker 1: and the electron and they're kind of entangled and that's

913
00:44:08,880 --> 00:44:11,879
Speaker 1: no big deal, whereas in like a strict theory where

914
00:44:11,920 --> 00:44:14,640
Speaker 1: you have collapse, then you have to wonder like, well,

915
00:44:14,880 --> 00:44:18,279
Speaker 1: did the photon collapse or not, because if I don't

916
00:44:18,360 --> 00:44:21,120
Speaker 1: destroy the information and I measure it, then the photon

917
00:44:21,160 --> 00:44:23,160
Speaker 1: has to collapse because I knew which way it went.

918
00:44:23,440 --> 00:44:26,000
Speaker 1: But if I do destroy the information, then how am

919
00:44:26,040 --> 00:44:28,960
Speaker 1: I getting an interference pattern later on? Because for that

920
00:44:29,000 --> 00:44:31,239
Speaker 1: to happen, it has to stay a wave. And so

921
00:44:31,280 --> 00:44:34,440
Speaker 1: this is sort of troublesome for the collapse theories of

922
00:44:34,520 --> 00:44:37,239
Speaker 1: quantum mechanics, not so much trouble for other theories like

923
00:44:37,280 --> 00:44:40,319
Speaker 1: many worlds and relational quantum mechanics. Doesn't it just mean

924
00:44:40,320 --> 00:44:43,000
Speaker 1: that maybe like the wave collapse in one direction but

925
00:44:43,080 --> 00:44:46,080
Speaker 1: not the other direction, Like couldn't you still you know,

926
00:44:46,280 --> 00:44:49,080
Speaker 1: use the coping dating interpretation and just say that it

927
00:44:49,080 --> 00:44:51,600
Speaker 1: collapse in like one direction and not the other. Well,

928
00:44:51,600 --> 00:44:54,080
Speaker 1: the electron is the one that has these multiple directions

929
00:44:54,080 --> 00:44:57,000
Speaker 1: of information that spin up down versus spin left right.

930
00:44:57,239 --> 00:45:00,600
Speaker 1: The photon is either interfering or it's not, you know,

931
00:45:00,719 --> 00:45:03,320
Speaker 1: and it either collapses and it's just like a single

932
00:45:03,360 --> 00:45:06,160
Speaker 1: source which gives you the smooth pattern, or it doesn't

933
00:45:06,160 --> 00:45:09,440
Speaker 1: collapse and you have the wave function which does interfere.

934
00:45:09,480 --> 00:45:13,000
Speaker 1: There aren't multiple directions there, and so it's hard to

935
00:45:13,080 --> 00:45:16,080
Speaker 1: understand how it collapse theory can really work because that

936
00:45:16,160 --> 00:45:18,640
Speaker 1: does kind of require going back in time and like

937
00:45:18,920 --> 00:45:22,120
Speaker 1: uncollapsing the way of function. To me, collapsing the way

938
00:45:22,160 --> 00:45:24,719
Speaker 1: function makes no sense at all it's not even consistent

939
00:45:24,760 --> 00:45:27,640
Speaker 1: with quantum mechanics because it destroys quantum information in a

940
00:45:27,680 --> 00:45:31,240
Speaker 1: way that we know violates basic principles and it violates

941
00:45:31,239 --> 00:45:33,719
Speaker 1: the like time continuity of quantum mechanics that says you

942
00:45:33,719 --> 00:45:36,120
Speaker 1: should be able to run experiments forward and backwards. So

943
00:45:36,160 --> 00:45:38,400
Speaker 1: the collapse theory never made any sense to me, really,

944
00:45:38,600 --> 00:45:41,480
Speaker 1: and I think this experiment really highlights how it's sort

945
00:45:41,480 --> 00:45:44,200
Speaker 1: of nonsense. But then the only other interpretation that we

946
00:45:44,239 --> 00:45:47,799
Speaker 1: have is the multi world theory, right, which this multiverse

947
00:45:47,920 --> 00:45:50,600
Speaker 1: theory that every time a quantum object makes a decision,

948
00:45:51,120 --> 00:45:54,560
Speaker 1: that the two universes are created. Yeah, that's another interpretation,

949
00:45:54,640 --> 00:45:57,480
Speaker 1: and that one is pretty happy with this experiment. There

950
00:45:57,520 --> 00:46:00,520
Speaker 1: are other interpretations that you can use the are consistent

951
00:46:00,560 --> 00:46:03,279
Speaker 1: with this experiment, Like relational quantum mechanics works well with

952
00:46:03,320 --> 00:46:05,480
Speaker 1: this because it says that like, hey, everything in the

953
00:46:05,560 --> 00:46:07,719
Speaker 1: universe has its own measurement of these things, and so

954
00:46:07,760 --> 00:46:11,000
Speaker 1: it doesn't matter what you measure, there is no reality anyway.

955
00:46:11,080 --> 00:46:13,600
Speaker 1: And then there are also like other variants of collapse

956
00:46:13,640 --> 00:46:16,040
Speaker 1: theories that are not as strict. You know, let's say, well,

957
00:46:16,080 --> 00:46:18,360
Speaker 1: collapse happens in this way or in that way, so

958
00:46:18,400 --> 00:46:20,560
Speaker 1: there's a whole spectrum of them, but this is troublesome

959
00:46:20,600 --> 00:46:23,759
Speaker 1: for like the most hardcore collapse theories. All right, well,

960
00:46:23,800 --> 00:46:25,440
Speaker 1: then I guess to answer the question what is a

961
00:46:25,520 --> 00:46:27,920
Speaker 1: quantum eraser? I feel like the answer to that is

962
00:46:27,960 --> 00:46:31,080
Speaker 1: sort of straightforward, but it's sort of the implications of

963
00:46:31,120 --> 00:46:34,160
Speaker 1: what quantum eraser can do. That's really sort of what

964
00:46:34,280 --> 00:46:37,560
Speaker 1: we spend in our check on and that's really hard

965
00:46:37,640 --> 00:46:39,800
Speaker 1: to sort of get your head around. So a quantum

966
00:46:39,840 --> 00:46:43,760
Speaker 1: razer is just taking quantum information from something and erasing

967
00:46:43,800 --> 00:46:45,320
Speaker 1: it in a way. Right, Like, if I have quantum

968
00:46:45,320 --> 00:46:48,359
Speaker 1: information stored in one direction of an electron spin, by

969
00:46:48,360 --> 00:46:51,520
Speaker 1: measuring it in the other direction, I can destroy that

970
00:46:51,640 --> 00:46:54,719
Speaker 1: quantum information. Right, that's the idea of a quantum razor.

971
00:46:54,880 --> 00:46:57,640
Speaker 1: And if that electron happens to be entangled with photons

972
00:46:57,680 --> 00:47:00,640
Speaker 1: which may or may not be interfering, then whether or

973
00:47:00,680 --> 00:47:03,560
Speaker 1: not you erase that information or not can determine whether

974
00:47:03,640 --> 00:47:06,560
Speaker 1: or not you can see interference in those photons. Well,

975
00:47:06,560 --> 00:47:08,200
Speaker 1: it doesn't determine whether or not you can see it

976
00:47:08,280 --> 00:47:10,680
Speaker 1: the way it tells you how to look for that interference. Well,

977
00:47:10,719 --> 00:47:13,600
Speaker 1: if you measure the which way of the photons using

978
00:47:13,600 --> 00:47:16,680
Speaker 1: those electrons, then you cannot see interference. The only way

979
00:47:16,719 --> 00:47:19,480
Speaker 1: to see interference is to destroy that information and then

980
00:47:19,920 --> 00:47:22,640
Speaker 1: use the results of destroying that information to pick out

981
00:47:22,680 --> 00:47:25,440
Speaker 1: the interference patterns from the photons. You can't do that

982
00:47:25,840 --> 00:47:28,520
Speaker 1: if you measure which way the photon went right, right,

983
00:47:28,520 --> 00:47:30,920
Speaker 1: But you're sort of still measuring the electron, and then

984
00:47:30,960 --> 00:47:33,160
Speaker 1: that's telling you how to look for the interference in

985
00:47:33,200 --> 00:47:35,759
Speaker 1: the photon parent right. Yes, you're measuring the electron, but

986
00:47:35,800 --> 00:47:38,719
Speaker 1: you're not measuring which way the original photon went. You're

987
00:47:38,719 --> 00:47:41,960
Speaker 1: measuring something else about the electron, which destroys that information.

988
00:47:42,160 --> 00:47:47,080
Speaker 1: All right, it sounds like we erase people's brain and

989
00:47:47,200 --> 00:47:50,360
Speaker 1: hopefully not their time for the last hour. Thanks very

990
00:47:50,440 --> 00:47:53,560
Speaker 1: much for going on this journey into the weird quantum world.

991
00:47:53,840 --> 00:47:55,840
Speaker 1: I love these thought experiments, the ones people think of

992
00:47:55,880 --> 00:47:59,000
Speaker 1: and say, whoa, what would actually happen? Because that's the

993
00:47:59,040 --> 00:48:02,040
Speaker 1: fun thing about experien mental physics is confronting the universe

994
00:48:02,040 --> 00:48:04,439
Speaker 1: and saying, all right, universe, show us what you got.

995
00:48:04,560 --> 00:48:06,920
Speaker 1: We set up a situation that forced you to reveal

996
00:48:07,040 --> 00:48:11,160
Speaker 1: what's happening, and the quantum universe always responds with something crazy.

997
00:48:11,400 --> 00:48:13,360
Speaker 1: And that's why we're here. To uh talk about the

998
00:48:13,360 --> 00:48:15,960
Speaker 1: craziness and to hopefully get you to wrap your mind

999
00:48:16,000 --> 00:48:19,000
Speaker 1: around all the different and interesting implications about what it

1000
00:48:19,000 --> 00:48:21,440
Speaker 1: means about the things around you that you see in

1001
00:48:21,520 --> 00:48:23,680
Speaker 1: touch or maybe don't see your touch. And so, if

1002
00:48:23,719 --> 00:48:26,080
Speaker 1: you're a person who likes questions and maybe even answers,

1003
00:48:26,160 --> 00:48:29,360
Speaker 1: check out our book Frequently Asked Questions about the Universe,

1004
00:48:29,440 --> 00:48:32,600
Speaker 1: available now and coming out in just a couple of weeks.

1005
00:48:32,719 --> 00:48:35,080
Speaker 1: You can find the links at Universe f a Q

1006
00:48:35,400 --> 00:48:38,040
Speaker 1: dot com. All right, well, thanks for joining us. We

1007
00:48:38,120 --> 00:48:49,120
Speaker 1: hope you enjoyed that. See you next time. Thanks for listening,

1008
00:48:49,160 --> 00:48:51,920
Speaker 1: and remember that Daniel and Jorge explained. The Universe is

1009
00:48:51,960 --> 00:48:55,440
Speaker 1: a production of I Heart Radio or more podcast from

1010
00:48:55,440 --> 00:48:58,560
Speaker 1: my Heart Radio. Visit the I Heart Radio Apple Apple

1011
00:48:58,600 --> 00:49:01,680
Speaker 1: Podcasts or where every you listen to your favorite shows.