Speaker 1: If you've done some traveling, then you know the experience
Speaker 1: of feeling out of place. There are still some places
Speaker 1: on Earth where the culture is sufficiently different from ours
Speaker 1: or from yours, then you can no longer sort of
Speaker 1: trust your instincts. You don't intuitively know how to behave
Speaker 1: and how to get around. Maybe you don't know what
Speaker 1: people are saying, and you can't read the street signs,
Speaker 1: and things just seem to work differently. The street food
Speaker 1: has weird eyeballs in it where looks like it's fried scorpions. Hey,
Speaker 1: maybe it is. Well, that's the experience physicists have when
Speaker 1: we discover that the rules of the universe are totally
Speaker 1: different from what we are familiar with. That's exactly the
Speaker 1: feeling we're going for, and that's the feeling we get
Speaker 1: very often when we travel to the quantum realm. Yeah. Hi,
Speaker 1: I'm Daniel. I'm a particle physicist and I'm one half
Speaker 1: of the dynamic duo known as Daniel and Jorge, hosts
Speaker 1: of this podcast, Daniel and Jorge Explain the Universe, brought
Speaker 1: to you by I Heart Radio. My co host today,
Speaker 1: Jorge Champ can't be with us, so I'll be talking
Speaker 1: to you myself about the amazing secrets of the universe.
Speaker 1: This podcast is dedicated to revealing truths about the universe,
Speaker 1: to taking things that seem amazing and mystifying that maybe
Speaker 1: you've heard about or people talk about when they want
Speaker 1: to sound smart, but you never really understood. Well, we
Speaker 1: are here to break it down and make sure you
Speaker 1: can walk away actually understanding it. You can talk about
Speaker 1: it like an intelligent person. So if you have questions
Speaker 1: about what you heard today, please send them to us
Speaker 1: two questions at Daniel and Jorge dot com. It is
Speaker 1: our goal that you actually understand everything we are talking
Speaker 1: about and hopefully maybe even get a chuckle along the way.
Speaker 1: Since it's just me today, we'll probably have fewer jokes
Speaker 1: than usual. It's often Hooree that injects the humor into
Speaker 1: these conversations. So we're little humans. We're here on Earth,
Speaker 1: and we're used to a certain kind of experience. We
Speaker 1: used to things a certain size and moving on a
Speaker 1: certain speed. But the universe is a big place and
Speaker 1: there are lots of things out there that are not
Speaker 1: like our experience, that are really big or moving really fast.
Speaker 1: And what we've learned as humans is that most of
Speaker 1: the universe is different from what we expected, and that
Speaker 1: it follows rules that are different. And when we try
Speaker 1: to understand the rest of the universe, the things that
Speaker 1: are not balls rolling down planes or how water flows
Speaker 1: down a hill, that we need to do something mentally difficult.
Speaker 1: We need to do some sort of extrapolation. And that's
Speaker 1: the job of physics is to take us from the
Speaker 1: known into the unknown, to say, well, we understand how
Speaker 1: these things happen here on Earth. Can we also understand
Speaker 1: how the Earth moves around the sun. Can we understand
Speaker 1: the origins of the universe. Can we peel back layers
Speaker 1: of reality and see how things are built underneath? And
Speaker 1: in order to do that, we have to describe things
Speaker 1: we haven't seen in terms of things that we have seen.
Speaker 1: We talk about particles, and we like to describe them
Speaker 1: as kind of like waves and kind of like little
Speaker 1: spinning balls, because those are familiar mental constructs. Those are
Speaker 1: things we understand, so we can talk to each other
Speaker 1: about them. It's like if you drink a new wine
Speaker 1: and you try to describe it to your friends and
Speaker 1: you say, oh, it has flavors of oak and maybe BlackBerry.
Speaker 1: It's a way to describe things that you don't know
Speaker 1: in terms of things that you know. It's a basic
Speaker 1: mental strategy for understanding things. But what happens when you
Speaker 1: run into something fundamentally different, something unlike anything you've seen before,
Speaker 1: something where all of your mental constructs fail or are limited,
Speaker 1: when our experience has just not prepared us for something.
Speaker 1: That's the topic of today's podcast, Can we ever understand
Speaker 1: quantum mechanics? And before we dive in, I want to
Speaker 1: give a music shout out to Casey Hagman who sent
Speaker 1: in that alternative quest in music. Thank you very much.
Speaker 1: We loved it. And quantum mechanics is one of the
Speaker 1: most difficult things for people to grasp, one of the
Speaker 1: most intimidating topics, because people feel like it just can't
Speaker 1: make sense to them, and they hear eminent scientists, researchers, philosophers,
Speaker 1: physicists talking about quantum mechanics as if they don't understand
Speaker 1: it either. And it's true, there's a lot left to
Speaker 1: be understood about quantum mechanics. Folks like Sean Carroll talk
Speaker 1: about it very intelligently, and there's lots of lively discussion
Speaker 1: about what quantum mechanics really means, But I want to
Speaker 1: talk about something simpler. Just what does quantum mechanics say?
Speaker 1: What have we learned about the universe via quantum mechanics?
Speaker 1: Is it possible for us to develop an intuition to
Speaker 1: understand quantum mechanics? Is? Will it always forever be foreign
Speaker 1: to us? Or can we become comfortable with it? Will
Speaker 1: we by spending enough time? And it by thinking? Is
Speaker 1: it possible that by spending enough time marinating in these
Speaker 1: concepts and thinking about them in the right way, that
Speaker 1: we could eventually become familiar with them, sort of the
Speaker 1: way Chinese might seem impenetrable to a Western person at first,
Speaker 1: but you spend enough time there and it becomes part
Speaker 1: of your brain. Your brain develops sort of new ways
Speaker 1: of thinking, new patterns, new ideas flow through it, so
Speaker 1: that the new language and the topics and the strategies
Speaker 1: and the tones of that language eventually become familiar. They
Speaker 1: become how you think. So the strategy there, of course,
Speaker 1: is immerging. You don't learn a new language effectively by
Speaker 1: learning vocabulary and studying in the classroom. The best way
Speaker 1: I've always found really learned to speak a new language,
Speaker 1: to think in a new language. Is to spend time
Speaker 1: doing it, is to go to that country and be
Speaker 1: part of those people and be there and use that
Speaker 1: as part of your life. And that's how your brain works.
Speaker 1: So our strategy today for helping you understand quantum mechanics
Speaker 1: is not to give you the math medical basics, but
Speaker 1: to spend some time in the quantum realm until we
Speaker 1: develop an intuition for how it works. But before we
Speaker 1: go there, I wanted to know what people understood about
Speaker 1: quantum mechanics and also whether they thought they understood quantum mechanics.
Speaker 1: Is quantum mechanics something that everybody out there feels like
Speaker 1: is impossible to understand or are most people under the
Speaker 1: impression that they have it figured out. I walked around
Speaker 1: campus that you see Irvine, and I asked people if
Speaker 1: they thought they could understand quantum mechanics. Before you hear
Speaker 1: these answers, think to yourself, how good is your grasp
Speaker 1: on the basic tenets of quantum mechanics. Do you feel fluent?
Speaker 1: Can you explain this stuff? Do you feel like you
Speaker 1: can navigate that strange quantum realm? Here's what people on
Speaker 1: the street that you see Irvine had to say. Do
Speaker 1: you feel like you understand quantum mechanics? Not anyone who
Speaker 1: answers yes to that doesn't really have an understanding of quantum.
Speaker 1: So no, very basically, I have working knowledge of quantum mechanics,
Speaker 1: but I would not say that I understand it at
Speaker 1: the a PhD physicist, I say I understand it decently. Well.
Speaker 1: Are you familiar with the double slit experiment? Yes, um
Speaker 1: show that particles. I think it was electrons that they use,
Speaker 1: and they move in waves, So if you're to fire
Speaker 1: the electrons and observe them, they'd only go on like
Speaker 1: a line. But then if you cover it you can
Speaker 1: see a wave like pattern in the back or something
Speaker 1: like that. I honestly don't even know about quantum mechanics.
Speaker 1: I'm just the goal nature of light. It's both a
Speaker 1: wave and a particle. So, as usual, we've got a
Speaker 1: very nice breath of answers from people who felt like
Speaker 1: they hardly understood anything about quantum mechanics the people who
Speaker 1: clearly had some working knowledge. So that's great, and we're
Speaker 1: hoping to bring everyone who listens to this podcast up
Speaker 1: to at least the very basic level of being able
Speaker 1: to intelligently think about and understand quantum topics, and I
Speaker 1: think the best way to start off is to hear
Speaker 1: a listener question. Here's a question from a young listener. Hi,
Speaker 1: Daniel Lynn Whole. Hey, my name is Robin, and I
Speaker 1: would love to know what the key differences between quantum
Speaker 1: and classical mechanics are and to what extent they agree
Speaker 1: with each other. This is a great way to start
Speaker 1: because classical physics is what we're familiar with. Classical physics
Speaker 1: is what describes how basketball moves, or how rocks roll
Speaker 1: down hills, or how things move in the atmosphere, things
Speaker 1: that we are familiar with that we have spent hundreds
Speaker 1: or thousands of years developing an intuition for things that
Speaker 1: probably our brains have evolved to be good at understanding,
Speaker 1: so that in some way they are natural objects to
Speaker 1: our mental worlds. Right, So the question really is what's
Speaker 1: the biggest difference between the classical world and the quantum world.
Speaker 1: What is it when you go to the quantum world
Speaker 1: that you can no longer assume that is obviously true
Speaker 1: in the classical world. The basic difference between classical objects
Speaker 1: and quantum mechanical objects is that quantum mechanical objects do
Speaker 1: not have a path. They don't have a trajectory through space,
Speaker 1: a well defined stead of where they were at any
Speaker 1: given time. That's the key, l Us and I want
Speaker 1: you to come away from today's podcast with. So let's
Speaker 1: talk about what that means. If you have a baseball
Speaker 1: and it's at one location now and ten seconds later
Speaker 1: it's across the baseball field, you imagine that it went
Speaker 1: from where it was to where it is now. You say, well,
Speaker 1: if it was over there before and it's over here
Speaker 1: and now, how did it get there. It must have
Speaker 1: gone between those two locations. You have two pieces of data,
Speaker 1: and you naturally interpolate because you make this assumption that
Speaker 1: a classical object has a location at every time, and
Speaker 1: that you can stitch those together into a path that
Speaker 1: traverses space time in some continuous way. And you might
Speaker 1: be thinking, well, of course everything does. Everything has a
Speaker 1: location at a given time, and only one location. But
Speaker 1: that's an intuition you've developed from experiencing the natural world,
Speaker 1: from playing baseball, or from being chased by people throwing
Speaker 1: rocks at you. You're used to things moving through the
Speaker 1: world in a responsible and understandable way. So you've made
Speaker 1: this assumption. Quantum objects do not have paths like that.
Speaker 1: They don't have locations that translate through time. For a
Speaker 1: quantum object like an electron, you can measure it at
Speaker 1: one location A and then later measure it at another
Speaker 1: location B, but it doesn't mean that it's flown through
Speaker 1: the universe from A to B in that intervening time.
Speaker 1: It can be at A later be at B, but
Speaker 1: it's not true that it's moved from A to B,
Speaker 1: meaning that if you've made a measurement halfway, you would
Speaker 1: have found it going from A to B. Now that's
Speaker 1: very confusing, that's very hard to understand. How can an
Speaker 1: object be here and later be there and never be
Speaker 1: in between? And the reason is that quantum objects are
Speaker 1: just different from the classical objects. They are not the
Speaker 1: same kind of thing. And this assumption that you have
Speaker 1: made about the universe comes from only experiencing classical objects
Speaker 1: things on our scale, but the smaller level the universe
Speaker 1: just does not work that way, and that's pretty hard
Speaker 1: to digest. But don't worry. There are things at the
Speaker 1: quantum realm which do follow your intuition in which you
Speaker 1: can use to help understand and develop an intuitive sense
Speaker 1: of the of the universe, and that's the quantum wave function.
Speaker 1: So how do the quantum object work. Well, a quantum
Speaker 1: object is here and later it's there. What determines that.
Speaker 1: It's not like quantum objects don't follow the laws of physics.
Speaker 1: And you probably know the quantum objects could also be random,
Speaker 1: that you can do the same thing to a quantum
Speaker 1: object twice and get different outcomes. And that's true. All
Speaker 1: those things are true. But the quantum object is ruled
Speaker 1: by something, and that's this quantum wave function. You don't
Speaker 1: have to know a lot of complicated math. All you
Speaker 1: need to know is that the wave function tells you
Speaker 1: where a quantum object is most likely to be. You
Speaker 1: probably have heard that electrons don't have a specific location
Speaker 1: to have a probability cloud around an atom. That's an
Speaker 1: extrapolation of the wave function. The wave function tells you
Speaker 1: where the electron is most like it to be. If
Speaker 1: it's large over here, has it's more likely to be there.
Speaker 1: If it's small somewhere else, it's less likely to be there.
Speaker 1: If it's zero somewhere, it means the particle cannot be there.
Speaker 1: Where is the particle? Actually, we don't know, and there's
Speaker 1: a lot of discussion about whether the wave function means
Speaker 1: the particle isn't actually anywhere or just we don't know
Speaker 1: where it is, but we do know that quantum mechanics
Speaker 1: says that that information does not need to exist. It's
Speaker 1: not necessarily the case that there's hidden knowledge that the
Speaker 1: electron is actually somewhere and we don't know. So the
Speaker 1: best way to think about the wave function is that
Speaker 1: it's physics saying what's allowed and what's more likely, and
Speaker 1: this wave function is what we can grab onto, what
Speaker 1: we can understand because the wave function does follow rules,
Speaker 1: and the wave function has a path that makes sense.
Speaker 1: It flows from one spot to the other. It doesn't
Speaker 1: just disappear and appear somewhere else. So you have to
Speaker 1: let go of your intuitive desire to understand the path
Speaker 1: of of an electron the path of a quantum object,
Speaker 1: because those things don't exist. But instead, when you let
Speaker 1: that go, you can grasp onto the next layer. The
Speaker 1: next layer is this wave function, the thing that determines
Speaker 1: where the electron is, the thing that determines where the
Speaker 1: electron is likely to be somewhere else. And that's the
Speaker 1: key thing, is that the wave function tells you where
Speaker 1: the electron is, and it responds to stimulation, it responds
Speaker 1: to the world, and it flows just like a wave,
Speaker 1: and you can use that you can understand how that
Speaker 1: wave flows through time to understand where the electron is
Speaker 1: likely to be in the future. Even if the electron
Speaker 1: itself doesn't have a path from A to B, it's
Speaker 1: wave function does. So the key to being comfortable with
Speaker 1: quantum mechanics is to get comfortable with the objects wave
Speaker 1: function rather than the object. All right, So the best
Speaker 1: way to learn a new language is to get practice,
Speaker 1: is to immerse yourself in it. So the next thing
Speaker 1: we're gonna do is a bunch of exercises of diving
Speaker 1: into the quantum realm and being comfortable with the wave
Speaker 1: function understanding how it flows. And to do that, we're
Speaker 1: going to explore in some detail the famous double slit
Speaker 1: experiment that shows us the crazy behavior of quantum mechanics.
Speaker 1: And at the end of it, I hope it all
Speaker 1: makes sense and the weird results of the double slit
Speaker 1: experiment feel totally natural. But first, let's take a quick break.
Speaker 1: All right, So we are talking about quantum mechanics and
Speaker 1: we're trying to get an intuitive grasp of quantum mechanics.
Speaker 1: We want not just that you say things about quantum
Speaker 1: mechanics that sound intelligent. We want to actually get an
Speaker 1: understanding how do things work at the lowest level. What
Speaker 1: intuition do we have that we can apply? And the
Speaker 1: key thing, again, remember, is the wave function. You have
Speaker 1: quantum mechanical objects like electrons and protons and photons. They
Speaker 1: are difficult to describe using our classical ideas of balls
Speaker 1: and objects that move through space because quantum objects don't
Speaker 1: have paths. And said, we're gonna focus on their wave function,
Speaker 1: the thing that says where they are likely to be,
Speaker 1: and we're gonna understand how that wave function changes through time.
Speaker 1: And we're gonna grab onto that instead of having a
Speaker 1: classical path with the object, we're gonna understand how the
Speaker 1: wave function changes through time, because that's the thing that's
Speaker 1: most like a classical path. All right, So we're gonna
Speaker 1: start very simple, and we're gonna work up to more
Speaker 1: complicated situations. Imagine that you are a photon. You are
Speaker 1: a tiny little quantum particle, and you're shot out of
Speaker 1: a laser with a bunch of your friends towards the screen.
Speaker 1: What happens while you leave the laser right later you
Speaker 1: hit the screen. Does that mean that you flew through
Speaker 1: the room from the laser to the screen. Not necessarily, right,
Speaker 1: You do not have a classical path. Just because you
Speaker 1: left the laser and later hit the screen doesn't mean
Speaker 1: you have a trajectory. Doesn't mean that you necessarily can
Speaker 1: be found in between things in the quantum realm. Don't go, however,
Speaker 1: your wave function does. When you are created inside the laser,
Speaker 1: just at the aperture of the laser, your wave function
Speaker 1: says where you're likely to be, And then your wave
Speaker 1: function flows through the room. If it isn't interacting with everything,
Speaker 1: it just flows nicely through the room, showing the most
Speaker 1: likely place to find your location. This is not the
Speaker 1: same as having a classical path. This is sort of
Speaker 1: the path of your wave function. Then it hits the screen,
Speaker 1: and the screen essentially measures it. It says, okay, wave function,
Speaker 1: where is this photon? And that's the moment when the
Speaker 1: universe has to decide. Instead of having a probability distribution
Speaker 1: about where you are, it says where is this actual photon?
Speaker 1: So you have two measurements. When you leave the laser
Speaker 1: and when you hit the screen. What happens in between
Speaker 1: we don't know, but the wave function tells us, given
Speaker 1: that you've left the laser, what's the likely place to
Speaker 1: land on the screen. And then when you land on
Speaker 1: the screen, the universe rolls a die and says where
Speaker 1: you actually are. Okay, that's easy. Now let's do the
Speaker 1: experiment again. A bunch of photons flying out from a
Speaker 1: laser towards the wall. Let's bring in some black walls
Speaker 1: so that instead of having a totally clear path of
Speaker 1: the screen, you have sort of a narrow gap, not
Speaker 1: too small, you know, a few inches. So what's going
Speaker 1: to happen, Well, some of the photons that leave the
Speaker 1: laser are going to go right through that gap and
Speaker 1: hit the screen, just like before. Some of them are
Speaker 1: gonna hit the walls that block a fraction of the
Speaker 1: laser and so on the screen behind. What you'll get
Speaker 1: is sort of a geometric shadow places where the photon
Speaker 1: made it through the gap. You're gonna hit the screen
Speaker 1: at the back, places where they hit the walls that
Speaker 1: we introduced to make this a little gap. They're going
Speaker 1: to hit those walls and they're not going to make
Speaker 1: it to the screen. All right, So instead of having
Speaker 1: just the laser splash on the back screen, now we
Speaker 1: have something called a geometric shadow. Geometric because it has
Speaker 1: crisp edges and it follows the shape of the of
Speaker 1: the thing that the laser went through. No big deal. Now,
Speaker 1: let's narrow the gap. Let's squeeze those two walls that
Speaker 1: can find the laser. Two very very small, something like
Speaker 1: hundreds of nanometers approximately the wavelength of light that's going
Speaker 1: through it. Now what we see on the back wall changes.
Speaker 1: Instead of seeing a geometric shadow, what we see is
Speaker 1: sort of a spray of light. The light is spreading
Speaker 1: out a little bit. It doesn't just become as narrow
Speaker 1: as the gap it's shining through. It spreads out a
Speaker 1: little bit. This is a wave effect. It happens anytime
Speaker 1: a wave passes through a slit that's narrow compared to
Speaker 1: its wavelength. We could do a deep dive under fraction,
Speaker 1: and maybe we will in a future podcast, but it's
Speaker 1: not critical to understand here. The only thing you need
Speaker 1: to understand is at anytime a wave meaning sound or
Speaker 1: light or bathtub splashes or wave functions pass through a
Speaker 1: narrow slit, they spread out a little bit. You can
Speaker 1: do the same kind of experiment in a bathtub. If
Speaker 1: you send water waves through a very narrow gap, you
Speaker 1: notice that when they come out of that gap they
Speaker 1: spread out. Then we'll just shoot out in a narrow column,
Speaker 1: all right. So now we have a very narrow gap,
Speaker 1: and on the backscreen we have a sort of a
Speaker 1: spray of results. We have a spray, not just a
Speaker 1: very narrow geometric shadow. We have a spray of results
Speaker 1: with a light can land, all right, and now we're
Speaker 1: going to make a second narrow gap. So we have
Speaker 1: sources of light in the back maybe lasers, maybe flashlights,
Speaker 1: it doesn't matter. And they shoot light out, and then
Speaker 1: we have two narrow gaps for the light to fly through.
Speaker 1: And so we have light coming through both of those
Speaker 1: gaps towards the wall. What do you see on the
Speaker 1: back screen? You might think, oh, I have two copies
Speaker 1: of what I saw when I had one narrow gap,
Speaker 1: because now I have two narrow gaps, So how hard
Speaker 1: can this be? That's not quite true. What you see
Speaker 1: is an interference pattern. An interference pattern is what happens
Speaker 1: when two waves collide. Because remember that waves are oscillation.
Speaker 1: For example, if you have waves in your bathtub, those
Speaker 1: waves are just the motion of the water moving up
Speaker 1: and down. They're not their own thing. There a motion
Speaker 1: of the water. Same way sound waves, These sounds that
Speaker 1: I'm making into the microphone that you're hearing in your
Speaker 1: earbuds or wherever you're listening, those are oscillations of the air.
Speaker 1: The air itself is shaking, and because it's the shaking
Speaker 1: of a medium, if something else comes along and shakes
Speaker 1: it in another way, those that shaking can interfere, and
Speaker 1: you can have two kinds of interference. You can have
Speaker 1: constructive interference two things are shaking it the same way,
Speaker 1: so you cant sort of double shaking. Or you can
Speaker 1: have destructive interference where two things are shaking it in
Speaker 1: the opposite way and they cancel each other out. For example,
Speaker 1: this is how noise canceling headphones work. They very rapidly
Speaker 1: hear the noises that are around you and admit exactly
Speaker 1: the right sound to cancel out the sound. You can
Speaker 1: create sound which will negate the other sound by shaking
Speaker 1: it the opposite direction. So when the two shakings add up,
Speaker 1: they add up to zero. So this is an interference
Speaker 1: pattern places where things add up to be stronger in
Speaker 1: place where things can cancel each other out. If you
Speaker 1: look on that screen, what you'll see is a very
Speaker 1: bright band in the middle where the two sources of
Speaker 1: light from the two narrow gaps are adding up on
Speaker 1: top of each other. And to the left of it
Speaker 1: is a dark band, a band where the things are
Speaker 1: canceling each other out. And as you move along the
Speaker 1: screen you get bands of light and bands of dark,
Speaker 1: bands of light and bands of dark, and the exact
Speaker 1: width of those things depends on the wavelength of the
Speaker 1: thing that's interfering with itself. Now you're probably thinking, oh, well,
Speaker 1: the light is a wave, right, and if light is
Speaker 1: a wave, it makes perfect sense for light to interfere
Speaker 1: with itself the same way sound does, in the same
Speaker 1: way waves in your bathtub night. That intuition makes sense,
Speaker 1: But unfortunately that is not what's happening, and the next
Speaker 1: experiment reveals that it's something much weirder, much more fascinating,
Speaker 1: which reveals the true quantum nature of light. So we
Speaker 1: have two narrow gaps and we're shining light through both gaps,
Speaker 1: and we see an interference pattern on the back screen,
Speaker 1: and we think well, that's fascinating. I bet it's because
Speaker 1: light is interfering. That light is coming through both gaps,
Speaker 1: and when it comes out the other side, either it
Speaker 1: adds up constructively we get a bright patch, or it
Speaker 1: can't as itself out and we get a dark patch.
Speaker 1: So now let's do an experiment to discover if that's
Speaker 1: actually true. What we're gonna do is we're gonna dial
Speaker 1: down the source of the light. Maybe we had a laser,
Speaker 1: maybe we had a flash light, doesn't matter too much,
Speaker 1: but let's turn down the source of the light. And
Speaker 1: as we talked about in a podcast very recently, light
Speaker 1: is not continuous. It's not just the smooth oscillations of
Speaker 1: the electromagnetic waves. Light is made of packets, and those
Speaker 1: packets are photons. So you can't actually turn a light
Speaker 1: down to any arbitrary value. You have to turn it
Speaker 1: down for example, one photon a second, or two photons
Speaker 1: a second, or three photons a second. You can't have
Speaker 1: one point five photons a second. But what we can
Speaker 1: do is something really fascinating. We can turn the light
Speaker 1: down so we're shooting out one photon at a time,
Speaker 1: maybe one photon, and then you wait five seconds, you
Speaker 1: shoot out another photon. The idea is that one photon
Speaker 1: has had plenty of time to go through the whole
Speaker 1: experiment before the next photon comes through. Now, if the
Speaker 1: interference comes from light coming through both of those narrow
Speaker 1: gaps and then interfering, then the interference pattern should vanish
Speaker 1: when we slow the experiment down to one photon at
Speaker 1: a time. Because there's only one photon in the experiment
Speaker 1: at a time, there's no other photon to interfere. That's
Speaker 1: what you would expect if the interference pattern came from
Speaker 1: the interference of the light waves. But it doesn't, because
Speaker 1: what happens when we do this experiment is that we
Speaker 1: still get an interference pattern. I remember learning about this
Speaker 1: in college and it blew my mind. You shoot one
Speaker 1: photon at a time, one photon goes here, another one
Speaker 1: goes there. And remember, quantum mechanics is random, So there's
Speaker 1: a wave function for each photon, and that wave function
Speaker 1: says the probability of any given photon going somewhere. But
Speaker 1: two photons shot in exactly the same direction under the
Speaker 1: same conditions don't have to land in the same place.
Speaker 1: The universe throws a new quantum number for every photon,
Speaker 1: So the first one might land here and the next
Speaker 1: one might land there. And what you do as you
Speaker 1: do this experiment, which now takes longer because you're shooting
Speaker 1: a lot of individual photons, what you see is that
Speaker 1: you very gradually build up the interference pattern. Again. You
Speaker 1: get a lot more photons landing where the interference pattern
Speaker 1: was bright, and you get no photons landing where the
Speaker 1: screen was previously dark, and you've got a few landing
Speaker 1: where the where the screen was a little bit bright.
Speaker 1: So the photons follow this interference pattern. It's like for
Speaker 1: each one it says, all right, well we gotta have
Speaker 1: a bunch over here and a few over there and
Speaker 1: none over there, so let's roll a die and see
Speaker 1: where you're gonna go. And eventually you just build up
Speaker 1: exactly the same pattern that you saw before. So how
Speaker 1: is that possible? How is it possible to have an
Speaker 1: interference pattern if you don't have two photons in the
Speaker 1: experiment at once, What is doing the interfering? Well, the
Speaker 1: problem with this thinking is that you're thinking of the
Speaker 1: photons as flying through the experiment. But remember they don't
Speaker 1: have quantum paths. These things ex this when they leave
Speaker 1: the light source, and then they exist when they hit
Speaker 1: the screen. But in between they don't necessarily have a
Speaker 1: location that we can think about sensibly. But what they
Speaker 1: do have is a wave function. So instead of trying
Speaker 1: to follow the photon through the experiment and making sense
Speaker 1: of it, let's follow the wave function. That's the thing
Speaker 1: we're gonna grab onto and try to make sense of
Speaker 1: things that it's something that we can actually understand. So
Speaker 1: let's start from the beginning. The photon is shot out
Speaker 1: from this laser or this flashlight whatever, and has a
Speaker 1: certain way function to be there. Now, the wave function
Speaker 1: flies across the room and it spreads out a little
Speaker 1: bit because that's the source of the light, and then
Speaker 1: it hits the wall, the wall that has two narrow
Speaker 1: gaps in it. Then what happens, Well, nobody has made
Speaker 1: a measurement yet. The photon is interacting with this wall,
Speaker 1: but nobody's looking, nobody's asked where is the photon? So
Speaker 1: it has chance to go through one slit and chance
Speaker 1: to go through the other slit. So what happens when
Speaker 1: the wave function hits the wall? And remember it's hey
Speaker 1: to talk about the wave function moving through space. The
Speaker 1: wave function has a path that we can think about.
Speaker 1: So when the wave function hits the wall, it splits
Speaker 1: into half of it goes through one slit and half
Speaker 1: of it goes through the other slid. And that reflects
Speaker 1: the fact that the photon itself has a fifty percent
Speaker 1: chance of hitting one slit and a fifty percent chance
Speaker 1: of hitting the other slid. And if nobody has asked
Speaker 1: which slip did it go through, it just has fifty
Speaker 1: percent chance of going through one and a fifty pc
Speaker 1: chance of going through the other. Now, let's follow the
Speaker 1: wave function. The wave function comes out the other side
Speaker 1: of these slits, and there's a little bit of source
Speaker 1: of wave function from one slit and source of wave
Speaker 1: function of the other slit. And a wave function, of course,
Speaker 1: is a wave. So what happens when you get two
Speaker 1: sources of a wave coming out from the wall towards
Speaker 1: the screen, of course you get interference and so on
Speaker 1: the screen. What's actually happening is that the wave function
Speaker 1: is interfering with itself, and it gives the photon a
Speaker 1: high probability to be in some locations and a low
Speaker 1: probability to be in other locations, and a zero probability
Speaker 1: to be in some locations. So for the wave function,
Speaker 1: it determines where that photon, that individual photon that we
Speaker 1: put into our experiment is likely to land. And so
Speaker 1: the thing that's doing the interfering is not light because
Speaker 1: you have a single photon in the experiment at once.
Speaker 1: And if you're not convinced that this is the wave
Speaker 1: function interfering, because you think maybe it's really just light interfering,
Speaker 1: wouldn't that be simpler. But remember that we're sending a
Speaker 1: single photon through a time in order to prevent the
Speaker 1: photons from interacting, from interfering with each other. But the
Speaker 1: real killer is that they did this experiment not with photons,
Speaker 1: but later with electrons and they got the same result.
Speaker 1: But what's doing the interfering is the wave function. Another
Speaker 1: way to think about it is that the photon has
Speaker 1: equal probability to go through one slit and the other,
Speaker 1: and that probability is doing the interfering because remember the
Speaker 1: way functions what controls where the partal has a probability
Speaker 1: to be. So because it has a probability to be
Speaker 1: through slit one and through slit two, those probabilities interfere
Speaker 1: with each other, and those probabilities determine where the photon
Speaker 1: can land on the back screen, and that's what we see,
Speaker 1: and that's why it builds up one at a time
Speaker 1: because for photon number one, it follows that distribution, and
Speaker 1: the universe rolls a die only when it gets to
Speaker 1: the back screen. That's when we're measuring it. That's when
Speaker 1: we're interacting with it. That's when we're saying, Okay, a photon,
Speaker 1: where are you? So remember that the quantum wave function
Speaker 1: determines where you are most likely you the photon are
Speaker 1: most likely to be found, and there are parts of
Speaker 1: the screen where the photon has a high chance of
Speaker 1: landing because the wave function interferes constructively and places on
Speaker 1: the screen where has no chance of landing because the
Speaker 1: two halves of its wave function are interfering with each
Speaker 1: other destructively. They're canceling each other out. And you might
Speaker 1: be thinking, well, is the wave function of physical thing?
Speaker 1: Is it just a tool where you using to calculate things,
Speaker 1: or is it something that's real and part of the universe.
Speaker 1: That's a hard question to answer. It's philosophical, but here
Speaker 1: we're seeing real physical effects of the existence of the
Speaker 1: wave function. The wave function really does act like a wave,
Speaker 1: and a wave that you can grasp on the way
Speaker 1: that you can use your intuition to understand. You can
Speaker 1: think of this wave function the same way you think
Speaker 1: of waves in water, flowing through things and diffracting and
Speaker 1: interfering and doing all those wave like things. And this
Speaker 1: is the key to understanding quantum mechanics is grabbing onto
Speaker 1: the wave function because it flows and it moves just
Speaker 1: like a classical wave. It's just that it determines the
Speaker 1: performance and the behavior in the location of a crazy
Speaker 1: quantum object. So while that sinks into your brain, you're
Speaker 1: understanding your classical intuition for the quantum wave function and
Speaker 1: how a single photons wave function can interfere with itself
Speaker 1: to give you this crazy pattern on the screen. While
Speaker 1: that's in your mind, let's take a quick break before
Speaker 1: we think about the last, the craziest, the most amazing
Speaker 1: part of this double slit experiment. Just after this break,
Speaker 1: all right, So we are spending time in the quantum
Speaker 1: realm trying to become familiar, trying to develop an intuition.
Speaker 1: And the key point I'm trying to make today is
Speaker 1: that your intuition cannot be applied to a quantum object
Speaker 1: because it's just different from the kind of things you're
Speaker 1: familiar with. It is not a tiny spinning ball, It
Speaker 1: is not a wave. It is neither. It is both
Speaker 1: into something new and weird that possibly we will never understand.
Speaker 1: But in my view, the best chance to understanding it
Speaker 1: is to spend time with it, to develop an intuition
Speaker 1: by immersion. So that's what we're doing today. We are
Speaker 1: flying our way through experiments conducted by a physicist trying
Speaker 1: to reveal the true nature of the universe. So we
Speaker 1: started out just shooting photons against the screen to get
Speaker 1: familiar with the idea that the photons don't move move
Speaker 1: from one side of the screen to the other. They
Speaker 1: have a probability to be in a certain place where
Speaker 1: they only exist where they are measured and in between.
Speaker 1: They do not necessarily have a path. You measure something,
Speaker 1: you see the photon coming out of the light source
Speaker 1: and later you see it on the screen. Doesn't mean
Speaker 1: you can draw a straight line between those and say
Speaker 1: the light was here. But what you can do is
Speaker 1: talk about its wave function. The wave function leaves the
Speaker 1: light source and later hits the screen, where eventually the
Speaker 1: universe demands a measurement, and so the universe has to
Speaker 1: decide based on where the wave function is, where to
Speaker 1: actually put the photon. But that wave function is something
Speaker 1: you can grasp, something you can follow through space in
Speaker 1: a way that your intuition will be satisfied with. So
Speaker 1: then we added these barriers. So instead of just splashing
Speaker 1: light on the back screen, we saw a geometric shadow.
Speaker 1: Then we narrowed the barriers until we saw the effects
Speaker 1: of waves. We saw that the fact that waves coming
Speaker 1: through a very narrow gap will spread out a little bit,
Speaker 1: and then we added a second narrow gap, so we
Speaker 1: had to sources of waves. And those waves apparently were interfering,
Speaker 1: and your intuition was suggested that maybe it was light
Speaker 1: doing the interfering, because we like to think of light
Speaker 1: as a wave. But then we played a trick. We said,
Speaker 1: let's slow down the experiment, only shoot one photon at
Speaker 1: a time. And this, in theory, if light was a
Speaker 1: wave and it was waves doing the interacting, this should
Speaker 1: destroy the interference pattern because only one photon was going
Speaker 1: through the experiment at a time. But it didn't. It
Speaker 1: slowed down the interference pattern and it showed us that
Speaker 1: there was still something they're doing the interfering, and that's
Speaker 1: the key is letting go of this idea that the
Speaker 1: light is flowing through the experiment, because quantum objects don't flow,
Speaker 1: they don't go, and they don't have they don't have paths. Instead,
Speaker 1: what's flowing through the experiment is the quantum wave of
Speaker 1: the photon, and the quantum wave of the photon can
Speaker 1: go through either slit as a fifty percent chance to
Speaker 1: go through one and a fifty percent chance to go
Speaker 1: through the other, and then it interferes with itself. It
Speaker 1: gives us this interference pattern, and it's hard to get
Speaker 1: your mind around what it means for the photon to
Speaker 1: have a chance to go through both slits at once.
Speaker 1: Most likely you think of it like this. You think, well,
Speaker 1: the photon either went through one slit or the other.
Speaker 1: We just don't know. And it's true that often in
Speaker 1: science and in physics, we use probability to describe our
Speaker 1: lack of knowledge. We say the universe is thirteen point
Speaker 1: eight billion years old plus or minus a hundred million,
Speaker 1: and it reflects not the fact that the universe doesn't
Speaker 1: have a specific age, but just the fact that we
Speaker 1: don't know it well enough that we haven't been able
Speaker 1: to measure it. But this is different. It's not true
Speaker 1: that the photon went through one slit or went through
Speaker 1: the other and we just don't know. The truth is
Speaker 1: that its wave went through both. It needed to go
Speaker 1: through both in order to give us the interference pattern. Remember,
Speaker 1: we didn't collapse the wave, We didn't interfere, We didn't
Speaker 1: interact with the photon um when it's going through the slit.
Speaker 1: We just let it fly through one slit or the other.
Speaker 1: We don't pay attention. We only are looking at the
Speaker 1: back screen. So le stick into that. Let's try to
Speaker 1: probe that. What if we try to figure out which
Speaker 1: slit the photon actually went through, because we are a
Speaker 1: hardcore classicist and we want to know did it go
Speaker 1: through one or the other. So we build a little
Speaker 1: detector detector that doesn't change the direction of the photon
Speaker 1: in a measurable way, but just tells us whether a
Speaker 1: photon went through a slit, and we attach it to
Speaker 1: one of the slits, and we do our experiment again.
Speaker 1: And the idea here is just to confirm our intuition,
Speaker 1: our classical intuition that the photon went through one slit
Speaker 1: or the other. We turn on the experiment again. We
Speaker 1: shoot one single photon at a time, and for every photon,
Speaker 1: our detector tells us whether it went through slit one
Speaker 1: because it beeps, or whether it went through slit two
Speaker 1: because it doesn't beep. Do we get the same result
Speaker 1: on the back screen. The answer is we do not.
Speaker 1: We do not get the interference pattern on the back screen. Instead,
Speaker 1: what we get our two geometrical shadows. And at first
Speaker 1: this might be nonsensical. You might think, well, but the
Speaker 1: detector is just telling you whether the photon went through
Speaker 1: one or the other. It's not changing the photon in
Speaker 1: any measurable way. What's the issue? How could it possibly
Speaker 1: change what we're seeing in the back screen. But that's
Speaker 1: because you're thinking about the photon is having a path,
Speaker 1: is flying through When you think, well, it either went
Speaker 1: through one slit or the other, it doesn't matter if
Speaker 1: I know that, don't shouldn't change what happens. It's like
Speaker 1: if you're watching a horse race, just watching what happens
Speaker 1: around the first bend shouldn't change who wins the race? Right, Well,
Speaker 1: that's not the case, because remember what's flying through your
Speaker 1: experiment is not a photon. Photons don't fly through things.
Speaker 1: They're not classical objects with paths. What's flying through the
Speaker 1: experiment is a quantum wave, and the quantum wave is
Speaker 1: sensitive to being watched. When you watch a quantum wave,
Speaker 1: when you interact with it, when you say, okay, quantum
Speaker 1: wave of this photon, where's the photon now? Then it
Speaker 1: changes it it collapses and it says, okay, the photon
Speaker 1: is here, and then the quantum wave can continue. But
Speaker 1: you've narrowed it down. You pinned it down and said, okay,
Speaker 1: it's right here, and then the quantum wave continues from
Speaker 1: that location. So in the first scenario, when we didn't
Speaker 1: have the detector, the quantum wave flies, it hits the
Speaker 1: two slits, and it splits in half, and some of
Speaker 1: the quantum wave goes through both slits. Each slit emits
Speaker 1: some portion of the quantum wave, and those two halves
Speaker 1: interfere with each other. In the version where you have
Speaker 1: the detector on, then you're asking the universe to decide
Speaker 1: which slit the photon went through, not just to reveal,
Speaker 1: but to decide which slit the photon went through. So
Speaker 1: the quantum wave can only go through one of the
Speaker 1: slits and not the other one. Because the detector tells
Speaker 1: you which slit it went through, it has no probability
Speaker 1: to go through the other slit, So all it can
Speaker 1: do is then emits some quantum wave from one of
Speaker 1: the slits. If you hadn't looked, then it's free to
Speaker 1: emit quantum wave from both slits, which can then interfere.
Speaker 1: But if you look, if you demand an answer, if
Speaker 1: you want to know which slit it went through, then
Speaker 1: the quantum wave can only emit from the other slit
Speaker 1: and then there's no interference pattern. There's just it's just
Speaker 1: like as if you had one slit. And this is
Speaker 1: the thing that blows most people's minds that asking questions
Speaker 1: of the universe changes the answer. And it's true because
Speaker 1: quantum waves respond to measurement. They like to be uncertain.
Speaker 1: They're happy to fly through the universe keeping their uncertainty
Speaker 1: their probability distribution until they are asked, and that moment
Speaker 1: that you ask it, then it collapses and it says
Speaker 1: the photon is here, the photon is there. So this
Speaker 1: is an object we can grasp onto because it helps
Speaker 1: us understand how things move through the universe, but also
Speaker 1: is something new and something weird and something we have
Speaker 1: to get familiar with. But the only way to do that,
Speaker 1: of course, is to spend some time with it. So
Speaker 1: I hope that's helped you understand a little bit about
Speaker 1: quantum mechanics. The first step in becoming familiar with the
Speaker 1: quantum realm is to abandon your idea of a quantum path,
Speaker 1: that things have to move through the universe in a
Speaker 1: continuous manner, that if you're over here and later you're
Speaker 1: over there, you have to have somehow moved from one
Speaker 1: to the other. But instead, instead of grabbing onto this
Speaker 1: quantum path, there is something else you can grab onto,
Speaker 1: this quantum wave function, which behaves in very understandable ways,
Speaker 1: and we have equations that govern exactly how it moves.
Speaker 1: And those equations, like the Shortinger wave equation, treat these
Speaker 1: things like waves. Waves that we can understand that we
Speaker 1: can actually apply our wave like intuition too, So if
Speaker 1: you want to develop an intuition for quantum mechanics, get
Speaker 1: cozy with the wave function. Now. The wave function, of
Speaker 1: course has other mysteries, mysteries that continue to confound us
Speaker 1: and the biggest one is this business about measurement. How
Speaker 1: is it the wave function knows to collapse that We've
Speaker 1: asked it a question and it's given us an answer
Speaker 1: that when the wave function hits the screen, it says
Speaker 1: the photon has various probabilities to be in various locations.
Speaker 1: But then it actually makes a decision. When does the
Speaker 1: universe decide to roll this dice and say, all right,
Speaker 1: photon number seventy four, you are over here. Phot to
Speaker 1: number seventy eight, you are over there. That is a
Speaker 1: deep and recurring mystery of quantum mechanics that nobody really knows.
Speaker 1: The answer to this description I've given you today is
Speaker 1: sometimes as the Copenhagen interpretation of quantum mechanics, and it
Speaker 1: has deep flaws in it. Flaws like who's doing the measuring?
Speaker 1: You might ask, for example, if we if the photons
Speaker 1: hit the screen but nobody looks, if there are no
Speaker 1: humans in that universe, no scientists to do the observing,
Speaker 1: then does the universe collapse the way of function or
Speaker 1: does it keep it vague. We don't know the answer
Speaker 1: to that question because we can't do the experiment in
Speaker 1: which nobody looks and also know the answer, So that's frustrating,
Speaker 1: and that's the source of a lot of discussion about
Speaker 1: quantum mechanics. And there are other interpretations out there, like
Speaker 1: the many Worlds interpretations that clever people like Sean Carroll
Speaker 1: find much more natural, but they require you to accept
Speaker 1: the existence of a huge number of other alternative universes.
Speaker 1: So every interpretation of quantum mechanics comes with some sort
Speaker 1: of cognitive load. And today we're not going to understand
Speaker 1: all of the nuances and open questions of quantum mechanics.
Speaker 1: We just wanted to spend some time becoming familiar with
Speaker 1: the quantum realm. So the next time you read about
Speaker 1: quantum mechanics, or think about quantum computers, or even just
Speaker 1: are on a trip somewhere weird and unusual, remember you
Speaker 1: can become familiar with something strange, something out of your
Speaker 1: experience as long as you spent enough time immersing it,
Speaker 1: marinating in the mathematics and the logic of it. Eventually
Speaker 1: it would become part of who you are. Thanks for
Speaker 1: listening to this explanation of our amazing and crazy and
Speaker 1: totally bonkers universe. And if you have things that you'd
Speaker 1: like us to discuss and break down in an accessible way.
Speaker 1: Please send them to us at feedback at Daniel and
Speaker 1: Joge dot com. Thanks for tuning in. If you still
Speaker 1: have a question after listening to all these explanations, please
Speaker 1: drop us the line. We'd love to hear from you.
Speaker 1: You can find us at Facebook, Twitter, and Instagram at
Speaker 1: Daniel and Jorge That's one word, or email us at
Speaker 1: feedback at Daniel and Jorge dot com. Thanks for listening,
Speaker 1: and remember that Daniel and Jorge Explaining the Universe is
Speaker 1: a production of High Heart Reading More podcast for my
Speaker 1: Heart Radio, visit the I Heart Radio, Apple podcasts, or
Speaker 1: wherever you listen to your favorite shows. Yeah