October 24, 2019 • 41 mins
Learn about the concept of quantum mechanics and what our understanding of it is.
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Episode Transcript
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Speaker 1 (00:08):
If you've done some traveling, then you know the experience
of feeling out of place. There are still some places
on Earth where the culture is sufficiently different from ours
or from yours, then you can no longer sort of
trust your instincts. You don't intuitively know how to behave
and how to get around. Maybe you don't know what
people are saying, and you can't read the street signs,
(00:29):
and things just seem to work differently. The street food
has weird eyeballs in it where looks like it's fried scorpions. Hey,
maybe it is. Well, that's the experience physicists have when
we discover that the rules of the universe are totally
different from what we are familiar with. That's exactly the
feeling we're going for, and that's the feeling we get
very often when we travel to the quantum realm. Yeah. Hi,
(01:08):
I'm Daniel. I'm a particle physicist and I'm one half
of the dynamic duo known as Daniel and Jorge, hosts
of this podcast, Daniel and Jorge Explain the Universe, brought
to you by I Heart Radio. My co host today,
Jorge Champ can't be with us, so I'll be talking
to you myself about the amazing secrets of the universe.
(01:30):
This podcast is dedicated to revealing truths about the universe,
to taking things that seem amazing and mystifying that maybe
you've heard about or people talk about when they want
to sound smart, but you never really understood. Well, we
are here to break it down and make sure you
can walk away actually understanding it. You can talk about
it like an intelligent person. So if you have questions
(01:51):
about what you heard today, please send them to us
two questions at Daniel and Jorge dot com. It is
our goal that you actually understand everything we are talking
about and hopefully maybe even get a chuckle along the way.
Since it's just me today, we'll probably have fewer jokes
than usual. It's often Hooree that injects the humor into
these conversations. So we're little humans. We're here on Earth,
(02:14):
and we're used to a certain kind of experience. We
used to things a certain size and moving on a
certain speed. But the universe is a big place and
there are lots of things out there that are not
like our experience, that are really big or moving really fast.
And what we've learned as humans is that most of
the universe is different from what we expected, and that
(02:34):
it follows rules that are different. And when we try
to understand the rest of the universe, the things that
are not balls rolling down planes or how water flows
down a hill, that we need to do something mentally difficult.
We need to do some sort of extrapolation. And that's
the job of physics is to take us from the
known into the unknown, to say, well, we understand how
these things happen here on Earth. Can we also understand
(02:57):
how the Earth moves around the sun. Can we understand
the origins of the universe. Can we peel back layers
of reality and see how things are built underneath? And
in order to do that, we have to describe things
we haven't seen in terms of things that we have seen.
We talk about particles, and we like to describe them
as kind of like waves and kind of like little
spinning balls, because those are familiar mental constructs. Those are
(03:20):
things we understand, so we can talk to each other
about them. It's like if you drink a new wine
and you try to describe it to your friends and
you say, oh, it has flavors of oak and maybe BlackBerry.
It's a way to describe things that you don't know
in terms of things that you know. It's a basic
mental strategy for understanding things. But what happens when you
run into something fundamentally different, something unlike anything you've seen before,
(03:44):
something where all of your mental constructs fail or are limited,
when our experience has just not prepared us for something.
That's the topic of today's podcast, Can we ever understand
(04:04):
quantum mechanics? And before we dive in, I want to
give a music shout out to Casey Hagman who sent
in that alternative quest in music. Thank you very much.
We loved it. And quantum mechanics is one of the
most difficult things for people to grasp, one of the
most intimidating topics, because people feel like it just can't
make sense to them, and they hear eminent scientists, researchers, philosophers,
(04:26):
physicists talking about quantum mechanics as if they don't understand
it either. And it's true, there's a lot left to
be understood about quantum mechanics. Folks like Sean Carroll talk
about it very intelligently, and there's lots of lively discussion
about what quantum mechanics really means, But I want to
talk about something simpler. Just what does quantum mechanics say?
What have we learned about the universe via quantum mechanics?
(04:47):
Is it possible for us to develop an intuition to
understand quantum mechanics? Is? Will it always forever be foreign
to us? Or can we become comfortable with it? Will
we by spending enough time? And it by thinking? Is
it possible that by spending enough time marinating in these
concepts and thinking about them in the right way, that
(05:08):
we could eventually become familiar with them, sort of the
way Chinese might seem impenetrable to a Western person at first,
but you spend enough time there and it becomes part
of your brain. Your brain develops sort of new ways
of thinking, new patterns, new ideas flow through it, so
that the new language and the topics and the strategies
and the tones of that language eventually become familiar. They
(05:30):
become how you think. So the strategy there, of course,
is immerging. You don't learn a new language effectively by
learning vocabulary and studying in the classroom. The best way
I've always found really learned to speak a new language,
to think in a new language. Is to spend time
doing it, is to go to that country and be
part of those people and be there and use that
(05:52):
as part of your life. And that's how your brain works.
So our strategy today for helping you understand quantum mechanics
is not to give you the math medical basics, but
to spend some time in the quantum realm until we
develop an intuition for how it works. But before we
go there, I wanted to know what people understood about
quantum mechanics and also whether they thought they understood quantum mechanics.
(06:14):
Is quantum mechanics something that everybody out there feels like
is impossible to understand or are most people under the
impression that they have it figured out. I walked around
campus that you see Irvine, and I asked people if
they thought they could understand quantum mechanics. Before you hear
these answers, think to yourself, how good is your grasp
on the basic tenets of quantum mechanics. Do you feel fluent?
(06:36):
Can you explain this stuff? Do you feel like you
can navigate that strange quantum realm? Here's what people on
the street that you see Irvine had to say. Do
you feel like you understand quantum mechanics? Not anyone who
answers yes to that doesn't really have an understanding of quantum.
So no, very basically, I have working knowledge of quantum mechanics,
(06:57):
but I would not say that I understand it at
the a PhD physicist, I say I understand it decently. Well.
Are you familiar with the double slit experiment? Yes, um
show that particles. I think it was electrons that they use,
and they move in waves, So if you're to fire
the electrons and observe them, they'd only go on like
(07:20):
a line. But then if you cover it you can
see a wave like pattern in the back or something
like that. I honestly don't even know about quantum mechanics.
I'm just the goal nature of light. It's both a
wave and a particle. So, as usual, we've got a
very nice breath of answers from people who felt like
they hardly understood anything about quantum mechanics the people who
clearly had some working knowledge. So that's great, and we're
(07:42):
hoping to bring everyone who listens to this podcast up
to at least the very basic level of being able
to intelligently think about and understand quantum topics, and I
think the best way to start off is to hear
a listener question. Here's a question from a young listener. Hi,
Daniel Lynn Whole. Hey, my name is Robin, and I
would love to know what the key differences between quantum
and classical mechanics are and to what extent they agree
(08:05):
with each other. This is a great way to start
because classical physics is what we're familiar with. Classical physics
is what describes how basketball moves, or how rocks roll
down hills, or how things move in the atmosphere, things
that we are familiar with that we have spent hundreds
or thousands of years developing an intuition for things that
probably our brains have evolved to be good at understanding,
(08:26):
so that in some way they are natural objects to
our mental worlds. Right, So the question really is what's
the biggest difference between the classical world and the quantum world.
What is it when you go to the quantum world
that you can no longer assume that is obviously true
in the classical world. The basic difference between classical objects
(08:46):
and quantum mechanical objects is that quantum mechanical objects do
not have a path. They don't have a trajectory through space,
a well defined stead of where they were at any
given time. That's the key, l Us and I want
you to come away from today's podcast with. So let's
talk about what that means. If you have a baseball
and it's at one location now and ten seconds later
(09:09):
it's across the baseball field, you imagine that it went
from where it was to where it is now. You say, well,
if it was over there before and it's over here
and now, how did it get there. It must have
gone between those two locations. You have two pieces of data,
and you naturally interpolate because you make this assumption that
(09:30):
a classical object has a location at every time, and
that you can stitch those together into a path that
traverses space time in some continuous way. And you might
be thinking, well, of course everything does. Everything has a
location at a given time, and only one location. But
that's an intuition you've developed from experiencing the natural world,
(09:52):
from playing baseball, or from being chased by people throwing
rocks at you. You're used to things moving through the
world in a responsible and understandable way. So you've made
this assumption. Quantum objects do not have paths like that.
They don't have locations that translate through time. For a
quantum object like an electron, you can measure it at
(10:13):
one location A and then later measure it at another
location B, but it doesn't mean that it's flown through
the universe from A to B in that intervening time.
It can be at A later be at B, but
it's not true that it's moved from A to B,
meaning that if you've made a measurement halfway, you would
(10:34):
have found it going from A to B. Now that's
very confusing, that's very hard to understand. How can an
object be here and later be there and never be
in between? And the reason is that quantum objects are
just different from the classical objects. They are not the
same kind of thing. And this assumption that you have
made about the universe comes from only experiencing classical objects
(10:56):
things on our scale, but the smaller level the universe
just does not work that way, and that's pretty hard
to digest. But don't worry. There are things at the
quantum realm which do follow your intuition in which you
can use to help understand and develop an intuitive sense
of the of the universe, and that's the quantum wave function.
(11:16):
So how do the quantum object work. Well, a quantum
object is here and later it's there. What determines that.
It's not like quantum objects don't follow the laws of physics.
And you probably know the quantum objects could also be random,
that you can do the same thing to a quantum
object twice and get different outcomes. And that's true. All
those things are true. But the quantum object is ruled
(11:37):
by something, and that's this quantum wave function. You don't
have to know a lot of complicated math. All you
need to know is that the wave function tells you
where a quantum object is most likely to be. You
probably have heard that electrons don't have a specific location
to have a probability cloud around an atom. That's an
extrapolation of the wave function. The wave function tells you
(11:58):
where the electron is most like it to be. If
it's large over here, has it's more likely to be there.
If it's small somewhere else, it's less likely to be there.
If it's zero somewhere, it means the particle cannot be there.
Where is the particle? Actually, we don't know, and there's
a lot of discussion about whether the wave function means
the particle isn't actually anywhere or just we don't know
(12:20):
where it is, but we do know that quantum mechanics
says that that information does not need to exist. It's
not necessarily the case that there's hidden knowledge that the
electron is actually somewhere and we don't know. So the
best way to think about the wave function is that
it's physics saying what's allowed and what's more likely, and
this wave function is what we can grab onto, what
(12:42):
we can understand because the wave function does follow rules,
and the wave function has a path that makes sense.
It flows from one spot to the other. It doesn't
just disappear and appear somewhere else. So you have to
let go of your intuitive desire to understand the path
of of an electron the path of a quantum object,
(13:02):
because those things don't exist. But instead, when you let
that go, you can grasp onto the next layer. The
next layer is this wave function, the thing that determines
where the electron is, the thing that determines where the
electron is likely to be somewhere else. And that's the
key thing, is that the wave function tells you where
the electron is, and it responds to stimulation, it responds
(13:23):
to the world, and it flows just like a wave,
and you can use that you can understand how that
wave flows through time to understand where the electron is
likely to be in the future. Even if the electron
itself doesn't have a path from A to B, it's
wave function does. So the key to being comfortable with
quantum mechanics is to get comfortable with the objects wave
(13:44):
function rather than the object. All right, So the best
way to learn a new language is to get practice,
is to immerse yourself in it. So the next thing
we're gonna do is a bunch of exercises of diving
into the quantum realm and being comfortable with the wave
function understanding how it flows. And to do that, we're
going to explore in some detail the famous double slit
experiment that shows us the crazy behavior of quantum mechanics.
(14:07):
And at the end of it, I hope it all
makes sense and the weird results of the double slit
experiment feel totally natural. But first, let's take a quick break.
(14:27):
All right, So we are talking about quantum mechanics and
we're trying to get an intuitive grasp of quantum mechanics.
We want not just that you say things about quantum
mechanics that sound intelligent. We want to actually get an
understanding how do things work at the lowest level. What
intuition do we have that we can apply? And the
key thing, again, remember, is the wave function. You have
quantum mechanical objects like electrons and protons and photons. They
(14:50):
are difficult to describe using our classical ideas of balls
and objects that move through space because quantum objects don't
have paths. And said, we're gonna focus on their wave function,
the thing that says where they are likely to be,
and we're gonna understand how that wave function changes through time.
And we're gonna grab onto that instead of having a
(15:10):
classical path with the object, we're gonna understand how the
wave function changes through time, because that's the thing that's
most like a classical path. All right, So we're gonna
start very simple, and we're gonna work up to more
complicated situations. Imagine that you are a photon. You are
a tiny little quantum particle, and you're shot out of
a laser with a bunch of your friends towards the screen.
(15:32):
What happens while you leave the laser right later you
hit the screen. Does that mean that you flew through
the room from the laser to the screen. Not necessarily, right,
You do not have a classical path. Just because you
left the laser and later hit the screen doesn't mean
(15:52):
you have a trajectory. Doesn't mean that you necessarily can
be found in between things in the quantum realm. Don't go, however,
your wave function does. When you are created inside the laser,
just at the aperture of the laser, your wave function
says where you're likely to be, And then your wave
function flows through the room. If it isn't interacting with everything,
(16:15):
it just flows nicely through the room, showing the most
likely place to find your location. This is not the
same as having a classical path. This is sort of
the path of your wave function. Then it hits the screen,
and the screen essentially measures it. It says, okay, wave function,
where is this photon? And that's the moment when the
universe has to decide. Instead of having a probability distribution
(16:36):
about where you are, it says where is this actual photon?
So you have two measurements. When you leave the laser
and when you hit the screen. What happens in between
we don't know, but the wave function tells us, given
that you've left the laser, what's the likely place to
land on the screen. And then when you land on
the screen, the universe rolls a die and says where
(16:56):
you actually are. Okay, that's easy. Now let's do the
experiment again. A bunch of photons flying out from a
laser towards the wall. Let's bring in some black walls
so that instead of having a totally clear path of
the screen, you have sort of a narrow gap, not
too small, you know, a few inches. So what's going
to happen, Well, some of the photons that leave the
(17:17):
laser are going to go right through that gap and
hit the screen, just like before. Some of them are
gonna hit the walls that block a fraction of the
laser and so on the screen behind. What you'll get
is sort of a geometric shadow places where the photon
made it through the gap. You're gonna hit the screen
at the back, places where they hit the walls that
we introduced to make this a little gap. They're going
(17:37):
to hit those walls and they're not going to make
it to the screen. All right, So instead of having
just the laser splash on the back screen, now we
have something called a geometric shadow. Geometric because it has
crisp edges and it follows the shape of the of
the thing that the laser went through. No big deal. Now,
let's narrow the gap. Let's squeeze those two walls that
(18:00):
can find the laser. Two very very small, something like
hundreds of nanometers approximately the wavelength of light that's going
through it. Now what we see on the back wall changes.
Instead of seeing a geometric shadow, what we see is
sort of a spray of light. The light is spreading
out a little bit. It doesn't just become as narrow
as the gap it's shining through. It spreads out a
(18:20):
little bit. This is a wave effect. It happens anytime
a wave passes through a slit that's narrow compared to
its wavelength. We could do a deep dive under fraction,
and maybe we will in a future podcast, but it's
not critical to understand here. The only thing you need
to understand is at anytime a wave meaning sound or
light or bathtub splashes or wave functions pass through a
(18:42):
narrow slit, they spread out a little bit. You can
do the same kind of experiment in a bathtub. If
you send water waves through a very narrow gap, you
notice that when they come out of that gap they
spread out. Then we'll just shoot out in a narrow column,
all right. So now we have a very narrow gap,
and on the backscreen we have a sort of a
spray of results. We have a spray, not just a
(19:02):
very narrow geometric shadow. We have a spray of results
with a light can land, all right, and now we're
going to make a second narrow gap. So we have
sources of light in the back maybe lasers, maybe flashlights,
it doesn't matter. And they shoot light out, and then
we have two narrow gaps for the light to fly through.
And so we have light coming through both of those
gaps towards the wall. What do you see on the
(19:25):
back screen? You might think, oh, I have two copies
of what I saw when I had one narrow gap,
because now I have two narrow gaps, So how hard
can this be? That's not quite true. What you see
is an interference pattern. An interference pattern is what happens
when two waves collide. Because remember that waves are oscillation.
For example, if you have waves in your bathtub, those
(19:47):
waves are just the motion of the water moving up
and down. They're not their own thing. There a motion
of the water. Same way sound waves, These sounds that
I'm making into the microphone that you're hearing in your
earbuds or wherever you're listening, those are oscillations of the air.
The air itself is shaking, and because it's the shaking
of a medium, if something else comes along and shakes
it in another way, those that shaking can interfere, and
(20:10):
you can have two kinds of interference. You can have
constructive interference two things are shaking it the same way,
so you cant sort of double shaking. Or you can
have destructive interference where two things are shaking it in
the opposite way and they cancel each other out. For example,
this is how noise canceling headphones work. They very rapidly
hear the noises that are around you and admit exactly
(20:32):
the right sound to cancel out the sound. You can
create sound which will negate the other sound by shaking
it the opposite direction. So when the two shakings add up,
they add up to zero. So this is an interference
pattern places where things add up to be stronger in
place where things can cancel each other out. If you
look on that screen, what you'll see is a very
(20:53):
bright band in the middle where the two sources of
light from the two narrow gaps are adding up on
top of each other. And to the left of it
is a dark band, a band where the things are
canceling each other out. And as you move along the
screen you get bands of light and bands of dark,
bands of light and bands of dark, and the exact
width of those things depends on the wavelength of the
(21:13):
thing that's interfering with itself. Now you're probably thinking, oh, well,
the light is a wave, right, and if light is
a wave, it makes perfect sense for light to interfere
with itself the same way sound does, in the same
way waves in your bathtub night. That intuition makes sense,
But unfortunately that is not what's happening, and the next
experiment reveals that it's something much weirder, much more fascinating,
(21:37):
which reveals the true quantum nature of light. So we
have two narrow gaps and we're shining light through both gaps,
and we see an interference pattern on the back screen,
and we think well, that's fascinating. I bet it's because
light is interfering. That light is coming through both gaps,
and when it comes out the other side, either it
adds up constructively we get a bright patch, or it
(21:59):
can't as itself out and we get a dark patch.
So now let's do an experiment to discover if that's
actually true. What we're gonna do is we're gonna dial
down the source of the light. Maybe we had a laser,
maybe we had a flash light, doesn't matter too much,
but let's turn down the source of the light. And
as we talked about in a podcast very recently, light
is not continuous. It's not just the smooth oscillations of
(22:22):
the electromagnetic waves. Light is made of packets, and those
packets are photons. So you can't actually turn a light
down to any arbitrary value. You have to turn it
down for example, one photon a second, or two photons
a second, or three photons a second. You can't have
one point five photons a second. But what we can
do is something really fascinating. We can turn the light
(22:43):
down so we're shooting out one photon at a time,
maybe one photon, and then you wait five seconds, you
shoot out another photon. The idea is that one photon
has had plenty of time to go through the whole
experiment before the next photon comes through. Now, if the
interference comes from light coming through both of those narrow
gaps and then interfering, then the interference pattern should vanish
(23:06):
when we slow the experiment down to one photon at
a time. Because there's only one photon in the experiment
at a time, there's no other photon to interfere. That's
what you would expect if the interference pattern came from
the interference of the light waves. But it doesn't, because
what happens when we do this experiment is that we
still get an interference pattern. I remember learning about this
(23:29):
in college and it blew my mind. You shoot one
photon at a time, one photon goes here, another one
goes there. And remember, quantum mechanics is random, So there's
a wave function for each photon, and that wave function
says the probability of any given photon going somewhere. But
two photons shot in exactly the same direction under the
(23:49):
same conditions don't have to land in the same place.
The universe throws a new quantum number for every photon,
So the first one might land here and the next
one might land there. And what you do as you
do this experiment, which now takes longer because you're shooting
a lot of individual photons, what you see is that
you very gradually build up the interference pattern. Again. You
(24:11):
get a lot more photons landing where the interference pattern
was bright, and you get no photons landing where the
screen was previously dark, and you've got a few landing
where the where the screen was a little bit bright.
So the photons follow this interference pattern. It's like for
each one it says, all right, well we gotta have
a bunch over here and a few over there and
(24:32):
none over there, so let's roll a die and see
where you're gonna go. And eventually you just build up
exactly the same pattern that you saw before. So how
is that possible? How is it possible to have an
interference pattern if you don't have two photons in the
experiment at once, What is doing the interfering? Well, the
problem with this thinking is that you're thinking of the
(24:53):
photons as flying through the experiment. But remember they don't
have quantum paths. These things ex this when they leave
the light source, and then they exist when they hit
the screen. But in between they don't necessarily have a
location that we can think about sensibly. But what they
do have is a wave function. So instead of trying
to follow the photon through the experiment and making sense
(25:15):
of it, let's follow the wave function. That's the thing
we're gonna grab onto and try to make sense of
things that it's something that we can actually understand. So
let's start from the beginning. The photon is shot out
from this laser or this flashlight whatever, and has a
certain way function to be there. Now, the wave function
flies across the room and it spreads out a little
bit because that's the source of the light, and then
(25:36):
it hits the wall, the wall that has two narrow
gaps in it. Then what happens, Well, nobody has made
a measurement yet. The photon is interacting with this wall,
but nobody's looking, nobody's asked where is the photon? So
it has chance to go through one slit and chance
to go through the other slit. So what happens when
the wave function hits the wall? And remember it's hey
(26:00):
to talk about the wave function moving through space. The
wave function has a path that we can think about.
So when the wave function hits the wall, it splits
into half of it goes through one slit and half
of it goes through the other slid. And that reflects
the fact that the photon itself has a fifty percent
chance of hitting one slit and a fifty percent chance
of hitting the other slid. And if nobody has asked
(26:22):
which slip did it go through, it just has fifty
percent chance of going through one and a fifty pc
chance of going through the other. Now, let's follow the
wave function. The wave function comes out the other side
of these slits, and there's a little bit of source
of wave function from one slit and source of wave
function of the other slit. And a wave function, of course,
is a wave. So what happens when you get two
(26:43):
sources of a wave coming out from the wall towards
the screen, of course you get interference and so on
the screen. What's actually happening is that the wave function
is interfering with itself, and it gives the photon a
high probability to be in some locations and a low
probability to be in other locations, and a zero probability
(27:04):
to be in some locations. So for the wave function,
it determines where that photon, that individual photon that we
put into our experiment is likely to land. And so
the thing that's doing the interfering is not light because
you have a single photon in the experiment at once.
And if you're not convinced that this is the wave
function interfering, because you think maybe it's really just light interfering,
(27:26):
wouldn't that be simpler. But remember that we're sending a
single photon through a time in order to prevent the
photons from interacting, from interfering with each other. But the
real killer is that they did this experiment not with photons,
but later with electrons and they got the same result.
But what's doing the interfering is the wave function. Another
(27:48):
way to think about it is that the photon has
equal probability to go through one slit and the other,
and that probability is doing the interfering because remember the
way functions what controls where the partal has a probability
to be. So because it has a probability to be
through slit one and through slit two, those probabilities interfere
(28:09):
with each other, and those probabilities determine where the photon
can land on the back screen, and that's what we see,
and that's why it builds up one at a time
because for photon number one, it follows that distribution, and
the universe rolls a die only when it gets to
the back screen. That's when we're measuring it. That's when
we're interacting with it. That's when we're saying, Okay, a photon,
(28:31):
where are you? So remember that the quantum wave function
determines where you are most likely you the photon are
most likely to be found, and there are parts of
the screen where the photon has a high chance of
landing because the wave function interferes constructively and places on
the screen where has no chance of landing because the
two halves of its wave function are interfering with each
(28:52):
other destructively. They're canceling each other out. And you might
be thinking, well, is the wave function of physical thing?
Is it just a tool where you using to calculate things,
or is it something that's real and part of the universe.
That's a hard question to answer. It's philosophical, but here
we're seeing real physical effects of the existence of the
wave function. The wave function really does act like a wave,
(29:14):
and a wave that you can grasp on the way
that you can use your intuition to understand. You can
think of this wave function the same way you think
of waves in water, flowing through things and diffracting and
interfering and doing all those wave like things. And this
is the key to understanding quantum mechanics is grabbing onto
the wave function because it flows and it moves just
like a classical wave. It's just that it determines the
(29:37):
performance and the behavior in the location of a crazy
quantum object. So while that sinks into your brain, you're
understanding your classical intuition for the quantum wave function and
how a single photons wave function can interfere with itself
to give you this crazy pattern on the screen. While
that's in your mind, let's take a quick break before
we think about the last, the craziest, the most amazing
(30:01):
part of this double slit experiment. Just after this break,
all right, So we are spending time in the quantum
realm trying to become familiar, trying to develop an intuition.
(30:23):
And the key point I'm trying to make today is
that your intuition cannot be applied to a quantum object
because it's just different from the kind of things you're
familiar with. It is not a tiny spinning ball, It
is not a wave. It is neither. It is both
into something new and weird that possibly we will never understand.
But in my view, the best chance to understanding it
is to spend time with it, to develop an intuition
(30:45):
by immersion. So that's what we're doing today. We are
flying our way through experiments conducted by a physicist trying
to reveal the true nature of the universe. So we
started out just shooting photons against the screen to get
familiar with the idea that the photons don't move move
from one side of the screen to the other. They
have a probability to be in a certain place where
they only exist where they are measured and in between.
(31:07):
They do not necessarily have a path. You measure something,
you see the photon coming out of the light source
and later you see it on the screen. Doesn't mean
you can draw a straight line between those and say
the light was here. But what you can do is
talk about its wave function. The wave function leaves the
light source and later hits the screen, where eventually the
universe demands a measurement, and so the universe has to
(31:29):
decide based on where the wave function is, where to
actually put the photon. But that wave function is something
you can grasp, something you can follow through space in
a way that your intuition will be satisfied with. So
then we added these barriers. So instead of just splashing
light on the back screen, we saw a geometric shadow.
Then we narrowed the barriers until we saw the effects
(31:50):
of waves. We saw that the fact that waves coming
through a very narrow gap will spread out a little bit,
and then we added a second narrow gap, so we
had to sources of waves. And those waves apparently were interfering,
and your intuition was suggested that maybe it was light
doing the interfering, because we like to think of light
as a wave. But then we played a trick. We said,
(32:11):
let's slow down the experiment, only shoot one photon at
a time. And this, in theory, if light was a
wave and it was waves doing the interacting, this should
destroy the interference pattern because only one photon was going
through the experiment at a time. But it didn't. It
slowed down the interference pattern and it showed us that
there was still something they're doing the interfering, and that's
(32:34):
the key is letting go of this idea that the
light is flowing through the experiment, because quantum objects don't flow,
they don't go, and they don't have they don't have paths. Instead,
what's flowing through the experiment is the quantum wave of
the photon, and the quantum wave of the photon can
go through either slit as a fifty percent chance to
go through one and a fifty percent chance to go
(32:55):
through the other, and then it interferes with itself. It
gives us this interference pattern, and it's hard to get
your mind around what it means for the photon to
have a chance to go through both slits at once.
Most likely you think of it like this. You think, well,
the photon either went through one slit or the other.
We just don't know. And it's true that often in
science and in physics, we use probability to describe our
(33:18):
lack of knowledge. We say the universe is thirteen point
eight billion years old plus or minus a hundred million,
and it reflects not the fact that the universe doesn't
have a specific age, but just the fact that we
don't know it well enough that we haven't been able
to measure it. But this is different. It's not true
that the photon went through one slit or went through
the other and we just don't know. The truth is
(33:39):
that its wave went through both. It needed to go
through both in order to give us the interference pattern. Remember,
we didn't collapse the wave, We didn't interfere, We didn't
interact with the photon um when it's going through the slit.
We just let it fly through one slit or the other.
We don't pay attention. We only are looking at the
back screen. So le stick into that. Let's try to
(34:01):
probe that. What if we try to figure out which
slit the photon actually went through, because we are a
hardcore classicist and we want to know did it go
through one or the other. So we build a little
detector detector that doesn't change the direction of the photon
in a measurable way, but just tells us whether a
photon went through a slit, and we attach it to
one of the slits, and we do our experiment again.
(34:22):
And the idea here is just to confirm our intuition,
our classical intuition that the photon went through one slit
or the other. We turn on the experiment again. We
shoot one single photon at a time, and for every photon,
our detector tells us whether it went through slit one
because it beeps, or whether it went through slit two
because it doesn't beep. Do we get the same result
(34:42):
on the back screen. The answer is we do not.
We do not get the interference pattern on the back screen. Instead,
what we get our two geometrical shadows. And at first
this might be nonsensical. You might think, well, but the
detector is just telling you whether the photon went through
one or the other. It's not changing the photon in
any measurable way. What's the issue? How could it possibly
(35:04):
change what we're seeing in the back screen. But that's
because you're thinking about the photon is having a path,
is flying through When you think, well, it either went
through one slit or the other, it doesn't matter if
I know that, don't shouldn't change what happens. It's like
if you're watching a horse race, just watching what happens
around the first bend shouldn't change who wins the race? Right, Well,
(35:25):
that's not the case, because remember what's flying through your
experiment is not a photon. Photons don't fly through things.
They're not classical objects with paths. What's flying through the
experiment is a quantum wave, and the quantum wave is
sensitive to being watched. When you watch a quantum wave,
when you interact with it, when you say, okay, quantum
wave of this photon, where's the photon now? Then it
(35:46):
changes it it collapses and it says, okay, the photon
is here, and then the quantum wave can continue. But
you've narrowed it down. You pinned it down and said, okay,
it's right here, and then the quantum wave continues from
that location. So in the first scenario, when we didn't
have the detector, the quantum wave flies, it hits the
two slits, and it splits in half, and some of
(36:07):
the quantum wave goes through both slits. Each slit emits
some portion of the quantum wave, and those two halves
interfere with each other. In the version where you have
the detector on, then you're asking the universe to decide
which slit the photon went through, not just to reveal,
but to decide which slit the photon went through. So
the quantum wave can only go through one of the
(36:29):
slits and not the other one. Because the detector tells
you which slit it went through, it has no probability
to go through the other slit, So all it can
do is then emits some quantum wave from one of
the slits. If you hadn't looked, then it's free to
emit quantum wave from both slits, which can then interfere.
But if you look, if you demand an answer, if
you want to know which slit it went through, then
(36:52):
the quantum wave can only emit from the other slit
and then there's no interference pattern. There's just it's just
like as if you had one slit. And this is
the thing that blows most people's minds that asking questions
of the universe changes the answer. And it's true because
quantum waves respond to measurement. They like to be uncertain.
They're happy to fly through the universe keeping their uncertainty
(37:14):
their probability distribution until they are asked, and that moment
that you ask it, then it collapses and it says
the photon is here, the photon is there. So this
is an object we can grasp onto because it helps
us understand how things move through the universe, but also
is something new and something weird and something we have
to get familiar with. But the only way to do that,
of course, is to spend some time with it. So
(37:36):
I hope that's helped you understand a little bit about
quantum mechanics. The first step in becoming familiar with the
quantum realm is to abandon your idea of a quantum path,
that things have to move through the universe in a
continuous manner, that if you're over here and later you're
over there, you have to have somehow moved from one
to the other. But instead, instead of grabbing onto this
(37:57):
quantum path, there is something else you can grab onto,
this quantum wave function, which behaves in very understandable ways,
and we have equations that govern exactly how it moves.
And those equations, like the Shortinger wave equation, treat these
things like waves. Waves that we can understand that we
can actually apply our wave like intuition too, So if
you want to develop an intuition for quantum mechanics, get
(38:19):
cozy with the wave function. Now. The wave function, of
course has other mysteries, mysteries that continue to confound us
and the biggest one is this business about measurement. How
is it the wave function knows to collapse that We've
asked it a question and it's given us an answer
that when the wave function hits the screen, it says
the photon has various probabilities to be in various locations.
(38:41):
But then it actually makes a decision. When does the
universe decide to roll this dice and say, all right,
photon number seventy four, you are over here. Phot to
number seventy eight, you are over there. That is a
deep and recurring mystery of quantum mechanics that nobody really knows.
The answer to this description I've given you today is
sometimes as the Copenhagen interpretation of quantum mechanics, and it
(39:03):
has deep flaws in it. Flaws like who's doing the measuring?
You might ask, for example, if we if the photons
hit the screen but nobody looks, if there are no
humans in that universe, no scientists to do the observing,
then does the universe collapse the way of function or
does it keep it vague. We don't know the answer
to that question because we can't do the experiment in
(39:24):
which nobody looks and also know the answer, So that's frustrating,
and that's the source of a lot of discussion about
quantum mechanics. And there are other interpretations out there, like
the many Worlds interpretations that clever people like Sean Carroll
find much more natural, but they require you to accept
the existence of a huge number of other alternative universes.
So every interpretation of quantum mechanics comes with some sort
(39:46):
of cognitive load. And today we're not going to understand
all of the nuances and open questions of quantum mechanics.
We just wanted to spend some time becoming familiar with
the quantum realm. So the next time you read about
quantum mechanics, or think about quantum computers, or even just
are on a trip somewhere weird and unusual, remember you
can become familiar with something strange, something out of your
(40:06):
experience as long as you spent enough time immersing it,
marinating in the mathematics and the logic of it. Eventually
it would become part of who you are. Thanks for
listening to this explanation of our amazing and crazy and
totally bonkers universe. And if you have things that you'd
like us to discuss and break down in an accessible way.
Please send them to us at feedback at Daniel and
(40:28):
Joge dot com. Thanks for tuning in. If you still
have a question after listening to all these explanations, please
drop us the line. We'd love to hear from you.
You can find us at Facebook, Twitter, and Instagram at
Daniel and Jorge That's one word, or email us at
(40:51):
feedback at Daniel and Jorge dot com. Thanks for listening,
and remember that Daniel and Jorge Explaining the Universe is
a production of High Heart Reading More podcast for my
Heart Radio, visit the I Heart Radio, Apple podcasts, or
wherever you listen to your favorite shows. Yeah
If you've done some traveling, then you know the experience
of feeling out of place. There are still some places
on Earth where the culture is sufficiently different from ours
or from yours, then you can no longer sort of
trust your instincts. You don't intuitively know how to behave
and how to get around. Maybe you don't know what
people are saying, and you can't read the street signs,
(00:29):
and things just seem to work differently. The street food
has weird eyeballs in it where looks like it's fried scorpions. Hey,
maybe it is. Well, that's the experience physicists have when
we discover that the rules of the universe are totally
different from what we are familiar with. That's exactly the
feeling we're going for, and that's the feeling we get
very often when we travel to the quantum realm. Yeah. Hi,
(01:08):
I'm Daniel. I'm a particle physicist and I'm one half
of the dynamic duo known as Daniel and Jorge, hosts
of this podcast, Daniel and Jorge Explain the Universe, brought
to you by I Heart Radio. My co host today,
Jorge Champ can't be with us, so I'll be talking
to you myself about the amazing secrets of the universe.
(01:30):
This podcast is dedicated to revealing truths about the universe,
to taking things that seem amazing and mystifying that maybe
you've heard about or people talk about when they want
to sound smart, but you never really understood. Well, we
are here to break it down and make sure you
can walk away actually understanding it. You can talk about
it like an intelligent person. So if you have questions
(01:51):
about what you heard today, please send them to us
two questions at Daniel and Jorge dot com. It is
our goal that you actually understand everything we are talking
about and hopefully maybe even get a chuckle along the way.
Since it's just me today, we'll probably have fewer jokes
than usual. It's often Hooree that injects the humor into
these conversations. So we're little humans. We're here on Earth,
(02:14):
and we're used to a certain kind of experience. We
used to things a certain size and moving on a
certain speed. But the universe is a big place and
there are lots of things out there that are not
like our experience, that are really big or moving really fast.
And what we've learned as humans is that most of
the universe is different from what we expected, and that
(02:34):
it follows rules that are different. And when we try
to understand the rest of the universe, the things that
are not balls rolling down planes or how water flows
down a hill, that we need to do something mentally difficult.
We need to do some sort of extrapolation. And that's
the job of physics is to take us from the
known into the unknown, to say, well, we understand how
these things happen here on Earth. Can we also understand
(02:57):
how the Earth moves around the sun. Can we understand
the origins of the universe. Can we peel back layers
of reality and see how things are built underneath? And
in order to do that, we have to describe things
we haven't seen in terms of things that we have seen.
We talk about particles, and we like to describe them
as kind of like waves and kind of like little
spinning balls, because those are familiar mental constructs. Those are
(03:20):
things we understand, so we can talk to each other
about them. It's like if you drink a new wine
and you try to describe it to your friends and
you say, oh, it has flavors of oak and maybe BlackBerry.
It's a way to describe things that you don't know
in terms of things that you know. It's a basic
mental strategy for understanding things. But what happens when you
run into something fundamentally different, something unlike anything you've seen before,
(03:44):
something where all of your mental constructs fail or are limited,
when our experience has just not prepared us for something.
That's the topic of today's podcast, Can we ever understand
(04:04):
quantum mechanics? And before we dive in, I want to
give a music shout out to Casey Hagman who sent
in that alternative quest in music. Thank you very much.
We loved it. And quantum mechanics is one of the
most difficult things for people to grasp, one of the
most intimidating topics, because people feel like it just can't
make sense to them, and they hear eminent scientists, researchers, philosophers,
(04:26):
physicists talking about quantum mechanics as if they don't understand
it either. And it's true, there's a lot left to
be understood about quantum mechanics. Folks like Sean Carroll talk
about it very intelligently, and there's lots of lively discussion
about what quantum mechanics really means, But I want to
talk about something simpler. Just what does quantum mechanics say?
What have we learned about the universe via quantum mechanics?
(04:47):
Is it possible for us to develop an intuition to
understand quantum mechanics? Is? Will it always forever be foreign
to us? Or can we become comfortable with it? Will
we by spending enough time? And it by thinking? Is
it possible that by spending enough time marinating in these
concepts and thinking about them in the right way, that
(05:08):
we could eventually become familiar with them, sort of the
way Chinese might seem impenetrable to a Western person at first,
but you spend enough time there and it becomes part
of your brain. Your brain develops sort of new ways
of thinking, new patterns, new ideas flow through it, so
that the new language and the topics and the strategies
and the tones of that language eventually become familiar. They
(05:30):
become how you think. So the strategy there, of course,
is immerging. You don't learn a new language effectively by
learning vocabulary and studying in the classroom. The best way
I've always found really learned to speak a new language,
to think in a new language. Is to spend time
doing it, is to go to that country and be
part of those people and be there and use that
(05:52):
as part of your life. And that's how your brain works.
So our strategy today for helping you understand quantum mechanics
is not to give you the math medical basics, but
to spend some time in the quantum realm until we
develop an intuition for how it works. But before we
go there, I wanted to know what people understood about
quantum mechanics and also whether they thought they understood quantum mechanics.
(06:14):
Is quantum mechanics something that everybody out there feels like
is impossible to understand or are most people under the
impression that they have it figured out. I walked around
campus that you see Irvine, and I asked people if
they thought they could understand quantum mechanics. Before you hear
these answers, think to yourself, how good is your grasp
on the basic tenets of quantum mechanics. Do you feel fluent?
(06:36):
Can you explain this stuff? Do you feel like you
can navigate that strange quantum realm? Here's what people on
the street that you see Irvine had to say. Do
you feel like you understand quantum mechanics? Not anyone who
answers yes to that doesn't really have an understanding of quantum.
So no, very basically, I have working knowledge of quantum mechanics,
(06:57):
but I would not say that I understand it at
the a PhD physicist, I say I understand it decently. Well.
Are you familiar with the double slit experiment? Yes, um
show that particles. I think it was electrons that they use,
and they move in waves, So if you're to fire
the electrons and observe them, they'd only go on like
(07:20):
a line. But then if you cover it you can
see a wave like pattern in the back or something
like that. I honestly don't even know about quantum mechanics.
I'm just the goal nature of light. It's both a
wave and a particle. So, as usual, we've got a
very nice breath of answers from people who felt like
they hardly understood anything about quantum mechanics the people who
clearly had some working knowledge. So that's great, and we're
(07:42):
hoping to bring everyone who listens to this podcast up
to at least the very basic level of being able
to intelligently think about and understand quantum topics, and I
think the best way to start off is to hear
a listener question. Here's a question from a young listener. Hi,
Daniel Lynn Whole. Hey, my name is Robin, and I
would love to know what the key differences between quantum
and classical mechanics are and to what extent they agree
(08:05):
with each other. This is a great way to start
because classical physics is what we're familiar with. Classical physics
is what describes how basketball moves, or how rocks roll
down hills, or how things move in the atmosphere, things
that we are familiar with that we have spent hundreds
or thousands of years developing an intuition for things that
probably our brains have evolved to be good at understanding,
(08:26):
so that in some way they are natural objects to
our mental worlds. Right, So the question really is what's
the biggest difference between the classical world and the quantum world.
What is it when you go to the quantum world
that you can no longer assume that is obviously true
in the classical world. The basic difference between classical objects
(08:46):
and quantum mechanical objects is that quantum mechanical objects do
not have a path. They don't have a trajectory through space,
a well defined stead of where they were at any
given time. That's the key, l Us and I want
you to come away from today's podcast with. So let's
talk about what that means. If you have a baseball
and it's at one location now and ten seconds later
(09:09):
it's across the baseball field, you imagine that it went
from where it was to where it is now. You say, well,
if it was over there before and it's over here
and now, how did it get there. It must have
gone between those two locations. You have two pieces of data,
and you naturally interpolate because you make this assumption that
(09:30):
a classical object has a location at every time, and
that you can stitch those together into a path that
traverses space time in some continuous way. And you might
be thinking, well, of course everything does. Everything has a
location at a given time, and only one location. But
that's an intuition you've developed from experiencing the natural world,
(09:52):
from playing baseball, or from being chased by people throwing
rocks at you. You're used to things moving through the
world in a responsible and understandable way. So you've made
this assumption. Quantum objects do not have paths like that.
They don't have locations that translate through time. For a
quantum object like an electron, you can measure it at
(10:13):
one location A and then later measure it at another
location B, but it doesn't mean that it's flown through
the universe from A to B in that intervening time.
It can be at A later be at B, but
it's not true that it's moved from A to B,
meaning that if you've made a measurement halfway, you would
(10:34):
have found it going from A to B. Now that's
very confusing, that's very hard to understand. How can an
object be here and later be there and never be
in between? And the reason is that quantum objects are
just different from the classical objects. They are not the
same kind of thing. And this assumption that you have
made about the universe comes from only experiencing classical objects
(10:56):
things on our scale, but the smaller level the universe
just does not work that way, and that's pretty hard
to digest. But don't worry. There are things at the
quantum realm which do follow your intuition in which you
can use to help understand and develop an intuitive sense
of the of the universe, and that's the quantum wave function.
(11:16):
So how do the quantum object work. Well, a quantum
object is here and later it's there. What determines that.
It's not like quantum objects don't follow the laws of physics.
And you probably know the quantum objects could also be random,
that you can do the same thing to a quantum
object twice and get different outcomes. And that's true. All
those things are true. But the quantum object is ruled
(11:37):
by something, and that's this quantum wave function. You don't
have to know a lot of complicated math. All you
need to know is that the wave function tells you
where a quantum object is most likely to be. You
probably have heard that electrons don't have a specific location
to have a probability cloud around an atom. That's an
extrapolation of the wave function. The wave function tells you
(11:58):
where the electron is most like it to be. If
it's large over here, has it's more likely to be there.
If it's small somewhere else, it's less likely to be there.
If it's zero somewhere, it means the particle cannot be there.
Where is the particle? Actually, we don't know, and there's
a lot of discussion about whether the wave function means
the particle isn't actually anywhere or just we don't know
(12:20):
where it is, but we do know that quantum mechanics
says that that information does not need to exist. It's
not necessarily the case that there's hidden knowledge that the
electron is actually somewhere and we don't know. So the
best way to think about the wave function is that
it's physics saying what's allowed and what's more likely, and
this wave function is what we can grab onto, what
(12:42):
we can understand because the wave function does follow rules,
and the wave function has a path that makes sense.
It flows from one spot to the other. It doesn't
just disappear and appear somewhere else. So you have to
let go of your intuitive desire to understand the path
of of an electron the path of a quantum object,
(13:02):
because those things don't exist. But instead, when you let
that go, you can grasp onto the next layer. The
next layer is this wave function, the thing that determines
where the electron is, the thing that determines where the
electron is likely to be somewhere else. And that's the
key thing, is that the wave function tells you where
the electron is, and it responds to stimulation, it responds
(13:23):
to the world, and it flows just like a wave,
and you can use that you can understand how that
wave flows through time to understand where the electron is
likely to be in the future. Even if the electron
itself doesn't have a path from A to B, it's
wave function does. So the key to being comfortable with
quantum mechanics is to get comfortable with the objects wave
(13:44):
function rather than the object. All right, So the best
way to learn a new language is to get practice,
is to immerse yourself in it. So the next thing
we're gonna do is a bunch of exercises of diving
into the quantum realm and being comfortable with the wave
function understanding how it flows. And to do that, we're
going to explore in some detail the famous double slit
experiment that shows us the crazy behavior of quantum mechanics.
(14:07):
And at the end of it, I hope it all
makes sense and the weird results of the double slit
experiment feel totally natural. But first, let's take a quick break.
(14:27):
All right, So we are talking about quantum mechanics and
we're trying to get an intuitive grasp of quantum mechanics.
We want not just that you say things about quantum
mechanics that sound intelligent. We want to actually get an
understanding how do things work at the lowest level. What
intuition do we have that we can apply? And the
key thing, again, remember, is the wave function. You have
quantum mechanical objects like electrons and protons and photons. They
(14:50):
are difficult to describe using our classical ideas of balls
and objects that move through space because quantum objects don't
have paths. And said, we're gonna focus on their wave function,
the thing that says where they are likely to be,
and we're gonna understand how that wave function changes through time.
And we're gonna grab onto that instead of having a
(15:10):
classical path with the object, we're gonna understand how the
wave function changes through time, because that's the thing that's
most like a classical path. All right, So we're gonna
start very simple, and we're gonna work up to more
complicated situations. Imagine that you are a photon. You are
a tiny little quantum particle, and you're shot out of
a laser with a bunch of your friends towards the screen.
(15:32):
What happens while you leave the laser right later you
hit the screen. Does that mean that you flew through
the room from the laser to the screen. Not necessarily, right,
You do not have a classical path. Just because you
left the laser and later hit the screen doesn't mean
(15:52):
you have a trajectory. Doesn't mean that you necessarily can
be found in between things in the quantum realm. Don't go, however,
your wave function does. When you are created inside the laser,
just at the aperture of the laser, your wave function
says where you're likely to be, And then your wave
function flows through the room. If it isn't interacting with everything,
(16:15):
it just flows nicely through the room, showing the most
likely place to find your location. This is not the
same as having a classical path. This is sort of
the path of your wave function. Then it hits the screen,
and the screen essentially measures it. It says, okay, wave function,
where is this photon? And that's the moment when the
universe has to decide. Instead of having a probability distribution
(16:36):
about where you are, it says where is this actual photon?
So you have two measurements. When you leave the laser
and when you hit the screen. What happens in between
we don't know, but the wave function tells us, given
that you've left the laser, what's the likely place to
land on the screen. And then when you land on
the screen, the universe rolls a die and says where
(16:56):
you actually are. Okay, that's easy. Now let's do the
experiment again. A bunch of photons flying out from a
laser towards the wall. Let's bring in some black walls
so that instead of having a totally clear path of
the screen, you have sort of a narrow gap, not
too small, you know, a few inches. So what's going
to happen, Well, some of the photons that leave the
(17:17):
laser are going to go right through that gap and
hit the screen, just like before. Some of them are
gonna hit the walls that block a fraction of the
laser and so on the screen behind. What you'll get
is sort of a geometric shadow places where the photon
made it through the gap. You're gonna hit the screen
at the back, places where they hit the walls that
we introduced to make this a little gap. They're going
(17:37):
to hit those walls and they're not going to make
it to the screen. All right, So instead of having
just the laser splash on the back screen, now we
have something called a geometric shadow. Geometric because it has
crisp edges and it follows the shape of the of
the thing that the laser went through. No big deal. Now,
let's narrow the gap. Let's squeeze those two walls that
(18:00):
can find the laser. Two very very small, something like
hundreds of nanometers approximately the wavelength of light that's going
through it. Now what we see on the back wall changes.
Instead of seeing a geometric shadow, what we see is
sort of a spray of light. The light is spreading
out a little bit. It doesn't just become as narrow
as the gap it's shining through. It spreads out a
(18:20):
little bit. This is a wave effect. It happens anytime
a wave passes through a slit that's narrow compared to
its wavelength. We could do a deep dive under fraction,
and maybe we will in a future podcast, but it's
not critical to understand here. The only thing you need
to understand is at anytime a wave meaning sound or
light or bathtub splashes or wave functions pass through a
(18:42):
narrow slit, they spread out a little bit. You can
do the same kind of experiment in a bathtub. If
you send water waves through a very narrow gap, you
notice that when they come out of that gap they
spread out. Then we'll just shoot out in a narrow column,
all right. So now we have a very narrow gap,
and on the backscreen we have a sort of a
spray of results. We have a spray, not just a
(19:02):
very narrow geometric shadow. We have a spray of results
with a light can land, all right, and now we're
going to make a second narrow gap. So we have
sources of light in the back maybe lasers, maybe flashlights,
it doesn't matter. And they shoot light out, and then
we have two narrow gaps for the light to fly through.
And so we have light coming through both of those
gaps towards the wall. What do you see on the
(19:25):
back screen? You might think, oh, I have two copies
of what I saw when I had one narrow gap,
because now I have two narrow gaps, So how hard
can this be? That's not quite true. What you see
is an interference pattern. An interference pattern is what happens
when two waves collide. Because remember that waves are oscillation.
For example, if you have waves in your bathtub, those
(19:47):
waves are just the motion of the water moving up
and down. They're not their own thing. There a motion
of the water. Same way sound waves, These sounds that
I'm making into the microphone that you're hearing in your
earbuds or wherever you're listening, those are oscillations of the air.
The air itself is shaking, and because it's the shaking
of a medium, if something else comes along and shakes
it in another way, those that shaking can interfere, and
(20:10):
you can have two kinds of interference. You can have
constructive interference two things are shaking it the same way,
so you cant sort of double shaking. Or you can
have destructive interference where two things are shaking it in
the opposite way and they cancel each other out. For example,
this is how noise canceling headphones work. They very rapidly
hear the noises that are around you and admit exactly
(20:32):
the right sound to cancel out the sound. You can
create sound which will negate the other sound by shaking
it the opposite direction. So when the two shakings add up,
they add up to zero. So this is an interference
pattern places where things add up to be stronger in
place where things can cancel each other out. If you
look on that screen, what you'll see is a very
(20:53):
bright band in the middle where the two sources of
light from the two narrow gaps are adding up on
top of each other. And to the left of it
is a dark band, a band where the things are
canceling each other out. And as you move along the
screen you get bands of light and bands of dark,
bands of light and bands of dark, and the exact
width of those things depends on the wavelength of the
(21:13):
thing that's interfering with itself. Now you're probably thinking, oh, well,
the light is a wave, right, and if light is
a wave, it makes perfect sense for light to interfere
with itself the same way sound does, in the same
way waves in your bathtub night. That intuition makes sense,
But unfortunately that is not what's happening, and the next
experiment reveals that it's something much weirder, much more fascinating,
(21:37):
which reveals the true quantum nature of light. So we
have two narrow gaps and we're shining light through both gaps,
and we see an interference pattern on the back screen,
and we think well, that's fascinating. I bet it's because
light is interfering. That light is coming through both gaps,
and when it comes out the other side, either it
adds up constructively we get a bright patch, or it
(21:59):
can't as itself out and we get a dark patch.
So now let's do an experiment to discover if that's
actually true. What we're gonna do is we're gonna dial
down the source of the light. Maybe we had a laser,
maybe we had a flash light, doesn't matter too much,
but let's turn down the source of the light. And
as we talked about in a podcast very recently, light
is not continuous. It's not just the smooth oscillations of
(22:22):
the electromagnetic waves. Light is made of packets, and those
packets are photons. So you can't actually turn a light
down to any arbitrary value. You have to turn it
down for example, one photon a second, or two photons
a second, or three photons a second. You can't have
one point five photons a second. But what we can
do is something really fascinating. We can turn the light
(22:43):
down so we're shooting out one photon at a time,
maybe one photon, and then you wait five seconds, you
shoot out another photon. The idea is that one photon
has had plenty of time to go through the whole
experiment before the next photon comes through. Now, if the
interference comes from light coming through both of those narrow
gaps and then interfering, then the interference pattern should vanish
(23:06):
when we slow the experiment down to one photon at
a time. Because there's only one photon in the experiment
at a time, there's no other photon to interfere. That's
what you would expect if the interference pattern came from
the interference of the light waves. But it doesn't, because
what happens when we do this experiment is that we
still get an interference pattern. I remember learning about this
(23:29):
in college and it blew my mind. You shoot one
photon at a time, one photon goes here, another one
goes there. And remember, quantum mechanics is random, So there's
a wave function for each photon, and that wave function
says the probability of any given photon going somewhere. But
two photons shot in exactly the same direction under the
(23:49):
same conditions don't have to land in the same place.
The universe throws a new quantum number for every photon,
So the first one might land here and the next
one might land there. And what you do as you
do this experiment, which now takes longer because you're shooting
a lot of individual photons, what you see is that
you very gradually build up the interference pattern. Again. You
(24:11):
get a lot more photons landing where the interference pattern
was bright, and you get no photons landing where the
screen was previously dark, and you've got a few landing
where the where the screen was a little bit bright.
So the photons follow this interference pattern. It's like for
each one it says, all right, well we gotta have
a bunch over here and a few over there and
(24:32):
none over there, so let's roll a die and see
where you're gonna go. And eventually you just build up
exactly the same pattern that you saw before. So how
is that possible? How is it possible to have an
interference pattern if you don't have two photons in the
experiment at once, What is doing the interfering? Well, the
problem with this thinking is that you're thinking of the
(24:53):
photons as flying through the experiment. But remember they don't
have quantum paths. These things ex this when they leave
the light source, and then they exist when they hit
the screen. But in between they don't necessarily have a
location that we can think about sensibly. But what they
do have is a wave function. So instead of trying
to follow the photon through the experiment and making sense
(25:15):
of it, let's follow the wave function. That's the thing
we're gonna grab onto and try to make sense of
things that it's something that we can actually understand. So
let's start from the beginning. The photon is shot out
from this laser or this flashlight whatever, and has a
certain way function to be there. Now, the wave function
flies across the room and it spreads out a little
bit because that's the source of the light, and then
(25:36):
it hits the wall, the wall that has two narrow
gaps in it. Then what happens, Well, nobody has made
a measurement yet. The photon is interacting with this wall,
but nobody's looking, nobody's asked where is the photon? So
it has chance to go through one slit and chance
to go through the other slit. So what happens when
the wave function hits the wall? And remember it's hey
(26:00):
to talk about the wave function moving through space. The
wave function has a path that we can think about.
So when the wave function hits the wall, it splits
into half of it goes through one slit and half
of it goes through the other slid. And that reflects
the fact that the photon itself has a fifty percent
chance of hitting one slit and a fifty percent chance
of hitting the other slid. And if nobody has asked
(26:22):
which slip did it go through, it just has fifty
percent chance of going through one and a fifty pc
chance of going through the other. Now, let's follow the
wave function. The wave function comes out the other side
of these slits, and there's a little bit of source
of wave function from one slit and source of wave
function of the other slit. And a wave function, of course,
is a wave. So what happens when you get two
(26:43):
sources of a wave coming out from the wall towards
the screen, of course you get interference and so on
the screen. What's actually happening is that the wave function
is interfering with itself, and it gives the photon a
high probability to be in some locations and a low
probability to be in other locations, and a zero probability
(27:04):
to be in some locations. So for the wave function,
it determines where that photon, that individual photon that we
put into our experiment is likely to land. And so
the thing that's doing the interfering is not light because
you have a single photon in the experiment at once.
And if you're not convinced that this is the wave
function interfering, because you think maybe it's really just light interfering,
(27:26):
wouldn't that be simpler. But remember that we're sending a
single photon through a time in order to prevent the
photons from interacting, from interfering with each other. But the
real killer is that they did this experiment not with photons,
but later with electrons and they got the same result.
But what's doing the interfering is the wave function. Another
(27:48):
way to think about it is that the photon has
equal probability to go through one slit and the other,
and that probability is doing the interfering because remember the
way functions what controls where the partal has a probability
to be. So because it has a probability to be
through slit one and through slit two, those probabilities interfere
(28:09):
with each other, and those probabilities determine where the photon
can land on the back screen, and that's what we see,
and that's why it builds up one at a time
because for photon number one, it follows that distribution, and
the universe rolls a die only when it gets to
the back screen. That's when we're measuring it. That's when
we're interacting with it. That's when we're saying, Okay, a photon,
(28:31):
where are you? So remember that the quantum wave function
determines where you are most likely you the photon are
most likely to be found, and there are parts of
the screen where the photon has a high chance of
landing because the wave function interferes constructively and places on
the screen where has no chance of landing because the
two halves of its wave function are interfering with each
(28:52):
other destructively. They're canceling each other out. And you might
be thinking, well, is the wave function of physical thing?
Is it just a tool where you using to calculate things,
or is it something that's real and part of the universe.
That's a hard question to answer. It's philosophical, but here
we're seeing real physical effects of the existence of the
wave function. The wave function really does act like a wave,
(29:14):
and a wave that you can grasp on the way
that you can use your intuition to understand. You can
think of this wave function the same way you think
of waves in water, flowing through things and diffracting and
interfering and doing all those wave like things. And this
is the key to understanding quantum mechanics is grabbing onto
the wave function because it flows and it moves just
like a classical wave. It's just that it determines the
(29:37):
performance and the behavior in the location of a crazy
quantum object. So while that sinks into your brain, you're
understanding your classical intuition for the quantum wave function and
how a single photons wave function can interfere with itself
to give you this crazy pattern on the screen. While
that's in your mind, let's take a quick break before
we think about the last, the craziest, the most amazing
(30:01):
part of this double slit experiment. Just after this break,
all right, So we are spending time in the quantum
realm trying to become familiar, trying to develop an intuition.
(30:23):
And the key point I'm trying to make today is
that your intuition cannot be applied to a quantum object
because it's just different from the kind of things you're
familiar with. It is not a tiny spinning ball, It
is not a wave. It is neither. It is both
into something new and weird that possibly we will never understand.
But in my view, the best chance to understanding it
is to spend time with it, to develop an intuition
(30:45):
by immersion. So that's what we're doing today. We are
flying our way through experiments conducted by a physicist trying
to reveal the true nature of the universe. So we
started out just shooting photons against the screen to get
familiar with the idea that the photons don't move move
from one side of the screen to the other. They
have a probability to be in a certain place where
they only exist where they are measured and in between.
(31:07):
They do not necessarily have a path. You measure something,
you see the photon coming out of the light source
and later you see it on the screen. Doesn't mean
you can draw a straight line between those and say
the light was here. But what you can do is
talk about its wave function. The wave function leaves the
light source and later hits the screen, where eventually the
universe demands a measurement, and so the universe has to
(31:29):
decide based on where the wave function is, where to
actually put the photon. But that wave function is something
you can grasp, something you can follow through space in
a way that your intuition will be satisfied with. So
then we added these barriers. So instead of just splashing
light on the back screen, we saw a geometric shadow.
Then we narrowed the barriers until we saw the effects
(31:50):
of waves. We saw that the fact that waves coming
through a very narrow gap will spread out a little bit,
and then we added a second narrow gap, so we
had to sources of waves. And those waves apparently were interfering,
and your intuition was suggested that maybe it was light
doing the interfering, because we like to think of light
as a wave. But then we played a trick. We said,
(32:11):
let's slow down the experiment, only shoot one photon at
a time. And this, in theory, if light was a
wave and it was waves doing the interacting, this should
destroy the interference pattern because only one photon was going
through the experiment at a time. But it didn't. It
slowed down the interference pattern and it showed us that
there was still something they're doing the interfering, and that's
(32:34):
the key is letting go of this idea that the
light is flowing through the experiment, because quantum objects don't flow,
they don't go, and they don't have they don't have paths. Instead,
what's flowing through the experiment is the quantum wave of
the photon, and the quantum wave of the photon can
go through either slit as a fifty percent chance to
go through one and a fifty percent chance to go
(32:55):
through the other, and then it interferes with itself. It
gives us this interference pattern, and it's hard to get
your mind around what it means for the photon to
have a chance to go through both slits at once.
Most likely you think of it like this. You think, well,
the photon either went through one slit or the other.
We just don't know. And it's true that often in
science and in physics, we use probability to describe our
(33:18):
lack of knowledge. We say the universe is thirteen point
eight billion years old plus or minus a hundred million,
and it reflects not the fact that the universe doesn't
have a specific age, but just the fact that we
don't know it well enough that we haven't been able
to measure it. But this is different. It's not true
that the photon went through one slit or went through
the other and we just don't know. The truth is
(33:39):
that its wave went through both. It needed to go
through both in order to give us the interference pattern. Remember,
we didn't collapse the wave, We didn't interfere, We didn't
interact with the photon um when it's going through the slit.
We just let it fly through one slit or the other.
We don't pay attention. We only are looking at the
back screen. So le stick into that. Let's try to
(34:01):
probe that. What if we try to figure out which
slit the photon actually went through, because we are a
hardcore classicist and we want to know did it go
through one or the other. So we build a little
detector detector that doesn't change the direction of the photon
in a measurable way, but just tells us whether a
photon went through a slit, and we attach it to
one of the slits, and we do our experiment again.
(34:22):
And the idea here is just to confirm our intuition,
our classical intuition that the photon went through one slit
or the other. We turn on the experiment again. We
shoot one single photon at a time, and for every photon,
our detector tells us whether it went through slit one
because it beeps, or whether it went through slit two
because it doesn't beep. Do we get the same result
(34:42):
on the back screen. The answer is we do not.
We do not get the interference pattern on the back screen. Instead,
what we get our two geometrical shadows. And at first
this might be nonsensical. You might think, well, but the
detector is just telling you whether the photon went through
one or the other. It's not changing the photon in
any measurable way. What's the issue? How could it possibly
(35:04):
change what we're seeing in the back screen. But that's
because you're thinking about the photon is having a path,
is flying through When you think, well, it either went
through one slit or the other, it doesn't matter if
I know that, don't shouldn't change what happens. It's like
if you're watching a horse race, just watching what happens
around the first bend shouldn't change who wins the race? Right, Well,
(35:25):
that's not the case, because remember what's flying through your
experiment is not a photon. Photons don't fly through things.
They're not classical objects with paths. What's flying through the
experiment is a quantum wave, and the quantum wave is
sensitive to being watched. When you watch a quantum wave,
when you interact with it, when you say, okay, quantum
wave of this photon, where's the photon now? Then it
(35:46):
changes it it collapses and it says, okay, the photon
is here, and then the quantum wave can continue. But
you've narrowed it down. You pinned it down and said, okay,
it's right here, and then the quantum wave continues from
that location. So in the first scenario, when we didn't
have the detector, the quantum wave flies, it hits the
two slits, and it splits in half, and some of
(36:07):
the quantum wave goes through both slits. Each slit emits
some portion of the quantum wave, and those two halves
interfere with each other. In the version where you have
the detector on, then you're asking the universe to decide
which slit the photon went through, not just to reveal,
but to decide which slit the photon went through. So
the quantum wave can only go through one of the
(36:29):
slits and not the other one. Because the detector tells
you which slit it went through, it has no probability
to go through the other slit, So all it can
do is then emits some quantum wave from one of
the slits. If you hadn't looked, then it's free to
emit quantum wave from both slits, which can then interfere.
But if you look, if you demand an answer, if
you want to know which slit it went through, then
(36:52):
the quantum wave can only emit from the other slit
and then there's no interference pattern. There's just it's just
like as if you had one slit. And this is
the thing that blows most people's minds that asking questions
of the universe changes the answer. And it's true because
quantum waves respond to measurement. They like to be uncertain.
They're happy to fly through the universe keeping their uncertainty
(37:14):
their probability distribution until they are asked, and that moment
that you ask it, then it collapses and it says
the photon is here, the photon is there. So this
is an object we can grasp onto because it helps
us understand how things move through the universe, but also
is something new and something weird and something we have
to get familiar with. But the only way to do that,
of course, is to spend some time with it. So
(37:36):
I hope that's helped you understand a little bit about
quantum mechanics. The first step in becoming familiar with the
quantum realm is to abandon your idea of a quantum path,
that things have to move through the universe in a
continuous manner, that if you're over here and later you're
over there, you have to have somehow moved from one
to the other. But instead, instead of grabbing onto this
(37:57):
quantum path, there is something else you can grab onto,
this quantum wave function, which behaves in very understandable ways,
and we have equations that govern exactly how it moves.
And those equations, like the Shortinger wave equation, treat these
things like waves. Waves that we can understand that we
can actually apply our wave like intuition too, So if
you want to develop an intuition for quantum mechanics, get
(38:19):
cozy with the wave function. Now. The wave function, of
course has other mysteries, mysteries that continue to confound us
and the biggest one is this business about measurement. How
is it the wave function knows to collapse that We've
asked it a question and it's given us an answer
that when the wave function hits the screen, it says
the photon has various probabilities to be in various locations.
(38:41):
But then it actually makes a decision. When does the
universe decide to roll this dice and say, all right,
photon number seventy four, you are over here. Phot to
number seventy eight, you are over there. That is a
deep and recurring mystery of quantum mechanics that nobody really knows.
The answer to this description I've given you today is
sometimes as the Copenhagen interpretation of quantum mechanics, and it
(39:03):
has deep flaws in it. Flaws like who's doing the measuring?
You might ask, for example, if we if the photons
hit the screen but nobody looks, if there are no
humans in that universe, no scientists to do the observing,
then does the universe collapse the way of function or
does it keep it vague. We don't know the answer
to that question because we can't do the experiment in
(39:24):
which nobody looks and also know the answer, So that's frustrating,
and that's the source of a lot of discussion about
quantum mechanics. And there are other interpretations out there, like
the many Worlds interpretations that clever people like Sean Carroll
find much more natural, but they require you to accept
the existence of a huge number of other alternative universes.
So every interpretation of quantum mechanics comes with some sort
(39:46):
of cognitive load. And today we're not going to understand
all of the nuances and open questions of quantum mechanics.
We just wanted to spend some time becoming familiar with
the quantum realm. So the next time you read about
quantum mechanics, or think about quantum computers, or even just
are on a trip somewhere weird and unusual, remember you
can become familiar with something strange, something out of your
(40:06):
experience as long as you spent enough time immersing it,
marinating in the mathematics and the logic of it. Eventually
it would become part of who you are. Thanks for
listening to this explanation of our amazing and crazy and
totally bonkers universe. And if you have things that you'd
like us to discuss and break down in an accessible way.
Please send them to us at feedback at Daniel and
(40:28):
Joge dot com. Thanks for tuning in. If you still
have a question after listening to all these explanations, please
drop us the line. We'd love to hear from you.
You can find us at Facebook, Twitter, and Instagram at
Daniel and Jorge That's one word, or email us at
(40:51):
feedback at Daniel and Jorge dot com. Thanks for listening,
and remember that Daniel and Jorge Explaining the Universe is
a production of High Heart Reading More podcast for my
Heart Radio, visit the I Heart Radio, Apple podcasts, or
wherever you listen to your favorite shows. Yeah
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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