August 22, 2024 • 36 mins
What is light made of? A particle, a wave, both, neither? Little puppies?
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Speaker 1 (00:08):
Is it possible to be two different things at the
same time?
Speaker 2 (00:13):
Can you like dogs and cats?
Speaker 1 (00:16):
Can you be a horse and a giraffe at the
same time?
Speaker 2 (00:19):
Can something taste salty and sweet? Can a dress be
black and blue and white and gold?
Speaker 1 (00:26):
In today's podcast, we talk about the centuries old scientific
debate about light.
Speaker 2 (00:32):
Is light a particle or a wave?
Speaker 1 (00:35):
Or is it both? Hello, I'm Jorge and I'm Daniel.
Speaker 2 (00:57):
Welcome to Daniel and Jorge Explain the Universe.
Speaker 1 (01:00):
In which we try to explain the whole universe and
everything in it, including light.
Speaker 2 (01:05):
Now, I'm a cartoonist. I draw something called PhD comics.
Speaker 1 (01:08):
And I'm a particle physicist. During the day, I smash
particles together at the large Hadron Collider.
Speaker 2 (01:14):
Yeah. Well, today on the program, we're going to talk
about the nature of light.
Speaker 1 (01:22):
That's right. People have been arguing for centuries what is light?
Is it made out of particles? Is it made out
of waves? It's something else? Is it tiny little puppies
screaming through space? People have gone back and forth on
the issue, and today even the topic is not yet
totally settled. So we're going to be taking you through
that history and breaking it down.
Speaker 2 (01:41):
It's one of the most mind blowing questions in human
scientific history.
Speaker 1 (01:45):
That's right, what is light made out of? So, as usual,
before we dig into it, we went out and we
asked people on the street. What do you think light
is made out of? What do people know about light?
Speaker 2 (01:54):
Is light a particle or is it a wave?
Speaker 1 (01:57):
Here's what people had to say. Do you think light
is made out of particles or waves or both or neither? Photons? Yeah? Photons? Yeah,
so you think light it's a particle. I think it's waves,
youth waves. Yeah, cool, it's both, I think because it
moves like a wave, but it also has properties of
a particle and there's nothing saying it can't people. Okay, light,
(02:23):
I think they're made of wavelength.
Speaker 2 (02:27):
Yeah all right. Well it's interesting because I think all
of the answers are right.
Speaker 1 (02:35):
Or none of them are or both.
Speaker 2 (02:39):
Yeah.
Speaker 1 (02:39):
Well, it seems like a lot of people reflected the
fact that there is a controversy like that. You know,
it's not really well described, but either those some people
went all in, you know, like it's a photon or
it's a wave, or it's a wave.
Speaker 2 (02:51):
Length right, Yeah, that was my favorite one.
Speaker 1 (02:54):
I want to be a wavelength.
Speaker 2 (02:55):
Like I've heard of this word. It sounds really cool
and scientific. I'm just going to throw it up.
Speaker 1 (03:00):
There, that's right. Yeah, maybe I get some points. We
award no points to people, no points, that's right. There's
no price. Your price is you get to be on
our podcast and maybe we even make fun of you. Yeah.
Speaker 2 (03:12):
Yeah, but yeah, I guess what you mean is nobody
sort of fell for the trap, right, Like nobody said,
oh of course it's a particle, or nobody said, oh
of course it's a wave. Most people sort of knew
that there's some sort of duality there, something weird going on.
Speaker 1 (03:24):
That's right. That science is having some trouble, some difficulty
coming up with a way to describe what light is.
And that might seem surprising you because light is everywhere, right,
and it runs the universe. It's streaming through the Solar
system from the Sun, illuminating our lives, empowering everything on Earth.
So you think this would be sort of a high
priority topic to figure out, like what is this stuff?
(03:45):
What is it made out of?
Speaker 2 (03:47):
Yeah? I mean, like, what are we paying you for, Daniel,
if not to figure these kinds of questions out.
Speaker 1 (03:52):
I was just about to figure out what light was
when you called and said it's time to do this podcast.
Speaker 2 (03:57):
Sorry, science was totally you're a train of thought there,
that's right.
Speaker 1 (04:02):
Reflect on that for a minute.
Speaker 2 (04:03):
Or but no, Yeah, I'm a California taxpayer. Part of
my salary goes to paying your salary, like you know,
one million of a percent.
Speaker 1 (04:13):
That's true. Yeah, so you're saying you did pay taxes
last year.
Speaker 2 (04:17):
There you go again.
Speaker 1 (04:18):
That's another topic.
Speaker 2 (04:19):
Seekers on air, Daniel. Anyway, So that's an interesting question,
like is light a waver particle? And it's weird that
we don't know, But maybe let's break it down a
little bit. What is it? Like, what are we actually
talking about when we say that light could be a
particle or light could be a wave, like you know,
most people probably think of light as just like just
(04:42):
like brightness, right.
Speaker 1 (04:43):
Yeah. The thing to understand here is that we try
to describe light in terms of things we know, and
that's what science is. Right. You see something weird and
new and you wonder, oh, is it like this other
thing I know? So we've observed different kinds of phenomena
in the world, like you've seen waves, right, you go
to the beach, you see waves and water, you drop
a rock in a small puddle, you see waves. We
(05:06):
know what waves are, and we see different phenomena. We
try to categorize them in terms of things we know, right,
So like when people were studying sound, they discovered, oh,
sound is actually a wave. You know, it's a compression
wave in the air. And that's cool because he says, oh,
I already know how the math for waves works. Right,
I've seen waves and water. I've seen waves and other stuff.
Speaker 2 (05:26):
You can describe it with like equations, right, yeah, wavy equations.
Speaker 1 (05:31):
That's right, very solid unwavy physics to describe waves. And
there's a lot of science that's gone into understanding waves.
So if you can cram it into that box and say, oh,
this is just another example of something we already know,
then you're taking a huge leap forward. Right. So that's
something people try to do, is say, like, look, can
we describe this in terms of other things we know?
Speaker 2 (05:50):
So we need like, you know, we know about light,
but we want to know how it behaves and what
makes it work.
Speaker 1 (05:56):
Yeah, and just on a more general level, you try
to see something new, try to describe in terms of
things you know, Like say you taste a new kind
of fruit and you'd be like, Oh, it's a little
bit like a cherry, and a little bit like an apple,
and a little bit like you know, it's got a
hint of smokiness to it or whatever. You know.
Speaker 2 (06:10):
You're like, it's a chapel.
Speaker 1 (06:12):
It's a chapel. How has nobody ever invented that? The
cherry apple chapel? Oh my gosh, somebody, if our lawyer
is listening, get on that right away. Copyright that idea.
Speaker 2 (06:22):
I'll reserve ww dot chapel dot com. Up.
Speaker 1 (06:24):
That's right. So that's the basic idea is we have
these things we've seen. You see something new, you don't
want to create a whole new category. You want to
fit into one of the existing categories.
Speaker 2 (06:33):
So we sort of knew about light. It came from
the sun. It you know, if you light a fire,
it spreads out into a room. And so we're like,
what's going on? Like what what best describes how this
light you know, comes from a source and bounces off
the walls and stuff.
Speaker 1 (06:49):
Exactly exactly, that's the question. And so we'd seen things
like waves, So what do we mean when we say
a wave? Like how could a light be a wave?
Speaker 2 (06:55):
Well, how can anything be a wave?
Speaker 1 (06:58):
Yeah? How can anything be a wave? A wave is
a funny thing because it's not a thing itself. It's
a property of some medium.
Speaker 2 (07:05):
Like it's the words like a ripple on something.
Speaker 1 (07:07):
Yeah, that's right. Like if you do the wave at
a baseball game, you know, there's nothing to the wave itself.
It's just a bunch of people moving up and down
and waving their hands, right.
Speaker 2 (07:16):
Or like a sound wave is just like air molecules
kind of bumping forward.
Speaker 1 (07:21):
That's right, yeah, exactly. Or a wave in the ocean
is just it's an arrangement of the water, right, It's
a way the water gets compressed and then stretched out
and compressed and then gets stretched out. So that's an
important thing about a wave is that it moves in
this way through a medium.
Speaker 2 (07:35):
M Okay, so that's a wave. It's like a propagation,
it's like a ripple through something. But then, so then
what what would you call a particle? A particle is
different than that.
Speaker 1 (07:44):
A particle is different than that, and it's a totally
different kind of thing, you know, And to be a
particle physicist, it's kind of odd, but The concept of
a particle is not that really well defined, you know.
But when I think of a particle, I think of
taking matter and breaking it down to its smallest pieces.
If something's made out of particles, it means that at
its smallest level, it's made out of these little bits
(08:05):
that can't be chopped into smaller bits, and that they're localized.
They're like small and contained. Right. If you discover that
something is made of particles, you expect it to be
like mostly empty space, but with these little dots of matter, right.
Speaker 2 (08:21):
Like you would take something and then you smash it
to bit and just keep smashing, and at some point
you're going to get to these little baby balls or
like little tiny pellets that you can't break down anymore.
Speaker 1 (08:31):
That's right. Yeah, It's like seeing a picture on your
computer screen and discovering it's made out of pixels, right,
and that those pixels are the basic elements and they
come together to make the whole picture. So figuring out
that something is made of particles means that there's made
out these these little bits that are not connected to
each other, right, they're separated. So a wave and a
(08:53):
particle in nature are totally different. Kinds of things. Right. Now,
water of course is made of particles, but can have
waves in it.
Speaker 2 (09:00):
Right, But I think maybe what's important here is that,
you know, particles we tend to think of as little
tiny bits that can bounce around, right, and like go
in a street line and then hit something else and
then bounce back, or you know, kind of fly through
space right in a discrete little package exactly.
Speaker 1 (09:19):
That's exactly the right way to say. It's a discrete
little package. Right. So things that are made of particles
we think of as being discrete little bits, and they
they're broken up into these little pieces, and you're right,
they move in straight lines. Right, Like you throw a rock,
you roll a smooth ball across the surface, you expect
it to move in a straight line.
Speaker 2 (09:38):
So that's kind of what we mean by a wave
and a particle, that's right.
Speaker 1 (09:41):
Yeah.
Speaker 2 (09:42):
And so the question is like, is light a ripple
on a medium? Is that what light is? Or is
it like actual little things and move around in space?
Speaker 1 (09:50):
Right? Does it have its own stuff to it? Right?
Or is it just a way something else moves right?
That's sort of another way to phrase the question.
Speaker 2 (09:58):
Right, And those are two pretty different pictures. Is of reality, right, Yeah,
light could be little pellets flying around, or it could
be some sort of ripple on a medium. To us
in our intuitive sense, it couldn't be any more different, right.
Speaker 1 (10:10):
That's right. Yeah. It's like you can't be a Democrat
and a Republican, you know, just you have to pick one,
you know. Yeah, if you vote, you can be or
you could be neither. I suppose you shouldn't be both though, Yeah,
that would be a violation of some election law not
recommended to violate elections, right, that's right. Yeah. So, speaking
(10:31):
of political shouting matches, this one, this historical scientific shouting match,
began all the way back with the Greeks, right, Democratus
he's the guy sort of the first atomist. He's the
first person to look at the world and to say,
you know, maybe everything's made out of tiny little bits,
not just light, but also matter. And that was sort
of the birth of that idea that maybe everything around
(10:52):
us that seems macroscopic is made out of tiny little things,
smaller than we can see. And you know, as usual,
when somebody comes up with a good idea, they overextended.
They're like, well, maybe if rocks were made out of stuff.
Then water is also made out of particles, and maybe
even light is made out of particles. You know, at
the time, seemed like a totally a crazy reach, And.
Speaker 2 (11:12):
That makes sense, right, because light seems to go in
a straight line. It seems to bounce off of things.
So why couldn't light just be like little tiny little
pellets that bounce around the room and then eventually hit
your eye and then that's how you see something.
Speaker 1 (11:24):
Yeah, it certainly seems to have some of those particle
like properties, right, it moves in straight lines, it certainly
would be going really really fast. At the time, people
thought that light traveled instantly, right. They thought that light
instantaneously went from like the sun to the Earth, or
or if you started a fire, that the light would
immediately illuminate the room. Now, we of course know that
(11:45):
it just happens super duper crazy fast, too fast for
those folks to ever measure, so it's almost like it's instantaneous.
But they thought that these things just moved instantly through
space and filled up the room. Okay, and I want
to talk a little bit more about that, but first
a quick break.
Speaker 2 (12:12):
So, initially we thought light was or the Greeks thought
that light was a.
Speaker 1 (12:16):
Particle, right, And I think we have to qualify that
because it makes the Greeks sound really smart. Just come
up with this idea of atoms and all that stuff.
Speaker 2 (12:25):
And you say this before you're really down into Greeks.
Speaker 1 (12:27):
Well, I think people give the Greeks too much credit
for that, because, as I've probably said to you before,
the Greeks had lots and lots of ideas. You know,
they had like thousands of these ideas about how the
way the world works, and yeah, one of them was
close to true. But like, if we're going to do
some accounting, let's also remember the nine hundred and ninety
nine ones that were totally off base, you know, and
give them credit for those.
Speaker 2 (12:48):
Yeah, find that Greek who thought life was just little puppies,
and be like seeing you guys also thought they were puppies.
You can't be that.
Speaker 1 (12:54):
Smart, that's right. But he's a cool idea. So give
them credit for having that idea. Know what they were
smoking when they came up with it, But I'd like
to figure out where to find some. And then it
was thousands of years later before people had another idea.
It was a Descartes, the guy who's famous for. You know,
I think therefore I am he thought about He was
one of the earliest scientists, not just philosopher, but a scientist,
(13:16):
back in the day when you know, science really was
part of philosophy, and he thought that light was waves.
Speaker 2 (13:22):
What made him think it was waves?
Speaker 1 (13:23):
You know, I don't think he had much justification for it.
This is back in the early days when science wasn't
really an empirical study where you didn't like go out
and do experiments to test your hypothesis. It just made
more sense to him for light to be like these
wavelike disturbances.
Speaker 2 (13:39):
Right, which kind of makes sense, right, Like if you
have a speaker in a room emitting sound waves, it's
not that different from like a light bulb in the
middle of the room emitting light all around it.
Speaker 1 (13:50):
Right. Yeah. And there's some things that light does that
don't really that don't really seem consistent with particles, you know,
like the way light bends through a lens, right, it's
called we call it in science, we call that refraction.
You know, with light changes from going through air to glass,
it bends in this weird way. That's something that's very
common for waves, right.
Speaker 2 (14:10):
And a particle wouldn't bend inside of a lens.
Speaker 1 (14:15):
No, No, a particle that's definitely a wave like behavior. Yeah, oh,
not particle like behavior. And so Descartes saw that and
he's like, oh, you know, we have optics, we have
these lenses, so maybe light is a wave. But if
light is a wave, then it opens this other question.
What's doing the waving? Right? I mean with sound, you
know it's the air and the water waves. Obviously it's
the water. But if light is a wave, then what
(14:37):
is waving?
Speaker 2 (14:38):
Meaning? Like, if light is a ripple, what is it a.
Speaker 1 (14:41):
Ripple of that's right? Yeah, what's doing the rippling? Right?
If it's a wave, it has to be a wave
in something, because a wave is just a description of
some other form of matter rippling, right.
Speaker 2 (14:52):
It couldn't just be like stuff that we can't see.
Speaker 1 (14:56):
Yeah, and so you have to invent some stuff that
we can't see. Right. To explain light being a wave,
you have to invent this universe filled with stuff, or
there has to be that stuff between us and the
sun for example, right, which is a huge amount of
this new stuff you're inventing. And if you're looking at
the stars, there has to be that stuff between you
and the stars. Right, So now we're talking about billions
of miles of this new stuff, and Dick Cart, you know,
(15:17):
didn't know, so he just gave it a name. He's
called I don't even know how to pronounce it, but
he called it plenum. And he thought, well, there must
be if light is a wave, there must be some
stuff that's doing the waving, and we'll just give it
a name and maybe we'll be right, and then we'll
be famous forever.
Speaker 2 (15:31):
Isn't it is that different than the ether?
Speaker 1 (15:33):
It's similar in concept, right, It's a different idea, but
it's similar in concept that like, if light is a wave,
it must be waving through something, and we don't know
what it is, which is invents something to and give
it a name as a placeholder so when later people
do the hard work of actually discovering it, we'll still
get credit.
Speaker 2 (15:49):
Okay, So it was a particle. Light was a particle,
then was a wave, and then what happened?
Speaker 1 (15:53):
Well, then Newton came along, right, And Newton's a really
smart guy, and everybody knows that he's famous for thinking
about gravity, but he also like to think about optics
and lenses and he thought for sure that light was
a particle because he saw it moving in straight lines
and he saw distinct shadows. But you know, Newton also
did a lot of experiments with optics. He studied prisms,
(16:14):
and he saw light bending, and he saw light splitting
into colors. And you can't explain that if light is
a particle, but he tried. You know, he's like, well,
maybe when a particle hits the glass, it gets some
sort of weird sideways force and that makes it bend.
But you know, that's not really an explanation. That's just
sort of like a I don't really understand it. But
(16:34):
maybe it's something like this.
Speaker 2 (16:36):
Like, if light is a particle, why does it split
into the rainbow kind of thing?
Speaker 1 (16:40):
Yeah, exactly. And you know this is again back in
the day when empirical studies of science weren't the main
way to answer questions. It was mostly thinking in your
head about things that made sense to you, and then
they would argue about them. Right. A lot of the
way scientific disputes used to be resolved was people would
argue about it and then say, well, that makes no sense,
so it can't be true. And we know now, of
(17:02):
course that the universe doesn't always make sense to us.
What's real isn't necessarily the things that we would have
accepted as true or accepted as a reasonable way to
describe the universe. But you know, if that's the way
nature works, that's the way nature works, you have to
accept it. But that's the sort of primacy of experimental
results came later on. So back in the day, people
(17:22):
just sort of used to argue for an explanation that
made sense to them.
Speaker 2 (17:25):
Right, Well, it was kind of hard for them to
build a particle collider, right.
Speaker 1 (17:29):
That's right, Yeah, exactly, They didn't have the massive government
funding to do that. These were men of leisure studying
science and the spare time.
Speaker 2 (17:38):
In fact, it was called like natural philosophy, right, It
wasn't called science at the time, was it.
Speaker 1 (17:43):
Yeah, that's right exactly. All science grew out of philosophy.
It was called these folks were natural philosophers. Okay, But
you know, later on then people started doing experiments, and
there were a bunch of French guys who did a
bunch of experiments and some of some English folks, and
they they were studying how light behaved and refraction and reflection,
and they saw it doing these things, and they thought,
(18:05):
there's no way Newton's right, this has to be a wave,
you know. They saw things like interference patterns, right. Interference
patterns is when you have two waves and sometimes one
is rippling up at the same time another one is
rippling down. Right. So imagine, for example, you have a
bathtub of water in front of you, and you slap
(18:25):
it with two hands at once, right, each one is
going to send waves out, and then when those end,
those waves are either rippling up or rippling down. And
when they reach each other, if they're both rippling up
at the same time, then they constructively interfere to get
a double wave. If they're both rippling down at the
same time, they constructively interfere to get a double down wave.
(18:47):
If one is rippling up and the other's rippling down,
then then they cancel each other.
Speaker 2 (18:51):
Out right, And so you would see no light.
Speaker 1 (18:54):
Yeah, exactly, And so you can do this kind of stuff,
you know, in your bathtub. You can see interference patterns.
And what happens if you have two sources like that,
like one from each of your hands, is you get
some areas where the waves are high, in some areas
where the waves are low in some areas where there
are no waves, and so as you say, if you
do it with light, then you see these patterns of
(19:14):
dark and light, these stripes.
Speaker 2 (19:16):
And you couldn't do that with particles, right, like a
particle wouldn't cancel another particle.
Speaker 1 (19:20):
Yeah, there's no way to explain that with particles. People thought, well, look,
this is something that waves do and light is doing it,
and there's no way to explain it with particles, so
light must be a wave. Right. In fact, there's even
famous cases where they said, well, you know, if light
is a wave, then you know, if you set up
this various experiment, you would get this crazy effect and
(19:40):
so that's absurd and so it definitely can't be true.
And then they went and did the experiment and saw
the crazy wave effect and they're like, oh, that is true.
You know this is a I love that because it's
the primacy of experiment experimentalism, right, like, go and check
the data, Go and actually get some data and see
what the universe tells you.
Speaker 2 (19:59):
Yeah, Like you're like, a doughnut can't possibly be a
croissant at the same time, But it turns out that
you can bake something called a cronat.
Speaker 1 (20:08):
Yeah exactly. I think that's a big debate in pastry
science still though. Yeah, is it from a donut like
that's like a croissant or is it a croissant that's
like a donut.
Speaker 2 (20:16):
Yeah. I'm getting my degree in a particle baking.
Speaker 1 (20:21):
Yeah, the large Pastry Collider. I'm looking forward to the
construction of that project.
Speaker 2 (20:29):
But that's kind of what you mean. It's like you,
people don't think it's possible until they actually see it.
And waves and light has been doing this to people
for hundreds of years, or they're like, they can't possibly
be doing this, or they can't it can't possibly be
doing that, but it just keeps doing all these weird things.
Speaker 1 (20:43):
Yeah, exactly. And that was the experiment it's called the
double slit experiment, the one that really convinced people that
light is a wave because they've shone a strong light
and they had just two little narrow slits which act
like as sources like slapping your hands in the bath
to water, and then on a screen behind it. They
saw these interference patterns, right, is that you could definitely
(21:04):
only get if light was a wave. And so that
was the early eighteen hundreds, and everybody was absolutely certain
light was totally a wave. The question was settled. We
knew forever light was a wave, and we still didn't
know what was it waving through.
Speaker 2 (21:19):
But how did they explain all those particle experiments?
Speaker 1 (21:21):
Well, this was before we even really knew about particles, right,
No real particles had been discovered at this point. With
this idea from the Greeks of thousands of years ago
that maybe things were made out of particles, and chemistry
was getting warmed up, and you know, people are starting
to think about atoms and molecules and stuff, but they
hadn't really seen any actual particles yet. And it was
(21:41):
decades later when the electron was discovered that people started
to think about the particle model again. But you know,
the wave theory was definitely ascended, right, Everybody definitely looked
at these double slit experiments and saw light doing all
this wavy stuff, and they were sure that light was
a wave.
Speaker 2 (21:57):
Now did people extend that to other things, like, you know,
they thought, oh, light is this weird wavy thing, but
surely us, we're made out of little tiny atoms.
Speaker 1 (22:05):
Yeah, that's a good question. I wonder if people thought,
hm light's a wave. Maybe we're a wave too, right.
Speaker 2 (22:11):
Yeah, or like everything is just like a wave.
Speaker 1 (22:14):
Yeah, probably not, because nobody thought that light had any
mass to it, right, whereas we definitely know that we
have mass. Right, we feel pretty heavy sometimes after a
big meal. Even before the discovery particles, though, there was
a huge advance in the theory of light, which was
a Scottish guy named Maxwell. He was working on electricity
(22:35):
and magnetism, and he put together all these equations to
describe electricity magnetism, and he just sort of wrote them
down in a new way. This is like the way
you could do theoretical physics back in the days. You
just take existing ideas and you find a new way
to write them down. But he wrote them down in
this way that looked like the mathematics of a wave.
We have this equation, it's called a wave equation, and
(22:57):
it describes how waves moved through a medium.
Speaker 2 (23:00):
Meaning like it could be described by equations that looked
like sine waves and cosine waves, right, I mean, just
in case anyone remembers high school math, that's kind of
what we mean by mathematical equations. So you can describe
it as a sine wave ors cosine wave.
Speaker 1 (23:15):
Right, that's right. Yeah, The solution to these equations are
sine waves and cosine waves. These are differential equations to
describe how things move through the medium, and if things
follow these equations, then they're waves, right, okay. And so
he looked at the equations for electricity and for magnetism
and he rewrote them and he realized, you can rewrite
them in a way that looks just like the wave equation. Right,
(23:38):
So he said, oh, electricity magnetism has the same equation
as waves moving through water or waves moving through air.
And in fact, if you write it in terms of
this wave equation, you can pull out what the speed
of those waves must be. And the speed that he
pulled out from this from these equations was the speed
of light. So he had this moment of epift. He
(24:00):
must have been like in his office late one night,
rearranged these equations and realized, oh, my gosh, light is
a wave and it's a wave of electromagnetism.
Speaker 2 (24:10):
So like a light bulb turned on on top of
his head, emitting waves.
Speaker 1 (24:14):
Exactly the first appropriate light bulb ever.
Speaker 2 (24:17):
Yeah, so then that seems pretty definitive. The double slit
experiment shows it light interferes with itself. And also this
guy figured out that it's mathematically describable by sine ways
and cosine waves right.
Speaker 1 (24:35):
Right right, That light is waves of electromagnetism. Yeah, exactly.
So then it all seems really nice and tidy. But
then the particle revolution comes, right. People discover the electron,
people discover the neutron, people discovering all these particles. But
then they were doing experiments where they were shining light
onto materials and trying to get it to kick off electrons.
(24:57):
So you shine a really bright light at something and
you hope that some of the electrons in the material
absorb that light and get enough energy to be free
to run away. And so this is called the photoelectric effect.
You shine light at something and you measure the electrons
that come off. So what they saw in this experiment
only made sense if the energy of the light comes
(25:17):
in little packets rather than a continuous stream like waves.
So they turned up the intensity of the light and
they made it brighter, but that didn't increase the energy
the electrons that were coming off, which doesn't make sense
if it's a wave. It only makes sense if photons
come in little packets, so that increasing the intensity of
the light means more photons, but it doesn't give more
(25:38):
energy to any one electron because each electron can only
absorb one photon. And nobody understood this at all. This
made no sense to anybody. It was a huge puzzle.
We totally believe that it acted like a wave. We
had the double slit experiment told us it was a wave.
Maxwell's equations told us it was a wave. But then
we had the photoelectric effect, which didn't quite make sense
(25:59):
to anybody. Okay, And then Einstein said, well, what if
light comes in these little packets like you were saying before,
What if light is not this continuous stream of energy
like a wave is right, a wave is continuous stream
of energy. What if it comes in these little bits?
And that explained everything if you if you thought that
light was came in these little packets, it explained the
(26:20):
photo electric effect, explained these all these other mysteries in physics,
and that was the birth of quantum mechanics.
Speaker 2 (26:27):
Did they think that maybe it was little packets of waves?
Do you know what I mean? Like little short bursts
of ripples, you know, do you know what I mean?
Like could that explain how it's both things that run
through his brain.
Speaker 1 (26:38):
Yes, absolutely, I think that's probably the first way he
thought about it. Is like a little localized ripple, right,
like a little Yeah, that's the best way to put it,
a little localized ripple, like the way you can send
a little ripple of water, Yeah, through a swimming pool,
or something.
Speaker 2 (26:54):
Like a chirp or like a little sound burst.
Speaker 1 (26:57):
Yeah, exactly, like a little chirp. But it's strange because
you know, you can make a chirp of any size.
You can make a big one, a little one, a
long one, a fat one. But light, for some reason,
wanted to come only these in these little distinct chirps
of a specific size, and the size of those chirps
was controlled by their color or their frequency. And so
that was the birth of quantum mechanics, which we could
(27:19):
spend a whole other podcast talking about, but it was
the first clue that maybe light did come in these
distinct little packages.
Speaker 2 (27:26):
Yeah, let's talk about that, but first let's take a
quick break.
Speaker 1 (27:41):
And that's what we talked about, Like, what is a particle.
It's a distinct little package. And then here's the part
that blew my mind. Is that. Then they went back
and they did that double slit experiment again, but they
slowed it down. Instead of shining a really big beam
of light, they just shown one photon at a time, right, Okay,
because they wanted to see what's going to happen. Right,
(28:02):
if light comes in these little packets, how does that
explain the interference effect? How can light interfere if it's
a particle?
Speaker 2 (28:08):
So like, instead of like pointing the hose of water
at these two little holes and just seeing what happens
on the other side, they were throwing one droplet of
water at a.
Speaker 1 (28:18):
Time, yes, exactly. And what they expected to see was
that there'ld be no interference pattern, right, because the interference
comes from having two sources. Right. You have interference when
you have two waves that are either adding up or
canceling out.
Speaker 2 (28:31):
Meaning like a huge stream of light is going through
these two little slits. Then the two little slits act
like little sources, like little sorts of ripples, which can
cancel az exactly. But if you throw one drop at time,
it's either going to go in one slit or it's
going to go on the other slid, right.
Speaker 1 (28:47):
That's right. Yeah, And so there should be nothing to interfere, right,
So that's what they expected. But what they what they
saw blew their minds. Right. What happens if you slow
the experiment down, you send one photon at a time,
is that you'll get an interference pattern. It's just that
it builds up piece by piece. So you used to
throw one photon through and it lands someplace on the screen.
(29:08):
Throw another photon through, it lands somewhere else on the screen.
After you add up a million photons, you rebuild the
original interference pattern you saw.
Speaker 2 (29:16):
That's crazy.
Speaker 1 (29:17):
What light is a particle, but it's acting like a wave? Right?
How is that? How can that even be? Right?
Speaker 2 (29:23):
It's not just that it's like it's a particle that's
acting like a wave, as if it was in a
huge stream of other particles.
Speaker 1 (29:31):
Right, that's right, And this blew everybody's mind. And the answer,
of course, is that light is a particle. But like
every kind of matter, like every particle, how it moves
is governed by mathematics of wave equations. So every particle
carries with it some quantum mechanical wave that determines where
(29:53):
it goes. But what was happening in that experiment was
that a particle photon was approaching the experiment and then
it could either go through the left hand side or
the right hand side slit right, And because it's quantum mechanical,
it did both. It had a chance to do both,
and what was interfering was the probability to go through
(30:13):
the left slit or the right slit.
Speaker 2 (30:15):
So that's interesting. I don't think i've heard that explanation before,
that it's a particle and a wave in the sense
that it is a particle, but it moves according to
wave equations.
Speaker 1 (30:26):
Yes, everything moves according to wave equations. It's just that
the wavelength for things depends on how much energy they have.
So that was this guy, Tobroguely. He came up with
this equation. And maybe you've heard the expression to broguely wavelength.
Speaker 2 (30:39):
I've heard the expression wavelength. That seems to be a.
Speaker 1 (30:41):
Yeah, everything is wavelengths. We were making fun of that guy.
Turns out he was right, oh twist ending, No, everything
has a wavelength. Right, you can describe the motion of
anything in terms of a wave. Now, the wavelength depends
on the mass and the momentum, and for most things
(31:02):
like me or you or a cantalope. The wavelength of
its quantum mechanical wave function is tiny, and so you
can't even notice, right, the wave effects of you and
your son walking down the hallway and interfering with each
other are basically negligible. But on the scale of particles,
these wave functions interfere with each other.
Speaker 2 (31:23):
Yeah, that's a crazy thodd that, you know. We I
think people think quantum is something that doesn't affect their lies,
but quantum ideas and concepts are everywhere, right, Like you
have a sort of like a quantum superposition, or you
you're not really there. You sort of there's a cloud
of you that is.
Speaker 1 (31:43):
I'm not really here, I'm just an AI on the internet.
But that's that's differently cloud. Yeah, there is this quantum
mechanical certainly and everything.
Speaker 2 (31:53):
Guess yeah, yeah, it's just that you can't notice.
Speaker 1 (31:55):
That really blew people's minds. This concept that like, okay,
light is a particle, but it's sort of acts like
a wave. We can use these wave equations to describe it.
And you know, there's another layer to that experiment which
is even crazier, right, which is if what's interfering is
the probability to go through the left slit or the
right slit right. When the photon approaches the experiment, it
(32:18):
can go through one or the other. The interference pattern
comes from the uncertainty of which it's going to go through.
So what you can do is you can add a
little detector to one slit that like gives you a
ping if it goes through that slit right, so you
know for sure if it goes through one slit or
the other. If you do that, the interference pattern disappears.
WHOA why does it disappear? It disappears because the interference
(32:41):
only came from the interference of the possibility of the
particles to go through the left slit or the right slit,
our lack of knowledge. Once you know it goes through
the right slit of left slit, there's no more uncertainty.
There's nothing to interfere. It just goes through the left
or it goes through the right.
Speaker 2 (32:56):
It's like you're throwing a box is full of cats
that are either dead or alive, and you see what
happens on the other side. It's different if you take
a peek inside the box before it gets.
Speaker 1 (33:08):
There, exactly exactly, and no cats were harmed in the
making of this podcast, I now feel energe to point
out that's sort of where we are today, is that
we know that light is a particle and then it
comes in these little discrete packets become photons, right, Yeah,
But we also know that, like everything else, light is
(33:31):
determined by how its wave function moves. Every particle and
every object has this wave function, and how it moves
is controlled by wave equations.
Speaker 2 (33:40):
It's not like it's both a particle and a wave
and people don't really know which one it is, or
people are still confused about that. But it sort of
sounds like you're not that confused about it, right, It
sort of sounds like you know it's a particle, but
it moves around like a wave.
Speaker 1 (33:54):
Yeah, but it's still confusing. I mean, I think you could.
It's totally reasonable to say it's both. It's a particle
but it acts like a wave. Right. It's also totally
reasonable to say it's neither. It's not a particle, it's
not a wave, it's something else. It's something weird, something
totally strange, we've never seen before. It's a war we can.
Speaker 2 (34:17):
Or a pave.
Speaker 1 (34:18):
You are on fire.
Speaker 2 (34:20):
I am simple spelling.
Speaker 1 (34:25):
But that's that's a joke. But it's also serious because
sometimes we discover things which are unlike anything else we've seen,
and how do you describe them?
Speaker 2 (34:33):
I mean, we should stop using these words. We should
maybe come up with a new word to describe what
it is, because it's not, yes, not described by either
word particle.
Speaker 1 (34:42):
That's right, it's a chapel, it's a cherry apple combination.
Speaker 2 (34:45):
Yeah, let's not call it a particle or a wave.
Let's just make up anyw word that embodies these two
ways to behave that's right.
Speaker 1 (34:52):
But here we've discovered something which is different from anything
in our microscopic world. There's nothing in our world particles, wave,
little puppies that is a good analogy for what light is.
So we have to try to sort of describe it
in terms of sometimes it's like this, sometimes like this.
My personal belief is that it's it's not like anything else,
and that these are approximations. But you know, like we
(35:15):
were talking about earlier, you can be different contradictory things
like how would you describe yourself? You know, sometimes your husband,
sometimes your father, Sometimes you're a cartoonist, sometimes you're just asleep.
You know, like all these things describe you, they're contradictory.
There are different facets of who you are at your core,
and none of them define you.
Speaker 2 (35:32):
Right right, But if you don't have to have the
right label, you make up and you know that's right.
Speaker 1 (35:37):
Yes, we need a new thing, right. Light is definitely
its own weird kind of thing.
Speaker 2 (35:44):
All right, Well, until next time.
Speaker 1 (35:54):
If you still have a question after listening to all
these explanations, please drop us a 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 Feedback at Danielanorge dot com
Is it possible to be two different things at the
same time?
Speaker 2 (00:13):
Can you like dogs and cats?
Speaker 1 (00:16):
Can you be a horse and a giraffe at the
same time?
Speaker 2 (00:19):
Can something taste salty and sweet? Can a dress be
black and blue and white and gold?
Speaker 1 (00:26):
In today's podcast, we talk about the centuries old scientific
debate about light.
Speaker 2 (00:32):
Is light a particle or a wave?
Speaker 1 (00:35):
Or is it both? Hello, I'm Jorge and I'm Daniel.
Speaker 2 (00:57):
Welcome to Daniel and Jorge Explain the Universe.
Speaker 1 (01:00):
In which we try to explain the whole universe and
everything in it, including light.
Speaker 2 (01:05):
Now, I'm a cartoonist. I draw something called PhD comics.
Speaker 1 (01:08):
And I'm a particle physicist. During the day, I smash
particles together at the large Hadron Collider.
Speaker 2 (01:14):
Yeah. Well, today on the program, we're going to talk
about the nature of light.
Speaker 1 (01:22):
That's right. People have been arguing for centuries what is light?
Is it made out of particles? Is it made out
of waves? It's something else? Is it tiny little puppies
screaming through space? People have gone back and forth on
the issue, and today even the topic is not yet
totally settled. So we're going to be taking you through
that history and breaking it down.
Speaker 2 (01:41):
It's one of the most mind blowing questions in human
scientific history.
Speaker 1 (01:45):
That's right, what is light made out of? So, as usual,
before we dig into it, we went out and we
asked people on the street. What do you think light
is made out of? What do people know about light?
Speaker 2 (01:54):
Is light a particle or is it a wave?
Speaker 1 (01:57):
Here's what people had to say. Do you think light
is made out of particles or waves or both or neither? Photons? Yeah? Photons? Yeah,
so you think light it's a particle. I think it's waves,
youth waves. Yeah, cool, it's both, I think because it
moves like a wave, but it also has properties of
a particle and there's nothing saying it can't people. Okay, light,
(02:23):
I think they're made of wavelength.
Speaker 2 (02:27):
Yeah all right. Well it's interesting because I think all
of the answers are right.
Speaker 1 (02:35):
Or none of them are or both.
Speaker 2 (02:39):
Yeah.
Speaker 1 (02:39):
Well, it seems like a lot of people reflected the
fact that there is a controversy like that. You know,
it's not really well described, but either those some people
went all in, you know, like it's a photon or
it's a wave, or it's a wave.
Speaker 2 (02:51):
Length right, Yeah, that was my favorite one.
Speaker 1 (02:54):
I want to be a wavelength.
Speaker 2 (02:55):
Like I've heard of this word. It sounds really cool
and scientific. I'm just going to throw it up.
Speaker 1 (03:00):
There, that's right. Yeah, maybe I get some points. We
award no points to people, no points, that's right. There's
no price. Your price is you get to be on
our podcast and maybe we even make fun of you. Yeah.
Speaker 2 (03:12):
Yeah, but yeah, I guess what you mean is nobody
sort of fell for the trap, right, Like nobody said,
oh of course it's a particle, or nobody said, oh
of course it's a wave. Most people sort of knew
that there's some sort of duality there, something weird going on.
Speaker 1 (03:24):
That's right. That science is having some trouble, some difficulty
coming up with a way to describe what light is.
And that might seem surprising you because light is everywhere, right,
and it runs the universe. It's streaming through the Solar
system from the Sun, illuminating our lives, empowering everything on Earth.
So you think this would be sort of a high
priority topic to figure out, like what is this stuff?
(03:45):
What is it made out of?
Speaker 2 (03:47):
Yeah? I mean, like, what are we paying you for, Daniel,
if not to figure these kinds of questions out.
Speaker 1 (03:52):
I was just about to figure out what light was
when you called and said it's time to do this podcast.
Speaker 2 (03:57):
Sorry, science was totally you're a train of thought there,
that's right.
Speaker 1 (04:02):
Reflect on that for a minute.
Speaker 2 (04:03):
Or but no, Yeah, I'm a California taxpayer. Part of
my salary goes to paying your salary, like you know,
one million of a percent.
Speaker 1 (04:13):
That's true. Yeah, so you're saying you did pay taxes
last year.
Speaker 2 (04:17):
There you go again.
Speaker 1 (04:18):
That's another topic.
Speaker 2 (04:19):
Seekers on air, Daniel. Anyway, So that's an interesting question,
like is light a waver particle? And it's weird that
we don't know, But maybe let's break it down a
little bit. What is it? Like, what are we actually
talking about when we say that light could be a
particle or light could be a wave, like you know,
most people probably think of light as just like just
(04:42):
like brightness, right.
Speaker 1 (04:43):
Yeah. The thing to understand here is that we try
to describe light in terms of things we know, and
that's what science is. Right. You see something weird and
new and you wonder, oh, is it like this other
thing I know? So we've observed different kinds of phenomena
in the world, like you've seen waves, right, you go
to the beach, you see waves and water, you drop
a rock in a small puddle, you see waves. We
(05:06):
know what waves are, and we see different phenomena. We
try to categorize them in terms of things we know, right,
So like when people were studying sound, they discovered, oh,
sound is actually a wave. You know, it's a compression
wave in the air. And that's cool because he says, oh,
I already know how the math for waves works. Right,
I've seen waves and water. I've seen waves and other stuff.
Speaker 2 (05:26):
You can describe it with like equations, right, yeah, wavy equations.
Speaker 1 (05:31):
That's right, very solid unwavy physics to describe waves. And
there's a lot of science that's gone into understanding waves.
So if you can cram it into that box and say, oh,
this is just another example of something we already know,
then you're taking a huge leap forward. Right. So that's
something people try to do, is say, like, look, can
we describe this in terms of other things we know?
Speaker 2 (05:50):
So we need like, you know, we know about light,
but we want to know how it behaves and what
makes it work.
Speaker 1 (05:56):
Yeah, and just on a more general level, you try
to see something new, try to describe in terms of
things you know, Like say you taste a new kind
of fruit and you'd be like, Oh, it's a little
bit like a cherry, and a little bit like an apple,
and a little bit like you know, it's got a
hint of smokiness to it or whatever. You know.
Speaker 2 (06:10):
You're like, it's a chapel.
Speaker 1 (06:12):
It's a chapel. How has nobody ever invented that? The
cherry apple chapel? Oh my gosh, somebody, if our lawyer
is listening, get on that right away. Copyright that idea.
Speaker 2 (06:22):
I'll reserve ww dot chapel dot com. Up.
Speaker 1 (06:24):
That's right. So that's the basic idea is we have
these things we've seen. You see something new, you don't
want to create a whole new category. You want to
fit into one of the existing categories.
Speaker 2 (06:33):
So we sort of knew about light. It came from
the sun. It you know, if you light a fire,
it spreads out into a room. And so we're like,
what's going on? Like what what best describes how this
light you know, comes from a source and bounces off
the walls and stuff.
Speaker 1 (06:49):
Exactly exactly, that's the question. And so we'd seen things
like waves, So what do we mean when we say
a wave? Like how could a light be a wave?
Speaker 2 (06:55):
Well, how can anything be a wave?
Speaker 1 (06:58):
Yeah? How can anything be a wave? A wave is
a funny thing because it's not a thing itself. It's
a property of some medium.
Speaker 2 (07:05):
Like it's the words like a ripple on something.
Speaker 1 (07:07):
Yeah, that's right. Like if you do the wave at
a baseball game, you know, there's nothing to the wave itself.
It's just a bunch of people moving up and down
and waving their hands, right.
Speaker 2 (07:16):
Or like a sound wave is just like air molecules
kind of bumping forward.
Speaker 1 (07:21):
That's right, yeah, exactly. Or a wave in the ocean
is just it's an arrangement of the water, right, It's
a way the water gets compressed and then stretched out
and compressed and then gets stretched out. So that's an
important thing about a wave is that it moves in
this way through a medium.
Speaker 2 (07:35):
M Okay, so that's a wave. It's like a propagation,
it's like a ripple through something. But then, so then
what what would you call a particle? A particle is
different than that.
Speaker 1 (07:44):
A particle is different than that, and it's a totally
different kind of thing, you know, And to be a
particle physicist, it's kind of odd, but The concept of
a particle is not that really well defined, you know.
But when I think of a particle, I think of
taking matter and breaking it down to its smallest pieces.
If something's made out of particles, it means that at
its smallest level, it's made out of these little bits
(08:05):
that can't be chopped into smaller bits, and that they're localized.
They're like small and contained. Right. If you discover that
something is made of particles, you expect it to be
like mostly empty space, but with these little dots of matter, right.
Speaker 2 (08:21):
Like you would take something and then you smash it
to bit and just keep smashing, and at some point
you're going to get to these little baby balls or
like little tiny pellets that you can't break down anymore.
Speaker 1 (08:31):
That's right. Yeah, It's like seeing a picture on your
computer screen and discovering it's made out of pixels, right,
and that those pixels are the basic elements and they
come together to make the whole picture. So figuring out
that something is made of particles means that there's made
out these these little bits that are not connected to
each other, right, they're separated. So a wave and a
(08:53):
particle in nature are totally different. Kinds of things. Right. Now,
water of course is made of particles, but can have
waves in it.
Speaker 2 (09:00):
Right, But I think maybe what's important here is that,
you know, particles we tend to think of as little
tiny bits that can bounce around, right, and like go
in a street line and then hit something else and
then bounce back, or you know, kind of fly through
space right in a discrete little package exactly.
Speaker 1 (09:19):
That's exactly the right way to say. It's a discrete
little package. Right. So things that are made of particles
we think of as being discrete little bits, and they
they're broken up into these little pieces, and you're right,
they move in straight lines. Right, Like you throw a rock,
you roll a smooth ball across the surface, you expect
it to move in a straight line.
Speaker 2 (09:38):
So that's kind of what we mean by a wave
and a particle, that's right.
Speaker 1 (09:41):
Yeah.
Speaker 2 (09:42):
And so the question is like, is light a ripple
on a medium? Is that what light is? Or is
it like actual little things and move around in space?
Speaker 1 (09:50):
Right? Does it have its own stuff to it? Right?
Or is it just a way something else moves right?
That's sort of another way to phrase the question.
Speaker 2 (09:58):
Right, And those are two pretty different pictures. Is of reality, right, Yeah,
light could be little pellets flying around, or it could
be some sort of ripple on a medium. To us
in our intuitive sense, it couldn't be any more different, right.
Speaker 1 (10:10):
That's right. Yeah. It's like you can't be a Democrat
and a Republican, you know, just you have to pick one,
you know. Yeah, if you vote, you can be or
you could be neither. I suppose you shouldn't be both though, Yeah,
that would be a violation of some election law not
recommended to violate elections, right, that's right. Yeah. So, speaking
(10:31):
of political shouting matches, this one, this historical scientific shouting match,
began all the way back with the Greeks, right, Democratus
he's the guy sort of the first atomist. He's the
first person to look at the world and to say,
you know, maybe everything's made out of tiny little bits,
not just light, but also matter. And that was sort
of the birth of that idea that maybe everything around
(10:52):
us that seems macroscopic is made out of tiny little things,
smaller than we can see. And you know, as usual,
when somebody comes up with a good idea, they overextended.
They're like, well, maybe if rocks were made out of stuff.
Then water is also made out of particles, and maybe
even light is made out of particles. You know, at
the time, seemed like a totally a crazy reach, And.
Speaker 2 (11:12):
That makes sense, right, because light seems to go in
a straight line. It seems to bounce off of things.
So why couldn't light just be like little tiny little
pellets that bounce around the room and then eventually hit
your eye and then that's how you see something.
Speaker 1 (11:24):
Yeah, it certainly seems to have some of those particle
like properties, right, it moves in straight lines, it certainly
would be going really really fast. At the time, people
thought that light traveled instantly, right. They thought that light
instantaneously went from like the sun to the Earth, or
or if you started a fire, that the light would
immediately illuminate the room. Now, we of course know that
(11:45):
it just happens super duper crazy fast, too fast for
those folks to ever measure, so it's almost like it's instantaneous.
But they thought that these things just moved instantly through
space and filled up the room. Okay, and I want
to talk a little bit more about that, but first
a quick break.
Speaker 2 (12:12):
So, initially we thought light was or the Greeks thought
that light was a.
Speaker 1 (12:16):
Particle, right, And I think we have to qualify that
because it makes the Greeks sound really smart. Just come
up with this idea of atoms and all that stuff.
Speaker 2 (12:25):
And you say this before you're really down into Greeks.
Speaker 1 (12:27):
Well, I think people give the Greeks too much credit
for that, because, as I've probably said to you before,
the Greeks had lots and lots of ideas. You know,
they had like thousands of these ideas about how the
way the world works, and yeah, one of them was
close to true. But like, if we're going to do
some accounting, let's also remember the nine hundred and ninety
nine ones that were totally off base, you know, and
give them credit for those.
Speaker 2 (12:48):
Yeah, find that Greek who thought life was just little puppies,
and be like seeing you guys also thought they were puppies.
You can't be that.
Speaker 1 (12:54):
Smart, that's right. But he's a cool idea. So give
them credit for having that idea. Know what they were
smoking when they came up with it, But I'd like
to figure out where to find some. And then it
was thousands of years later before people had another idea.
It was a Descartes, the guy who's famous for. You know,
I think therefore I am he thought about He was
one of the earliest scientists, not just philosopher, but a scientist,
(13:16):
back in the day when you know, science really was
part of philosophy, and he thought that light was waves.
Speaker 2 (13:22):
What made him think it was waves?
Speaker 1 (13:23):
You know, I don't think he had much justification for it.
This is back in the early days when science wasn't
really an empirical study where you didn't like go out
and do experiments to test your hypothesis. It just made
more sense to him for light to be like these
wavelike disturbances.
Speaker 2 (13:39):
Right, which kind of makes sense, right, Like if you
have a speaker in a room emitting sound waves, it's
not that different from like a light bulb in the
middle of the room emitting light all around it.
Speaker 1 (13:50):
Right. Yeah. And there's some things that light does that
don't really that don't really seem consistent with particles, you know,
like the way light bends through a lens, right, it's
called we call it in science, we call that refraction.
You know, with light changes from going through air to glass,
it bends in this weird way. That's something that's very
common for waves, right.
Speaker 2 (14:10):
And a particle wouldn't bend inside of a lens.
Speaker 1 (14:15):
No, No, a particle that's definitely a wave like behavior. Yeah, oh,
not particle like behavior. And so Descartes saw that and
he's like, oh, you know, we have optics, we have
these lenses, so maybe light is a wave. But if
light is a wave, then it opens this other question.
What's doing the waving? Right? I mean with sound, you
know it's the air and the water waves. Obviously it's
the water. But if light is a wave, then what
(14:37):
is waving?
Speaker 2 (14:38):
Meaning? Like, if light is a ripple, what is it a.
Speaker 1 (14:41):
Ripple of that's right? Yeah, what's doing the rippling? Right?
If it's a wave, it has to be a wave
in something, because a wave is just a description of
some other form of matter rippling, right.
Speaker 2 (14:52):
It couldn't just be like stuff that we can't see.
Speaker 1 (14:56):
Yeah, and so you have to invent some stuff that
we can't see. Right. To explain light being a wave,
you have to invent this universe filled with stuff, or
there has to be that stuff between us and the
sun for example, right, which is a huge amount of
this new stuff you're inventing. And if you're looking at
the stars, there has to be that stuff between you
and the stars. Right, So now we're talking about billions
of miles of this new stuff, and Dick Cart, you know,
(15:17):
didn't know, so he just gave it a name. He's
called I don't even know how to pronounce it, but
he called it plenum. And he thought, well, there must
be if light is a wave, there must be some
stuff that's doing the waving, and we'll just give it
a name and maybe we'll be right, and then we'll
be famous forever.
Speaker 2 (15:31):
Isn't it is that different than the ether?
Speaker 1 (15:33):
It's similar in concept, right, It's a different idea, but
it's similar in concept that like, if light is a wave,
it must be waving through something, and we don't know
what it is, which is invents something to and give
it a name as a placeholder so when later people
do the hard work of actually discovering it, we'll still
get credit.
Speaker 2 (15:49):
Okay, So it was a particle. Light was a particle,
then was a wave, and then what happened?
Speaker 1 (15:53):
Well, then Newton came along, right, And Newton's a really
smart guy, and everybody knows that he's famous for thinking
about gravity, but he also like to think about optics
and lenses and he thought for sure that light was
a particle because he saw it moving in straight lines
and he saw distinct shadows. But you know, Newton also
did a lot of experiments with optics. He studied prisms,
(16:14):
and he saw light bending, and he saw light splitting
into colors. And you can't explain that if light is
a particle, but he tried. You know, he's like, well,
maybe when a particle hits the glass, it gets some
sort of weird sideways force and that makes it bend.
But you know, that's not really an explanation. That's just
sort of like a I don't really understand it. But
(16:34):
maybe it's something like this.
Speaker 2 (16:36):
Like, if light is a particle, why does it split
into the rainbow kind of thing?
Speaker 1 (16:40):
Yeah, exactly. And you know this is again back in
the day when empirical studies of science weren't the main
way to answer questions. It was mostly thinking in your
head about things that made sense to you, and then
they would argue about them. Right. A lot of the
way scientific disputes used to be resolved was people would
argue about it and then say, well, that makes no sense,
so it can't be true. And we know now, of
(17:02):
course that the universe doesn't always make sense to us.
What's real isn't necessarily the things that we would have
accepted as true or accepted as a reasonable way to
describe the universe. But you know, if that's the way
nature works, that's the way nature works, you have to
accept it. But that's the sort of primacy of experimental
results came later on. So back in the day, people
(17:22):
just sort of used to argue for an explanation that
made sense to them.
Speaker 2 (17:25):
Right, Well, it was kind of hard for them to
build a particle collider, right.
Speaker 1 (17:29):
That's right, Yeah, exactly, They didn't have the massive government
funding to do that. These were men of leisure studying
science and the spare time.
Speaker 2 (17:38):
In fact, it was called like natural philosophy, right, It
wasn't called science at the time, was it.
Speaker 1 (17:43):
Yeah, that's right exactly. All science grew out of philosophy.
It was called these folks were natural philosophers. Okay, But
you know, later on then people started doing experiments, and
there were a bunch of French guys who did a
bunch of experiments and some of some English folks, and
they they were studying how light behaved and refraction and reflection,
and they saw it doing these things, and they thought,
(18:05):
there's no way Newton's right, this has to be a wave,
you know. They saw things like interference patterns, right. Interference
patterns is when you have two waves and sometimes one
is rippling up at the same time another one is
rippling down. Right. So imagine, for example, you have a
bathtub of water in front of you, and you slap
(18:25):
it with two hands at once, right, each one is
going to send waves out, and then when those end,
those waves are either rippling up or rippling down. And
when they reach each other, if they're both rippling up
at the same time, then they constructively interfere to get
a double wave. If they're both rippling down at the
same time, they constructively interfere to get a double down wave.
(18:47):
If one is rippling up and the other's rippling down,
then then they cancel each other.
Speaker 2 (18:51):
Out right, And so you would see no light.
Speaker 1 (18:54):
Yeah, exactly, And so you can do this kind of stuff,
you know, in your bathtub. You can see interference patterns.
And what happens if you have two sources like that,
like one from each of your hands, is you get
some areas where the waves are high, in some areas
where the waves are low in some areas where there
are no waves, and so as you say, if you
do it with light, then you see these patterns of
(19:14):
dark and light, these stripes.
Speaker 2 (19:16):
And you couldn't do that with particles, right, like a
particle wouldn't cancel another particle.
Speaker 1 (19:20):
Yeah, there's no way to explain that with particles. People thought, well, look,
this is something that waves do and light is doing it,
and there's no way to explain it with particles, so
light must be a wave. Right. In fact, there's even
famous cases where they said, well, you know, if light
is a wave, then you know, if you set up
this various experiment, you would get this crazy effect and
(19:40):
so that's absurd and so it definitely can't be true.
And then they went and did the experiment and saw
the crazy wave effect and they're like, oh, that is true.
You know this is a I love that because it's
the primacy of experiment experimentalism, right, like, go and check
the data, Go and actually get some data and see
what the universe tells you.
Speaker 2 (19:59):
Yeah, Like you're like, a doughnut can't possibly be a
croissant at the same time, But it turns out that
you can bake something called a cronat.
Speaker 1 (20:08):
Yeah exactly. I think that's a big debate in pastry
science still though. Yeah, is it from a donut like
that's like a croissant or is it a croissant that's
like a donut.
Speaker 2 (20:16):
Yeah. I'm getting my degree in a particle baking.
Speaker 1 (20:21):
Yeah, the large Pastry Collider. I'm looking forward to the
construction of that project.
Speaker 2 (20:29):
But that's kind of what you mean. It's like you,
people don't think it's possible until they actually see it.
And waves and light has been doing this to people
for hundreds of years, or they're like, they can't possibly
be doing this, or they can't it can't possibly be
doing that, but it just keeps doing all these weird things.
Speaker 1 (20:43):
Yeah, exactly. And that was the experiment it's called the
double slit experiment, the one that really convinced people that
light is a wave because they've shone a strong light
and they had just two little narrow slits which act
like as sources like slapping your hands in the bath
to water, and then on a screen behind it. They
saw these interference patterns, right, is that you could definitely
(21:04):
only get if light was a wave. And so that
was the early eighteen hundreds, and everybody was absolutely certain
light was totally a wave. The question was settled. We
knew forever light was a wave, and we still didn't
know what was it waving through.
Speaker 2 (21:19):
But how did they explain all those particle experiments?
Speaker 1 (21:21):
Well, this was before we even really knew about particles, right,
No real particles had been discovered at this point. With
this idea from the Greeks of thousands of years ago
that maybe things were made out of particles, and chemistry
was getting warmed up, and you know, people are starting
to think about atoms and molecules and stuff, but they
hadn't really seen any actual particles yet. And it was
(21:41):
decades later when the electron was discovered that people started
to think about the particle model again. But you know,
the wave theory was definitely ascended, right, Everybody definitely looked
at these double slit experiments and saw light doing all
this wavy stuff, and they were sure that light was
a wave.
Speaker 2 (21:57):
Now did people extend that to other things, like, you know,
they thought, oh, light is this weird wavy thing, but
surely us, we're made out of little tiny atoms.
Speaker 1 (22:05):
Yeah, that's a good question. I wonder if people thought,
hm light's a wave. Maybe we're a wave too, right.
Speaker 2 (22:11):
Yeah, or like everything is just like a wave.
Speaker 1 (22:14):
Yeah, probably not, because nobody thought that light had any
mass to it, right, whereas we definitely know that we
have mass. Right, we feel pretty heavy sometimes after a
big meal. Even before the discovery particles, though, there was
a huge advance in the theory of light, which was
a Scottish guy named Maxwell. He was working on electricity
(22:35):
and magnetism, and he put together all these equations to
describe electricity magnetism, and he just sort of wrote them
down in a new way. This is like the way
you could do theoretical physics back in the days. You
just take existing ideas and you find a new way
to write them down. But he wrote them down in
this way that looked like the mathematics of a wave.
We have this equation, it's called a wave equation, and
(22:57):
it describes how waves moved through a medium.
Speaker 2 (23:00):
Meaning like it could be described by equations that looked
like sine waves and cosine waves, right, I mean, just
in case anyone remembers high school math, that's kind of
what we mean by mathematical equations. So you can describe
it as a sine wave ors cosine wave.
Speaker 1 (23:15):
Right, that's right. Yeah, The solution to these equations are
sine waves and cosine waves. These are differential equations to
describe how things move through the medium, and if things
follow these equations, then they're waves, right, okay. And so
he looked at the equations for electricity and for magnetism
and he rewrote them and he realized, you can rewrite
them in a way that looks just like the wave equation. Right,
(23:38):
So he said, oh, electricity magnetism has the same equation
as waves moving through water or waves moving through air.
And in fact, if you write it in terms of
this wave equation, you can pull out what the speed
of those waves must be. And the speed that he
pulled out from this from these equations was the speed
of light. So he had this moment of epift. He
(24:00):
must have been like in his office late one night,
rearranged these equations and realized, oh, my gosh, light is
a wave and it's a wave of electromagnetism.
Speaker 2 (24:10):
So like a light bulb turned on on top of
his head, emitting waves.
Speaker 1 (24:14):
Exactly the first appropriate light bulb ever.
Speaker 2 (24:17):
Yeah, so then that seems pretty definitive. The double slit
experiment shows it light interferes with itself. And also this
guy figured out that it's mathematically describable by sine ways
and cosine waves right.
Speaker 1 (24:35):
Right right, That light is waves of electromagnetism. Yeah, exactly.
So then it all seems really nice and tidy. But
then the particle revolution comes, right. People discover the electron,
people discover the neutron, people discovering all these particles. But
then they were doing experiments where they were shining light
onto materials and trying to get it to kick off electrons.
(24:57):
So you shine a really bright light at something and
you hope that some of the electrons in the material
absorb that light and get enough energy to be free
to run away. And so this is called the photoelectric effect.
You shine light at something and you measure the electrons
that come off. So what they saw in this experiment
only made sense if the energy of the light comes
(25:17):
in little packets rather than a continuous stream like waves.
So they turned up the intensity of the light and
they made it brighter, but that didn't increase the energy
the electrons that were coming off, which doesn't make sense
if it's a wave. It only makes sense if photons
come in little packets, so that increasing the intensity of
the light means more photons, but it doesn't give more
(25:38):
energy to any one electron because each electron can only
absorb one photon. And nobody understood this at all. This
made no sense to anybody. It was a huge puzzle.
We totally believe that it acted like a wave. We
had the double slit experiment told us it was a wave.
Maxwell's equations told us it was a wave. But then
we had the photoelectric effect, which didn't quite make sense
(25:59):
to anybody. Okay, And then Einstein said, well, what if
light comes in these little packets like you were saying before,
What if light is not this continuous stream of energy
like a wave is right, a wave is continuous stream
of energy. What if it comes in these little bits?
And that explained everything if you if you thought that
light was came in these little packets, it explained the
(26:20):
photo electric effect, explained these all these other mysteries in physics,
and that was the birth of quantum mechanics.
Speaker 2 (26:27):
Did they think that maybe it was little packets of waves?
Do you know what I mean? Like little short bursts
of ripples, you know, do you know what I mean?
Like could that explain how it's both things that run
through his brain.
Speaker 1 (26:38):
Yes, absolutely, I think that's probably the first way he
thought about it. Is like a little localized ripple, right,
like a little Yeah, that's the best way to put it,
a little localized ripple, like the way you can send
a little ripple of water, Yeah, through a swimming pool,
or something.
Speaker 2 (26:54):
Like a chirp or like a little sound burst.
Speaker 1 (26:57):
Yeah, exactly, like a little chirp. But it's strange because
you know, you can make a chirp of any size.
You can make a big one, a little one, a
long one, a fat one. But light, for some reason,
wanted to come only these in these little distinct chirps
of a specific size, and the size of those chirps
was controlled by their color or their frequency. And so
that was the birth of quantum mechanics, which we could
(27:19):
spend a whole other podcast talking about, but it was
the first clue that maybe light did come in these
distinct little packages.
Speaker 2 (27:26):
Yeah, let's talk about that, but first let's take a
quick break.
Speaker 1 (27:41):
And that's what we talked about, Like, what is a particle.
It's a distinct little package. And then here's the part
that blew my mind. Is that. Then they went back
and they did that double slit experiment again, but they
slowed it down. Instead of shining a really big beam
of light, they just shown one photon at a time, right, Okay,
because they wanted to see what's going to happen. Right,
(28:02):
if light comes in these little packets, how does that
explain the interference effect? How can light interfere if it's
a particle?
Speaker 2 (28:08):
So like, instead of like pointing the hose of water
at these two little holes and just seeing what happens
on the other side, they were throwing one droplet of
water at a.
Speaker 1 (28:18):
Time, yes, exactly. And what they expected to see was
that there'ld be no interference pattern, right, because the interference
comes from having two sources. Right. You have interference when
you have two waves that are either adding up or
canceling out.
Speaker 2 (28:31):
Meaning like a huge stream of light is going through
these two little slits. Then the two little slits act
like little sources, like little sorts of ripples, which can
cancel az exactly. But if you throw one drop at time,
it's either going to go in one slit or it's
going to go on the other slid, right.
Speaker 1 (28:47):
That's right. Yeah, And so there should be nothing to interfere, right,
So that's what they expected. But what they what they
saw blew their minds. Right. What happens if you slow
the experiment down, you send one photon at a time,
is that you'll get an interference pattern. It's just that
it builds up piece by piece. So you used to
throw one photon through and it lands someplace on the screen.
(29:08):
Throw another photon through, it lands somewhere else on the screen.
After you add up a million photons, you rebuild the
original interference pattern you saw.
Speaker 2 (29:16):
That's crazy.
Speaker 1 (29:17):
What light is a particle, but it's acting like a wave? Right?
How is that? How can that even be? Right?
Speaker 2 (29:23):
It's not just that it's like it's a particle that's
acting like a wave, as if it was in a
huge stream of other particles.
Speaker 1 (29:31):
Right, that's right, And this blew everybody's mind. And the answer,
of course, is that light is a particle. But like
every kind of matter, like every particle, how it moves
is governed by mathematics of wave equations. So every particle
carries with it some quantum mechanical wave that determines where
(29:53):
it goes. But what was happening in that experiment was
that a particle photon was approaching the experiment and then
it could either go through the left hand side or
the right hand side slit right, And because it's quantum mechanical,
it did both. It had a chance to do both,
and what was interfering was the probability to go through
(30:13):
the left slit or the right slit.
Speaker 2 (30:15):
So that's interesting. I don't think i've heard that explanation before,
that it's a particle and a wave in the sense
that it is a particle, but it moves according to
wave equations.
Speaker 1 (30:26):
Yes, everything moves according to wave equations. It's just that
the wavelength for things depends on how much energy they have.
So that was this guy, Tobroguely. He came up with
this equation. And maybe you've heard the expression to broguely wavelength.
Speaker 2 (30:39):
I've heard the expression wavelength. That seems to be a.
Speaker 1 (30:41):
Yeah, everything is wavelengths. We were making fun of that guy.
Turns out he was right, oh twist ending, No, everything
has a wavelength. Right, you can describe the motion of
anything in terms of a wave. Now, the wavelength depends
on the mass and the momentum, and for most things
(31:02):
like me or you or a cantalope. The wavelength of
its quantum mechanical wave function is tiny, and so you
can't even notice, right, the wave effects of you and
your son walking down the hallway and interfering with each
other are basically negligible. But on the scale of particles,
these wave functions interfere with each other.
Speaker 2 (31:23):
Yeah, that's a crazy thodd that, you know. We I
think people think quantum is something that doesn't affect their lies,
but quantum ideas and concepts are everywhere, right, Like you
have a sort of like a quantum superposition, or you
you're not really there. You sort of there's a cloud
of you that is.
Speaker 1 (31:43):
I'm not really here, I'm just an AI on the internet.
But that's that's differently cloud. Yeah, there is this quantum
mechanical certainly and everything.
Speaker 2 (31:53):
Guess yeah, yeah, it's just that you can't notice.
Speaker 1 (31:55):
That really blew people's minds. This concept that like, okay,
light is a particle, but it's sort of acts like
a wave. We can use these wave equations to describe it.
And you know, there's another layer to that experiment which
is even crazier, right, which is if what's interfering is
the probability to go through the left slit or the
right slit right. When the photon approaches the experiment, it
(32:18):
can go through one or the other. The interference pattern
comes from the uncertainty of which it's going to go through.
So what you can do is you can add a
little detector to one slit that like gives you a
ping if it goes through that slit right, so you
know for sure if it goes through one slit or
the other. If you do that, the interference pattern disappears.
WHOA why does it disappear? It disappears because the interference
(32:41):
only came from the interference of the possibility of the
particles to go through the left slit or the right slit,
our lack of knowledge. Once you know it goes through
the right slit of left slit, there's no more uncertainty.
There's nothing to interfere. It just goes through the left
or it goes through the right.
Speaker 2 (32:56):
It's like you're throwing a box is full of cats
that are either dead or alive, and you see what
happens on the other side. It's different if you take
a peek inside the box before it gets.
Speaker 1 (33:08):
There, exactly exactly, and no cats were harmed in the
making of this podcast, I now feel energe to point
out that's sort of where we are today, is that
we know that light is a particle and then it
comes in these little discrete packets become photons, right, Yeah,
But we also know that, like everything else, light is
(33:31):
determined by how its wave function moves. Every particle and
every object has this wave function, and how it moves
is controlled by wave equations.
Speaker 2 (33:40):
It's not like it's both a particle and a wave
and people don't really know which one it is, or
people are still confused about that. But it sort of
sounds like you're not that confused about it, right, It
sort of sounds like you know it's a particle, but
it moves around like a wave.
Speaker 1 (33:54):
Yeah, but it's still confusing. I mean, I think you could.
It's totally reasonable to say it's both. It's a particle
but it acts like a wave. Right. It's also totally
reasonable to say it's neither. It's not a particle, it's
not a wave, it's something else. It's something weird, something
totally strange, we've never seen before. It's a war we can.
Speaker 2 (34:17):
Or a pave.
Speaker 1 (34:18):
You are on fire.
Speaker 2 (34:20):
I am simple spelling.
Speaker 1 (34:25):
But that's that's a joke. But it's also serious because
sometimes we discover things which are unlike anything else we've seen,
and how do you describe them?
Speaker 2 (34:33):
I mean, we should stop using these words. We should
maybe come up with a new word to describe what
it is, because it's not, yes, not described by either
word particle.
Speaker 1 (34:42):
That's right, it's a chapel, it's a cherry apple combination.
Speaker 2 (34:45):
Yeah, let's not call it a particle or a wave.
Let's just make up anyw word that embodies these two
ways to behave that's right.
Speaker 1 (34:52):
But here we've discovered something which is different from anything
in our microscopic world. There's nothing in our world particles, wave,
little puppies that is a good analogy for what light is.
So we have to try to sort of describe it
in terms of sometimes it's like this, sometimes like this.
My personal belief is that it's it's not like anything else,
and that these are approximations. But you know, like we
(35:15):
were talking about earlier, you can be different contradictory things
like how would you describe yourself? You know, sometimes your husband,
sometimes your father, Sometimes you're a cartoonist, sometimes you're just asleep.
You know, like all these things describe you, they're contradictory.
There are different facets of who you are at your core,
and none of them define you.
Speaker 2 (35:32):
Right right, But if you don't have to have the
right label, you make up and you know that's right.
Speaker 1 (35:37):
Yes, we need a new thing, right. Light is definitely
its own weird kind of thing.
Speaker 2 (35:44):
All right, Well, until next time.
Speaker 1 (35:54):
If you still have a question after listening to all
these explanations, please drop us a 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 Feedback at Danielanorge dot com
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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