Is Light a Particle or a Wave?

What is light made of? A particle, a wave, both, neither? Little puppies?

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Episode Transcript

Speaker 1 (00:08):
Is it possible to be two different things at the
same time? Can you like dogs and cats? Can you
be a horse and a giraffe at the same time?
Can something taste salty and sweet? Can address be black
and blue and white and gold? In today's podcast, we
talked about the centuries old scientific debate about light. Is

(00:32):
light a particle or a wave? Or is it both? Hello?

(00:55):
I'm Organ and I'm Daniel. Welcome to Daniel and Jorge
Explain the Universe, in which we try to explain the
whole universe and everything in it, including light. Now, I'm
a cartoonist. I draw something called PhD comics, and I'm
a particle physicist. During the day, I smash particles together
with the large hadron collider. Yeah. Well, today on the program,

(01:15):
we're going to talk about the nature of light. 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

(01:38):
we're gonna be taking you through that history and breaking
it down it's one of the most mind blowing questions
in human scientific history. 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? Is light a particle or
is it a wave? Here's what people had to say.

(01:58):
Do you think light? Is it out of particles or waves?
Or both? Are neither? Photos? Yeah? Photons? Yeah, so you
think it's a particle. I think it's waves. Yeah, it's both,
I think because it moves like a wave, but it
also has properties of a particle and there's nothing saying

(02:19):
it can't people. Okay, um, light, I think they're made
of wavelength Yeah all right. Well, um, it's interesting because
I think all of the answers are right or none
of them, or or both. Yeah. Well, it seems like

(02:40):
a lot of people reflected the fact that there is
a controversy like that. You know, it's not really well
described by either those. Some people went all in, you know,
like it's a photon or it's a wave, or it's
a wave length right, Yeah, that was my favorite one.
I want to be a wavelength like I've heard of
this word. It sounds really cool and scientific. I'm just
gonna throw it out there, that's right. Yeah, maybe I

(03:02):
get some points. We award no points, people, no points.
That's right. There's no prize, your prices. You get to
be on our podcast, and maybe we even make fun
of you. 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
needed that there's some sort of duality there, something weird

(03:24):
going on. 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 to 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

(03:44):
what is this stuff? What is it made out of? Yeah?
I mean, like, what are we paying you for, Daniel,
if not to figure these kinds of questions out. I
was just about to figure out what light was when
you called and said it's time to do this podcast.
Totally about your thin train of thought there, that's right,
reflect on that for a minute or but no, Yeah,

(04:06):
I'm a California taxpayer. Part of my salary goes to
being your salary, like you know, one million of a present.
That's true. Yeah, so you're you're saying you did pay
taxes very against another topics on air Daniel. Anyway, So
that's an interesting question, like is light a waiver particle?

(04:28):
And it's weird that we don't know, um, but maybe
let's bring 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 is
just like just like brightness, right. Yeah. The thing to
understand here is that we try to describe light in

(04:48):
terms of things we know, and that's what science is. Right.
You see something weird and new and you wonder, is
it like this other thing I know? So we've observed
different kinds of phenomenon in the world. Like you see waves, right,
you go to the beach, you see waves and water.
You drop a rock in a small puddle. You see waves.
We know what waves are, and we see different phenomena.

(05:09):
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.
You can describe it with like equations, right, yeah, wavy equations,

(05:31):
that's right, very solid unwavy physics to describe waves. And
there's a lot of science that's gone into understanding ways.
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?
So we need like we you know, we know about light,

(05:54):
but we want to know how it behaves and what
makes it work. Yeah, and just on a more general level,
you try to see something new you are 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. You're like, it's a chapel. It's a chapel.
How has nobody ever invented that? The cherry apple chapel?

(06:16):
Oh my gosh, somebody, if our lawyer is listening, get
on that right away. Copyright that idea chapel dot com.
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 in into one of the existing categories. So we
set a new about light. It came from the sun.
You know, if you light a fire, it spreads out

(06:38):
into a room. And so we're like, what's going on?
Like what? Um? What best describes how this light you know,
comes from a source and bounces off the walls and stuff?
Exactly exactly, that's the question. And so we've seen things
like waves, So what do we mean when we say
a wave? Like, how could a light be a wave? Well?
How can anything be a wave? Yeah? How can anything

(06:58):
be a wave. A wave is a funny thing because
it's not a thing itself. It's a property of some medium.
Like it's like a ripple on something. 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 waving their hands, right,
Or like a sound wave is just like air molecules

(07:19):
kind of bumping forward. That's right, yeah, exactly. Or a
wave in the in the ocean, it's just it's an
arrangement of the water, right, It's it's the way the
water gets compressed and then stretched out and compressed and
then get stretched out. So that's the important thing about
a wave is that it moves in this way through
a medium. Okay, so that's a wave. It's like a propagation,
it's like a ripple through something. But then so then

(07:41):
what what would you call a particle? Particle is different
than that. A particle is different than that, and it's
a totally different kind of thing, you know, and uh,
to be a particle physicist, it's kind of odd, but
the concept of a particle is not that really well
to find you know, um, But when I think of
a particle, I think of taking matter and breaking it
down to its smallest pieces. Like, if something is made
out of particles, it means that at its smallest level,

(08:03):
it's made out of this these little bits that can't
be chopped into smaller bits, and that they're localized. They're
like um, small and contained. Right. If 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, Like it would take something and then you
smash it to bits and just keep smashing, and at

(08:24):
some point you're going to get to these little like
baby balls or like little tiny pellets that you can't
break down anymore, that's right. Yeah, It's like seeing a
picture on your 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. Um.
So figuring out this something is made of particles means

(08:46):
that there's made of these these little bits that are
not um connected to each other, right, they're separated. So
a wave and a particle in nature are totally different
kinds of things, right Now, water of course is made
of particles, but can have waves in it, right, But
I think maybe maybe what's important here is that, you know,
particles we tend to think of as little tiny bits.

(09:06):
They can bounce around, right, and like go in a
straight 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. That's exactly the right way
to say. It is a discrete little package. Right. So
things that have made of particles we think of as
being discrete little bits um and they they've broken up

(09:27):
into these little little pieces, and you're right, they move
in straight lines. Right, Like you throw a rock, your
your roll a smooth ball across the surface, you expected
to move in a straight line. So that's kind of
what we mean by a wave and a particle, that's right. Yeah.
And so the question is is like is light a
ripple on a medium? Is that what light is? Or
is it like actually little things and move around in space? Right?

(09:50):
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, right, And
those are two pretty different picture is a reality? Right? Yeah?
The 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,
that's right. Yeah, it's like you can't be a Democrat

(10:12):
and a Republican, you know, just you have to pick one,
you know. Yeah, if you can be or you could
be neither as opposed, Um, you shouldn't be both though, Yeah,
that would be a violation of some some election law
not recommended to violate election that's right. Yeah. So, speaking
of political shouting matches, this one, this historical scientific shouting match,

(10:35):
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
us that seems macroscopic is made out of tiny little things,

(10:55):
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 are made out of stuff,
then water is also made out of particles, and maybe
even light is made out of particles. You know. It's
at the time seemed like a totally a crazy reach.
And that makes sense, right, because light seems to go
in a straight line. It seems to bounce off of things.

(11:17):
So why couldn't light just be like a little tiny
little pellets that bounce around the room and then eventually
hit your eye and then that's how you see something. Yeah,
it certainly seems to have some of those particles like properties, right,
it moves in straight lines. Uh, it certainly would be
going really really fast. At the time, people thought that
light traveled instantly, right. They thought that light um instantaneously

(11:37):
went from like the sun to the earth, or or
um if you started a fire, that the light would
immediately illuminate the room. Now, we of course know that
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

(11:58):
a quick break. So, initially we thought light was or
the Greeks thought that light was a particle, right, And

(12:18):
I think we have to qualify that because it makes
the Greek sound really smart. Just come up with this
idea of atoms and all that stuff, and you say
this before you're really really down in the Greeks. Well,
I think people give the Greeks too much credit for that, because,
as I've probably said to you before, Um, 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.

(12:39):
But like, if we're going to do some accounting, let's
also remember the n that were totally off base, you know,
and and give them credit for those. Yeah, find that
Greek we thought life was just little puppies, and be like, see,
you guys also thought they were puppies. You can't be
that smart, that's right. But it's a cool idea. So
give them credit for having that idea and know what

(13:00):
they were smoking when they came up with it. But
I'd like to figure out where to find something. Um.
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 early scientists, not
just philosopher, but a scientist, back in the day when
you know, science really was part of philosophy, and he
thought that light was waves. What made him think it

(13:23):
was waves? 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. Um,
it just made more sense to him for light to
be like these wave like disturbances, which kind of makes sense, right,
Like if you have a speaker in a room emitting

(13:43):
sound waves, Um, it's not that different from like a
light bulb in the middle of the room emitting light
all around it. 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 in science, we
call that refraction. You know, with light changes from going

(14:04):
through air to glass, it bends in this weird way.
That's something that's very common for waves, right, And a
particle wouldn't bend inside of a lens. No particle, that's
definitely a wave like behavior, yeah, 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

(14:26):
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
in water waves obviously it's the water. But if light
is a wave, then what is waving? Meaning? Like, if
light is a ripple, what is it a 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,

(14:46):
because a wave is just a description of some other
form of matter rippling, right, It couldn't just be like
a stuff that we can't see. Yeah, and so you
have to invent some stuff that we can't see, right,
So 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,

(15:06):
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 Decart, you know, 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

(15:27):
give it a name and maybe we'll be right, and
then we'll be famous forever. Isn't it is that different
under either 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
give it a name as a placeholder so when later
people do the hard work of actually discovering it will

(15:48):
still get credit. So it was a particle. Light was
a particle, then it was a wave, and then what happened, Well,
then Newton came along, right, And Newton is 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

(16:11):
did a lot of experiments with optics. He did he
studied prisms, 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, um, and that makes it bend.
But you know, that's not really an explanation. That's just

(16:32):
sort of like a I don't really understand it. But
maybe it's something like this, like, if light is a particle,
why does it split into the rainbow kind of thing? 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

(16:52):
argue about them. Right. A lot of a way scientific
disputes he used to be resolved was people would argue
about it and then say, well, that makes no sense,
so it can't be true. Um. And we know now,
of course, that the universe doesn't always make sense to us.
What's real doesn't isn't necessarily the things that we would
have accepted as true or or accepted as a reasonable
way to describe the universe. But you know, if that's

(17:13):
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 just sort of used to argue for an explanation
that made sense to them. Right, Well, it was kind
of hard for them to build a particle collider, right,
that's right, yeah, exactly. They they didn't have the massive

(17:34):
government funding to do that. These were men of leisure
studying science in the spare time. In fact, it was
called like natural philosophy, right, It wasn't called science at
the time, wasn't. Yeah, that's right exactly. Science All science
grew out of philosophy. Um, it was called these folks
were natural philosophers. But you know, later on then people
started doing experiments. And there were a bunch of French

(17:55):
guys um who did a bunch of experiments, and some
of some English folks, and they were studying how life
behaved and refraction and reflection, and they saw it doing
these things, and they thought, there's no way Newton's right,
this has to be a wave. Um. You know, they
saw things like interference patterns. Right. Interference patterns is when
you have two waves and sometimes one is rippling up

(18:17):
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 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,

(18:38):
then they constructively interfere to get a double wave. If
they're both rippling down at the same time, they constructively
interfered to get a double down wave. If one is
rippling up and the others rippling down, then they then
they cancel each other out right, And so you would
see no light, yeah, exactly, And so you can do
this kind of stuff in you know, in your bathtub,

(18:59):
you can see interfe Auran's patterns, um. 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 are low, in some areas where there are no waves.
And so as you say, if you do with light,
then you see these patterns of dark and light, these stripes,
And you couldn't do that with particles, right, like a
particle wouldn't cancel another particle. Yeah, there's no way to

(19:21):
explain that with particles. People thought, well, look, um, 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. In fact, there's even famous
cases where they said, well, you know, um, if light
is a wave, then you know, if you set up
this various experiment, you would get this crazy effect and
so that's absurd and so it definitely can't be true.

(19:43):
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 experimental experimentalism, right, like, go and check
the data, Go and actually get some data and see
what the universe tells you. Yeah, Like you're like a
doughnut can't possibly be a croissant at the same time,

(20:04):
But it turns out that you can bake something called
the cronut. Yeah exactly. I think that's a big debate
in pastry science still though. And is it from a
donut like that's like a croissant or is it a
croissant that's like a donut. Yeah. I'm getting my degree
and I'm particle baking. Yeah, the large Pastry Collider. I'm
looking forward to the construction of that project. But that's

(20:29):
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. Yeah, exactly.
And and that was the experiment, it was called the
double slit experiment, the one that really convinced people that

(20:50):
light is a wave because they shone a strong light
and they had just two little narrow slits which act
like sources like slapping your hands in the bathtub water
and and on the screen behind it, they saw these
interference patterns, right, is that you could definitely only get
if light was a wave. And so that was the
early eighteen hundreds, and everybody was absolutely certain light was

(21:10):
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. But how did they explain all
those particle experiments? 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

(21:32):
of particles and the chemistry was getting warmed up, and
you know, people are starting to think about atoms and
molecules and stuff, um, but they hadn't really seen any
actual particles yet. It was decades later when the electron
was discovered, um, 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

(21:52):
experiments and saw light doing all this wavy stuff, and
they were sure that light was a wave. Now that
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. Yeah, that's a good question.
I wonder if people thought lights a wave. Maybe we're
a wave too, right yeah, or like everything is just

(22:13):
like a wave. Yeah, probably not because um, 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. Um. Even before the
discovery particles, though, there was a huge advance in the
theory of light, which was a Scottish guy named Maxwell.

(22:33):
He was working on electricity 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. Um.
But he wrote them down in this way that looked
like the mathematics of a wave. We have this equation,

(22:56):
it's called a wave equation, and it describes how waves
moved medium I meaning like it could be described by
the equations that looked like sign waves and co sine waves, right,
I mean, just in case anyone remembers high school math,
that's kind of that's kind of what we mean by
mathematical equations that you can describe it as a sign
wave ver cosine waves. Right, that's right. Yeah, the solution

(23:17):
of these equations are sign waves and cosine waves. These
are differential equations to describe how things move through the medium.
And if things follow these equations, then their waves. Right.
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, So he said, oh, electricity magnetism

(23:40):
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 a pi any. He must have been like

(24:01):
in his office late one night, rearrange these equations and realized, oh,
my gosh, light is a wave and it's a wave
of electromagnetism. So like a light bulb turn on on
top of his head, emitting waves exactly the first appropriate
light bulb ever. Yeah, So then that seems pretty definitive um.

(24:23):
The double slid experiment shows it light interferes with itself.
And also this guy figured out that it's mathematically describable
by sign ways and cosign waves right 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,

(24:47):
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. 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 right to run away.
And so this is called the photo electric effect. You

(25:08):
shine light is 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 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

(25:29):
it's a wave. It only makes sense if photons come
in little packets, So then increasing the intensity of the
light means more photons, but it doesn't give more energy
to any one electron because each electron can only absorb
one photon. And nobody understood this at all. There's made
no sense to anybody. Was a huge puzzle. We totally

(25:49):
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 photo electric effect, which didn't quite make sense
to anybody, Okay, and then Einstein said, well, what if
light comes in these little packets like you were saying before.
But if light is not this continuous stream of energy

(26:10):
like a wave, is right, a wave is continuous stream
of energy. What if it comes in these little bits
and um? And that explained everything if you if you
thought that light was came into little packets, it explained
the photo electric effect, explained these all these other mysteries
in physics, and that was the birth of quantum mechanics.
Did they think that maybe it was little packets of waves?
Do you know what I mean? Like little short bursts

(26:32):
of ripples? You know, you know what I mean, Like,
could that explain how it's both things that run through
his brain? Yes? Absolutely, I think that's probably the first
way he thought about it. Is like a little localized ripple, right,
like um, 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 um through swimming pool,

(26:54):
or something like a chirp or like a little sound burst. 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, along 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

(27:15):
their color or their frequency. And so that was the
birth of quantum mechanics, which we could spend a whole
other podcast talking about um. But it was the first
clue that maybe light did come in these distinct little packages. Yeah,
let's talk about that, but first let's take a quick break.

(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 it light comes in these little packets, How does
that explain the interference effect? How can light interfere if
it's a particle? 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 time, yes, exactly, and
they what they expected to see was that there would

(28:22):
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,
meaning like a huge stream of light is going through
these two little slids. Then the two little slits act
like little sources, like little sorts of ripples, which can
cancel that exactly. But if you throw one drop at time,

(28:43):
it's either going to go in one slid or it's
going to go on the other slid. Right, 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 will get an interference pattern. It's just that
it builds up piece by piece. So used to throw

(29:04):
one photon through and it lands someplace on the screen.
Through another photon through lands somewhere else on the screen.
After you add up a million photons, you rebuild the
original interference pattern you saw. What Light is a particle,
but it's acting like a wave? Right, How is that?
How can that even be? Right? It's not just that
it's like it's a particle that's acting like a wave,

(29:27):
as if it was in a huge stream of other particles. 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

(29:49):
carries with it's some quantum mechanical wave that determines where
it goes. But what was happening in that experiment was
that a particle photon was approaching the experiment and then
you 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,

(30:10):
and what was interfering was the probability to go through
the left slit or the right slit. So that's interesting.
I don't think i've heard that explanation before that it's
a it's a particle and a wave in the sense
that it is a particle, but it moves according to
wave equations. Yes, everything moves according to wave equations. It's
just the wavelength for things depends on how much energy

(30:32):
they have. So that was this guy to Brogueli. He
came up with this equation and maybe you've heard the
expressions and Brogueli wavelength. I've heard the expression wavelength. That
seems to be everything is wavelength. We were making fun
of that guy. Turns out he was right, twist ending, No, um,
everything has a wavelength, right, Um, you can describe the

(30:56):
motion of anything in terms of a wave. Now, the
wavelength pends on the mass and the momentum and for
most things like me or you or cantelope, the wavelength
of its quantum mechanical um wave function is tiny, and
so you can't even notice, right, there's the wave effects
of you and your son walking down the hallway and
interferring with each other are basically negligible. But on the

(31:19):
scale of particles, these wave functions interfere with each other. Yeah,
that's a crazy thodd that you know. We I think
people think quantum is something that doesn't affect their lives,
but quantum ideas and concepts are everywhere, right, Like you
have a sort of like a quantum superposition, or you

(31:39):
you're not really there, you sort of there's a cloud
of you that I'm not really here. I'm just an
a on on the internet. But that's a that's definitely cloud. Yeah,
there is this quantum mechanical certainty and everything. Yes, yeah, yeah,
it's just you can't notice. That really blew people's minds.
This concept that like, okay, light is a particle, but
it's so acts like a wave. We can use these

(32:02):
wave equations to describe it um. 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. Then
when the when the photon approaches the experiment, it can
go through one or the other. The interference pattern comes
from the uncertainty of which is going to go through.

(32:24):
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.
Why does it disappear. It disappears because the interference only
came from the interference of the possibility of the particle

(32:45):
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 goes through the right. It's it's like you're throwing
boxes full of hats that are either dead or alive,
and you see what happens on the other side. It's

(33:05):
different if you take a peek inside the box before
it gets there exactly exactly, and no cats were harmed
in the making of this podcast. I now feeling hers
to point out, Um, 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,

(33:28):
But we also know that, like everything else, light is
determined by how it's wave function moves. Every particle and
every object has this wave function and how it moves
is controlled by wave equations. It's not like, uh, it's
both 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

(33:48):
about it, right, It sort of sounds like you know
it's a particle, but it moves around like a wave. Yeah,
and 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

(34:10):
totally strange we've never seen before, or a pave you
are on fire. The simple spelling. But 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

(34:32):
do you describe them? 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 not
described by either word particle. That's right, it's a chapel,
it's a cherry apple combination. Yeah, let's not call it
a particle or a wave. Let's just make up a
new word that embodies these two ways to behave. That's right.
But here we've discovered something which is different from anything

(34:55):
in our macroscopic world. There's nothing in our world particles, waves,
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
were talking about earlier, you can be different contradictory things

(35:17):
like how would you describe yourself? You know, sometimes your husband,
sometimes your father, sometimes your cartoonist. Sometimes you're just asleep.
You know, like all these things describe you their contradictory
There are different facets of who you are at your core,
and none of them define you right right, But if
you don't have to have the right label, you make
up and you do right. Yes, we need a new thing.

(35:39):
Right light is definitely its own weird kind of thing.
All right, Well, until next time. 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

(36:00):
can find us at Facebook, Twitter, and Instagram at Daniel
and Jorge That's one word, or email us at Feedback
at Daniel and Jorge dot com.