How do we know the photon is real?

What is a photon and how do we know it exists? Particle Physicist Professor Daniel Whiteson explains the photon.

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Episode Transcript

Speaker 1 (00:09):
Hey, they're listeners. You're into physics. So here's a tribua
question for you. Do you know who won a Nobel
Prize for relativity? That might feel like it's your question
because you want to say Einstein, because they think relativity
and Einstein will I'll tell you it's not Einstein. Now,
maybe you're scrambling through your mind to think about the
names of other physicists. You might know. How many physicists

(00:31):
can you name? Anyway, you got Einstein, you got me. Well,
I'll give you a clue. It's neither Einstein nor me.
So who was it? Right? Well, some folks wanted for
proving that relativity was correct. There were Nobel Prizes for
gravitational waves and for binary pulsars. But the answer is

(00:51):
that nobody wanted for relativity. Nobody who came up with
this incredible earth shattering idea that now frames all of
modern physics won the Nobel Prize for it. But you
might be thinking, hold on, didn't Einstein win a Nobel Prize?
And he did, but he wanted essentially for quantum mechanics.

(01:26):
Hello everyone, I'm Daniel. I'm a particle physicist, and I'm
the co host of this podcast together with Korge cham
who can't be here this week, So you're just listening
to me talking about the joys of physics and trying
to simulate Jorge in my mind. Every time i'm talking,
I'm thinking, here's what Jorge would say. At this moment,
I'm trying to interject a little Jorge Ism for you,

(01:49):
since we all miss him and you were listening to
our podcast, Daniel and Jorge Explain the Universe, a production
of I Heart Radio in which we zoom all around
the universe and try to find interesting, fascinating, cool little
nuggets of physics that would blow your mind, but take
them apart so they don't actually explode your head and
cause your brains displatter anywhere. Instead, we want to smoothly

(02:13):
and calmly insert them into your mind so you understand them,
so you can talk to your friends about them, so
you can actually comprehend these amazing, wonderful facts that we
have learned about the universe, and also understand all the
things we don't know about the universe, which is my
favorite part of physics, and that's why Jorge and I
wrote the book We Have No Idea, A Guide to

(02:35):
the Unknown universe, which takes you on an amazing tour
of all the big and basic questions about the universe
that we still have no idea what the answers are.
And on the podcast, we've been doing something fun, which
is taking a little tour of how we know what
we know, and specifically how we know anything about particle physics.
It's still incredible to me when I look around at

(02:56):
the world that everything is made out of these tiny
microscopic object so we can't see that we've taken thousands
of years to even discover that they exist. Yet we
have this really complex, really elaborate, really amazingly effective model
of what's happening down there at the microscopic scale, all
these tiny quantum particles interacting and zooming around. Physicists can

(03:18):
do calculations to tell you exactly what's going to happen
when this particle hits that particle. It's really incredibly complex
and mature. Though of course we have lots of questions,
but I think a lot of times people think of
this as sort of like an idea, something people came
up with a description of the universe. But it's critical
that everybody understand that this isn't just an idea that

(03:40):
came out of our heads. This is something born out
of desperation. This is our attempt to grapple with the
weird and bizarre and counterintuitive and frankly mind blowing experiments
that have shattered our perceptions of reality. We thought the
universe worked a certain way. We thought everything was smooth,
You could cut objects as many times as you wanted

(04:03):
to infinitely small pieces, but you can't. We thought the
universe was deterministic, that if you did the same experiment
twice you would get the same outcome. Right, that would
make sense, But it's not. It's fundamentally random. And the
core of that is particle physics, because it attempts to
describe the entire universe in terms of these tiny, weird,
nondeterministic little particles, in terms of these tiny, little, weird,

(04:27):
nondeterministic particles that seem to follow rules that just do
not describe the world that we are familiar with. So
my goal is to take you on a tour of
those experiments, the ones that change the way we think
about the universe, that showed us that the universe is
different from what we imagined, because it's not just the
final idea that you want that I want you to understand.
I want you to know what the evidence is. How

(04:49):
do we know what we know? Now? Recently we talked
about the discovery of the first particle, the first experiment
that revealed this incredible revelation that the Uni verse is
made out of time to little dots. And so today
we are continuing that tour. We are talking about how

(05:12):
do we know the photon is a thing? You're familiar
with photons? To you, photon is a very normal word
you hear bandied about here, talked about but how do
we know that photons are there? How do we know
that light is made out of photons that is chopped
up into these little pieces that can't be cut down
even further. What is the actual experiment that proves to

(05:32):
us that photons are a thing, that light is not
just electromagnetic waves, but it does these other weird things
that you have to give it particle status to explain. So,
as usual, I was wondering how many people out there
know why we think the photon is a thing, Why
we don't just think about light as electromagnetic waves. So
I walked around the campus of U see Irvine and

(05:53):
accosted a bunch of friendly and unsuspecting students, and I
asked them, do you know how the photon was discovered?
Do you have an idea of why we think the
photon is a thing. So before you listen to these answers,
think to yourself, or pause the podcast, or just take
a moment. How do you know photons are a thing?
Are you just believing physicists when they tell you, or

(06:14):
do you know what the data says? I'm not entirely sure.
I feel I shouldn't have that. I I'm sorry, I
probably should know. But it was the slit experiments, wasn't it.
And they projected a laser beam onto a single slit
or double slits and it diffracted the beam and that's

(06:35):
how they discovered it. Particle wave duality. Yeah, the photo
electrical facts. You shone a light on a metal and
then the metal you cross. Uh, you start for Einstein ninth.
I don't remember the year. Yeah, I don't remember who
did it, but I remember that you shine a light
on a metal, do you give the electron enough energy
to start conducting it's particles? Well, we know it's a

(06:59):
wave because it travels through vacuum, and we know that
it's a particle because you can transfer energy from it. Right, Yeah,
it has it hasn't defined momentum, even though it has
no mass. But the slit experiment double slip one showed
that it was a wave, like a single slit showed
that a particle. Well, it's not necessarily a particle. It's

(07:25):
both a particle and a wave. And for a really
long time we thought it was just a wave. But
I believe the first time we figured out that it
was a particle had to do um exciting metals to
release photons and realized that the distributions were discreet. So

(07:49):
I was really impressed with these answers. A lot of
understanding here that photons are particles and that they're part
of this larger idea of light being a wave and
a particle. Even some discussion of the double slit experiment,
which I'm dying to get into in a future podcast
and talk all about the amazing facts of quantum mechanics.
But the double slit experiment actually shows you that the

(08:10):
photon is a wave. But there was somebody out there
who talked about the photoelectric effect, and that's the key
that was the experiment that showed us that photons were
a thing. But before we talk about the crazy experiment
to prove that quantum mechanics is our reality, that showed
us that the universe is probably sliced up into little
bits and not infinitely smooth. Let's set the stage. Okay,

(08:33):
let's you remember how people thought about light, and to
get the context of the story, you have to rewind
all the way back to Isaac Newton. Isaac Newton, of
course very famous not just for the cookies, but also
for his discovery of his theory of gravity, which unified
motion of objects here on Earth with motion of objects
in the heavens. Really gave us access to the whole
universe to imagine, Wow, maybe physics can actually describe things

(08:58):
not just here in front of us, but out there
in the universe. Those are things out there that follow
laws of physics. Incredible accomplishments. But Newton also also made
amazing discoveries in the field of optics. He spent a
lot of time with lenses and with prisms, and he
was convinced that light was a particle, and he thought
a lot about how light traveled. He saw it moving

(09:19):
in straight lines, except when he was bent by these
lenses and he was convinced that light was a particle.
And because he was a genius and he's a staggering
influence on the field of physics, people listen to him
and rightly so. And for hundreds of years people were
convinced that it was a particle, even though other folks
had really nice theories of light as a wave. And

(09:39):
it wasn't until the eighteen hundreds when people started observing,
like doing things that particles couldn't do, that they had
to adapt their mindset. And that's the key. There you
see experiment rearing its uncomfortable head again saying, oh no, no,
you thought you understood the universe. You have an idea
in your mind, you have a mental model of how

(09:59):
this is working, but it can't describe what's actually happening.
And that's why I'm an experimentalist. That's why I think
experiment is the place to be, because experimentalists are the
ones who make the discoveries. They are on the forefront
of knowledge. They're out there exploring the universe, discovering things
that don't make sense. Theorists, of course, do an incredible job.

(10:19):
They tied all together they understand they predict future phenomenon.
But for me, the bit about physics that's wonderful is
the experimental side, is making those discoveries, is asking nature
a question and demanding an answer, pinning nature in a
corner so that nature has to tell you is the
universe this way or that way? And so the thing
that told people that photons couldn't just be a particle

(10:42):
were wave like effects, things like interference. And you're familiar
with interference, maybe you have noise canceling headphones. Now is
canceling headphones work via interference? Sound is a wave. It's
a shaking of air, and the air comes towards your head,
and if you can create waves shake in the other
direction at the same time, they basically cancel out those

(11:04):
waves that are coming in your head. So sound canceling
headphones are proof that sound is a wave because they
can do this wave like thing that particles just cannot do.
In the same way, people saw light behaving in a
way that could only be described by a wave, and
so you had interference effects, and you had all sorts
of theories sort of built momentum until you get to
James Clerk Maxwell his incredible genius pulled together lots of

(11:27):
ideas about electricity and magnetism into his unified theory of
electromagnetism that described light as oscillations of electromagnetic fields. And
when he pulled all these equations together, he saw the
equations fit together in a way to describe the oscillations
of electromagnetic fields moving at a certain speed, a speed
he could calculate, and that speed came out to be boom,

(11:48):
exactly the speed of light. What a moment of epiphany
that must have been for him. He pulls together all
of this knowledge, he gets new insight, he looks at
the world in a new way, and then it pops
out this obvious, amazing prediction that light moves at this
speed of light, this number that we had already known.
So what amazing confirmation for him. So that was dominant,

(12:09):
and people thought, okay, well, light's definitely a wave, right,
does all these wave like things we have this beautiful theory,
it's got to be a wave. Okay. So if light
is a wave, right, we think about it in terms
of electromagnetic radiation. It's just the waving of the field,
just the same way sound is waving of the air.
Different kinds of waves, but that doesn't really matter. And

(12:29):
the key thing to understand if light is just electromagnetic radiation,
it's just oscillations of electromagnetic fields. That means they can
have any value. You can just turn up the intensity
of the light right to make the light brighter. What
happens when you make light brighter in the wave theory
is to just increase how much the waves are shaking, right,

(12:49):
They're just shaking more so they have more energy. So
that's sort of the classical theory of electromagnetic radiation of
light as just these wiggling of the waves that can
have any value at all. You can turn it up,
you can turn it down, just the same way you
can make music louder or softer, and you can have
essentially any value to that volume. So that was the
sort of prevailing thinking at the time before the photon

(13:10):
was discovered. But then, of course, an experiment came along
that couldn't be explained, and experiment came along that just
had answers that did not make sense in the wave
theory of the universe. So we'll dig into what that
experiment was and how it worked. But first let's take
a quick break. So we're back and we're talking about

(13:43):
why we think the photon is a thing. What experiment
back there in history convinced people that photons had to
be a particle. And remember that in the context of
this experiment, light was thought to be a wave. It
was thought to be electromagnetic radiation, just this oscillation of
the fields. Somebody essentially shouting in the electromagnetic spectrum, and
then came along this crazy experiment. The name of the

(14:05):
experiment is not critical, but what it studied with something
called the photoelectric effect. Essentially, what you're doing here is
you're shining a really powerful beam of light at some surface.
And a surface, of course we know now is made
out of atoms. And what they observed is that if
you shone light at a surface, electrons would boil off
of it. You could pull them off by putting them

(14:27):
in an electric field, and then you can measure their energy.
People thought, oh, that's cool, we can boil particles off
of the surface by shooting light beams at it. What
would a physicist do in this scenario? She would probably think, Oh,
let me see what I can do and what happens
that I turn it up? What happens if I turn
it down? What happens? So I made the light purple?
What happens if I make the light green? Right, a

(14:47):
physicist would want to know if the results make sense
under all conditions. Sure, maybe we can understand how this
works in this scenario. But can we push our limits
of knowledge? Can we find some wrinkle, some corner of
the space in which it doesn't make sense? That's right,
experimentalists are always just trying to spoil everything for theorists.
That's not true at all. Actually, as Jorge would say,

(15:08):
because every time experimentalists do something and find a result
that doesn't make sense, that's an amazing clue. That's the
clue the theorists need to come up with a new
theory of the universe. Anyway, back to the photoelectric effect.
What happens when you shine light at the surface Electrons
come off. Now, if you're thinking of light as electromagnetic waves,

(15:29):
then what should happen if you turn up the intensity.
If you turn up the intensity, then electron should shoot
off with more energy. Because under the classical idea, the
original idea of light is a wave. Then if you
turn up the intensity of the light, the strength of
the light beam, then you're putting more energy. Is just
electromagnetic waves oscillating with more energy, and so there should

(15:51):
be more energy there to dump into the electrons, and
so the electron should boil off with more energy, and
there should be no dependence on the frequency. You can
just get the energy out of the electromagnetic waves. It
doesn't matter how fast they're shaking, as long as the
energy is there. The energy they're depending just on the intensity.
So that's the idea. They thought, if we turn up

(16:12):
the intensity of the light, we make the light brighter,
then you should get electrons coming off with more energy,
and there should be no dependence on the color. All right,
So that's what they thought makes perfect sense. And then
because their experimentalist, because they actually want to go out
and explore the universe, not just do thought experiments in
their head the way the old Greeks did, they went

(16:32):
out and they actually tried this, and what they found,
of course, blew their mind. Where they found is two
things that didn't make any sense at all. First of all,
the energy the electrons that came off the surface didn't
depend on the intensity at all. You could turn up
the intensity and the energy the electrons wouldn't change. You

(16:52):
could turn down the intensity and the energy the electrons
wouldn't change. Weirdly, if you turned up the intensity got
more electrons. You didn't get any electrons with more energy,
but you got more electrons boiling off. And if you
made the light dimmer, if you turned down the intensity again,
the energy didn't change, but the number of electrons dropped.

(17:13):
And this didn't make any sense at all in the
classical idea, if light is just a wave, if it's
just oscillation of the electromagnetic field, then it should depend
on the intensity, but there was no dependence on the
intensity at all. Instead, changing the intensity didn't change the
energy the electrons coming off. It only changed the number
of electrons we saw. So then they said, ah, that's weird,

(17:35):
So let's try changing the frequency of the light. So
they go from blue light down to red light and
back to purple light and just to see, and they
found that the energy to electrons, weirdly, did depend on
the frequency of the light. At higher frequencies, the electrons
had more energy, and at low enough frequencies you wouldn't
get any electrons at all. So this made no sense

(17:56):
to anybody. People who are thinking, who are confident that
light with just electromagnetic radiation could not explain either of
these effects. One the fact that the energy to electrons
didn't depend on the intensity of the radiation, which made
no sense because they thought these are just classical waves
and the intensity means more energy, so why aren't we
getting more energy out of the electrons? And number two

(18:18):
that the energy the electrons coming off did depend on
the color of the light. But it made no sense
to people because people were thinking about light as waves.
Now there was somebody thinking about light in other terms,
and that was Plunk. Plunk was studying a totally different problem,
another unsolved question in physics, which had to do with
black body radiation, which we'll talk about in another episode,

(18:42):
and he was trying to solve that problem and he
just couldn't. He was trying to explain why we didn't
see in the lab what we expected to see based
on the theory, and to solve his problem he had
to come up with a crazy idea. He said, well,
I don't know why, and I can justify this at all,
But if I assume whom that light comes in little

(19:03):
packets of energy that you can have like zero or
one or two little bits of energy, but you can't
have into your numbers in between. Then it solves my problem.
And for him it was sort of a mathematical thing
because like, I'm trying to do this calculation, it's not working.
Nobody can figure it out. Oh look if I make
this totally unjustified assumption that my calculation works and explains

(19:24):
the data, and that's cool. That's a totally valid way
to do theory and to do physics. And then you
got to go back and say, well, what does that mean? Right?
And it was Einstein who put it together. Einstein heard
about Plunk's idea, he said, that's fascinating, and he heard
about the photo electric effect and said, oh, interesting puzzle,
and he put them together. And so Einstein, who never

(19:45):
actually won the Nobel Prize for relativity, did win the
Nobel Prize later for putting these two ideas together. And
though he didn't do the experiments for the photo electric effect,
and he also didn't have the original idea to break
light down into little pieces, he just put the idea
in the right place to solve the problem and explain
this experiment. All right, So let's talk about how the

(20:07):
idea that photons might be little particles, little packets of
energy explains this experiment. But first, let's take another break.

(20:28):
All right, we're back and we're talking about why photons
are a thing. We reminded ourselves why people originally thought
that photons were waves, and then we talked about the
photo electric effect. This experiment with a weird result and
a result that could not be explained using classical theory
that could not be understood if you thought about light
as a wave. So how do we explain the photo

(20:50):
electric effect? How do we understand the weird results of
this experiment just by saying that light comes in little packets?
All right, Well, Einstein said, I'm gonna assume that likes
in these little packets, and that the energy inside one
packet is proportional to the frequency. That means that higher
frequencies things like blue, have more energy than photons at

(21:11):
lower frequencies, things like red. What that means is, if
you want more energy in your photon, you need purple
er photons. If you want less energy in your photons,
you need redder photons. His microscopic understanding, what's happening is
you have this surface of metal and it's got electrons
in it, and electrons need a certain amount of energy
in order to escape. They're bound to their atoms. They're happy,

(21:33):
they're they're circling the nuclei right, They don't necessarily want
to leave. In order for them to leave, they have
to get a certain minimum of energy. So what happens
when a photon comes and hits the surface. While photon
hits the electron and either it has enough energy to
kick the electron off or it doesn't. If it doesn't know,
electron is kicked off. And what that means is that

(21:54):
the frequency of the light has to be right high
enough frequency to have a high enough energy to kick
off any electrons. And that explains why when they turned
the frequency down on the light, no matter how bright
it was, if they turned the color down to deep
deep red, they just didn't see any electrons coming off.
And they couldn't explain that with their classical theory. With
their classical theory, they thought, well, lights a wave, the

(22:16):
color doesn't matter. We can make it red. As long
as we make it really really bright, electron should still
come off. But they didn't. And this theory explains why.
Because the photons in little chunks, and each electron can
only absorb energy from one photon at a time, and
that's the critical idea. You can only interact with one
photon at a time, so you if the photon doesn't

(22:39):
have enough energy because it's too low frequency, it's too red,
then it just can't get you out of your atom trap.
And you have there are other photons coming down the
pike if you have a really really intense beam, but
those don't help because once that first photon has failed
to get you out of the atom, then your back
on the atom again, and the next one is also
going to fail. The photons can't work together. So that's

(23:01):
the key idea, the fact that the beam of light
is not just one wave that's shaking the electrons so
that if you turn it up, you're shaking them more
and getting them enough energy to get out of those atoms.
But it's broken up into pieces, and each piece needs
enough energy on its own to get those electrons out
of the atom. So the way you do it, the
way you can get the electrons out of the atom

(23:22):
is by changing the frequency because that gets more energy
into each photon. And so if a purple one comes,
remember purple being very high frequency, it has enough energy
to get the electrons out of the atom and a
little bit left over. So as you increase the frequency
of the light, you're increasing the energy per photon, essentially
the energy that each electron has access to, and then

(23:45):
it has enough energy get out of the atom and
to zoom off with a good amount of speed. So
the higher the frequency of the light, the more energy
in each photon, the more energy these electrons come out at.
And that is exactly what they saw in the experiment,
and that can only be explained if electrons can only
interact with one particle of light at a time, and
the light is in fact a particle. It also explains

(24:08):
why the energy of electrons does not depend on the
intensity of the beam. You can have a really powerful
red beam, but it's too low frequency. All those photons
are wasted because none of them have enough energy to
get the electrons out. It doesn't matter how high you
turn it up. And even if you're turn it up
to green and you have enough energy to get the
electrons out of there, you don't get more energetic electrons

(24:31):
by increasing the intensity. Again, you have to change the
energy in each photon. That's hitting the electron. You can
only do that by changing the frequency. And this assumes
again that electrons can only interact with one photon at
a time, which is pretty solid assumption. So the amazing
thing is that this idea, which really came from Plank,
explain these experiments which really were done by other people.

(24:55):
But the unification of it, the bringing together the idea,
the moment of inside the explanation of this weird experiment,
was done by Einstein. And that's what Einstein won the
Nobel Prize for, not for doing the experiment, not for
having the idea, but for being sort of in the
right place at the right time to bring that idea
to solve this open problem. Now, the photon was not

(25:17):
named as a particle for decades later. All of this
happened just around the turn of the nineteenth century, and
Einstein won the Nobel Prize later for it, but it
wasn't until nineteen twenty six that people started calling these
things photons. And it comes from the Greek word for light.
But it also touches on something I think is really interesting,
which is the sort of concept of a particle. I

(25:38):
like to imagine what we're physicists thinking back then, what
did they think that the universe looked like at a
microscopic scale, Because to us, the notion of a particle
is kind of familiar. I mean, they're weird, they do
things that we don't understand. They follow rules and make
no sense to us. But we're comfortable with the idea
that the universe is atomic, meaning that's made up of

(25:59):
little bit it and all we have to do is
sort of figure out what those bids do. But at
the time, this whole concept of a particle was kind
of new. Remember where they had discovered the electron. That
was only recently. That was the first piece of evidence
that there was something as a particle. Sort of the
invention of the concept of a particle was the discovery
of the electron. And all he really did there was

(26:20):
identify something tiny that had both mass and charge, and
so he said, oh, look there's a thing there as
these two attributes. I'm going to call it a particle.
Actually he called it a corpus skule. But the concept
of intellectual groundwork was laid then for a particle. So
then you get to the photon. Now the photon has energy,
it has direction, but it doesn't have mass. It's not

(26:43):
a thing in that sense, there's no stuff to it,
So that immediately sort of bends your mind around what
is this concept of a particle. Anyway, we've created this
idea to accommodate the discovery the electron. We hope, oh,
maybe there are other particles, And later on the podcasts
will take a tour of the discoveries of other particles,
which have hilarious and amazing and dramatic stories to them.

(27:06):
But very early in the history of particles we had
to already bend the rules and say, oh, well, we
were talking about particles is a little bits of stuff.
But they can also be not stuff, right, they can
also just be energy. And so to me, it's amazing
that this field of particle physics was founded on such
crazy discoveries. So to me, it's wonderful that the field
of particle physics is founded on such crazy discoveries. And

(27:29):
you've got to give a lot of credit to the theorists,
of course, who put these ideas together and helped us
understand what we were seeing. But to me, the most
exciting moments are those moments of experimental surprise when the
universe does something that we don't understand when the unit,
when we predict the universe will do a and instead
it does be because that's the universe talking to us,

(27:50):
or that's the universe answering our questions, that's the universe
being the subject of our interrogation when we say we
want to know how this works, prove it to us,
or reveal to us the underlying mechanism. And that's what
experimental physics is about. Is about cornering the universe and
forcing it to reveal something new to you. And a
lot of times that revelation happens when you didn't expect that.

(28:11):
You thought, oh, we're just double checking this over here.
We're pretty sure we understand it. Just dotting the eyes
and crossing the teas and all of a sudden, oops,
you get something totally surprising. But those are the moments
that we learned something new about the universe, And those
are the moments I'm striving for my own personal research.
When I'm smashing particles together at the LHC. We think
we understand what's going to happen, but I'm always secretly

(28:32):
hoping that a student will come to me and say, hey, Daniel,
what's this. I found this weird thing in our data
that just doesn't make any sense, and that's only happened
once or twice in my entire career, and I look
forward to it happening again. So maybe one day we'll
be hearing about a crazy discovery we made at the
Large Adeon Collider. Until then, thanks for listening to this

(28:52):
description of how we know the photon is a thing,
and please, if you're interested in learning more about the
history of physics or understanding how we know how the
universe works and what we don't know, please send me
a suggestion to feedback at Daniel and Jorge dot com.
Thanks for tuning in. If you still have a question

(29:17):
after listening to all these explanations, please drop us a line.
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