trying to figure out how things glow,

like molten glass or hot lava.

It seems like a simple question and was

being pondered by such physicists

as Lord Rayleigh at the turn of the 20th century.

I give you a wooden skewer that, when touched to a flame,

glows orange-red.

Rayleigh knew that the color of this glow

depends on temperature, like the sun that

has a surface temperature of 5,500 degrees Celsius radiates

a bunch of different colors that together look white to us.

Or me-- at 37 degrees Celsius, I have this invisible halo

of infrared light.

Fun fact.

Infrared light is invisible to humans,

but snakes can actually sense it from up to a meter away

to detect prey.

Rayleigh wanted to understand where the light came from

and come up with a rule that would

predict how much of each color it would emit.

This is its spectrum.

Another fun fact.

Lord Rayleigh was the guy who figured out, mathematically,

why the sky is blue.

So Rayleigh thought of the simplest possible object,

something that absorbs all light.

Objects absorb, emit, and reflect light.

Rayleigh's object would only absorb and emit, but not

reflect.

So all you would see when you look at it

is its glow or its radiation.

Physicists call this a blackbody.

Technically, a blackbody isn't black, it's glowing.

But whatever, physicists.

A true blackbody doesn't exist in real life

because nothing absorbs all light

and radiates perfectly, although stars like the sun

come pretty close.

Kind of funny, right, that the brightest thing

we know in our solar system is the closest thing

we know to a blackbody.

Physicists.

Anyway, at that time, physicists didn't really

understand atoms and molecules.

They thought everything was made of particles

that vibrate like springs.

Rayleigh and his colleague, James Jeans,

imagined a blackbody made of these vibrating particles

where the vibration was continuously

converted to light.

From that model, they came up with a rule

to predict what colors a blackbody would radiate

at certain temperatures.

But the unfortunate happened.

Their reasoning implied that a blackbody

should emit infinite amounts of ultraviolet light.

They tried to apply their usual rules of Newtonian physics

and came up with nonsense.

We now call this the ultraviolet catastrophe.

Oh, eye of a Newton.

Something was very wrong with their work.

A blackbody can't emit infinite amounts of radiation

because that would violate laws of conservation of energy.

And just from, like, experience, when you put toast

in your toaster, it doesn't promptly

burn your toast to smithereens.

When observation contradicts theory like this,

it often means there's something missing in the theory.

Rayleigh and Jeans had found a glaring error

in physics theory.

Bum, bum, bum!

The same year that Rayleigh and Jeans published their work,

a German physicist, named Max Planck,

was studying radiation for a different .

Reason he wanted to understand why heat always

flows from a hot object to a cold object.

And in his quest to solve that problem,

he unintentionally fixed their error.

He assumed that a blackbody could only emit light

in discrete quantities.

Its energy comes in chunks, equal to the frequency

of the light multiplied by this-- a number we now

know as Planck's Constant.

This is weird.

It would be like if your faucet could only

pour full glasses of water at a time.

But when Planck assumed light was quantized,

theory matched with observation.

He solved the ultraviolet catastrophe.

But here's the crazy part.

Max Planck didn't immediately realize how big of a deal

this was.

Nobody did.

He was just doing the thing that we all

do with our math homework, when your answer doesn't match

the answer in the back of the book

so you just switch a plus for a minus

but you're not really sure why.

Planck didn't understand what his work

meant in the real world.

It wasn't until a couple years later that Einstein realized

that these packets of energy meant that light wasn't just

a wave.

It's made of particles that we now call photons.

So then physicists started thinking

about what this meant for atoms that emit those photons,

and The Theory of Quantum Mechanics began to unfold.

Atoms emit photons when their electrons lose energy,

and so their electrons must lose energy in chunks-- in quanta.

Quantum mechanics.

The consequences of Max Planck's discovery

that light comes in quanta, in packets,

snowballed into what we now know as quantum mechanics.

Quantum mechanics is bizarre.

The microscopic world just misbehaves.

Like, we're talking many billions of times smaller

than the width of a human hair.

For example, an electron can be in multiple places

at the same time.

In fact, an electron has no precise location.

But the weird thing is, when you measure the electron,

you find it in a specific place.

It's like it has no location until you measure it.

This popular image of an atom is wrong.

Instead, most of the time it looks

like a cloud of probability around the proton.

This cloud of probability is the electron.

Think about it.

The particles that make up your body

aren't a stack of solid objects.

They're a collection of probability clouds.

Quantum mechanics is unsatisfyingly unintuitive.

But time and again, it has proven to be real,

applied in computer chips, lasers, even LEDs.

An electron probability cloud is only

a nibble of the cookie that is quantum weirdness.

And after more than 100 years of study on the topic,

there's still more to learn, which

is why we're doing things like colliding particles

at the Large Hadron Collider.

Just as a small example.

I love this story because it shows how important it

is to be wrong.

Science isn't fixed facts and figures.

It's constantly being modified and improved.

Without Rayleigh uncovering this huge theoretical error that

was the ultraviolet catastrophe, physicists

may have never come up with the rules

to describe a brand new tiny, tiny universe.

Thank you so much for watching, and happy physicsing.

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