The Ultraviolet Catastrophe

Over 100 years ago, physicists were 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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