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I’ve lost count of how many times I’ve said this in SciShow videos over the years,
but quantum mechanics is weird.
There are particles that are waves, things in multiple places at once, cats that only
decide whether they’re alive or dead when you look at them…
Quantum physics is so weird that you’d think it was discovered because of some super-complicated
behavior of individual atoms or electrons or something, maybe involving a particle accelerator
or two.
But it was actually invented to explain ovens.
Physicists were trying to figure out why hot things glow, and glow differently at different
temperatures — like why a blacksmith’s iron glows red, or why the base of a candle
flame is blue.
It turned out that the best tool for studying those questions was an oven with a hole in
it.
And when physicists started to work out the equations, their theories matched the oven
measurements so badly that people started calling it the “ultraviolet catastrophe.”
Basically, ovens broke physics.
The scientist who finally solved the problem thought his own solution was just a mathematical
gimmick.
Only years later did people realize he’d stumbled onto quantum mechanics.
The story begins in the mid-19th century.
Scientists already knew that when you heat up an object, it gives off electromagnetic
radiation—energy in the form of visible light, radio, ultraviolet, or other types
of radiation on the electromagnetic spectrum.
The challenge was figuring out how much, and at which colors and wavelengths.
They had already observed that hotter objects glow bluer than colder objects—in other
words, they emit shorter-wavelength radiation.
But no one had captured that relationship with equations yet.
Of course, the exact intensity and mix of wavelengths you’ll see depends on what’s
being heated.
A molten glass rod will emit light, but at the same time, some incoming light will pass
through it or be reflected.
And if you twist that rod, even some of the radiation it’s emitting might get reflected
or scattered off other parts of the glass.
But physicists like to simplify.
They think first about idealized examples before they extrapolate to all the messy complications
of the real world — assume a spherical cow in a vacuum, and stuff like that.
So in 1860, a German physicist named Gustav Kirchhoff came up with the spherical cow of
radiation—the simplest possible case.
He imagined an object where the only light coming from it is the light emitted because
of its temperature.
In other words, it’s a perfect emitter.
No incoming light is transmitted or reflected — it’s all absorbed and converted into
heat.
And the heat within the object is translated back into radiation that’s emitted.
Kirchhoff called this imaginary object a black body, since it doesn’t reflect or transmit
any light.
Ironically, black bodies are terrible at being black.
They’re only black at absolute zero; the whole point is that when they’re heated,
they glow.
Anyway, Kirchhoff laid down a challenge to physicists: measure and explain how the radiation
given off by a black body, or black body radiation, varies by wavelength and temperature.
Over the next few decades, the scientific community assembled a few key facts.
One: heat causes the particles that make up matter to vibrate, or oscillate, back and
forth.
Two: matter contains lots of positive and negative electrical charges.
And three: when you vibrate electrical charges, they give off electromagnetic radiation.
So it was a reasonable guess that the reason hot things glow is that the charges inside
give off radiation as they vibrate from the heat.
That hypothesis led to some real breakthroughs in the 1890s—but also some big head-scratchers.
The first breakthrough came in 1896 from another German physicist, Wilhelm Wien, who made an
assumption that was highly controversial.
He suggested the particles doing the bouncing were — wait for it — molecules.
Molecular theory wasn’t fully accepted yet, so this was a bit of a leap.
But by applying the laws of thermodynamics, Wien came up with a plausible equation for
how much radiation at each wavelength should be given off by a bunch of hot molecules bouncing
around.
For any given temperature, as the wavelength decreases — and the frequency increases
— there’s a quick rise in the amount of radiation for that wavelength, then a gradual
drop off.
It matched all the empirical data…most of which, incidentally, had also been generated
by German scientists.
Everybody in this story is German.
By 1899, Wien’s distribution law was looking really solid!
He even won a Nobel prize for his work in 1911.
But there was a big catch: true black bodies don’t exist.
So all those nicely behaved measurements of hunks of copper or containers of gas could
have been messed up by complications from transmission and reflection.
Wien and his colleagues realized they could get better measurements with a closer approximation
of a real black body: the inside of a totally enclosed oven, with a hole punched in the
side.
Almost all light that enters through that hole will just bounce around inside until
it’s absorbed.
So nearly all the radiation that spills out from the hole must have been generated by
the heat inside.
In 1899, just as people were getting psyched about Wien’s distribution law, experimentalists
— still German — reported new high-precision measurements using the oven method…and the
news wasn’t good.
Wien’s law still more or less fit for visible and ultraviolet light.
But the further into the infrared the researchers looked, the further off Wien’s law was.
So it wasn’t so perfect after all.
Now, there was another idea out there—an equation developed by the one non-German in
this story.
Over in England, John William Strutt, 3rd Baron Rayleigh — or just “Lord Rayleigh”
for short — had already been questioning Wien’s law on theoretical grounds.
Rayleigh was fixated on a principle called the equipartition theorem.
That’s an idea from thermodynamics that energy is very egalitarian: it likes to distribute
itself equally among all available types of motion.
It’s kind of like what happens if you toss a ping-pong ball into a box full of other
ping-pong balls.
You probably won’t end up with all the balls jumping to the right, or spinning clockwise
in tandem.
Instead, you’ll see a pretty random distribution of directions of motion and spin.
Rayleigh applied that concept to how heat energy gets distributed between different
wavelengths of radiation.
He showed that according to equipartition, each wavelength where the waves fit perfectly
between the walls of the oven should get an equal share of energy.
Those wavelengths aren't evenly distributed — there are more short ones that fit perfectly
than long ones—so the long-wavelength end of the spectrum still ends up dimmer.
But the brightness of each wavelength should increase in lockstep with the temperature.
According to Wien's law, on the other hand, as the temperature rises, the brightness of
the color yellow or any other wavelength should increase at a slower and slower rate.
And longer wavelengths stop brightening sooner.
So the higher the temperature, the more longer wavelengths get cheated of their rightful
share of energy.
Rayleigh didn’t think the laws of physics would short-change longer wavelengths like
that — and as those precision measurements showed, he was right!
Instead, Rayleigh suggested a different formula based directly on equipartition.
According to his formula, a higher temperature always makes all wavelengths of radiation
brighter.
Rayleigh’s proposal, now known as the Rayleigh-Jeans Law, fit perfectly in exactly the part of
the spectrum where Wien’s law failed!
But even Rayleigh noticed there was a glaring problem.
The shorter the wavelengths you’re looking at, the easier it is to find wavelengths that
fit perfectly between the oven walls.
According to Rayleigh’s reasoning, that means there should be more wavelengths getting
equal slices of energy in the ultraviolet, which has shorter wavelengths, than there
are in the infrared, with its longer wavelengths.
And the number of active wavelengths should just keep growing the further you look into
the ultraviolet — which means the total amount of energy in those wavelengths should
just keep growing, too.
Take that to its logical conclusion, and there should be an infinite amount of ultraviolet
light coming out of every object above absolute zero!
That’s not what the experiments were finding.
And it’s just, like, also obviously wrong.
Like, you don’t turn on your stove and immediately sizzle into a sunburned crisp from an infinitely
large blast of UV radiation, you’ve just experienced.
This absurd prediction of the theory is what earned the title “the ultraviolet catastrophe.”
So physicists were stuck, which happens every once in a while.
On the one hand, they had Wien’s law, which seemed theoretically sound and worked well
at short wavelengths but didn’t match reality on the long end.
On the other hand, there was the Rayleigh-Jeans Law, which was theoretically sound and worked
for long wavelengths, but was catastrophically nonsensical for short wavelengths.
Not to mention that the two supposedly theoretically sound theories didn’t agree with each other!
Some assumptions somewhere had to be deeply broken.
Enter the hero of our story—Max Planck, the savior of physics.
Bet you can’t guess what country he was from.
Planck found a way to stitch the two curves together.
He didn’t have any particular justification for why the equation should look that way…he
literally just tossed in an extra minus-one because that way it looked like Wien’s law
at short wavelengths and like Rayleigh’s at longer ones.
It’s like when you have no idea what the clue for 16 Down is supposed to mean, but
if you squint at the words around it in the puzzle you can make a decent guess.
Planck’s guess fit the data perfectly.
For all wavelengths.
Scientists finally had their answer to Kirchhoff’s challenge!
Which left Planck…well, really disturbed, actually.
Because sure, his equation worked, but he had no idea why.
But after about two months of studying this, he realized that the formula made total sense
— if oscillating charges could only gain or lose energy in fixed-size chunks.
Planck noticed that his law had an interesting interpretation: it led to another equation
that seemed to express the number of ways you could distribute heat energy among all
the oscillating charges in the oven.
This interpretation relied on the math for how many ways there are to put, say, 10 balloons
into 4 groups.
But the equation for that only makes sense if you’re talking in whole numbers.
You can’t out one balloon into 2 different groups.
Because then you’ll just end up with a mess of shredded latex.
So for the equation to make any sense, some of the terms have to be whole numbers.
Planck wanted to interpret his law as describing how heat could be distributed among oscillating
charges.
But he realized that for it to make sense, he had to make an assumption like the one
with the balloons: that energy is grouped into discrete packets that have to stay whole
— you can’t have half a packet!
In other words, energy must be exchanged in small indivisible units, or quanta.
All that complicated stuff about wave-particles and dead cats derives from this one core fact.
Planck had just discovered quantum mechanics!
…And nobody, not even Planck, noticed.
No one appreciated how profound a shift this was.
In fact, Planck wasn’t even sure it was true.
He later wrote that his discovery was, quote, “an act of despair,” and said that “a
theoretical interpretation had to be found at any price, however high it might be.
”
His own solution seemed to him like just a mathematical cheat.
It wasn’t until Einstein took another look a few years later that he realized energy
really did come in packets.
Einstein conceived of radiation as a sort of cloud of little energy particles — what
we now know as photons.
And you might be thinking, Einstein wasn’t German!
You said they were all German!
He was born in Germany!
By extension, all the atoms and electrons and other stuff must be gaining and losing
energy by absorbing and emitting those particles.
And all of quantum mechanics unfolded from there.
It’s strange to think that something as mundane as an oven could totally overhaul
our understanding of the world.
But that’s just how science works: if some part of the world, even a simple one, says
no to your theory, you eventually have to stop fighting reality.
Some assumption your theory makes must be wrong.
Quantum mechanics saved physics from ovens—and now we just have to accept that the universe
is a weird, weird place.
And while ovens are an important part of our weird universe, they’re a REALLY important
part of baking delicious treats!
In this Skillshare class on Easy and Versatile Baking, cookbook author and baker Julia Turshen
teaches you how to make the one yeast dough you need to make everything from jam buns
to monkey bread.
It’s getting cold where we are in Montana, so I’m ready for some hearty homemade bread.
And right now, Skillshare is offering SciShow viewers 2 months of unlimited access to this
class, as well as over 20,000 others for free!
Just follow the link in the description to take advantage of this offer.
Make some bread!
Learn to paint!
Start a freelance career!
Whatever you’re interested in, Skillshare probably has a class for you.
So check it out, and know that you’re supporting SciShow when you do, so thank you!
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