- In December of 1900, a conservative
German scientist named Max Planck published the paper
which included according to the Nobel prize
winning physicist, Max Borne quote
"The most revolutionary idea which has ever shaken physics."
For in this paper,
Planck assumed that energy came in little energy packets.
Why did he do it?
And why was it so revolutionary?
Great questions.
Let's ask Planck.
No, not with a Ouija board, silly.
Why don't we just read his autobiography
and his other papers and see what Planck himself said?
And that's exactly what I did.
Ready to learn not only how,
but why quantum mechanics was started.
Let's go.
♪ Electricity, electricity, electricity, electricity ♪
It all started when Planck was just two years old
and another German scientist named Gustaf Kirchhoff came
up with a theoretical puzzle about heat and light.
See Kirchhoff had just accidentally discovered
that elements will absorb exactly the same frequency
of light that they emit.
Therefore in 1860, Kirchhoff published the
paper where he imagined the perfect
object that could completely absorb every frequency
of radiation that hit it.
And therefore could admit all frequency.
Kirchhoff called this object, a black body
and predicted that the amount of light
a black body will admit does not depend on the material,
but only on the temperature and the frequency of the light.
Don't get too distracted by the word black and black body.
It just means if it's cool, it's perfectly black.
Not that it can't emit any light.
Anyway, the black body radiation problem became one
of the great unanswered questions in science,
and every great scientist tried their hand at solving it.
However, it took over 30 years
till 1894 until a friend of Planck's named Wilhelm Wien
came up with an expression for black body radiation.
That seemed like it was a pretty good fit
and was generally accepted as true.
The only problem with Wien's equation is
that it was derived from experiment, not theory.
By this time, max Planck was the head of theoretical physics
at the University of Berlin and as basically Germany's
sole pure theoretician, Planck decided it was important
for him to derive Wien's equations
from the basic theories of physics.
This was not an easy task
and it took him five years, till 1899,
When he triumphantly produced his results.
He thought the case was closed.
And for a while this black body
radiation equation was called the Planck-Wien law.
However, nature had another idea.
For another friend of Planck's had found a way to measure
low energy infrared waves
and found that at relatively low energies
the Planck-Wien equation didn't work.
This friend gave Planck a heads
up and he quickly made a new equation quote,
"Which as far as I can see at the moment,
fits the observational data".
Planck politely titled his paper quote
an improvement of Wien's equation.
Although it was a totally new equation.
Although it does look like Wien's equation
at high frequencies.
This new Planck equation seemed to work perfectly
but Planck wasn't happy.
It was just a lucky intuition.
Just as Wien had done six years earlier and it wasn't based
on physics principles.
Planck recalled quote
"For this reason on the very day I formulated this law
I began to devote myself
to the task of investing it with true meaning.
For six years, I had struggled with a black body theory.
I knew the problem was fundamental and I knew the answer.
I had to find a theoretical explanation at any cost".
So in an act of desperation, Planck used something
he hated, statistical mechanics.
Now statistical mechanics have been around
since 1859 when James Clerk Maxwell
of Maxwell's equations,
decided to study gases by using statistics.
The general idea is gases are filled
with a ridiculous number of atoms
moving at different speeds in different directions.
And in order to study them,
you need to study what they're doing
on average, which is why you
need statistics and probability.
Five years after Maxwell wrote his paper
about statistical mechanics,
a 20 year old German scientists named Ludwig Bolzmann
read Maxwell's paper and decided to devote his life
to this research and became Germany's main champion
of the existence of atoms.
Max Planck admitted
that for most of his career, he had been quote
"Hostile to the atomic theory",
which was the foundation of Bolzmann's entire research.
Why did Planck hate statistical mechanics?
Well, it wasn't the math, like me.
It was because of what's statistical mechanics
said about something called the second law
of thermodynamics or entropy.
See Planck had gotten his PhD
at age 19 in the second law of thermodynamics.
The second law can be written
in many, many, many forms, but the basic idea
is things can't become more ordered
or less messy by themselves.
Physicist called the messiness of a system, the entropy.
So the second law of thermodynamics can also be written,
the entropy in a closed system cannot be less than zero.
However, if you believed in statistical mechanics,
then the atoms could become more ordered.
It was just really, really unlikely.
In other words, the second law wasn't as much a
law as a statistical certainty.
James Maxwell elegantly put it this way.
Quote, "The second law of thermodynamics
has the same degree of truth as a statement
that if you throw a couple of water into the sea,
you can't get the same cup of water out again."
To Planck, I think that statistical
certainty never seemed good enough.
However, desperate times called for desperate measures
and basically holding his nose,
he dived into the interrelationship
between probability and entropy.
Quote, "In other words,
to pursue the line of thought inaugurated by Bolzmann".
Now, Bolzmann had been writing about an idea that atoms
and gases can have different
arrangements to produce their average energy.
And he'd given the letter W
for the German word for probability, for the measure
of how many different arrangements there could be.
It makes sense that the entropy
or the messiness, which has given the letter S would relate
to the amount of ways you can arrange your materials.
W, but how?
Planck recalled quote
"Since the entropy, the S is an additive magnitude
but the probability W is a multiplicative one,
I simply postulated that S equals
K log W where K is a universal constant".
This became known as Bolzmann's entropy equation.
And K became known
as Bolzmann's constant,
even though both were defined by Planck.
In fact, this equation S equals K log W is actually written
on Bolzmann's tombstone.
Ironically Bolzmann didn't determine
his constant, Planck did.
But then Planck had a problem.
He couldn't seem to get the equation to work
without another constraint, because if the energy is quote
"Considered to be a continuously divisible quantity
this distribution is possible in infinitely many ways."
Planck therefore imagined that the energy came
in little energy elements with an energy that equal
to constant eight times, the frequency of light.
In his paper, Planck said it clearly quote
"The most essential point of the
whole calculation is to consider the energy E to be composed
of a very definite number of equal parts.
This is the delineation
between modern and classical physics.
Let's take a moment to appreciate what a strange
and radical idea this is.
Think of a water wave or a sound wave.
They do not come in little packages.
They're vibrations of water
or molecules where the more the water molecules vibrate,
the more energy the wave has.
Planck was creating a brand new thing in physics,
a wave packet.
Five years later, Einstein called them quanta of energy,
although they're currently called photons.
this is the birth of the quantum.
It's a really, really big deal.
However, Planck didn't realize what he'd done
to the whole nature of physics.
Quantizing energy was to Planck
quote "A purely formula assumption.
And I didn't give it much thought."
Instead he assumed that he could massage is new theory
into classical physics and spent well
over a decade trying together distance himself
from his own idea.
Other physicists politely ignored him.
And most believed
the competing theory that used statistical mechanics
in a different way,
but they require energy to be in little packages.
The only problem
with this other method, is that as the frequency went up
you get more energy states without any quantum limitation.
So that as you went to the ultraviolet range
the radiation went up exponentially.
A situation poetically called the ultraviolet catastrophe.
Some people tried to solve this problem with a fudge factor
or saying it didn't work at high frequencies for reasons.
One of the inventors of the equation said it was fine.
This is fine.
As a catastrophe was happening
but just unevenly and very, very slowly.
By the way, Planck thought that that man
was an idiot, writing Wien that he quote
"Is the model of the theorists as he should not be.
He just ignores the facts if they don't fit".
However, at the time a theory
that didn't fit the facts seemed far preferable
to a theory that didn't fit classical physics.
Even a young college graduate named
Albert Einstein had quote
"mixed feelings" about Planck's paper.
Then in may of 1901, Einstein read a paper
about something called the photoelectric effect
by Phillip Leonard.
And it all made sense.
Planck's equation was not a formal assumption.
It was a discovery about the fundamental nature of light.
Einstein wrote his girlfriend
quote, "I've just read a wonderful paper by Leonard
and under the influence of this beautiful piece I'm filled
with such happiness
and joy that I absolutely must share it with you."
By 1905 Einstein and his dear
Kitten became the first people to
actually use Planck's equation to describe what light was.
And well over a hundred years later,
this paper is still considered correct.
Einstein actually won his Nobel prize
due to the photoelectric effect.
And I will get to that.
But first I want to take a little step back
and talk about Phillip Lennon.
Why was he studying the photoelectric effect?
What is the photoelectric effect
and what did Leonard think the photoelectric effect was?
And that is next time on the "Lightening Tamers".
Thanks for watching my video.
If you haven't seen my last one about Kirchhoff
and Bunsen and spectroscopy and finding gold in the sun,
you should really check it out.
It's one of my favorite stories.
Also, I have another video
about James Clerk Maxwell, discovering Maxwell's equations.
And if you haven't seen that one,
I think you should check it out.
He's so charming.
I really like him.
And I think you will like him too,
even if you suffered with his equations.
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