Hello! Today’s episode is a bit special since

we’ll deal with an anecdote,

but not just any random anecdote.

One of the most famous story in the history of science,

which is special in that it is… completely false.

I mean: it’s an urban legend.

But it has a true meaning, since it will allow us to get started, quite easily,

with quantum mechanics.

The story takes place around the year 1905 in a university.

I really don’t know which one, quite logically, as it never existed.

A teacher of that university goes to a colleague,

— who happens to be Sir Ernest Rutherford,

the one we owe our knowledge about atomic nuclei to,

as mentioned in my La Preuve Par Vieux* about Mendeleev —
* Casting Out Lives

The teacher goes to Rutherford and

submits a problem he has with a student.

He’s like: “I have a problem with my student, who thinks he should get

the best grade for his work, where I think he should actually get the lowest.

We agreed that we need a third party to conciliate.

Would you do that?”

Rutherford is intrigued and replies: “Why not! Let’s go to your student!”

On a side note, the student is the one and only Niels Bohr.

Niels [F-WORD] Bohr.

Yup! He, who’ll later be one of the founding fathers of quantum mechanics.

That one.

So what’s the problem? The teacher asked Niels Bohr the following:

“Say you have a building and an barometer.

How do you measure the building’s height with the barometer?”

The expected answer would be something like:

“I measure the air pressure with my barometer at the bottom of the building first,

then the air pressure at the top of the building,

I calculate the difference between both pressures, and I use a formula

that lets us measure the height of the building

according to the pressure difference.


But Niels Bohr solves this problem as follows:

“I attach the barometer to a very long string and let it go down gently

until it touches the ground.

Then I only need to measure the required length of the string

in order to get the building’s height.”

That is technically correct,

but doesn’t let us know whether

the student has the required knowledge of physics.

That’s why Rutherford suggests to Bohr the following:

“Okay then, you have 10 minutes to prepare

a valid oral answer, that also shows that you know physics good enough.

You okay with this?”

And Bohr replies: “yeah, suits me well.”

During the first 5 minutes, Niels Bohr seems to rack his brain over it but doesn’t say a single word.

So Rutherford goes to him, wondering if he’s all right.

Niels Bohr says: “Sure, I’m all right. I just have so many valid answers

that I’m now evaluating which one would be the best.”

Rutherford then lets him keep on thinking

and a minute later, Bohr gives his answer:

“Super easy, I only need to drop the barometer from atop the building,

to measure its fall time, and with the free fall equation, I can calculate

the distance according to its fall time.”

Which is a totally correct answer as well.

Then Rutherford agrees with the other professor to give him the best mark, because

Bohr has proved his skills in physics.

But Rutherford is a slightly curious guy,

so he goes to Bohr, and asks for his other solutions.

“Well I could only wait for the sun to rise up and put the barometer on the ground.

Knowing the height of the barometer, and the length of the shadow it casts,

if I measure the length of the building’s shadow,

with Thales’ theorem, I can simply calculate the building’s height.”

Yes, this is another valid answer, it’s elegant,

and it was used in Antiquity to measure the height of the pyramids.

Another simple method, but sort of brute force, is:

“I can count how many times the barometer fits alongside the building.

Depending on the size of the barometer, I can tell you

the building is 140 barometers tall, hence its height…”


Another more technical method

would be to let the barometer hang with a looong long string,

from the top of the building

til the barometer only brushes the ground.

Then you swing it like a pendulum,

measure the oscillation time

— i.e. the time it takes the barometer to move back and forth exactly once —

and the pendulum equation gives you the length of the string.

It’s tough, but it works.

Lastly, if you like less twisted methods,

just knock at the door of the janitor and say:

“Sir, if you tell me the height of this building, I shall reward you with this wonderful barometer!”

It might not work… Anyway, Bohr is trolling here.


At some point, however entertained Rutherford was,

he has to ask Niels Bohr whether he knew the answer that was expected…

Bohr replies: “Naturally!

Of course I know what to do with a barometer.”

Rutherford asks: “Then, why didn’t you simply give this answer at first?”

And Bohr responds with: “Well, I’m just really sick of being told how to think.”

As I said, this story is not true,

despite the fact that many professors think it is and that

some school books still relate it.

It never happened.

Bohr and Rutherford first met only later,
a long time after Bohr had finished his studies.

So what’s the purpose of the story?

Why is it still so widespread today?

This story is still so widespread today, since

Niels Bohr’s behaviour in it is quite believable,

considering Niels Bohr’s actual personality.

Indeed, Niels Bohr was not the kind of guy who let others dictate him how to think.

And THAT is part of the fundamentals of quantum mechanics.

Quantum mechanics describes the universe at a scale, with which we are never faced.

Simply put,

any experience you can have about

reality, tangibility, matter, or solids…

all of this has absolutely *no* sense at all, when it come to quantum mechanics.

And the very first lesson given by quantum mechanics is:

“You must forget everything you know, or think you know, about reality.

None of this applies to quantum scale.”

One must have at least Niels Bohr’s stubborness

to un-comprehend the reality one experiences.

This certainly explains how Niels Bohr did challenge Einstein for so long about

the fundamental questions of quantum mechanics.

And right here comes the struggle, when you sink your teeth into quantum mechanics.

If you think you have just understood what it’s about,

then you have most likely just missed out on the concept.

Sad, isn’t it? But it’s so.

Thank you everyone, since you’re more and more willing to e-penser (e-think).

At last, a Quickie that deserves such a name.

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We can soon meet IRL at the Geek Faëries,

for thoses who want to come, links are in the description.

See you very soon for a new episode, and of course,

Until then, stay curious, and take some time to e-penser (e-think)

#Subtitles by Az’
#Proofread by Cyril & Edouard
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