A brief history of Quantum Mechanics - Part 1 - The Oven

Epistemologists and historians are going to kill me. Maybe fellow physicists too. But this is a true history of quantum mechanics – to be taken with a grain of salt.

It all starts with an oven. A great piano player of Prussian values was heating a special oven. Instead of a door, this oven had a tiny pinhole through which you could peek inside. The question was: Does it get red inside if I make it very hot? Or more specifically … what combination of colours happen inside when all the heat of the walls gets re-absorbed, and re-emitted and re-absorbed and re-emitted, ad infinitum by the walls.

The calculation goes something like this: the walls of the oven are made of tiny particles. Just like some people are smaller and some are taller … but most are average … the laws of statistics apply to particles. Some are a little cooler and some extremely hot, but most are around the average.

In physics, particles moving fast are hot (think of them bouncing hard on your
skin and warming it up) and particles moving slow are cold. Moving fast means they emit high energy light, and slow, low energy light. Perfect! So take pen and paper, measure the average temperature inside with a thermometer and pick a distribution of speeds around the average. This should describe all the speeds in the box and thus the colors and energies. Peek again, check the colors, … did you get it right?

Max Planck, the piano player, didn’t.

Max Planck around 1900

Max Planck around 1900, Archiv der Max-Planck-Gesellschaft

He was seeing colors all over the place. But even worse, his pen and paper calculation said that some particles would have infinite speeds to match the average and median temperature. With deep mathematical trickery and insight he invented a new probability distribution which perfectly matched the colors, the speeds and the temperature. The rub? He had to trash all previously known physics in the process. What was the gross violation?

particle speeds are quantised!

Heresy! How dare he?! Well … let’s see what quantised speeds actually mean because it’s a lot worse than it sounds.

Imagine a bicycle going at 3 mph. Now you want to go a little faster so you
push a little harder on the pedals. But nothing happens. You push, push, push … and yet you’re still going at 3mph. You try a little harder … and suddenly, BAM! You’re going at 5mph. You push a little harder, harder … and nothing happens. Then, all of a sudden, POW! You’re at 7mph. Interestingly you never went at 4mph or 6mph or even 4.32mph or 5.01mph. You went straight from 3 to 5 and straight to 7. You try to break … and KABOOM! you’re at 3mph again.

This was exactly how ‘quantum’ physics was born (quanta means a piece, a given quantity that is indivisible). The oven was giving funny colors for the given temperature. So a physicist invented some rules and the calculation turned out to match the colors exquisitely. However the rules sounded somewhat like this: particles can only go at 2mph, 4mph, 6mph, etc … but they never, ever go at 3mph, 1.45mph or 45.32mph. Those speeds are forbidden and never occur in nature.

It was 1900, and the whole world believed particles were tiny billiard balls … but something strange was happening in that oven that would change physics forever.

It was later found that speeds are not the only quantised magnitudes. Some movements were allowed but others were also ‘forbidden’. Also certain positions were forbidden: You know that a 100lb lead ball one inch above the ground has less destructive power than one 100ft above your head. It’s called the potential energy (the further an object is from the earth the more potential energy it has). But quantum ones had rules like: You can only be at 10in, 15in, 20in, 15in from the floor … but never at 11in or 15.3in 1. How could this be possible?

Plank, to be honest, wasn’t quite sure what to make of his hypothesis. The distances and speeds were so small, that he was probably missing some subtlety. It turns out he wasn’t … but it would take 8 years for anyone to notice.

We’ll explore that discovery in the second part of this story.

[1] note that these particles are not really at “inches” of each other. They are actually really, really, really close. (nano-meters. But they can only be at certain nano-meters from each other)

Radiation Distribution Planck vs Wien
Radiation as a function of the temperature in the oven, 115 years ago.

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