Welcome to Quantum Mechanics 101

Back in the 1890's, physicists were confident that they understood matter and energy. Atoms were understood to be particles of matter, and energy consisted of electromagnetic waves like light and heat.

One of the problem with this understanding of atoms as billiard balls and waves as energy was that it couldn't explain one of the most perplexing problems of the time, something known as "cavity radiation".

The problem of cavity radiation was that if you took ANY hollow solid object, drilled a hole through it's wall into the hollow interior, and then heated that object, the spectrum of the electromagnetic radiation that was emitted from the exterior of the object was dependant on both the temperature you heated the object to and the material the object was made of. That seemed to make perfect sense.

But, the spectrum of the radiation emitted from the hole in the object was solely dependant on the temperature, and was the same regardless of what material the object was made of.

WTF?

The radiation emitted from the hole originated at the interior surface of the body, which was exactly the same as the exterior surface of the body, so why would the spectrum of the radiation emitted from the hole be so uncharacteristically different?

At the time, a well established physicist by the name of Wilhelm Wiens was working on the problem of cavity radiation and was able to derive a theoretical equation based on the assumptions of atoms as particles and energy as waves that partially predicted cavity radiation at relatively low temperatures. The equation Wiens came up with was:

http://upload.wikimedia.org/math/3/5/e/35e692d3a95d53e219af39cabfe47021.png
And at the same time, a young Physicist in Germany named Max Planck was working with emperical formulae that predicted the spectrum of the radiation emitted from the cavity very precisely . These emperical formulae had no theoretical basis, but were simply equations generated which simply happened to match the results observed from laboratory measurements very accurately. The emperical equation Max Planck had was:

http://upload.wikimedia.org/math/d/e/4/de49247c3e38ac7ef9a3c713d822d308.png
Max Planck noticed that Wiens' theoretical equation bore a striking similarity to the emperical equations that he had been working with, and decided to try to work backward through Wiens' theoretical derivation using his emperical equation as a starting (er, ending) point to see if he could come up with a theoretically sound derivation of his emperical equation.

And, he found he could do it.

All he had to do was make the contentious assumption that the energy emitted from the cavity was "quantized", or in distinct individual packets. This is much like saying that you cannot have ANY amount of money in your wallet, only certain distinct quantized amounts. For example, you cannot have Pi dollars in your wallet. You can have #3.14 or you can have $3.15, but you cannot have $ Pi.

That is, if energy was money, Planck had discovered the penny.

Planck subsequently received the 1917 Nobel Prize in Physics for his discovery of the "Elemental Quanta", and this was the origin of the branch of physics we call "Quantum Mechanics".

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