The Birth of Quantum Mechanics

Louis De Broglie

Max Planck started Quantum Mechanics in 1900. One of the great ironies in Science History is that he never liked the field, indeed, fought against it for the majority of his life, like a Father whose standards for his son are so high that the child always disappoints him, although outside observers know the father is being too tough and stubborn.

Which is not to say Planck wasn't a nice man, because he was. His life had a great sadness though: He had four children, and all died before him, one each of his two sons in the two World Wars, and his two daughters in childbirth. They say the 3rd greatest sadness that can befall a person is Divorce, the second the death of a lifelong spouse, but the saddest of all is the loss of your child, and that happened to Max 4 times. :-(

But he also fathered QM, clearly the most unexpected and outstanding scientific discovery of his age, or any other.

Within 5 years a patent clerk in Switzerland added to the theory, and gave Planck his due. Planck wasn't completely in agreement with young Albert Einstein's interpretations, but he was grateful for the acknowledgment, and promoted Einstein within the Physics community. The rest is History.

Niels Bohr added still more to the theory in 1913, and the 3 first Greats in QM were established.

And there the theory stood, with more questions than answers, as new theories often do, and for a good long time.

Finally, a young Physics student named Louis De Broglie tackled a thorny problem in QM with his Doctoral Dissertation in 1924. Is his own words:

"The fundamental idea of [my 1924 thesis] was the following: The fact that, following Einstein's introduction of photons in light waves, one knew that light contains particles which are concentrations of energy incorporated into the wave, suggests that all particles, like the electron, must be transported by a wave into which it is incorporated... My essential idea was to extend to all particles the coexistence of waves and particles discovered by Einstein in 1905 in the case of light and photons."

... Louis-Victor-Pierre-Raymond, 7th duc de Broglie

In other words: If waves of light can act as particles, can particles then act as waves? Because they can, and in certain circumstances as we shall see: they do.

The significance of De Broglie's thesis is that it started the second revolution in Quantum Mechanics, indeed it opened the floodgates of discovery in the field, and within 5 years the other Greats in Quantum Mechanics would complete the theory, men such as Max Born, Pascual Jordan, Werner Heisenberg, Erwin Schrodinger, Wolfgang "The New Einstein" Pauli, and Paul Dirac, with a tremendous assist by mathematician Hermann Weyl and Group Representation Theory.

De Broglie's work is beautifully simple, easy enough to be understood by a bright 8th-grader science student, let alone a high-school-er. Which begs the question: WHY are we waiting until college to teach our children Quantum Mechanics? Well I don't know why. Because the teachers don't understand it, perhaps? Well, that's an issue and question for another time.

In any event, here's the theory, below. From Andrew Thomas' wonderful webpage: What is Reality?, from the first chapter: Quantum Mechanics: An Introduction:


Quantum mechanics could be said to have started in 1900 when Max Planck made the discovery that light, which was considered to be purely wave-like, was in fact composed of energy which came in discrete packets (called "quanta").


In the Planck formula, the energy of the packets, e, is proportional to the light frequency, f, the constant of proportionality being Planck's constant, h:



This result suggested that waves (light) were in fact composed of particles. The converse of this result came in 1923 when Louis de Broglie (pronounced to rhyme with "destroy") suggested that matter (particles) behaves as a wave (as is evident in the double-slit experiment), the wavelength, , being inversely proportional to the particle's momentum, p.


Here's the derivation:



We now know that absolutely everything in the known universe is made out of these strange particle/wave entities which obey these two formulae for quantum behaviour, given above.

Max Planck