Friday, June 13, 2014

Science Friday: De Broglie's Wavy Particles

On to chapter four of George Gamow's classic, Thirty Years That Shook Physics. Chapter 4 is on "L. de Broglie and Pilot Waves."  The previous posts were:

1a. Planck's Quantum
1b. Jumping Photons (Einstein and the Photoelectric Effect)
1c. The Compton Effect (Proof of Energy Packets)

2a. Thomson and Rutherford's Atoms
2b. Bohr's Contributions (How electrons fill the atom)

3a. Pauli Exclusion Principle (no two electrons at any one energy state)
3b. The Pauli Neutrino

4a. Now the plot thickens with de Broglie (pronounced, de-broy - he was French and of course they never pronounce words the way they look... much worse than English on that score). Again, Gamow's personal recollections of these individuals are priceless.

He recounts how he decided to spend Christmas in the late 20s in Paris and tried to meet up with de Broglie while he was there. De Broglie was from a family of some prominence, as it turns out, a family of diplomats. Gamow tells how when he arrived at the family mansion, he was corrected by the butler to know that he had not come to see "Professeur de Broglie" but "Monsieur le Duc de Broglie," the seventh Duke de Broglie.

Gamow assumed during the visit that de Broglie did not speak English, as he struggled to speak in French to him, made diagrams, etc. As it turned out, de Broglie spoke perfect English. But when you're in France...

De Broglie won the Nobel Prize in physics for an idea that he proposed in his PhD dissertation in 1925 and which was verified less than five years later. Einstein had proposed, following on Planck, that light was not just a wave (as James Clerk Maxwell had demonstrated in the 1800s) but that it also functioned as a particle. That is to say, if you break down a light stream into individual photons, you will see that the wave properties of light are a statistical phenomenon. The fewer the photons involved, the more light behaves like individual particles.

De Broglie, on the hunch that there would be a symmetry between energy and matter on this score, suggested that matter, which we think of as particles, might have wave properties as well. He used existing formula to predict how those waves would function if they were associated with particles like, say, electrons. When electrons were then later diffracted, sure enough, they showed wave light patterns.

For de Broglie, these waves were like pilot waves associated with the particles. But de Broglie had opened up a can of worms. Next week, we will see the transition from the "old quantum theory" of Einstein to the "quantum mechanics" of the Copenhagen school, which will soon become the new norm in physics.

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