Friday, May 30, 2014

Science Friday: Wolfgang Pauli's Exclusion Principle

I'll muster the will to make my Friday post on George Gamow's classic, Thirty Years That Shook Physics. This week I started chapter 3, "W. Pauli and the Exclusion Principle."  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)

Again, it's delightful to read someone with a playful personality who knew the key players in the quantum revolution. Before he gets down to business, Gamow talks of how Pauli, though German, was an anti-Nazi. He talks of how once with his arm in a cast that propped it up, he refused to be photographed with his arm in that position.

Pauli apparently laughed quite loudly and his round figure jiggled when he laughed.  Gamow also playfully speaks of the "Pauli Effect," whereby things tended to break when he was around. In words that reminded me of Sheldon Cooper on the Big Bang Theory, Gamow writes, "It is well known that theoretical physicists cannot handle experimental equipment; it breaks whenever they touch it" (64). [Might I add that in early high school I thought of going into theoretical physics...]

Forgive me for not going into detail on the exclusion principle. The problem that Pauli first addressed was the question of why electrons did not collapse toward the lowest quantum state. His finding was basically that there is a "quota" for each electron state. Once the lower levels are filled, electrons have to fill higher levels. No more than two electrons can exist on any one level.

It's like assigned seating in an auditorium. Only two electrons can sit on the first row, right by the nucleus platform. Then there are only two seats in the second row as well. Then it gets a little more complicated. Interestingly, the average diameter of the atom remains constant no matter how big the atom is.

With these basic principles in view, Pauli and Bohr were able to model all the atoms from hydrogen to uranium.

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