Friday, May 23, 2014

Science Friday: 2b. Bohr's Contributions (5)

I've been reading through George Gamow's classic, Thirty Years That Shook Physics. This week finishes chapter 2, "N. Bohr and Quantum Orbits."  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

The rest of chapter 2 was a delight, not because of the equations but because of the stories. The rest of the chapter primary deals with Niels Bohr, who took the notion of energy packets and applied it to the atom.

1. Why doesn't the energy of electrons dissipate away? Because it has an energy "ground state," lower than which its energy cannot go. Further, electrons can only gain energy in the discrete packets that Planck suggested.

At this point, Bohr pulled together the results of several different experiments. As hydrogen is excited, say by running electricity through it, it produces a characteristic set of lines if light from it is passed through a prism. This "line spectrum" is like hydrogen's signature.

What interested Bohr about hydrogen is that he supposed that it might only have one electron. It was thus perfect for exploring the possible energy states an electron could have. The real innovation was his hypothesis that the different line spectra of hydrogen might correspond to different circular orbits hydrogen's electron might have.

Several scientists had already derived equations for different parts of hydrogen's spectrum (Balmer, Lyman, Paschen, and Brackett). Bohr connected these different parts of hydrogen's spectrum into a model in which you had a number of energy states at one orbit and then a number at another orbit, then more at another, and so forth.

Here are two diagrams to depict his theory. The first shows the series of electrons he proposed at each orbit. The second relates these to the various series of line spectra found for hydrogen.



Sommerfeld would then, like Kepler, suggest elliptical orbits. Any student of chemistry today will recognize these hypotheses as the beginning of what we now know as the various orbitals of electrons within various shells (1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10...).

2. But the most enjoyable part of the chapter were the personal reminiscences of Gamow about Bohr. I had never gotten this far in the book before. I guess the Carlsberg Brewery of Denmark fame has an scientific institute in the center of its property, Blegdamsvej 15, at that time. The most famous Danish scientist of the time is supposed to live there, and Bohr took up his residence there for many years.

I guess many of the most famous physicists of those days spent at least some time there with Bohr, who apparently was a delightful, whimsical person who "fathered many scientific children" (51). Gamow contrasts Einstein, who generally worked alone, with Bohr, who apparently loved to mentor and work with others.

Gamow describes a community of geniuses that I suspect most of us interested in ideas dream of being a part of. But alas, it doesn't happen very often, and who of us would qualify? There was Cambridge in the 20s with philosophers and writers. There was Princeton for the relativists with Jim Peebles. There's the Chicago School in economics...

What a remarkable time to be a physicist! Gamow had managed to go to Germany from Russia for a couple months in the summer of 1928. He relates that the Soviet Union had not yet closed the door to such things yet at that time. He had come up with a brilliant idea about the nucleus in those months and had sent off a paper for consideration to be published.

On his way back to Russia, he had just enough money left to go by Copenhagen for a day and try to meet Bohr, unannounced, just a hope to meet the man. As an indication of the kind of man Bohr was, Gamow ended up staying for a year there on a fellowship. The chapter continues with reminiscences of Bohr's whimsy.

Gamow ends the chapter remembering the time in 1939 at a conference in DC, where he had gone to teach in 1933. Bohr was at the conference, as was Enrico Fermi (coming up in chapter 7). In the middle of the conference, they received word that an experiment had been conducted in Berlin in which uranium basically split in half when bombarded with neutrons.

The experiment was partially reproduced that evening in DC and would lead to the atomic bomb, six years later.

Next Week: Wolfgang Pauli

1 comment:

Martin LaBar said...

You should read _Copenhagen_, a prize-winning play about Bohr, Mrs. Bohr, and Heisenberg, or, possibly, see it. I have done both. It's not so much about physics as it is about the relationship between Bohr and Heisenberg, and about the responsibility of scientists to the public.