Friday, December 16, 2011

Advent calendar #16: Stern's cigar

This is a story one cannot escape if one studies physics in Frankfurt am Main.

In 1922 Otto Stern and Walther Gerlach demonstrated the directional quantization of angular momentum by sending silver atoms through an inhomogeneous magnetic field. Silver has only one electron in the valence shell, so the orbital angular momentum vanishes and only the electron spin contributes to the total angular momentum of the atom. Depending on the orientation of the spin relative to the magnetic field, the atom takes one out of two trajectories, leading to a discrete splitting of the beam after it passed the magnetic field. Classically, one would expect a smooth distribution. This experiment, conducted in Frankfurt am Main, is known today as the Stern-Gerlach experiment, and was one of the milestones on the way to quantum mechanics.

But it was not just the ingenuity of the experimenters that lead to success since originally Stern and Gerlach couldn't see anything on the screen that should be showing two discrete lines. Dudley Herschbach, who won the Nobel prize for Chemistry in 1986, retold Stern's description of the discovery as follows:

"After venting to release the vacuum, Gerlach removed the detector flange. But he could see no trace of the silver atom beam and handed the flange to me [Stern]. With Gerlach looking over my shoulder as I peered closely at the plate, we were surprised to see gradually emerge the trace of the beam... Finally we realized what [had happened]. I was then the equivalent of an assistant professor. My salary was too low to afford good cigars, so I smoked bad cigars. These had a lot of sulfur in them, so my breath on the plate turned the silver into silver sulfide, which is jet black, so easily visible. It was like developing a photographic film."

The complete story of Stern and Gerlach's experiment can be found in Physics Today 56 (December 2003) Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics (pdf), by Bretislav Friedrich and Dudley Herschbach. They also went on to test the plausibility of this story and repeated the original experiment at its 80st anniversary. The found that bad breath alone wouldn't do the trick, but that more likely Stern was actually puffing on a cigar when Gerlach handed him the invisible result.


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  2. Hi Bee & Stefan,

    Thanks as I’d never heard that story before and found the full story you pointed to even more interesting in discovering Stern was a post doctorial student of Albert Einstein who had him first become interested in such quantum experimentation. It was also interesting to learn that Born got involved in helping support the research of Stern and Gerlach by charging an admittance fee for series of public lectures he gave on Relativity. However the most interesting thing I learned in all this is that neither Stern , Gerlach or others at the time realized this experiment had demonstrated the spin character of the quanta. So then the next time someone tell me the Stern-Gerlach experiment was designed to have spin recognized as a real quality of the quanta I’ll ask them what is it that they’ve been smoking:-)

    ”A curious historical puzzle remains. In view of the interest aroused by the SGE in 1922, we would expect that the postulation of electron spin in 1925 should very soon have led to a reinterpretation of the SGE splitting as really due to spin. However, the earliest attribution of the splitting to spin that we have found did not appear until 1927, when Ronald Fraser noted that the ground-state orbital angular momentum and associated magnetic moments of silver, hydrogen, and sodium are zero. Practically all current textbooks describe the Stern-Gerlach splitting as demonstrating electron spin, without pointing out that the intrepid experimenters had no idea it was spin that they had discovered.”

    -Physics Today, “Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics”, December 2003, page 53



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  4. "So then the next time someone tell me the Stern-Gerlach experiment was designed to have spin recognized as a real quality of the quanta I’ll ask them what is it that they’ve been smoking"

    With all due respect, Phil, could you restate that in English please, colloquial dialect? Because I'm preeeety sure what you're saying, but not quite.

    Thanks for the memories of college daze, btw.

    Also Phil, you can't get mad at me because I did start this post off with "With all due respect...." :-)

  5. Official Truth says chemistry is merely a subset of physics. Ask a physicist for an ab initio aspirin. A discipline is blind to what it denies exists until its face is pushed into it.

    Blow a little smoke, see what develops. Physics denies the fundamental nature of physical chirality for its being an extrinsic emergent phenomenon. That's where the next dollop of fun resides. Somebody should look.

  6. Uncle Al,
    do You smoke?

  7. @Georg: Smokin'! in the lab, never liked tobacco, and anyone can do better than on the street,

    Adding a little disorder often packs things tighter. The trick is stumbling upon the correct strange orbit. TRIZ, the Theory of Inventive Problem-Solving That is one fine toolbox.

    Hey physics... opposite shoes. After all, it CAN'T work that way. You'd think they'd be happy to look.

  8. Robert Bunsen said one time:

    "Es gibt auch Chemiker, die nicht rauchen,
    die sind aber auch danach!"

    "There are some chemists who do not smoke,
    but those are like that!"

    The translation of the second part is not really good, but I do not know better.

  9. Hi Georg,

    Perhaps Bunsen meant while some Chemists smoke the work of some others presents as smoke as having little substance.




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