## Wednesday, August 01, 2012 ... /////

### Gerlach and Osheroff: birthdays

Two well-known enough physicists were born on August 1st: Walter Gerlach in 1889 and Douglas Osheroff in 1945.

Osheroff

Douglas Osheroff – congratulations to Stanford – was born to an originally Jewish Russian father and an originally Slovak mother in the Washington state.

As an undergrad, he would attend Feynman's lectures at Caltech, among other things. In the late 1960s, he would already investigate helium-3 at Cornell together with David Lee and Robert Richardson. Note that helium-3 is a fermion, whether we talk about the nucleus or the whole atom, so it has to pair with another helium-3 to produce a boson which may exhibit phenomena similar to superfluidity and/or related but inequivalent Bose-Einstein condensation.

The superfluidity of helium-4 had been known from the 1937 experiments by Kapitsa, Allen, and Misener, and it's debatable whether the superfluidity of yet another substance is such a radical discovery (although the pairing is needed, much like lower temperatures than for helium-4) – but many advances in the 1960s differ from their ancestors in the 1930s by similar "details'.

The three men later shared the 1996 Nobel prize for their discovery of the superfluidity of helium-3 – for work that Osheroff did many years before he received his PhD (in 1973).

Gerlach

Walter Gerlach was born in a pure enough German family; he's one of the physicists who highlight the dominance of the German physics at the beginning of the 20th century. I wonder whether he was a relative of the Gerlach after whom the highest peak of Slovakia (and former Czechoslovakia) is named (well, the peak is named after the village Gerlachov but I guess that this village's name also has some "human" origin).

His PhD would be about radiation. During the First World War, he served in the German Army under Max Wien (a physicist but you shouldn't confuse him with Wilhelm Wien after whom the displacement law in the black body radiation is named) but the service was physics-oriented – he did WiFi telegraphy. ;-)

Most importantly, he began to teach in Frankfurt in 1920 and in November 1921, he together with Otto Stern discovered the spin quantization in magnetic fields via their Stern-Gerlach experiment. Much later, he would probably be among the top 10 physicists who worked on the nuclear bomb for a rogue state, the Nazi Germany, but he wasn't punished for that in any way.

But let me return to the Stern-Gerlach experiment. A furnace was shooting silver atoms into a tube with some magnetic field. The silver atoms only produced two spots on the photographic plate, in disagreement with classical physics that would predict a whole line – with the destination determined by the orientation of the magnetic moment vector.

Now, we must realize that in 1921-1922, people wouldn't have modern quantum mechanics yet. They were interpreting the results in the framework of the "old quantum theory" – Bohr and Bohr-Sommerfeld models etc. Those models had "realist" trajectories that just happened to be quantized according to an ad hoc rule.

It seems clear to me that no classical model of this kind could ever describe similar experiments in a satisfactory way. The spin is one of the observables that make the need for quantum mechanics most directly obvious. By thinking about the Stern-Gerlach experiment and/or easily generalized gedanken or real experiments carefully enough, you must be able to figure out that the values of $J_z=\pm 1/2$ cannot be "classically determined" in advance because that would violate the rotational symmetry; a restriction saying that the projection of the spin with respect to any axis is quantized (and what holds for one axis must hold for all of them by the rotational symmetry) has no solutions.

There were several confusing enough experiments (for a classical physicist) that made the birth of quantum mechanics kind of inevitable – the smartest guys ultimately needed three more years to find its probabilistic-and-operator foundations. It could have been faster but it could have been slower, too.