I’m teaching undergrad solid-state for the first time, and it has served as a reminder of how condensed matter physics got off the ground. I suspect that one reason CM historically had not received a lot of respect in the early years (e.g. Pauli declaring that solid-state physics is the physics of dirt) is that it began very much as a grab bag of empirical observations, with the knowledge that the true underpinnings were well out of reach at the time. Remember the order of a few key discoveries:
- Dulong and Petit realize that the specific heats of solids are all about the same size near room temperature, in 1819.
- Seebeck discovers that applying a temperature gradient across metals generates a voltage, in 1822.
- Braun observes that a metal wire touching a lead sulfide crystal conducts much better in one current direction than the other (making what we now call a Schottky diode) in 1874.
- Hall finds that a magnetic field applied perpendicular to a current in a metal leads to a voltage mutually transverse to both the current and the field, in 1879. Two years later he also found that you can get a transverse voltage in a ferromagnet even in the absence of an applied magnetic field, the “anomalous Hall effect” which was not really understood until this century.
- Electrons are discovered in 1897.
A whole host of materials physics observations predate the discovery of the electron, let alone modern statistical physics and quantum mechanics. The early days of condensed matter had a lot of handwaving. The derivation of the Hall effect in the classical Drude picture (modeling electrons in a metal based on the kinetic theory of gases) was viewed as a triumph, even though it clearly was incomplete and got the sign wrong (!) for a bunch of materials. (Can you imagine trying to publish a result today and saying, ‘sure, it’s the wrong sign half the time, but it has to be sort of correct’?)
That we now actually understand so much about the physics of materials is one of the great intellectual accomplishments of the species, and the fact that so much of the explanation has real elegance is worth appreciating.
This history is a great example of why journal editors and referees should be more willing to publish experimental results without requiring a theoretical interpretation.
ReplyDeleteHonestly I wish my solid state teachers had exposed us to a list of experimental puzzles to motivate studying the subject in the first place! The best kind of physics comes out of an experimentalist walking up to you and saying "hey, we saw this weird effect in the lab, can you explain what's going on and whether it's useful?"
ReplyDeleteCondensed matter is one of the few fields of science where this beautiful type of interaction between theory and experiment happens daily. For many other fields, the time for such intimate interactions is long gone.
Well said
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