Classical Physics Books

“The development of quantum mechanics in the 1920s motivated physicists to tackle all the unsolved problems of physics with the new methods and see if they worked (they mostly did). But what was the evidence for any of this new way of thinking?

The evidence that was persuasive at the time was a number of rather abstract physics experiments concerning the nature of atomic spectra or the interaction between light and metal surfaces. Each was important in its own way, but what ought to have played an important role in retrospect was something far, far simpler: the observation that magnets work. The crucial step was made by an unknown Dutch scientist called Hendreka van Leeuwen, and what she showed was that magnets couldn’t exist if you just use classical (i.e. pre-quantum) physics. Hendreka van Leeuwen’s doctoral work in Leiden was done under the supervision of Lenz and the work was published in the Journal de Physique et le Radium in 1921. Unfortunately, it subsequently transpired that her main result had been anticipated by Niels Bohr, the father of quantum mechanics, but as it had only appeared in his 1911 diploma thesis, written in Danish, it was unsurprising she hadn’t known about it. Their contribution, though conceived independently, is now known as the Bohr–van Leeuwen theorem, which states that if you assume nothing more than classical physics, and then go on to model a material as a system of electrical charges, then you can show that the system can have no net magnetization; in other words, it will not be magnetic. Simply put, there are no lodestones in a purely classical Universe.”
― Stephen J. Blundell, Magnetism: A Very Short Introduction