I think it is (in this usage) meant as the gain in human understanding of the photoelectric effect (Einstein 1905), specific heat, diamagnetism, and so forth on

I think it is (in this usage) meant as the gain in human understanding of the photoelectric effect (Einstein 1905), specific heat, diamagnetism, and so forth once quantum mechanics began development. The leap from classical mechanical theories being fundamental to quantum mechanical theories being fundamental (with classical physics emerging from that), in other words, was a big shift even though subatomic particles themselves, and their bound states, are generally very small. Random doesn't really enter into it. One can get that in a firmly non-quantized theory, as in one of Einstein's other 1905 papers, https://en.wikipedia.org/wiki/Brownian_motion#Einstein's_theory https://en.wikipedia.org/wiki/Brownian_motion#Einstein's_the... https://history.aip.org/exhibits/einstein/essay-brownian.htm https://history.aip.org/exhibits/einstein/essay-brownian.htm https://physicsworld.com/a/einsteins-random-walk/ https://physicsworld.com/a/einsteins-random-walk/