As early as 1904 Hanturo Nagaoka postulated a "Saturnian" model of the atom with flat rings of electrons around a positively charged nucleus, and in 1935 Hideki Yukawa published the theory of mesons which he got the nobel prize for. Similarly Shinichiro Tomonaga.
I largely think of theoretical physics as a European and American thing. How were the Japanese able to get access to scientific articles? Did they have university laboratories? Did the Japanese already have there own study of physics and mathematics before they became aware of the European one? How were japanese physicists received by the Europeans?
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Since Meiji Restoration, Imperial Japan had tried to adapt diverse fields of science (humanities, social sciences, and natural sciences) from various European countries. As for physical science, Japan first took Germany, then USA as its own model. The switching point of this German to USA is probably set around WWI. The science academics of respective country might have some kind of connection with the academics in Japan (kind of a destination country of studying abroad).
As for Hantaro NAGAOKA, one of his first tutors was Scottish Scientist Cargil Gilston Knott (1856-1922) (Obituary Note) who was then invited as a 'foreigner professor' (Oyatoi Gaikokujin: lit. trans. hired foreigner) in Japan, but later he studied under von Hermholz and Bortzmann in Germany (1893-), so, so to speak, he belonged to a kind of 'German Generation' of the History of Science in Japan.