Was there someone who got a medical education, took an interest in physics and later became a known theoretical physicist?
I am a medical student studying graduate mathematics and consider becoming a theoretical physicist myself.
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Hermann von Helmholtz not only had a formal education in medicine but even made his living of it for some time. Further in the past this was quite common among naturalists, philosophers and mathematicians, but designation of profession as "theoretical physicist" narrows the time frame. There is no doubt however that Helmgoltz was a theoretical physicist, making seminal discoveries in physics.
He was a military doctor. When he published his paper "On preservation of living force" (vis viva in Latin, "energy" in modern language) he was praised by his military superiors for finally doing some useful research:-)
One name that comes to mind is Alfred Kleiner, Einstein's doctoral advisor, although he was more distinguished as an experimentalist than a theoretician. He graduated as a medical student from the University of Zurich in 1872, and obtained bis Dr. Med in 1874. During his Berlin studies in 1873-1879 he became attracted to physics, and worked as an assistant at the physical institute of Polytechnikum until 1885. Here is from The Historical Development of Quantum Theory, Volume 5, Part 1, p.280:
In 1885 he was promoted to full professor and director of the new physics institute in Ramistraße. He established his own experimental research programme in physics, systematically investigating the following topics: temperature dependence of specific heats and heats of solutions; dielectric behaviour of non-ideal insulators; frequency dependence of the resistance and inductance of coils; and the influence of intermediate matter on gravity. Kleiner was an experienced experimentalist, who constructed, for example, a quadrant electrometer of very high sensitivity; he also trained a number of talented students, such as Friedrich Adler (1879-1960), son of the Austrian Socialist Leader Victor Adler.
Einstein turned to Kleiner after his falling out with Weber in 1901, and changed his topic from thermoelectricity to molecular kinetics. A 1901 letter to Mileva Marić mentions that he had "spent the whole afternoon with Kleiner in Zurich and explained my ideas on the electrodynamics of moving bodies to him. ...He advised me to publish my ideas about the electromagnetic theory of light for moving bodies together with the experimental method." On the Electrodynamics of Moving Bodies was the paper where Einstein developed special relativity in 1905. However, when in 1908 Kleiner proposed an extraordinary professorship for theoretical physics, he thought that Adler was the best candidate. Adler declined, saying that Einstein's theoretical work was superior, and Einstein was then elected to the position. Einstein's overall characterization of Kleiner (in a letter to Laub) was ambivalent:
[...] "Professor Kleiner, the chief of our institute is perhaps not a magnificent physicist, but he is a wonderful man, whom I enjoy. It seems to me that scientific distinction and the worth of a personality do not always go together."