Theoretical Physics is the second branch of physics invented in modern times to combat the traditional branch of experimental physics. These two branches are ideological opposites on how to conduct physical enquiries and derive valid explanations of nature as revealed to be truth. The traditional experimental physics was the original method of physics for hundreds of years until the start of the 19th century, when the new theoretical physics began to become popular. This was initiated by Einstein and Bohr who corresponded with famous "Gedanken" "experiments". Gedanken is the German word for "thought". These were "thought experiments" although there was never any actual experiment conducted. This was simply Einstein and Bohr's way of creating a new conception of physics using the terms and language that they knew, namely the experimental physics. Gedanken experiments were just arguments between Einstein and Bohr on what the "correct" result of a hypothetical experiment should be. Bohr insisted on the probability quantum theory, and Einstein on Newton's determinant system. Bohr and other's including Heisenberg, Schroedinger and Born convinced the scientific community of the "correct" quantum "Copenhagen" interpretation of physics.
Nowadays, theoretical physics is much more popular (see TV programmes dedicated to the theoretical physics approach) and has overtaken the traditional experimental physics, even though this approach is very controversial and has been spoken against by many people.