Question: What is a theoretical physicist

Edoardo Vescovi answered on 18 Jun 2019: last edited 18 Jun 2019 9:55 am

Theoretical physics is the part of physics that has little to do with experiments. Hard to believe: the scientific method says to observe natural phenomena, make models and put them to a test against experiments. How can physics live without experiments?
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Our brain is wired up so to like explaining many facts with the least effort. In physics we find useful to find the key formula that explains many natural phenomena. The falling down of a stone and the moon’s orbit seem to have little in common, yet they are controlled by a single force (gravity) and a simple formula (invented by Newton). You never see “gravity”, unlike stones and the moon, but the “idea of gravity” in our mind explains millennia of sky observations in one shot and makes predictions for thousand years to come!
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Theoretical physics borrows tons of experiments and organise them under the rigour of mathematics. It defines abstract concepts (time, space, force, particle) that obey a small set of general rules (axioms). Nobody tells which ones: theoretical physics is a human invention. As formulas are nice and simple, their solutions need hard math. If you take those concepts and axioms and use similarities and a pinch of intuition, theoretical physics can say great things on nature: which particles should exist, how galaxies should move, how many dimensions the universe should have. Sometimes too great: current technology is not enough to set up precise experiments, but we know what to build at least! If experiments fail, we change those concepts and axioms, then repeat.
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Einstein was a theoretical physicist. When he wrote the theory of relativity, he didn’t want to solve a troublesome (Michelson-Morley) experiment. He was bugged instead by the general question: how do we measure the speed of light using clocks, rulers, and flashlights? In that he concluded that time slows down at high speeds. Experiments tested this 50 years later and knowing that to look for!