What is theoretical physics?

Theoretical physicists today still use a core technology that was developed in the 18th century out of the calculus pioneered by Isaac Newton and Gottfried von Leibniz.

saac Newton derived his three Laws of Motion through close, almost obsessive observation and experimentation, as well as mathematical reasoning. The relationship he discovered between force and acceleration, which he expressed in his own arcane notation of fluxions, has had the most impact on the world in the differential notation used by his professional rival, Wilhelm von Leibniz, as the familiar differential equation from freshman physics:

F = ma = m(d^2x/dt^2)