Michelson-Morley Experiment
Intuitive
Source: A Michelson-Morley resonant cavity test of Lorentz invariance by Stephen Parker
Concrete
Abstract
The motto in this section is: the higher the level of abstraction, the better.
Why is it interesting?
The Michelson-Morley experiment discovered that the speed of light is constant in all inertial frames of reference. This lead to the insight that Newtonian mechanics must be replaced with special relativity.