Michelson Morley experiment threw Aether theory out, Einstein suggested Light was constant, used his thought experiments to explain this, however thought experiments are limited to common sense. As time developed this theory was able to be proved true using
the flying of atomic clocks to determine the existence of time dilation
the dilated lifetimes of mesons penetrating the Earth’s atmosphere
the energy yield from converted mass in nuclear reactions
the observed increase in the mass of particles accelerated to near-light speed, in devices such as particle accelerators.
With Light becoming a constant, everything such as space and time became relative to it. Length becomes smaller for the observer, time appears slower to the observer, and mass dilates for the observer.