1. Einstein extended this principle so that it accounted for the constant speed of light, a phenomenon that had been recently observed in the Michelson–Morley experiment.

2. To explain the negative outcome of the Michelson–Morley experiment, George FitzGerald (1889) and Hendrik Lorentz (1892) introduced the contraction hypothesis, according to which a body is contracted during its motion through the stationary aether.