Einstein found a way out. He discovered that, whilst the distance in space between two events and the difference in time between two events each change, there is a quantity that does not change if we switch perspective between inertial frames: the distance in space and time, taken together in a very special way. If we call the distance between Monet’s easel and door Δx and the time difference between the dab of red paint and the click of the lock Δt, then the ‘distance’ Δs2 = c2Δt2 - Δx2 does not change. Both the aviator and Monet agree on Δs, even though they disagree on Δt and Δx. The
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