Space-Time: 4 dimensions
The difficulties of different appearances to different observers can be dealt with by considering “space-time” as one 4-dimensional space.
Define “length in space-time” by S given by S2 = L2 - (ct)2 = L02 (Note negative sign. This is the difference from usual Pythagorian Theorem)
c is the conversion factor between time and space
Even embedded in our international standards for length! The meter is now defined in terms of the speed of light!