"It is not easy for one person to be in two places simultaneously"
"It is not even easy for two people to be in two places simultaneously. Yours, Albert"

POSTULAT

1. All the laws of physics (and nature) have exactly the same form in all inertial reference frames. This is the principle of relativity
2. The speed of light in free space is the same for all observers in inertial frames (as seen in the experiment by Michelson and Morley)

Från dessa postulat bygger Einstein upp relativitetsteorin skakar våra uppfattningar om tid, rum och om samtidighet.