There can never be a clock at the center of the Universe to which everyone can set their watches. Your entire life can be the blink of an eye to an alien who leaves Earth traveling close to the speed of light, then returns an hour later to find that you have been dead for centuries.
Clifford A. Pickover, The Physics Book: From the Big Bang to Quantum Resurrection, 250 Milestones in the History of Physics
aliens, physics, special-relativity, universe