How a Pocket Watch Fits into an Expanding Universe
Time is a fickle thing. Inside a massive star, it flows calmly, like sand in an old clock on a mantelpiece. Outside, on the scale of the entire universe, it stretches along with space, as if the same clock were in an elevator that speeds up and slows down. For a long time, physicists puzzled over whether these two worlds could be glued together without visible seams.