Fast forward to Albert Einstein, whose special theory of relativity radically reshaped our conception of time. One key aspect of the theory is the relativity of simultaneity. If events A and B are separated in space (i.e., not causally related) people in relative motion (e.g., walking and standing still) won't agree whether A and B happened simultaneously, or A was followed by B. This lack of absolute simultaneity, which is a function of the speed of light being constant, has some pretty freaky consequences.