The universe is expanding, which means that distant galaxies appear to be moving away from us at a big fraction of the speed of light. Einstein showed us that the rate at which your clock ticks depends on your motion and rates that you see a clock ticking depends on the motion of that clock relative to you. So fast moving objects, you see that their clock appears to be ticking slower. And so we might see these quasars fluctuating. These are supermassive black holes that are each seeing a bunch of gas from their galaxy,. They splatter and they split over time. But if you're looking at something that's half a universe away, then this thing called relativistic time d