Is there a universal "now"?

There is no universal present moment, since no instrument reads one and no procedure produces one. What exists is an order of causes: one event comes after another when it read something the other wrote, and events with no such chain between them have no fact about which came first. In a recipe, the water must boil before the pasta goes in, while the cheese can be grated whenever, and nothing in the dinner says when. A timeline is that order with the ties broken, and ties can be broken many equally correct ways. In 1971 four atomic clocks flew around the world. Eastbound they came back 59 nanoseconds behind the clocks on the ground, westbound 273 ahead. Clocks compare only where they meet, since at a distance nothing measures how a signal's round trip splits between out and back.