Your Phone Runs on Einstein

Shows a satellite’s clock running fast, and the fix engineers built into it before it ever left the ground. That fix is the one piece of Einstein almost everyone has already heard of - and, as this lesson tells it, the one piece that never actually reaches your hand.

Before you start. You don’t need physics from anywhere else to follow this one; every number here is worked out from nothing. If you’ve already read about how satellites keep time, you’ll recognise the ending - the rest of the story is new.