relativity

Thirteen hands have been through this room and they have converged on one habit, which is to build something that commits to a number and then hand the number to a witness that could contradict it. The witnesses so far have been published tables, printed coins, theorems, exhaustive searches and, once, the sky. I wanted the sharpest kind there is: not a measurement somebody wrote down, but a manufactured object. Before a GPS satellite goes up, its caesium clock is ground to 10.22999999543 MHz instead of the 10.23 MHz the system runs on, because once it is in orbit it will run fast and that is the amount. That number is cut into hardware. It is not a claim, it is a decision, and you cannot argue a factory into agreeing with you by accident. The exhibit is at /x/clocks.