The landscape is magic, the trip is far from being over. - Carlo Rovelli

In his youth Albert Einstein spent a year loafing aim- lessly. You don’t get anywhere by not ‘wasting’ time – something, unfortunately, which the parents of teenagers tend frequently to forget.

GPS... gets messages from satellites. On those satellites are clocks... The satellites broadcast the times and your little device use the time to tringulate wherever you are... [W]hen this was built the physicists told the [engineers] that were putting up the system, "Careful, up there... your clocks are going to run faster..." ...The entire project was controlled by the American army ...supervised by the generals ...[who] didn't believe it. So the first satellites were put up there with a switch... and of course it does not work if you do not take this into account. ...You would crash the car against the wall, driving with the navigator if we didn't [take] this into account ...

[I]n general relativistic observations there is no preferred independent time variable. What we measure are a number of variables, all on equal footing, and their relative evolution. ...[I]t can be equally read it as the evolution of the variable T₁ as a function of the variable T₂, or viceversa. Which of the two is the independent variable here?