I'm currently reading a book by top physicist Kip Thorne called 'The Science of Interstellar.' He was the science advisor for the film 'Interstellar.'
In order to explain some of the weird stuff in the film with regard to the black hole featured therein he puts forward some mind boggling concepts.
In order to do this he gives an introduction to Einstein's General Theory of Relativity with regard to time dilation in gravitational fields, and our old friend the GPS comes in for quite a lot of the explanation.
Here's a typical sentence: "Time at a 20,000 kilometre height flows more rapidly on Earth by forty microseconds each day, and the satellites must correct for this. They measure time with their own clocks, then slow that time down to the rate of time flow on Earth before transmitting it to our phones."
You have to wonder what planet Albert Einstein came from ?
