Atomic Clocks

"The experiment began with a back-of-the-envelope calculation."https://spacedaily.com/d-a-1971-experiment-flew-four-atomic-clocks-around-the-world-on-commercial-airliners-first-heading-east-then-heading-west-and-when-the-clocks-were-brought-home-and-compared-with-sta/

Space and Time

"Space and time looked settled, at least in broad outline."https://www.thebrighterside.news/post/physicists-propose-that-our-universe-may-contain-three-dimensions-of-time/