After the discovery of gravitational waves, there’s a lot of talk about Einstein’s General Relativity. We usually talk about it in the context of black holes and other things we don’t quite see every day, but I bet you held a relativity experiment in your hand in the past 10 minutes. Indeed, if you used a smartphone or anything with a GPS, you effectively performed a general relativity experiment.
Indeed, the 31 GPS satellites actually spend their days broadcasting the time on the super-accurate atomic clock each has on board.
The signal takes a few hundredths of a second to reach you on the ground. So comparing very accurately the time on your watch to the signal from the satellite, you can calculate how far it is. Putting together the distance from enough satellites, you will find your own position on the planet.
It’s all nice and fine up to here, but what does this have to do with relativity?
According to the theory, higher up in gravitational fields (say, when you’re orbiting in space) time flows ever-so-slightly faster. A minute in orbit is a teensy weensy shorter than a minute on Earth. As usual with relativity, the effect is small, so small you wouldn’t notice.
Because the effect is so tiny, GPS satellites were first deployed with relativistic corrections turned off. Scientist figured it wouldn’t make a difference. Boy were they wrong: in a short time, the localization was off by kilometers.
Since they also thought this might happen, the scientists had made possible to switch on the corrections from the ground.
So every time your satnav tells you where to turn, every time Google Maps tells you accurately how far the nearest pub is, you’re actually confirming that General Relativity is correct.