Any sufficiently advanced technology is indistinguishable from magic. -- Arthur C. ClarkeWhen I ducked into DC last fall for a quick visit to one of my favorite museums after completing some business in the suburbs, my GPS served me well. It led me right to the Phillips Collection, taking me through a back alley shortcut and delivering me right to an available parking space in front of the building. It was like magic (except for the parking space, which was just a lucky accident).
For most of us, GPS technology -- whether on a cell phone or in a car -- does seem indistinguishable from magic. But GPS doesn't rely on magic; it relies on Relativity. GPS devices provide the most direct application of Einstein's General and Special Relativity most of us ever encounter directly in our lives.
I had been aware of this in a general sort of way, but awhile back Making Light provided this link to a great post by Richard C. Pogge at Ohio State that explains why the satellite-based navigation system had to be designed to take relativistic effects into account.
If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time. This kind of accumulated error is akin to measuring my location while standing on my front porch in Columbus, Ohio one day, and then making the same measurement a week later and having my GPS receiver tell me that my porch and I are currently about 5000 meters in the air somewhere over Detroit.There's more. Check it out here. Although there's no magic involved, the account of how Einstein's insights proved to be absolutely vital to modern electronic navigation is fascinating.