What the Global Positioning System Tells Us about Relativity

The first GPS satellites had a switch to enable the correction for relativistic effects because there was some skepticism if it was needed to do so.

It didn't take very long before it was decided that the switch needed to be permanently in the 'on' position, and it was dropped from subsequent models.

EDIT: makes you wonder if Einstein had not made his predictions regarding relativity, this would have been the first real opportunity to find out in a 'natural' way, how long it would have taken them based on the observation that things did not work as intended.

I've actually seen a BBC Horizon documentary a while ago, "Do You Know What Time Is?" [http://www.bbc.co.uk/programmes/b00fyl5z], that also talks about this. On top of all the theoretical background, they interview people at the US Army that manage the GPS sattelites, and show how they have to resync the internal clocks every day because of this.

Worth a watch if you like this kind of material.

EDIT: Sorry, I believe it was a different episode: "What On Earth Is Wrong With Gravity?" [http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband...]

Here are the online video links to both episodes:

http://www.youtube.com/watch?v=V3aYKAJEVfQ

http://video.google.com/videoplay?docid=-421765295887486158

The discussion of the twin paradox and SR seems to be lacking. There's a focus on only the time dilation effects and nothing on length contraction which would resolve the issue. Instead it becomes a mess of confusion.

No, there have been numerous other signs of relativity, going all the way back to the unexpected aberration of the orbit of Mercury that Einstein was aware of. Eventually somebody would have noticed the gravitational deflection of the stars during eclipses, too, though it could have been decades before that happened. (Not because of technology, of course, which clearly existed in Einstein's day, though just barely, but simply because nobody might have noticed.)

Einstein was definitely ahead of the curve, but wasn't running on zero data, even if we discount the purely mathematical issues, such as the inability to reconcile the lack of a reference frame in the Maxwell equations with the need for a reference frame for all other physics of the time.

(It might be worth pointing out here that relativity itself was not a new concept; now when we say "relativity" we mean Einsteinian relativity, but there is also, for instance, "Galilean relativity". Relativity wasn't a surprise, it's just nobody could make it all work out properly without contradictions until Einstein.)

I really recommend this online book: http://www.mathpages.com/rr/rrtoc.htm It is both a great (IMHO) introduction to relativity, Einsteinian and otherwise, and also has a lot about history of Einstein's theory and the general state of thought at the time. I do not mean to diminish Einstein's accomplishment, because like I said he legitimately was ahead of the curve, but it wasn't quite the "bolt from the blue" that it is often portrayed as. All the math pieces Einstein needed had been developed, and his genius was largely the combination of figuring out how to put them together in a way that explained reality, and doing so while discarding inappropriate assumptions that were holding back the physics community at large. I'm actually more impressed by the latter, lest you think I'm trying to diminish that accomplishment; that's a hard harder than that sentence may make it sound.

> The accuracy of this comparison is limited mainly because atomic clocks change frequencies by small, semi-random amounts (of order 1 ns/day) at unpredictable times for reasons that are not fully understood

What?