As far as I know, each GPS device sycs with several different satellites, or clocks, at least 3, and also corrections for gravitational effects from general relativity.
2011/10/14 Stephen A. Lawrence <[email protected]> > OMG -- of course! You can't synchronize (all) clocks on the Earth's > surface -- it's a rotating frame, and Sagnac comes around and bites you on > the bumm if you try! Yet by using the GPS satellite signals, which are > available everywhere, they were doing essentially that: using a "universal" > time value, which doesn't produce a sensible result on the Earth's surface. > > Using clocks in another frame (the GPS clocks) to synchronize the clocks in > the rotating frame (on the surface of the earth) just adds confusion, it > doesn't avoid the problem, which is fundamental. In particular, if you sync > your (rotating) clocks with an external source, then when you measure light > speed you find it's anisotropic -- it's faster one way than the other. > > The only way to deal with it an experiment like this it is to pick just two > clocks and E-sync them using point-to-point two-way light travel (or use > some other source, but then figure out what the E-sync times would have been > and use the computed values). Apparently, they didn't do that. > > When I say it's "fundamental", I mean that if you use two-way light signals > to sync pairs of clocks on the Earth's surface, and you do it for a chain of > pairs of clocks reaching all the way around the Earth, you will find the > clocks at the beginning and the end of the chain are stubbornly out of sync, > even though the clocks all the way back along the chain were in sync. And > you can't avoid it by using some other clock source. > > See, for example, > > http://www.physicsinsights.**org/sagnac_1.html<http://www.physicsinsights.org/sagnac_1.html> > > > > > On 11-10-14 01:42 PM, Terry Blanton wrote: > >> On Fri, Sep 23, 2011 at 10:30 AM, Terry Blanton<[email protected]> >> wrote: >> >>> Don't bury Einstein yet: >>> >>> http://www.newscientist.com/**article/dn20957-dimensionhop-** >>> may-allow-neutrinos-to-cheat-**light-speed.html<http://www.newscientist.com/article/dn20957-dimensionhop-may-allow-neutrinos-to-cheat-light-speed.html> >>> >>> "Sher also mentions a third option: that the measurement is correct. >>> Some theories posit that there are extra, hidden dimensions beyond the >>> familiar four (three of space, one of time). It's possible that the >>> speedy neutrinos tunnel through these extra dimensions, reducing the >>> distance they have to travel to get to the target. This would explain >>> the measurement without requiring the speed of light to be broken." >>> >>> Those neutrinos probably knew a short cut in the other 6 dimensions. >>> >> Well it wasn't extra dimensions. It was relativity itself. They >> needed entangled clocks! >> >> http://www.technologyreview.**com/blog/arxiv/27260/?p1=blogs<http://www.technologyreview.com/blog/arxiv/27260/?p1=blogs> >> >> "Faster-Than-Light Neutrino Puzzle Claimed Solved by Special Relativity" >> >> T >> >> >> >
