6 ms·

The last thing I heard about this was that the results could be explained by the researchers overlooking the relativistic motion of the GPS clocks, and when cor

The last thing I heard about this was that the results could be explained by the researchers overlooking the relativistic motion of the GPS clocks, and when corrected for it, the results were actually another confirmation of special relativity. See http://www.technologyreview.com/blog/arxiv/27260/ http://www.technologyreview.com/blog/arxiv/27260/ That said, a brief search turned up this paper ( https://facultystaff.richmond.edu/~ebunn/vanelburg.pdf https://facultystaff.richmond.edu/~ebunn/vanelburg.pdf ) which argues that that explanation is faulty (i.e., the paper made a mistake, and when corrected, the original researchers' results stand). (Disclaimer: although I once read a book on this stuff, I shouldn't be confused with an expert, and I have no idea who's right and who's wrong. But it is exciting.)

sliverstorm 15y ago
I seem to remember when that theory was discussed, it was explained that GPS clocks already take into account relativistic motion. Or something like that, don't quote me, I'm not a physicist.
- nobody31415926 15y ago
  GPS onboard clock is corrected for relativity due to it's orbit height and speed so it's clock time is approximately correct But measuring the simultaneity of an event between two different points with a moving clock is a lot trick.
  - bermanoid 15y ago
    That might have been an issue if they were measuring anything with the satellite clocks. But they weren't. By my understanding, they were merely using the satellite signals to synchronize the ground clocks in the ground frame, and after that was done, the satellite's were never heard from again.
    - nobody31415926 15y ago
      Synchronizing two clocks in different positions is very difficult since relativity always comes in as you are moving a third clock between them. It's essentially impossible if you don't know the gravity field encountered by the moving clock.
- Confusion 15y ago
  I am a physicist and GPS indeed takes relativistic effects into account. GPS would not just be slightly wrong if it didn't: it would be entirely worthless. See for example [1]. [1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
  - jahnu 15y ago
    Fun fact: When designing the original system there was disbelief amongst some involved that the relativistic effects were either real or relevant so they built in a switch to be able to remotely disable those parts of the calculations. They never did disable them.
    - Angostura 15y ago
      Actually, that gose down as the funnest fact that I've learned for at least a month, so thank you.
      - jahnu 15y ago
        I don't remember for sure but I think I learned that from one of these lectures... "Particle Physics for Non-Physicists: A Tour of the Microcosmos" http://www.thegreatcourses.com/tgc/courses/course_detail.aspx?cid=1247 http://www.thegreatcourses.com/tgc/courses/course_detail.asp...
bermanoid 15y ago
That paper got some wide circulation for some reason, but it was an extremely simple criticism (six or seven equations, no derivations involving anything more substantial than distance = rate x time) by a computer scientist that simply misinterpreted the way the GPS synchronization was used (none of the physics was wrong in the paper, FWIW, the problem was that the author's claims about it having any connection to the OPERA experiment were completely unfounded). IIRC someone from OPERA responded to questions about that paper by saying, basically, that it wasn't even worth debunking because they had real work to do, and publicly disproving every amateur with a theory would suck up all their time.
Arjuna 15y ago
This is an interesting paper on the topic: Determination of the CNGS Global Geodesy [1]. It discusses how the distance traveled by the neutrino beam was calculated. It covers GPS measurements, tidal effect considerations, the Sagnac effect [2], etc. [1] PDF: http://operaweb.lngs.infn.it/Opera/publicnotes/note132.pdf http://operaweb.lngs.infn.it/Opera/publicnotes/note132.pdf [2] http://en.wikipedia.org/wiki/Sagnac_effect http://en.wikipedia.org/wiki/Sagnac_effect

sliverstorm 15y ago

I seem to remember when that theory was discussed, it was explained that GPS clocks already take into account relativistic motion. Or something like that, don't quote me, I'm not a physicist.

nobody31415926 15y ago
GPS onboard clock is corrected for relativity due to it's orbit height and speed so it's clock time is approximately correct But measuring the simultaneity of an event between two different points with a moving clock is a lot trick.
- bermanoid 15y ago
  That might have been an issue if they were measuring anything with the satellite clocks. But they weren't. By my understanding, they were merely using the satellite signals to synchronize the ground clocks in the ground frame, and after that was done, the satellite's were never heard from again.
  - nobody31415926 15y ago
    Synchronizing two clocks in different positions is very difficult since relativity always comes in as you are moving a third clock between them. It's essentially impossible if you don't know the gravity field encountered by the moving clock.
Confusion 15y ago
I am a physicist and GPS indeed takes relativistic effects into account. GPS would not just be slightly wrong if it didn't: it would be entirely worthless. See for example [1]. [1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
- jahnu 15y ago
  Fun fact: When designing the original system there was disbelief amongst some involved that the relativistic effects were either real or relevant so they built in a switch to be able to remotely disable those parts of the calculations. They never did disable them.
  - Angostura 15y ago
    Actually, that gose down as the funnest fact that I've learned for at least a month, so thank you.
    - jahnu 15y ago
      I don't remember for sure but I think I learned that from one of these lectures... "Particle Physics for Non-Physicists: A Tour of the Microcosmos" http://www.thegreatcourses.com/tgc/courses/course_detail.aspx?cid=1247 http://www.thegreatcourses.com/tgc/courses/course_detail.asp...

bermanoid 15y ago

That paper got some wide circulation for some reason, but it was an extremely simple criticism (six or seven equations, no derivations involving anything more substantial than distance = rate x time) by a computer scientist that simply misinterpreted the way the GPS synchronization was used (none of the physics was wrong in the paper, FWIW, the problem was that the author's claims about it having any connection to the OPERA experiment were completely unfounded). IIRC someone from OPERA responded to questions about that paper by saying, basically, that it wasn't even worth debunking because they had real work to do, and publicly disproving every amateur with a theory would suck up all their time.

Arjuna 15y ago

This is an interesting paper on the topic: Determination of the CNGS Global Geodesy [1]. It discusses how the distance traveled by the neutrino beam was calculated. It covers GPS measurements, tidal effect considerations, the Sagnac effect [2], etc. [1] PDF: http://operaweb.lngs.infn.it/Opera/publicnotes/note132.pdf http://operaweb.lngs.infn.it/Opera/publicnotes/note132.pdf [2] http://en.wikipedia.org/wiki/Sagnac_effect http://en.wikipedia.org/wiki/Sagnac_effect