22 ms·

Disclaimer: It's been a few years since uni, and I haven't read the actual paper, just the linked article. From what I can tell, the quoted passage, "From the

Disclaimer: It's been a few years since uni, and I haven't read the actual paper, just the linked article. From what I can tell, the quoted passage, "From the perspective of the clock, the detector is moving towards the source and consequently the distance travelled by the particles as observed from the clock is shorter," is the core of the argument. I think this is what you need to consider/read into it: - There is relative rotation involved, so the frames are accelerating with respect to each other. - Since the times involved are very short with respect to the period of rotation, we can probably ignore the acceleration itself (?), BUT: - LHC and GS have differing velocity vectors at any given time due to their difference in longitude. This means we have to treat them as separate frames of reference. ("From the perspective of the clock, the detector is moving towards the source") - I think this is just about a single satellite (though since the satellites are in orbit, they too are accelerating wrt earth) - I'm not sure if the effect will vary slightly depending on the satellite's position, but as it has to be "visible" from both locations, the range of possible positions is fairly constrained. I'd have to work it through/read the paper. :-P And yeah, comparing to the time of a photon is going to be difficult through the earth. Even gamma radiation won't make it.

VladRussian 15y ago
>And yeah, comparing to the time of a photon is going to be difficult through the earth. Even gamma radiation won't make it. so instead of comparing to photon going through the fiber on the surface we're comparing against photon(s) going to the satellite and back. Even just using HAM radio bouncing from ionosphere would be less complicated and more straightforward than through GPS satellites here.
- nitrogen 15y ago
  1. The distance traveled by a photon going through a fiber optic cable will be different from the distance through the ground. 2. The distance traveled by radio waves bouncing off the ionosphere would have much more uncertainty than the ~60 feet in question. 3. Radio waves in the air and photons in a fiber will move noticeably slower than c due to the fact that they are not traveling through a vacuum. Neutrinos, however, minimally interact with matter, so they would travel faster than the test photon(s).
  - VladRussian 15y ago
    >1. The distance traveled by a photon going through a fiber optic cable will be different from the distance through the ground. it is still shorter than through the satellite. Also the fiber distance difference can be measured and accounted for - much simpler than to account for satellite position.
    - nitrogen 15y ago
      But they're not bouncing photons up to and down from a GPS satellite; they're only using GPS for clock synchronization and measuring the distance, which is accurate to a lot less than the distance light travels in 60ns.
      - VladRussian 15y ago
        >But they're not bouncing photons up to and down from a GPS satellite; they're only using GPS for clock synchronization by noting photons coming from GPS when neutrinos start at CERN and when neutrinos reach Gran Sasso. 2 downward legs (actually 3 as the photons at the start of neutrino run need to be noted at both points) instead of 1 upward and 1 downward that would constitute proper "bouncing" - i don't see much difference to warrant specific term in this case than talking about general approach of racing the neutrinos against photons. Compare to running through a fiber, which length (only 700km) and speed of light inside it can be easily accounted for, with GPS we have photons coming from the 12K miles distance (in the best case, ie. when the satellite is right overhead) which is measurable much less precisely than the fiber on the ground and these photons are coming through the atmosphere conditions of which (specifically speed of light in it) are much less precisely known than about the same fiber on the ground. >and measuring the distance, which is accurate to a lot less than the distance light travels in 60ns. until they used the military GPS they would have positioning error in the Earth surface tangential plane of a few feet at least (say 2 feet though it would be an extremely small error). Using 20000km/700km proportionality of that triangle, 2 feet on tangential plane would mean ~60 feet in GPS signal arrival precision, ie. 60ns+ of time of flight (it is of course oversimplification, only to demonstrate the scale of imprecision inherently present in the scheme with satellite flying over at 20K km) Very interesting reading btw, especially on atmospheric and Stagnac effect errors : http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_P...

VladRussian 15y ago

>And yeah, comparing to the time of a photon is going to be difficult through the earth. Even gamma radiation won't make it. so instead of comparing to photon going through the fiber on the surface we're comparing against photon(s) going to the satellite and back. Even just using HAM radio bouncing from ionosphere would be less complicated and more straightforward than through GPS satellites here.

nitrogen 15y ago
1. The distance traveled by a photon going through a fiber optic cable will be different from the distance through the ground. 2. The distance traveled by radio waves bouncing off the ionosphere would have much more uncertainty than the ~60 feet in question. 3. Radio waves in the air and photons in a fiber will move noticeably slower than c due to the fact that they are not traveling through a vacuum. Neutrinos, however, minimally interact with matter, so they would travel faster than the test photon(s).
- VladRussian 15y ago
  >1. The distance traveled by a photon going through a fiber optic cable will be different from the distance through the ground. it is still shorter than through the satellite. Also the fiber distance difference can be measured and accounted for - much simpler than to account for satellite position.
  - nitrogen 15y ago
    But they're not bouncing photons up to and down from a GPS satellite; they're only using GPS for clock synchronization and measuring the distance, which is accurate to a lot less than the distance light travels in 60ns.
    - VladRussian 15y ago
      >But they're not bouncing photons up to and down from a GPS satellite; they're only using GPS for clock synchronization by noting photons coming from GPS when neutrinos start at CERN and when neutrinos reach Gran Sasso. 2 downward legs (actually 3 as the photons at the start of neutrino run need to be noted at both points) instead of 1 upward and 1 downward that would constitute proper "bouncing" - i don't see much difference to warrant specific term in this case than talking about general approach of racing the neutrinos against photons. Compare to running through a fiber, which length (only 700km) and speed of light inside it can be easily accounted for, with GPS we have photons coming from the 12K miles distance (in the best case, ie. when the satellite is right overhead) which is measurable much less precisely than the fiber on the ground and these photons are coming through the atmosphere conditions of which (specifically speed of light in it) are much less precisely known than about the same fiber on the ground. >and measuring the distance, which is accurate to a lot less than the distance light travels in 60ns. until they used the military GPS they would have positioning error in the Earth surface tangential plane of a few feet at least (say 2 feet though it would be an extremely small error). Using 20000km/700km proportionality of that triangle, 2 feet on tangential plane would mean ~60 feet in GPS signal arrival precision, ie. 60ns+ of time of flight (it is of course oversimplification, only to demonstrate the scale of imprecision inherently present in the scheme with satellite flying over at 20K km) Very interesting reading btw, especially on atmospheric and Stagnac effect errors : http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_P...