5 ms·

But the GPS satellites and receivers already correct for these relativistic effects. Specifically: "The engineers who designed the GPS system included these r

But the GPS satellites and receivers already correct for these relativistic effects. Specifically: "The engineers who designed the GPS system included these relativistic effects when they designed and deployed the system. ... Further, each GPS receiver has built into it a microcomputer that (among other things) performs the necessary relativistic calculations when determining the user's location." [1] [1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....

themgt 15y ago
I think it's more subtle than that - it's due to the motion of the satellites relative to the experiment (i.e. the direction of the neutrino path), not to the individual ground stations
thegrossman 15y ago
Relativistic effects are only taken into account when determining the position of the satellite itself, and the rate of it's clock. It can't compensate for the effect in the linked-to article (namely, the fact that the distance and flight time between the neutrino source and destination is shorter according to the satellite, versus an observer on the ground) because that effect depends on the specifics of the experiment. Consider this: If you flipped the location of the neutrino source and destination, you'd actually get the reverse effect (neutrinos would appear to be going slower than light). So it's up to observers on the ground to compensate for relativistic effects of this nature. (As I mentioned in another comment, I would be shocked if they didn't already do that)
- onemoreact 15y ago
  Neutrino's are hard to detect and even harder to generate in detectable quantites. So for now we have exactly one location and one path in the world where we can both generate and detect them at a reasonable distance. Also, GPS satilites don't orbit in one direction you need 3 to quickly get an accurate posisiton so they use a wide range of orbits to minimize the number needed. http://www8.garmin.com/aboutGPS/ http://www8.garmin.com/aboutGPS/ Which is something the article seems to miss.
  - zb 15y ago
    1) You need 4. 2) They all orbit in the same direction. 3) There are a number of different orbits, but they are all inclined at the same angle. The illustration on that page was, I'm afraid, drawn by someone with no clue what the constellation actually looks like.
    - onemoreact 15y ago
      1) Technicaly, there are 2 possible locations that the same distance from each of 3 satilites, but the other point is far off in space. The 4th is vary useful for increased accruacy when you have a shitty ground clock, but it's not 'needed' it's it's loss simply results in lower accracy which can be compensated for by more readings over time. (Which can be vary useful when want accuracy and have a ground station that only get's to see a small chunk of the sky.) 2) They all have polar orbits that reach the same location 4 minutes earlyer each day. However, there horizontal ground speed relative to a stationary observer changes over the course of their orbit. (It's zero at the pole and maxes out at the equator.) And is also diffrent form the other satilites you are comparing it with. Which is why you get those S shaped paths: http://www.n2yo.com/satellites/?c=20 http://www.n2yo.com/satellites/?c=20 3)I agree but see 2. PS: Yea, that picture was fairly bad, I was looking for an easy to read article not a picture. The classic picture of a constilation with fixed polar obits is vary misleading once you take the earth's rotation into consideration. It's rare to find something that shows both the orbit's and their horisontal velocity fairly accuratly without making it look like they interweave. I once saw an animation showing the orbit of 4 satilites and the area they could cover area just those 4 satilites could give you coverage of but it's far harder with just a picture.
      - zb 15y ago
        You need four because you must solve for 4 unknowns, the fourth being time. Note that getting the time is the whole point in this case. None of the satellites are in a polar orbit. The orbits are inclined at 55° to the equatorial plane.
        15y ago
- pbhjpbhj 15y ago
  As I understand it, and I've only read the original OPERA experiment paper, they use the GPS signal to sync some caesium clocks (I'm assuming they're caesium based on their name CS-something) and that they've done a comparison by physically moving the clocks to check the synchronisation. The paper is pretty involved on the points of the GPS sync though so I could have misread that. I was impressed that they allow for continental drift in their analysis and indeed that they were apparently able to detect the crusts movement by an Earthquake with the apparatus. >Consider this: If you flipped the location of the neutrino source and destination, you'd actually get the reverse effect (neutrinos would appear to be going slower than light). // Flipped WRT what? They've run the experiment at differing times of the day when the experiment is effectively flipped WRT the helios. They also present a tentative energy relationship with the apparent superluminal speed which wouldn't, it seems, be accounted for by a simple relativistic [time-shift] systematic uncertainty.
  - cube13 15y ago
    >Flipped WRT what? They've run the experiment at differing times of the day when the experiment is effectively flipped WRT the helios. It's "flipped" only with respect to the center of the earth. Since this isn't a point of measurement(the satellites are the ones effectively holding the stopwatch), it's not really worth taking into account. If the neutrino stream was going west->east(i.e. with the satellites) rather than east->west, the receiving station would appear to be moving away from the starting point of the neutrino stream. That would effectively decrease the speed of neutrinos.

themgt 15y ago

I think it's more subtle than that - it's due to the motion of the satellites relative to the experiment (i.e. the direction of the neutrino path), not to the individual ground stations

thegrossman 15y ago

Relativistic effects are only taken into account when determining the position of the satellite itself, and the rate of it's clock. It can't compensate for the effect in the linked-to article (namely, the fact that the distance and flight time between the neutrino source and destination is shorter according to the satellite, versus an observer on the ground) because that effect depends on the specifics of the experiment. Consider this: If you flipped the location of the neutrino source and destination, you'd actually get the reverse effect (neutrinos would appear to be going slower than light). So it's up to observers on the ground to compensate for relativistic effects of this nature. (As I mentioned in another comment, I would be shocked if they didn't already do that)

onemoreact 15y ago
Neutrino's are hard to detect and even harder to generate in detectable quantites. So for now we have exactly one location and one path in the world where we can both generate and detect them at a reasonable distance. Also, GPS satilites don't orbit in one direction you need 3 to quickly get an accurate posisiton so they use a wide range of orbits to minimize the number needed. http://www8.garmin.com/aboutGPS/ http://www8.garmin.com/aboutGPS/ Which is something the article seems to miss.
- zb 15y ago
  1) You need 4. 2) They all orbit in the same direction. 3) There are a number of different orbits, but they are all inclined at the same angle. The illustration on that page was, I'm afraid, drawn by someone with no clue what the constellation actually looks like.
  - onemoreact 15y ago
    1) Technicaly, there are 2 possible locations that the same distance from each of 3 satilites, but the other point is far off in space. The 4th is vary useful for increased accruacy when you have a shitty ground clock, but it's not 'needed' it's it's loss simply results in lower accracy which can be compensated for by more readings over time. (Which can be vary useful when want accuracy and have a ground station that only get's to see a small chunk of the sky.) 2) They all have polar orbits that reach the same location 4 minutes earlyer each day. However, there horizontal ground speed relative to a stationary observer changes over the course of their orbit. (It's zero at the pole and maxes out at the equator.) And is also diffrent form the other satilites you are comparing it with. Which is why you get those S shaped paths: http://www.n2yo.com/satellites/?c=20 http://www.n2yo.com/satellites/?c=20 3)I agree but see 2. PS: Yea, that picture was fairly bad, I was looking for an easy to read article not a picture. The classic picture of a constilation with fixed polar obits is vary misleading once you take the earth's rotation into consideration. It's rare to find something that shows both the orbit's and their horisontal velocity fairly accuratly without making it look like they interweave. I once saw an animation showing the orbit of 4 satilites and the area they could cover area just those 4 satilites could give you coverage of but it's far harder with just a picture.
    - zb 15y ago
      You need four because you must solve for 4 unknowns, the fourth being time. Note that getting the time is the whole point in this case. None of the satellites are in a polar orbit. The orbits are inclined at 55° to the equatorial plane.
      - 15y ago
pbhjpbhj 15y ago
As I understand it, and I've only read the original OPERA experiment paper, they use the GPS signal to sync some caesium clocks (I'm assuming they're caesium based on their name CS-something) and that they've done a comparison by physically moving the clocks to check the synchronisation. The paper is pretty involved on the points of the GPS sync though so I could have misread that. I was impressed that they allow for continental drift in their analysis and indeed that they were apparently able to detect the crusts movement by an Earthquake with the apparatus. >Consider this: If you flipped the location of the neutrino source and destination, you'd actually get the reverse effect (neutrinos would appear to be going slower than light). // Flipped WRT what? They've run the experiment at differing times of the day when the experiment is effectively flipped WRT the helios. They also present a tentative energy relationship with the apparent superluminal speed which wouldn't, it seems, be accounted for by a simple relativistic [time-shift] systematic uncertainty.
- cube13 15y ago
  >Flipped WRT what? They've run the experiment at differing times of the day when the experiment is effectively flipped WRT the helios. It's "flipped" only with respect to the center of the earth. Since this isn't a point of measurement(the satellites are the ones effectively holding the stopwatch), it's not really worth taking into account. If the neutrino stream was going west->east(i.e. with the satellites) rather than east->west, the receiving station would appear to be moving away from the starting point of the neutrino stream. That would effectively decrease the speed of neutrinos.