3 ms·

The funny thing is that GPS satellites operate in the context of special relativity, where events can occur in reverse order depending on the observer's velocit

• mjburgess 4y ago
  Events can't occur in a "reverse" order. At sufficient distances, the evolution of two regions is entirely causally independent, so there is no sense of simultaneity. Otherwise, the causal order of events is always defined. Different observers may have "more information" about some events over others, but over ordinary (even multi-galactic) distances, events are well-ordered and sequential.
  • planede 4y ago
    Events are partially-ordered. Two events that can't have causal relationship between them are incomparable causality-wise.
  • moring 4y ago
    > Events can't occur in a "reverse" order. At sufficient distances, the evolution of two regions is entirely causally independent, so there is no sense of simultaneity. This discussion is essentially about whether "events can occur in reverse order for different observers" is the right way to say "the order of time coordinates of two spacelike-separated events is reversed for different observers". This was in reply to: > the overall outcome is a reasonably coherent notion of what the current time is The notion of an observer-independent "current time" implies simultaneity which doesn't exist in SR, and if you mistakenly assume an observer-independent "current time" which you'd likely assume to be the time axis of your own frame of reference, then two spacelike-separated events would be reversed for some observer who did the same but moves with almost speed-of-light relative to you. Please keep in mind that I wanted to point out the irony in using GPS for a universal reference time without giving an intro into special relativity.
• goodcanadian 4y ago
  Nitpick: GPS is affected by general relativity effects (different gravitational field) rather than special relativity effects (where they would have to be traveling a substantial proportion of the speed of light).
  • Keysh 4y ago
    GPS is affected by both general and special relativity, in opposite ways. The GR effect is larger, but if you don’t include the SR effect, you’ll get it wrong. https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gps.html https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gp...
    • defrost 4y ago
      I've worked in the field of high precision surveying since the 1980s or so, ever since being intrigued by reverse engineering NAVSTAR signals prior to their form being fully public. I absolutely agree that GPS is affected by both general and special relativity, however I do take issue with the close of your linked article. Relativity is not just some abstract mathematical theory: understanding it is absolutely essential for our global navigation system to work properly! I'll insist that while the corrections must be made for GPS to be useful, understanding the why and the model of the cause for those corrections isn't essential as is insisted. Suppose we have no knowledge of relativity but a general understanding of trigonometry and from there surveying and point fixing against mountain tops and base stations; we extend that to doing the same against stations in motion (our satellites) and we find that we are in error by some unknown function. We might posit that the cause is the effect of solar wind and the magnetic field against our satellites, we might guess that there's an unknown additional small force (or forces) that excites us to experiment further and expand our grasp of physics .. however ... In the meantime we can generate accurate corrections by developing the error f(t,parameters) against a series of fixed ground points that should return the same fixed location but instead inform us of our error. We can then solve for a Kalman filter that corrects in the near past and near future, and upload those corrections to our satellite fleet for inclusion in the broadcast data packets. In fact, this is what is already done, despite our understanding of relativity as there are indeed other factors that cause errors. In a similar manner aircraft performing geomagnetic surveying develop heading correction Kalman filters by flying figure eight patterns over quiet zones so that they can then fly patterns East-West | North-South and measure the reaction of the earth structure against the geomagnetic field in a manner independent of the aircraft heading and its own response (as a lump of metal moving through a field) (these being factored out by the correction filter).
• msla 4y ago
  To simplify what others have said, it's only possible to reorder two events A and B in Special Relativity if A can't possibly have caused B and vice-versa. Otherwise, SR maintains ordering. Look up timelike versus spacelike separation.