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Communication with satellites certainly requires knowing the speed of light. Beyond having that magic number, do relativistic phenomena affect communication with satellites? For example, would one be able to take an accurate photograph of a specific spot on a remote planet without knowledge of relativity?

A similar question can be raised in the context of GPS: would one get accurate transmissions based on knowledge of the speed of light alone, or is knowledge of the details of relativity required to get accurate transmissions?

In other words, consider the stage of physics at the end of the 19th century when the speed of light was known already, but the formalism of relativity did not exist yet. Would people have been able to communicate accurately and in time with space probes, and get accurate transmissions via GPS?

GE was confirmed experimentally long ago by Mercury precession (see links below) but beyond such fairly esoteric phenomena in astronomy I was wondering about the specific practical manifestations this has, as described above.

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  • $\begingroup$ Can you elaborate a little more on your question? What does communicating with a satellite have to do with photographing a planet? $\endgroup$ – DilithiumMatrix Nov 2 '16 at 18:11
  • $\begingroup$ You have to tell the camera to snap a picture at a specific moment if you wish a picture of a specific spot on the surface of the planet. Since there are delays involved in sending the command and its reception, the camera may not snap the picture at the right moment. $\endgroup$ – Mikhail Katz Nov 2 '16 at 18:16
  • $\begingroup$ Communicate with spacecraft in general? Sure. They'd figure it out. Get accurate GPS readings? No. That requires GR corrections $\endgroup$ – Jim Nov 3 '16 at 12:23
  • $\begingroup$ @Jim, fascinating. So you are saying that merely knowing the speed of light would be insufficient to get accurate GPS readings, and one would need the full formalism of relativity theory for this? Could you elaborate? $\endgroup$ – Mikhail Katz Nov 3 '16 at 13:15
  • $\begingroup$ Accurate GPS readings require being able to synchronize the clocks of all satellites involved in the reading in order to get accurate distance estimates from all of them. To achieve any useful precision, those clock synchronizations need to take into consideration even the relativistic time dilation effects from the satellites being at a different gravitational potential than us, not to mention special relativistic time dilation. Without relativity, the errors in the clocks for each satellite would be too large to pinpoint your location to better than a few kilometers $\endgroup$ – Jim Nov 3 '16 at 14:15
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Communication with satellites certainly requires knowing the speed of light. Beyond having that magic number, do relativistic phenomena affect communication with satellites? For example, would one be able to take an accurate photograph of a specific spot on a remote planet without knowledge of relativity?

Yes, almost certainly for planets such as Mars, where the gravitional influence of the Sun is tiny, (in the context of using GR or Newtonian Mechanics for communication) and we can use Newton's laws of motion with confidence.

It really depends on how far away you are from a large mass, say the Sun, or how close you orbit a rotating planet, (Frame Dragging). It also depends on the absence of any non trivial gravitational source between Earth and the "remote" planet.

But no, not as certainly for the planet Mercury at its closest point in its orbit to the Sun. I am guessing it would make little difference, to incorporate GR even here, but I am open to correction on this. This is covered here Perihilion of Mercury and provides a good example of the correct description of spacetime by Einstein, compared to the Newtonian prediction, which puzzled astronomers before 1915, as it could not be verified experimentally .

It was the difference in predictions using Newtonian physics as opposed to GR regarding Mercury's orbit that was one of the earliest verifications that Newtonian mechanics is a subset of GR.

One place closer to home, where GR needs to be incorporated, is in the GPS navigation system, where the satellite data would rapidly become erroneous as regards predicted versus true position unless GR effects are allowed for.

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GPS very famously requires general relativistic corrections to give accurate results, as the clocks on the satellites run at the 'wrong' rate as they are further up the Earth's gravity well than us. A reference for this is Relativity and the Global Positioning System, Neil Ashby, 2002, Physics Today, May 2002, 41. This is cited by this page, which has some other useful links. But just typing 'gps general relativity' into a search engine will find plenty of pointers.

See also this SE question (thanks to ACuriousMind for this).


[I'm adding this as an answer as you asked for a source in the comments: any reputation should go to Jim really.]

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  • $\begingroup$ A physics.SE question about that topic is here. $\endgroup$ – ACuriousMind Nov 8 '16 at 16:45
  • $\begingroup$ @ACuriousMind Thanks: I have added a link. It might be that this whole question is a duplicate, I guess. $\endgroup$ – tfb Nov 8 '16 at 18:02
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Don't forget stellar aberrations--the (special) relativistic treatment is certainly required for star trackers, and would be required for anything that requires accurate pointing--perhaps COM.

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  • $\begingroup$ Thanks! Could you elaborate? $\endgroup$ – Mikhail Katz Nov 9 '16 at 9:11
  • $\begingroup$ I know some people working on the Cassini spacecraft, and stellar aberrations needed to be considered for accurate data collection and data processing. Here is a paper: hou.usra.edu/meetings/lpsc2015/pdf/2719.pdf $\endgroup$ – JEB Jan 5 '17 at 2:25

protected by Qmechanic Nov 8 '16 at 17:49

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