Re: Does GR explain why orbiting clocks lose time
- From: Tom Roberts <tjroberts@xxxxxxxxxx>
- Date: Sun, 22 May 2005 19:09:11 GMT
Curious wrote:
[Does GR explain why orbiting clocks lose time] rather than the clocks on Earth?
Well first, orbiting clocks can either gain or lose time relative to clocks on earth, depending on their altitude. For low earth orbit (e.g. the Space Shuttle, Space station, Hubble), an orbiting clock does indeed lose time compared to a clock on earth. But for higher orbits (e.g. GPS, geosynchronous, moon) the orbiting clock actually gains time relative to a clock on earth.
By "loses" or "gains" time I mean nothing special: a clock which "loses time" is ticking slower than the other clock, when they are compared via electromagnetic signals.
Yes, GR explains this, in the usual manner of physical theories[#]. In a nutshell: this is a manifestation of the curvature of spacetime.
[#] That is, it is a "one level deep" explanation -- for an answer to "why is that so?" you'll have to venture outside science.
Does GR assume an absolute frame of reference?
No. In GR _any_ "frame of reference" can be used, as can coordinates for which "frame" makes no sense (e.g. null coordinates).
Sometimes people get confused about this because some particular solutions to the equations of GR do indeed exhibit an "absolute frame of reference", in that there is a globally-preferred coordinate system. All other coordinate systems are valid (as usual), but when they exist these globally-preferred coordinates directly implement a symmetry of the manifold and therefore are significantly simpler to use for many purposes. In particular, the FRW manifolds used for cosmological models have such a coordinate system, as does the Schwarzschild manifold used to model stars and planets. In fact, most _known_ solutions to the Einstein field equation have such coordinates, as such global symmetries are usually necessary to obtain a solution....
Tom Roberts tjroberts@xxxxxxxxxx .
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