Is time on earth, relative to everywhere else, dependent on the earth's speed? Earth rotates at a speed, it moves around the sun, the sun moves around the galaxy and the galaxy is also moving - is it that cumulative speed that give us our version of time? Setting aside the catastrophe of speeding up or slowing down the earth in anyone of these 'speeds', would a change in any one of them impact time relative to an observer not on earth? How fast would a rocket have to go in the opposite direction to stop moving relative to earth, the galaxy etc. and would time then stop for anything/one in that capsule?

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    $\begingroup$ You seem to believe that there exists an absolute referential, which is wrong. ("Galaxy is also moving" relative to what?) $\endgroup$ – Jeanbaptiste Roux Feb 22 at 17:44
  • $\begingroup$ I upvoted because it seems like a common misconception that doesn't seem to have a straight answer elswhere $\endgroup$ – yolo Feb 22 at 17:52
  • $\begingroup$ Your last question is a bit confusing. The Earth is moving relative to the galaxy, so how can your astronaut stop moving relative to both the Earth and to the galaxy? $\endgroup$ – PM 2Ring Feb 22 at 18:22

Short answer: It depends...

Long answer: ...on what you're measuring. Take your example about the earth moving and the sun moving through the moving galaxy. We can take our frame of reference to be, say, the Andromeda galaxy. The effect of time dilation differs between these two points. So by 'our version of time', it's more like a change in time relative to some other place; it's not an absolute value.

Think of it like this. Say you wanted to measure speed in the most direct way; measure the distance between point a and point b, and measure the rate at which it changes. The difference in time between those two points is related to that relative velocity. If an object c was orbiting a, then you would have to pick; am I measuring the speed relative to a or b. And when you're measuring time dilation (with special relativity), it is the difference between the time experienced between those to points.

While it may not be strictly true, I find it can be useful to think of time dilation as some kind of Δx, much like how distance is Δd (the change in position between two points).


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