Preferred Reference Frames

Nov 28, 2020
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Just a beginner here, but I’ve been struggling with something with special relativity.
Here’s my question. A travels away from B, who’s on Earth, at 90% SOL (or any arbitrarily high speed to make time dilation have a noticeable effect). B’s clock moves slower than A’s. But, if there are no preferred intertial reference frames, A could be said to be moving away from B at 90% SOL. Why does B’s clock still move slower.?
 

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