That is not clearly the case. The motion or rest state of a single object in empty space is indeterminate. When two objects are present they can only be in motion or at rest relative to each other. The state of motion of either object cannot be determined without reference to the other.
My initial question was in connection with what appears to be a paradox concerning time dilation as described by Ethan Siegel in his original article.
In a comment on another article I posed a similar question. I will transcribe that comment with the hope that it will clarify the issue that I seek to address:
Captain Picard peers through the rear portal of the Enterprise and says, “I am moving away from Adama over there in the…medium.com
There are two space ships; let’s call them the Enterprise and the Galactica. The Enterprise and the Galactica are moving directly away from each other at a constant speed of .99C. Each ship’s captain is exactly the same age.
Captain Picard peers through the rear portal of the Enterprise and says, “I am moving away from Adama over there in the Galactica at .99C; he will be much older than I at the end of this journey.”
Captain Adama, gazing at the Galactica’s viewing screen, sees that he is receding from Picard of the Enterprise at .99C and realizes that Picard will be much older than he at the end of the journey.
Who will be older — Picard or Adama?