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  • $\begingroup$ I think that this is by far the best answer to this question, but I can't get behind your first sentence, because that it seems to imply that special relativity is only concerned with changes in Lorentz frame, and that nothing "relativistic" ever happens that can be described within a single Lorentz frame. But phenomena like Lorentz contraction and time dilation certainly "have to do with relativity" even though "all measurements are made within a single frame." $\endgroup$
    – tparker
    Commented Oct 9, 2019 at 4:22
  • $\begingroup$ @tparker: Thank you for your kind words, but I want to defend my first sentence. An object is headed toward earth. It decelerates. What happens to its length, as measured from earth? Answer: It depends on the details of how it decelerates. That is the question at hand, and the correct answer is exactly the same as the answer you'd give if you'd never heard of relativity. If knowing about relativity doesn't change the answer then (by a reasonable definition) the question has nothing to do with relativity. $\endgroup$
    – WillO
    Commented Oct 9, 2019 at 4:44
  • $\begingroup$ @tparker: (Of course if you then go on to ask how things look in some other frame, then you're doing relatiivity.....) $\endgroup$
    – WillO
    Commented Oct 9, 2019 at 4:44
  • $\begingroup$ Okay, I'm starting to come around to your first sentence. Taken in isolation, it seems misleading, because you could imagine a different problem where Lorentz contraction really is conceptually crucial, in which cases this same sentence would be incorrect. But in the context of this particular question and the rest of the answer, I think it's okay. $\endgroup$
    – tparker
    Commented Oct 9, 2019 at 12:44
  • 1
    $\begingroup$ @tparker : If you're assuming that decleration is instanteous and uniform (in the earth frame) then it's natural to assume the same about the initial acceleration at the beginning of the journey. With that assumption, the traveler never contracted in the first place. $\endgroup$
    – WillO
    Commented Oct 9, 2019 at 16:49