Total energy & mass in different reference frames

Like you do it for anything else. The measure won't be as good as for a small object but you'll get some figure. Don't we seem to have a figure for the age of the universe already?
And I'm essentially talking about the principle, as in your own example.

Indeed. It's the basis for my distinction between substantial rest mass and unsubstantial relativist mass.

Same thing, I think, yes.
EB

 

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