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Oh boy .. simple question for mainstreamers but a can of worms for the 'non-conformists' !!
Einstein answered this question as part of the early stages of his General Relativity work, I think.
Special Relativity is really only meant to cover matter moving through space (as Mark has said).
But spacetime is not matter.
This subject is a topical one, given the impending Gravitational Wave detectors about to be launched into space.
Interesting.
Cheers
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