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arindamsinha
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DaleSpam said:...Yes, the rest mass is invariant, that is what makes it a useful quantity...
I have a broader question on this - is mass at all properly defined in GR context, including rest mass or so-called invariant mass?
Leaving aside concept of mass gain, I have been under the impression that mass itself is defined in different ways in different situations in GR, with caveats, such that there is really no common definition of mass, and that applies even to rest/invariant mass.
Ref: http://en.wikipedia.org/wiki/Mass_in_general_relativity
It is rather surprising. Any thoughts?