Rod: If of course we think E=mc^2 to be correct, then: Mass remains constant regardless of relative motion. The energy of a mass or mass less particle also remains unchanged regardless of relative motion.
If in it's rest frame a mass is measured as x, then all frames must agree.
If in it's rest frame a mass' energy is measured as x, then all frames must agree.
***What the above is saying is that a measurement is performed which will always involve relative motion between, the mass or it's energy, and the measuring devices. If another observer whizzes by, he cannot measure different results.***Rod Ryker... It is reasoning and faith that bind truth.