But isn't even mass relative to the speed of the observer?
I mean, imagine that you were to take a weight of 1Kg with you to the surface of a magical Earth-clone planet capable of reaching 99.999999% the speed of light.
Once you reach that speed you measure the weight on your Earth-clone.
Wouldn't you measure 1Kg as well?
The observer on the original planet Earth, if they could, would measure your weight to measure a much higher mass.
Therefore the concept of mass itself is relative, isn't it?
RE: QUICK PHYSICS INFO: Why Is The Speed Of Light Constant?