For SR time and length are not considered constants but involve change as a function of the speed of light.
The basic idea is an observer at rest with has a clock which shows rest time and with a rod which has a rest length. From his point of view a moving clock undergoes time dilution and a moving rod length contraction as described by the Lorentz transformations.
Mass in relation towards SR also uses the same mathematics. Mass also has a rest mass and a moving mass changes as described by the Lorentz transformations.
This causes a problem related to for example planet and star sized objects. How do you calculate the rest mass of each while they are always in movement?
The same problem exists also for the elementary particles. IMO it does not make sense to assume a rest mass, while any a sub-atomic particle is never at rest.
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