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Acceleration for an object of constant mass is always in a spacelike direction, never along a timelike axis.ESponge2000 said:When a long object accelerates rapidly, along the time-like axis does it hold the same length for an observer riding the object as it accelerates?
Regarding the length, that depends on how the acceleration is physically achieved. In the usual assumption, as @Ibix mentioned, you push from one end and the length is constant if the acceleration is constant. If you were able to push from each point in a pre-programmed pattern then it is possible to have “Born rigid” acceleration which would change over time but still preserve distances on the object.
Most of the recent back and forth seems to have nothing to do with your original question regarding mass.