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## Main Question or Discussion Point

Is the increase in mass/density of an object in motion purely relativistic?

What I mean is if you were in a spaceship approaching the speed of light would you be able to measure a change in your mass/density or would the change only be observable to someone at rest. I'm aware of the Lorentz formulas so I understand that mass/density changes are relativistic but are they purely relativistic like time?

What I mean is if you were in a spaceship approaching the speed of light would you be able to measure a change in your mass/density or would the change only be observable to someone at rest. I'm aware of the Lorentz formulas so I understand that mass/density changes are relativistic but are they purely relativistic like time?