Mass is energy, energy is mass. What's the difference? They are at different points in a curve that relates space to time. If you trade away all the time, you

Mass is energy, energy is mass. What's the difference? They are at different points in a curve that relates space to time. If you trade away all the time, you get a maximum velocity: c. So the things moving at c don't experience time, and have momentum but not mass. http://www.math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html http://www.math.ucr.edu/home/baez/physics/ParticleAndNuclear... In the same direction, if you take a mass and accelerate it, it experiences less time. There you have relativistic time dilation, as beloved by science fiction stories. https://en.wikipedia.org/wiki/Time_dilation https://en.wikipedia.org/wiki/Time_dilation And you simultaneously get length contraction in the direction of each velocity: https://en.wikipedia.org/wiki/Length_contraction https://en.wikipedia.org/wiki/Length_contraction These are all aspects of the same thing. Really not a dumb question at all.