"All three variables—length, time, mass—are covered by the same Lorentz contraction [ \scriptstyle \sqrt{1-\frac{v^2}{c^2}} ]... Length and the rate of clocks vary in the same direct proportion, so the formula is the same for each. Mass... varies in the inverse proportion... \scriptstyle \frac{1} {\sqrt{1-\frac{v^2}{c^2}}}."