Physics: Principles with Applications (7th Edition)

If the velocity of an object exceeded the speed of light, $\gamma$ would become an imaginary number.
Both the length contraction and time dilation formulas include the factor of $\gamma = \frac{1}{\sqrt{1-v^{2}/c^{2}}}$. This would be an imaginary number if v exceeded c, because you would be taking the square root of a negative number.