sr Lorentz factor γ For a velocity of 0.8c the value becomes ( )22 22 111 1 0.641/ 1 0.8 /vc cc = = −− − 11 0.6000.36 = = = 1.67 Thus, at the speed of eight-tenths of the speed of light the factor becomes quite significant

http://www.farmingdale.edu/faculty/peter-nolan/pdf/relativity/Ch02Rel.pdf
For a velocity of 0.8c the value becomes    ( )22 22 111 1 0.641/ 1 0.8 /vc cc = = −− − 11 0.6000.36 = = = 1.67  Thus, at the speed of eight-tenths of the speed of light the factor becomes quite significant