https://terrytao.wordpress.com/2012/10/02/einsteins-derivation-of-emc2-revisited/ https://terrytao.wordpress.com/2012/10/02/einsteins-derivati... It is one of

https://terrytao.wordpress.com/2012/10/02/einsteins-derivation-of-emc2-revisited/ https://terrytao.wordpress.com/2012/10/02/einsteins-derivati... It is one of the assumptions of Einstein, namely that there is a fixed max velocity c. To get Newton's 1/2mv^2 in the Taylor expansion requires a factor c^2. Something else would lead to measurable deviations in slowly moving objects.