# Undefined result of relativistic velocity addition formula [duplicate]

Isn't it impossible to estimate the velocity of framework through relativistic velocity addition formula when the event moves at speed of the light? $$u=\frac{v-v'}{1-vv'/c^2}$$

if $v=v'=c$ $u=\frac{0}{0}$ undefined,

Note that the $u$ denotes velocity of frame, $v$ denotes velocity of event and $v'$ denotes velocity of event from frame viewpoint.

(because when the event moves at speed of the light result of relativistic velocity addition formula is equal to zero devided by zero which is undefined and indeterminate)

## marked as duplicate by John Rennie, Kyle Kanos, Ben Crowell, ACuriousMind♦, Qmechanic♦Sep 20 '14 at 19:16

• That should be $u=(v+v')/(1+vv'/c^2)$. – lemon Sep 20 '14 at 10:29
• To see a acceptable limit, begin by choose one of the velocities (say $v$) to be $c$, then $u=c$ for all $v' < c$. Now, you may take the limit $v'=c$ – Trimok Sep 20 '14 at 11:44