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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)

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    $\begingroup$ That should be $u=(v+v')/(1+vv'/c^2)$. $\endgroup$
    – lemon
    Commented Sep 20, 2014 at 10:29
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    $\begingroup$ possible duplicate of Speed of light travel $\endgroup$
    – John Rennie
    Commented Sep 20, 2014 at 10:41
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    $\begingroup$ Not at all. The frame that moves at the speed of light is always the frame that moves at the speed of light. That's actually the main assumption of special relativity. The expression above isn't undefined, either. Use L'Hospital's rule. $\endgroup$
    – CuriousOne
    Commented Sep 20, 2014 at 11:05
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    $\begingroup$ 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 $v'=c$ $\endgroup$
    – Trimok
    Commented Sep 20, 2014 at 11:44
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    $\begingroup$ @Achmed - in physics, when you get into nonsensical math, always ask yourself what is the physical situation you are attempting to describe. Here it seems to be: "while traveling on a train passing a station, if I send a photon in the direction of travel, what would its speed be as seen from the perspective of a photon sent in the same direction from the platform?" The correct answer is that photons lack perspective... $\endgroup$
    – Johannes
    Commented Sep 20, 2014 at 15:00

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