On time-interval transformations in special relativity
  We revisit the problem of the Lorentz transformation of time-intervals in special relativity. We base our discussion on the time-interval transformation formula $ c\Delta t' = \gamma (c\Delta t - \vec{\beta} \cdot \Delta \vec{r}) $ in which $ \Delta t'$ and $ \Delta t $ are the time-intervals between a given pair of events, in two inertial frames $ S $ and $ S'$ connected by an general boost. We observe that the Einstein time-dilation-formula, the Doppler formula and the relativity of simultaneity, all follow when one the frames in the time-interval transformation formula is chosen as the canonical frame of the underlying event-pair. We also discuss the interesting special case $ \Delta t' = \gamma \Delta t $ of the time-interval transformation formula obtained by setting $ \vec{\beta} \cdot \Delta \vec{r}=0 $ in it and argue why it is really \textbf{not} the Einstein time-dilation formula. Finally, we present some examples which involve material particles instead of light rays, and highlight the utility of time-interval transformation formula as a calculational tool in the class room. 
A.V.Gopala Rao, K.S.Mallesh and K.N.Srinivasa Rao
physics.gen-ph