The Lorentz factor 
is a measure of relative speed between two observers. It is given by 
. In curved spacetime, it is defined “locally”, meaning the observers coincide at the same event (same place and time)
The Lorentz factor is a measure of relative speed between two observers. It is given by . In curved spacetime, it is defined “locally”, meaning the observers coincide at the same event (same place and time)