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Wox
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I'm trying to understand how the geometry of Minkowski space is related to physical observations, in particular, measurements of the velocity of an object. In the attempt below, I got stuck at the meaning of the relativistic 3-velocity. Can anyone get me back on track?
Consider a world line with parameterization [itex]\bar{w}(t)=(ct,\bar{x}(t))[/itex]. An observer with world line [itex]\bar{w}_{0}(t)=(ct,\bar{0})[/itex] measures a (proper) ellapsed time (since the origin) of [itex]\tau_{0}(t)=t[/itex]. Therefore, [itex]\bar{w}(t)[/itex] is parameterized by the proper time of the observer connected to the reference frame it is described in. As a result, the spatial part of the following 4-velocity, represents the velocity of an object with world line [itex]\bar{w}[/itex] as perceived by the observer [itex]\bar{w}_{0}[/itex] connected to the current reference frame:
[tex]\frac{d\bar{w}}{dt}=(c,\frac{d\bar{x}}{dt})[/tex]
When parameterizing the world line [itex]\bar{w}(\tau)[/itex] by its proper time [itex]\tau[/itex], we get the following 4-velocity, which spatial part represents the velocity of an object with world line [itex]\bar{w}[/itex] as perceived by ?:
[tex]\frac{d\bar{w}}{d\tau}=\frac{d\bar{w}}{dt}\frac{dt}{d\tau}=(c \gamma,\gamma\frac{d\bar{x}}{dt})[/tex]
So perceived by who? Is relativistic 3-velocity [itex]\gamma\frac{d\bar{x}}{dt}[/itex] something that can be measured at all?