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Relativistic Frames of Reference
From the Galilean description of motion for a frame of reference moving relative to another frame considered stationary we know that
Figure 2.1: Primed reference frame moving in the z direction with velocity v.
[math]t= t'[/math]
[math]x=x'[/math]
[math]y=y'[/math]
[math]z=z'+vt[/math]
Using Einstein's Theory of Relativity, we know that the speed of light is a constant, c, for all reference frames. In the unprimed frame, from the definition of speed:
[math]speed=\frac{\Delta Distance}{\Delta Time}[/math]
[math]c=\frac{\Delta d}{\Delta t}[/math]
where
[math]c=3\times 10^8\ m/s[/math]
Using the distance equation in a Cartesian coordinate system, the equation for the speed of light becomes
[math]c=\frac{\sqrt{\Delta x^2+\Delta y^2+\Delta z^2}}{t}[/math]
Following the postulate of Special Relativity, this implies for the primed frame
[math]c=\frac{\sqrt{\Delta x^{'2}+\Delta y^{'2}+\Delta z^{'2}}}{t}[/math]
We can rewrite this as
[math]\frac{\Delta x^{'2}+\Delta y^{'2}+\Delta z^{'2}}{\Delta t^{'2}}= c^2=\frac{\Delta x^2+\Delta y^2+\Delta z^2}{\Delta t^2}[/math]
This is possible since the ratios of distance to time are multiples of the same base, i.e. the square of the speed of light [math](\frac{3\times 10^8\ m}{s})^2[/math]. Therefore for the relative change in the time in one frame, the distance must change by the same factor to maintain the same constant. With this we can write
[math]c^2 \Delta t^{'2}=\Delta x^{'2}+\Delta y^{'2}+\Delta z^{'2}\ \ \ \ \ c^2 \Delta t^{2}=\Delta x^2+\Delta y^2+\Delta z^2[/math]
[math]\Rightarrow c^2 \Delta t^{'2}-\Delta x^{'2}-\Delta y^{'2}-\Delta z^{'2}= c^2 \Delta t^{2}-\Delta x^2-\Delta y^2-\Delta z^2[/math]
This quantity is known as the time space interval [math]ds^2[/math] when the change is infinitesimal
[math]s^2\equiv c^2 dt^{'2}-dx^{'2}-dy^{'2}-dz^{'2}= c^2 dt^{2}-dx^2-dy^2-dz^2[/math]
Since the speed of light is a constant for all frames of reference, this allows the space time interval to also be invariant for inertial frames.
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