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+Special Relativity
+.. warning:: This is a rough work in progress!! Likely to be factual errors,
+ poor grammar, etc.
+.. note:: Most of this content is based on a 2002 Caltech course taught by
+ Kip Thorn [PH237]_
+*See also `physics/general relativity</k/physics/generalrelativity/>`_*
+As opposed to general relativity, special relativity takes place in a *flat*
+Minkowski space time: a 4-space with three spatial dimensions and one time
+| Index notation | Variable | Type |
+| `$x^\0`:m: | `$t$`:m: | Time |
+| `$x^\1`:m: | `$x$`:m: | Spatial |
+| `$x^\2`:m: | `$y$`:m: | Spatial |
+| `$x^\3`:m: | `$z$`:m: | Spatial |
+The separation `$(\Delta s)^2`:m: between two events in space time, in a given
+Lorentzian/inertial frame, is defined
+:m:`$$ (\Delta s)^2 \equiv -(\Delta t)^2 + (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 $$`
+:m:`$$ (\Delta s)^2 \equiv -(\Delta x^0)^2 + \sum_{i,j} \delta_{ij} \Delta x^i \Delta x^j$$`
+where :m:`$\delta_{ij}$` is the Kronecker delta (unity or 1 when
+:m:`$i=j$`; zero otherwise), and the indices i and j are over the spatial
+dimensions 1,2,3 (corresponding to x,y,z). It can be shown that this separation
+is Lorentz-invariant; the scalar value of separation between two events does
+not depend on the inertial frame chosen.
+Note the negative sign in front of the time dimension. The are three types of
+separations: **space-like** when :m:`$(\Delta s)^2 > 0$`, **null-** or
+**light-like** when :m:`$(\Delta s)^2 = 0$`, and **time-like** when
+:m:`$(\Delta s)^2 < 0$`. When dealing with time-like separations, ignore the
+implication of an imaginary number. The difference in time :m:`$\Delta \Tau$`
+is always real: :m:`($\Delta \Tau)^2= -(\Delta s)^2$`.
+.. [PH237] `Gravitational Waves`:title: (aka ph237), a course taught by Kip Thorne at Caltech in 2002. See for notes and lecture videos.