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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
+dimension.
+
++----------------+--------------------+
+| 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 |
++----------------+--------------------+
+
+Separations
+-------------
+
+The separation `$(\Delta s)^2$`:m: between two events in space time, in a given
+Lorentzian/inertial frame, is defined
+as:
+
+:m:`$$ (\Delta s)^2 \equiv -(\Delta t)^2 + (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 $$`
+
+or
+
+: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:`($\Delta \Tau)^2= -(\Delta s)^2$`.
+
+
+References
+----------------
+
+.. [PH237] `Gravitational Waves`:title: (aka ph237), a course taught by Kip Thorne at Caltech in 2002. See http://elmer.tapir.caltech.edu/ph237/ for notes and lecture videos.