Document units

git-svn-id: http://svn.einsteintoolkit.org/cactus/EinsteinBase/HydroBase/trunk@19 57fe0bb3-ccba-405f-9b23-de0201f165b7

1 files changed, 36 insertions, 1 deletions

diff --git a/doc/documentation.tex b/doc/documentation.tex
index cdf33f7..601a123 100644
--- a/doc/documentation.tex
+++ b/doc/documentation.tex
@@ -170,7 +170,42 @@ In this way the initiation of the primitive variables, methods to recover the co
 variables, and basic atmosphere handling can be implemented in different thorns while allowing
 a central access point for analysis thorns.
 
-\subsection{Acknowledgments}
+\section{Units}
+
+HydroBase uses a specific set of units, so that there are no
+misunderstandings between thorns.  These units are derived from the
+conventions
+\begin{eqnarray}
+  M = M_\mathrm{sun} &;& c = G = 1
+\end{eqnarray}
+which are commonly used in astrophysics and in relativity.
+
+We assume the following definitions and constants of nature:
+\begin{eqnarray}
+  c             & = & 299792458\, \mathrm{m/s}
+  \\
+  G             & = & 6.67428\cdot 10^{-11}\, \mathrm{m^3/kg/s^2}
+  \\
+  \mu_0         & = & 4 \pi 10^{-7}\, \mathrm{N/A^2}
+  \\
+  \epsilon_0    & = & \frac{1}{\mu_0 c^2}
+  \\
+  M_\mathrm{sun} & = & 1.98892\cdot10^{30}\, \mathrm{kg}
+\end{eqnarray}
+
+This corresponds to the following units for mass, length, time, and
+magnetic field:
+\begin{eqnarray}
+  [M] & = & M_\mathrm{sun}
+  \\{}
+  [L] & = & [M]\; G/c^2
+  \\{}
+  [T] & = & [L]\; / c
+  \\{}
+  [B] & = & 1/[L]\; / \sqrt{\epsilon_0 G / c^2}
+\end{eqnarray}
+
+\section{Acknowledgments}
 
 This thorn was produced by Tanja Bode, Roland Haas, Frank Löffler, and Erik Schnetter.