Rapidity

\setlength{\unitlength}{1mm}
\begin{picture}(90,32)
  \put(0,15){\vector(1,0){57}} 
  \put(60,14){$\chi$ = Rapidit\"at}
  \put(25,0){\vector(0,1){32}} 
  \put(27,29){$\beta=v/c$}
  \multiput(0,5)(4,0){13}{\line(1,0){2}}
  \multiput(0,25)(4,0){13}{\line(1,0){2}} 
  \put(60,24){$\displaystyle\beta=\frac{v}{c}=\tanh\chi$}
  \qbezier(25,15)(34,24)(45,24.6)
  \qbezier(25,15)(16, 6)( 5,5.4)
\end{picture}