  Given a null geodesic $\gamma_0(\lambda)$ with a point $r$ in $(p,q)$
conjugate to $p$ along $\gamma_0(\lambda)$, there will be a variation of
$\gamma_0(\lambda)$ which will give a time-like curve from $p$ to $q$. This is
a well-known theory proved in the famous book\cite{2}. In the paper we prove
that the time-like curves coming from the above-mentioned variation have a
proper acceleration which approaches infinity as the time-like curve approaches
the null geodesic. This means no observer can be infinitesimally near the light
and begin at the same point with the light and finally catch the light. Only
separated from the light path finitely, does the observer can begin at the same
point with the light and finally catch the light.
