When a null geodesic γ0,orthogonal to a space-like two-surface φ,is from φ to q with a point r∈(φ,q)conjugate to φ along γ0(λ),there will be a variation of γ0(λ) which will give time-like curves from φ to q.This is a well-known theory in the famous bood.When the variation vector is not a generalized Jacobi field on the null geodesic γ0,the acceleration of these time-like curves approaches infinity as the time-like curves approach the null geodesic.