  The recent proposal on the correspondence between the ${\cal N}=4$ super
Yang-Mills theory and string theory in the Penrose limit of the
AdS$_5\times$S$^5$ geometry involves a few puzzles from the viewpoint of
holographic principle, especially in connection with the interpretation of
times. To resolve these puzzles, we propose to interpret the PP-wave strings on
the basis of tunneling null geodesics connecting boundaries of the AdS
geometry. Our approach predicts a direct and systematic identification of the
S-matrix of Euclidean string theory in the bulk with the short-distance
structure of correlation functions of super Yang-Mills theory on the AdS
boundary, as an extension of the ordinary relation in supergravity-CFT
correspondence. Holography requires an infinite number of contact terms for
interaction vertices of string field theory and constrains their forms in a way
consistent with supersymmetry.
