  Dynamical properties of a generic null surface are known to have a
thermodynamic interpretation. Such an interpretation is completely based on an
analogy between the usual law of thermodynamics and structure of gravitational
field equation on the surface. Here we materialise this analogy and show that
assigning a temperature on the null surface for a local observer is indeed
physically relevant. We find that for a local frame, chosen as outgoing
massless chargeless particle (or field mode), perceives a "{\it local unstable
Hamiltonian}" very near to the surface. Due to this it has finite quantum
probability to escape through acausal null path which is given by
Maxwell-Boltzmann like distribution, thereby providing a temperature on the
surface.
