Transient first-order interference of two independent thermal light beams

By analyzing the first-order interference of two independent thermal light beams in Feynman´s path integral theory, we conclude that it is impossible to observe the transient first-order interference pattern by superposing two independent thermal light beams if the initial phases of the photons in thermal light are random. The result suggests that the classical model of thermal light field within the coherence time may not be the same as the one of laser light field within the coherence time.