The interpretations of the delayed-choice experiments suggest that photons ¡°retroactively decide¡± to travel as particles or as waves from the time of its emission. To test the interpretations thoroughly, we propose and perform novel extended-delayed-choice experiments. The experiments are designed for easier to set up, carry out and reach meaningful results without ambiguity. The experimental results show the following: (A) The present does not decide the past retroactively, i.e., the causality holds; (B) Bohr¡¯s complementarity principle is challenged; (C) photons behave as particle before striking on a double-slit/cross-double-slit; (D) Particle nature is intrinsic.

delayed-choice experiment, causality, Mach-Zehnder-Interferometer, double-slit experiment, Complementarity principle, wave-particle duality