eng Science and Education Publishing International Journal of Physics 2333-4576 2021-01-17 9 1 48 52 10.12691/ijp-9-1-6 IJP2021916 article Hui Peng davidpeng1749@gmail.com, davidpeng1749@yahoo.com 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. http://pubs.sciepub.com/ijp/9/1/6/ijp-9-1-6.pdf delayed-choice experiment causality Mach-Zehnder-Interferometer double-slit experiment Complementarity principle wave-particle duality