On the origin of asymmetric aging in the twin problem: Comments on M. Harada's and M. Sachs's views

As a sequel to my recent article [Phys. Essays 12, 588 (1999)], comments are made on M. Harada's and M. Sachs's views, which negated the asymmetric aging effect in the twin problem. Posing the question "if the contraction factor introduces only the scale change and has nothing to do with physical changes, what is the origin of the asymmetric aging?" I give an answer related to the inertial-gravitational force and to the gravitational redshift of light. Finally, it is shown that Sachs's view is contrary to Einstein's own view, whether in or after 1905.