Many people, when they were students, had once asked their professors the question of why the speed of light is constant; many of these people, after becoming professors or instructors, are often continuously asked the same question by their students again and again. Can this long-standing, big question continue to baffle us? The answer from this book is: from now on, the long-standing question of why the speed of light is constant will no longer baffle those who want to know the answer to it, because the reason why the speed of light is constant has been brought to light in this book.... "Hi, dear special relativity, what makes you so happy?" a physicist saluted and chatted with the theory of special relativity."I will no longer be baffled or perplexed by the long-standing, big question of why the speed of light is constant from now on, because the reason why the speed of light is constant has been found, as shown in this book; because this reason turns out to be amazingly interesting and irresistibly attractive," special relativity replied excitedly."Wow! This is greatly important, also highly significant! Since 'the speed of light is constant' is one of the most important postulates in science, the reason why the speed of light is constant is certainly crucially or remarkably important; thus knowing this reason is definitely of profound and great significance," the physicist remarked positively and specifically."You are definitely correct!" special relativity responded approvingly.