The special theory of relativity consists of many theoretical results and empirical findings obtained by A.A. Michelson, H.A. Lorentz, H. Poincare, H. Minkowski and others. But in a well suitable form it was proposed by Albert Einstein in 1905. Einstein was not sure for the name, “Special Relativity”. Of course this adds a new episode in our scientific-world. In the words of Andrew Whitaker, “If I were to ask a number of people in the street what they think has been the most important new theory in physics this century, and who has been the greatest physicist, I am fairly sure that of those able to express an opinion at all a substantial majority would say that relativity has been the greatest theory, and Einstein the greatest physicist. Indeed Albert Einstein has probably achieved the remarkable feat of not just becoming the best-known practitioner of any branch of science among the general public, but retaining that position for seventy five years, forty of those years since his death in 1955”. Theory of relativity is a natural-scientific theory. The whole theory consists of the two parts, between which one part is special relativity, the other part is related with the problem of gravity and is known as “The General Theory of Relativity” in 1916. The general theory of relativity is a theory of gravitation, while the special theory of relativity does not consist the idea of gravitation. Special theory of relativity is thus for flat space-time; Whenever the general theory of relativity deals with a curved space time relativistic-universe. A critical test to check the bending of light form various stars as it passed the sun was undertaken by a team under Arthur Eddington on the island of „Principle‟ during the „Solar-Eclipse of 1919‟. “The news that, not only did the bending 8 take place, but the amount agreed with Einstein‟s predictions, was bound to make his position at the head of the community of physicists unassailable. Also, and much more surprisingly, it made him famous among ordinary people across the world. Einstein‟s special theory of relativity explains the null result of the Michelson-Morley experiment. In an ordinary way, we say that the special theory of relativity concludes that all is relative, Einstein proposed a fundamental condition on physics; Which is known as the principal of relativity and which states that all the laws of physics must be the same in all inertial frames. Of course, the second monumental postulate of the special theory of relativity is the principle of constancy of the speed of light according to which, the speed of light is „c‟ in all inertial frames. This postulate explains the Michelson-Morley result. Light travels at speed c along every path of the infer meter, so there is no phase difference between different light-paths, and hence no change in interference pattern when the apparatus is rotated. In a very interesting way, Einstein produced his special theory of relativity using the concepts of (a) observer-, (b) uniform-speed, (c) relative to the speed of light, (d) spacial-units & (e) temporal-units. These five aspects of science changed the way of classical (physics) mechanics & introduced the rectified mechanics. Which was mainly introduced by Einstein special relativity deals with an explanation for the question that how speed affects mass, time and space. This theory includes a way for the speed of light to define the relationship between 9 energy and matter. The theory of special relativity explains that small amounts of mass (m) can be interchangeable with enormous amounts of energy (E) special relativity applies to “special” cases. It is mostly used when discussing huge energies, ultrafast speeds and astronomical distances, all without the complications of gravity. The postulates of special relativity can be expressed very succinctly using the mathematical language of pseudo-Riemannian manifolds. The theory of Galilean relativity is the limiting case of special relativity, i.c., c- which is sometimes referred to as the non-relativistic limit In this book, a fair explanation regarding the concepts of special relativity with a proper criticism is done. As I have mentioned earlier that special relativity explains the theory of invariants in different inertial frames of reference. The book is very helpful for a critical explanation to the subject and for research work in associated field. To write this book, I have received many facts from different books; so I am also in praise with their honourable writers. I am also thankful to my respected Professor, Dr. Amarendra Nath Tripathy (Late), Department of Mathematics, B.R.A. Bihar University, Muzaffarpur; who taught the subject in a very unique style; by which I was automatically interested to the subject. I am thankful to my father satya Narain Mishra (Late), my mother, Smt Kunti Devi & my “mama jee”, Sri Vivekanand Dvivedi, Ex - Principal, Kedar pandey (+2) Inter college, Bettiah also for their valuable academic-spirit. In my academic-field, there are many burning personalities, among which Dr. Santosh Narain Pandey, Dr. Nishi Kant Pathak, Dr. Tarkeswar Upadhyay, Prof. M.M. Tripathy, Prof. M.B. Singh, Dr. T.K. Jha, Prof. Javed Ekbal, Prof. P.N. Tiwari & Prof. Mithileshwar Tiwari are always 10 remarkable and rememberable for this creation, so I am indebtful to these mentioned personalities. Arun Prakash Pandey (Late) was the founder secretary of the governing body of P.U.P. College, Motihari and was a man of the scientific spirit. He always questioned before me in different units of scientific-field, by whom I was bound to think about such a creation, of course, he is no more, but in the whole creation, he was along with my writings! This is just like a time-dilation concept. The speed of light is in a practical way unchallengeable. So, the speed of a particle, it greater than the speed of light, than……………………. I have no hesitation to say that I am much indebted and grateful to almighty „God‟ whose power always accelerated me to write something regarding the subject lastly, I am thankful to the whole publication team of “Scriptor Publication”, whose sincerity so acts that the book is in the running-way. I am wising to Mr. Bilal Ahmad and Mr. Divyansh Singh for their best academic – achievements due to their academic acceleration to publish this book.