Today our topic is about time travel . Now i am going to tell one sentence say that it is possible or not . There was a twins both age is 30 one is engineer and another one is a astronaut . one day the astronaut goes to space . The rocket goes in a speed of light . after 10 years one of  the twins comes out from space . his age is 30 , and his co twin age is 80 . is it possible ? . you can get a clear idea about this in end . ok lets go to our topic , our topic is Einstein special relativity theorem . what is relativity  ? imagine that you are observation 1 and a man in space is observation 2 . if i snap (  the sound comes from your hand when you fold ) using my both in different  time . if i ask you do i snap my both hand you will say yes , but the same when i ask to a astronaut he will say no because the time taken to snap one hand to another hand the earth will be moved . so the first snap is on one side and the other snap is little bit moved . this is called relativity . so let finish this session by this . let i see you with part 2