Doubt on FRAME of REF(Its killing me!!!!!!Help) Hello Guys I am really fascinated by Theory of Relativity and although its not in my educational syllabus(not a phy scholar) , I really study a lot about it. Now coming to the point , I have a doubt on FoR and it may be basic, so I hope you fellas can clear it out. Talking about TWIN paradox, lets assume Mr.STILL remains on earth while his brother ,Mr.MOVE moves in a spaceship at very high speed(say 87% of light). From my understanding this is what will happen from Mr.STILL's point of view(or Frame of Ref): Mr.MOVE is moving at 87% of Light's speed so time will slow down for Mr.MOVE. Mr. STILL waits and waits and after 6 days his brother ,Mr.MOVE returns after taking a trip at .87c. Mr.STILL will predict this:Since I am stationary , and my brother is moving at .87c, time will slow down for Mr.MOVE(my brother). At 87% of light , time slows by a factor of 2. Now my clock says that I have spent 6 days ,waiting. So when he will return ,Mr.MOVE won't feel as if he spent 6 days, infact he will think that he spent just 3 days up there. From Mr.MOVE's point of view: Now assume that Mr.MOVE don't know that he is Motion and thinks he is still, so he will see Mr.STILL moving at .87c in opposite direction(along with earth). After waiting for 3 days,as his clock will tell him(or will it be 3 days? I am confused) he will see Mr.STILL and earth returning.So this is what Mr.MOVE will predict:Since I am stationary , and my brother (Mr.STILL)is moving at .87c time will slow down for Mr.STILL(my brother). At 87% of light , time slows by a factor of 2. Now my clock says that I have spent 3 days ,waiting. So when he will return ,Mr.STILL won't feel as if he spent 3 days, infact he will think that he spent just 1.5days down there on earth. From my understanding, both are right since laws of physics is same in every frame of reference.And both are relativly in motion with each other so both can assume taht they are stationary. Now both Mr.STILL and Mr.MOVE thinks that Mr.MOVE has spent 3 days in ship(Mr.STILL does it by calculation while Mr.MOVE is told by his clock). But....... According to Mr.STILL he has spent 6 days on earth(clock will say) while Mr.MOVE tell by his calculations that Mr.STILL will spend only 1.5 days in earth and not 6. But both can't be true, Mr.STILL has spent a definite time up there.Where is the assumption wrong?. Please tell me.Its killing me.