Hi, Not sure how stupid my question might be. Could anyone please clarify me, why the atomic clock in a space ship is taken as a reference while explaining a special theory of relativity?. I always wonder how 2 people one outside the space ship and the other inside it feels about a time in 2 different manner. and this problem exists only when I carry an atomic clock inside a spaceship (for the reference of a time). If i carry an analog clock(run by a battery), both the people inside and outside the space ship can see the same time right.?