1. The problem statement, all variables and given/known data There are 2 particles(1,2) separated ∆x=L moving with the same velocity u_x in frame of reference S , there's an other reference S' moving at v . I have to calculate ∆x'. GAMMA(LORENTZ'S FACTOR) 2. Relevant equations 3. The attempt at a solution I have done x1=0 when t=0 So for particle 1 x1=ut Particle 2 x2=L+ut So ∆x=L=gamma(∆x') , and it's not correct The solution I have got from class is X1=x0+ut1 X2=x0+L+ut2 So when I measure the distance between them in S' the solution is completely different, the thing I don't understand is , how t2=\t1 and not t2=t1.