Say 2 particles A and B. Particle A is stationary in this inertial reference frame. Particle B is moving with constant velocity v=0.8c Both is a similar isotope of the same radioactive element.(hence, supposedly same half-life) Q1: Which will decay faster in this reference frame? In theory, it's A(stationary), B will decay slower due to time dilation. However, if we change our perspective and take a different reference frame, B is stationary in this refernce frame. Hence, A is moving in the opposite direction with v=0.8c relative to B. Q2: Which will decay faster this time? Not sure if there a paradox in this 1. Just a problem that occurred to me that troubles me a lot.