What happens when you take two entangled particles and put one on a spaceship that moves close to the speed of light? Now you measure one of the two particles. The state of the other should become decided at the exact moment you do the measurement. But there is no absolute simultaneity if you are dealing with two different reference frames. So relative to which reference frame is the process instantaneous? And what happens if you send one of the particles into the future using a spaceship that returns to earth after a while? Does a measurement on one of them still affect the other instantaneously?