According to Einstein nothing can move faster than c. But whereas c is constant in all frames of reference, the speed of massive objects is defined relative to other massive objects. So when we say nothing can go faster than c, we surely mean nothing can go faster than c relative to another object. So - I've heard of particles that travel quite close to c relative to the Earth. If I fire a particle like this in one direction, it will be travelling at a speed greater than 0.5 c relative to the Earth. If I then fire another particle of the same speed in the opposite direction, it will be travelling greater than 0.5c relative to the Earth and greater than c relative to the other particle. Correct? Is it then correct to state that, in one of these particles' frame of reference, the other particle will be going backwards in time?