Let's say Bob floats in space. Bob's identical twin Joe moves past Bob at relativistic speed. As Joe is passing by, Bob gives him more speed by pushing him with his hand. Somehow I just happen to know that Joe will feel a smaller force pushing him than what Bob feels. So my question is: Why is the proper force felt by Joe smaller than the proper force felt by Bob? (Bob and Joe are like superman. They are not breaking any laws of physics though) "Somehow I just happen to know" is perhaps somewhat obnoxious. So let's say that in a little bit different scenario Bob "pushes" Joe by shooting him with a laser gun, Bob feels the normal recoil of the gun, Joe feels an arbitrarily small force, depending on how much redshift there is.