In chapter one of Feynman's famous Lectures on Physics, he writes the following.
Mass is found to increase with velocity, but appreciable increases require velocities near that of light. A true law is: if an object moves with a speed of less than one hundred miles a second the mass is constant to within one part in a million.
Because speeds are relative to an observer, if I as an observer move quickly away from an object, is there some sense in which I change its mass?
This seems like it can't be right. Is the above even a well-posed statement? Namely, can an object even have an objective speed? If so, does a change in speed really change something's mass? If so, to which observers?