A question of invariable mass. In a inertial system, the invariable mass of a system never change with time. This system may not be an isolated system. Whether in any inertial system, the invariant mass of the system remains unchanged. Or, in a certain inertial system, what is the necessary and sufficient condition for the invariant mass of the system to be constant? The same time https://en.wikipedia.org/wiki/Mass_in_special_relativity proposed: Conservation of invariant mass also requires the system to be enclosed so that no heat and radiation (and thus invariant mass) can escape. As in the example above, a physically enclosed or bound system does not need to be completely isolated from external forces for its mass to remain constant, because for bound systems these merely act to change the inertial frame of the system or the observer. Where can I find the strict definition of bound-system? Thank you .