Well lately i have in mess for this. The problem is about the stress energy tensor. Well we know that T_mn = r0 U^m U^n where r0 is mass density and U is proper velocity. Ok now consider the local observer. For him except for U^0 other U^m will be jero. So for local observer. T_00 = r0 c^2 other component of this tensor will be zero. Surely for other observer almost all the component maybe nonzero. And T_00 component for other observer will be T_00 = r0 c^2 (dt/dTou)^2 here tou is proper time. And it will be greater than T_00 of local observer. So how can the T tensor remain invariant. Sure its trace can't be the same. For other observer T_00 it self is bigger let alone the other nonzero component. Please help me here. Its bugging me a lot. If u dnt understand something about my writing than tell me.