<P> In general relativity, energy--momentum conservation is not well - defined except in certain special cases . Energy - momentum is typically expressed with the aid of a stress--energy--momentum pseudotensor . However, since pseudotensors are not tensors, they do not transform cleanly between reference frames . If the metric under consideration is static (that is, does not change with time) or asymptotically flat (that is, at an infinite distance away spacetime looks empty), then energy conservation holds without major pitfalls . In practice, some metrics such as the Friedmann--Lemaître--Robertson--Walker metric do not satisfy these constraints and energy conservation is not well defined . The theory of general relativity leaves open the question of whether there is a conservation of energy for the entire universe . </P> <P> In quantum mechanics, energy of a quantum system is described by a self - adjoint (or Hermitian) operator called the Hamiltonian, which acts on the Hilbert space (or a space of wave functions) of the system . If the Hamiltonian is a time - independent operator, emergence probability of the measurement result does not change in time over the evolution of the system . Thus the expectation value of energy is also time independent . The local energy conservation in quantum field theory is ensured by the quantum Noether's theorem for energy - momentum tensor operator . Note that due to the lack of the (universal) time operator in quantum theory, the uncertainty relations for time and energy are not fundamental in contrast to the position - momentum uncertainty principle, and merely holds in specific cases (see Uncertainty principle). Energy at each fixed time can in principle be exactly measured without any trade - off in precision forced by the time - energy uncertainty relations . Thus the conservation of energy in time is a well defined concept even in quantum mechanics . </P>

Who state the law of conservation of energy