What does "now" mean in physics?

In physics, "now" is surprisingly problematic. In classical mechanics it seemed universal and absolute, but Einstein's special relativity showed that simultaneity is relative to the observer's reference frame. Two events that are simultaneous for one observer may occur at different times for another moving observer. There is no single, universal "now" shared across the cosmos. In general relativity the situation is even more complex due to curved spacetime. Physicists can define local "now" slices called spacelike hypersurfaces, but the choice is arbitrary. Some physicists, following the block universe interpretation, argue that past, present, and future all equally exist, making "now" merely a subjective label rather than a physically privileged moment.