The concept of spacetime in modern physics arises from the combination of space and time into a four-dimensional framework. In the theory of general relativity, spacetime is treated as a unified entity, where space and time are intertwined and cannot be separated. However, the question of whether space and time truly have the same fundamental nature at the most fundamental level of reality, such as in quantum mechanics or a quantum theory of gravity, remains an open question.
In the framework of quantum mechanics, certain quantities, such as energy or momentum, are quantized, meaning they can only take on discrete values rather than continuous ones. However, it's important to note that the quantization of these quantities in quantum mechanics does not necessarily imply that space and time themselves have discrete quantum levels. Instead, it is the fields and particles within spacetime that exhibit quantized behaviors.
The possibility of spacetime having discrete quantum levels or being fundamentally discrete is an area of active research in theoretical physics, particularly in the field of quantum gravity. Various approaches, such as loop quantum gravity or string theory, propose different ways in which spacetime might have a discrete or granular structure at the smallest scales. However, these theories are still under development, and there is currently no consensus on the exact nature of spacetime at the quantum level.
In summary, while space and time are unified in the concept of spacetime, the question of their fundamental nature at the quantum level is still an open and active area of research. It is possible that future theories will provide a more detailed understanding of how space and time emerge and behave at the most fundamental scales.