The reason we consider space and time as separate entities, rather than just space, is based on the principles and observations of physics, particularly as described by Einstein's theory of relativity.
In classical physics, space and time were treated as separate and independent entities. The concept of space was associated with three dimensions (length, width, and height), while time was considered as a separate dimension, progressing uniformly and independently.
Einstein's theory of relativity introduced a revolutionary concept that unified space and time into a four-dimensional framework known as spacetime. According to this theory, the fabric of the universe is not simply three-dimensional space, but a four-dimensional continuum that combines both space and time.
The theory of relativity showed that space and time are intimately connected and cannot be considered independently. Spacetime is not a fixed and absolute framework but is influenced by the presence of mass and energy, causing it to curve and deform. Objects moving through spacetime experience this curvature, leading to phenomena such as time dilation and gravitational effects.
The unification of space and time in spacetime allows for a consistent description of physical phenomena, particularly those involving high speeds and massive objects. It provides a framework in which the laws of physics can be formulated more accurately and consistently.
By considering space and time as integrated components of spacetime, physicists have been able to develop theories and models that explain a wide range of phenomena, from the behavior of subatomic particles to the dynamics of galaxies and the universe as a whole.