(Phys.org)—The question of why space is three-dimensional (3D) and not some other number of dimensions has puzzled philosophers and scientists since ancient Greece. Space-time overall is four-dimensional,or (3 + 1)-dimensional, where time is the fourth dimension. It's famous that the time dimension is related to the second law of thermodynamics: time has one direction (forward) because entropy (a degree of disorder) never decreases in a closed system such as the universe.
Source: phys.org