It is because: there are 5 degrees of freedom in quantum mechanical mono waves, if one is not pre-occupied with Minkowski spacetime, then the spacetime which accommodates all mono waves most elegantly would be 5d.
It is because. There are 5 degrees of freedom in quantum mechanical mono waves. One is not pre-occupied with Minkowski spacetime. The spacetime elegantly would be 5d. This spacetime accommodates all mono waves most.