Gravity from the constructive approach standpoint

The general theory of relativity was first formulated as geometrodynamics, i.e., as a theory where the space–time metric is a dynamic quantity. But it is currently obvious that from the standpoint of particle physics, gravity is simply the theory of a massless spin-$2$ particle. We show that many important properties of gravity itself and of its possible extensions and modifications can be understood starting from the simple physical requirement that the number of physical degrees of freedom is correct and there are no nonphysical degrees of freedom.