In the framework of general relativity, spacetime itself is not described in terms of frequency and wavelength. Instead, spacetime is a four-dimensional manifold that provides a geometric framework for describing the curvature caused by massive objects or the presence of energy.
The concept of frequency and wavelength is typically associated with waves that propagate through a medium or field. For example, in electromagnetic waves, frequency refers to the number of wave cycles per unit of time, and wavelength represents the distance between two consecutive points in the wave with the same phase.
In the case of "perfectly flat" or Minkowski spacetime, which is the spacetime associated with no massive objects or gravitational fields, there are no waves propagating through it. In this context, there is no characteristic frequency or wavelength to measure.
However, it's worth mentioning that gravitational waves, which are ripples in the fabric of spacetime, do possess frequencies and wavelengths. These waves are generated by the acceleration of massive objects and propagate through spacetime, carrying energy away from the source. Gravitational waves have been detected and measured, and they have specific frequencies and corresponding wavelengths associated with them. But these waves are perturbations of the underlying flat spacetime rather than defining its intrinsic properties.
In summary, the notion of frequency and wavelength is not directly applicable to the "perfectly flat" spacetime itself. These concepts are more relevant in the study of waves propagating through a medium or field, such as electromagnetic or gravitational waves.