Planck scale
The Planck scale is the energy scale at which the effects of quantum gravity become dominant, approximately 1.22 × 1019 GeV or equivalently a length of 1.6 × 10−35 meters and a time of 5.4 × 10−44 seconds. It is constructed from the three fundamental constants of nature — the gravitational constant G, the reduced Planck constant ℏ, and the speed of light c — and represents the scale at which the classical concepts of space and time themselves are expected to break down. No experiment currently conceived can probe the Planck scale directly, making it the boundary between the known and the conjectured in fundamental physics.
The Planck scale is where the Einstein field equations of general relativity must be replaced by a theory of quantum gravity, and it serves as the natural ultraviolet cutoff in estimates of the vacuum energy density that produce the cosmological constant problem. The fact that our most successful theories fail at this scale is not an embarrassment but a signal: the Planck scale marks the edge of the map, and what lies beyond it requires new mathematics, not just new parameters.