Planck Time

The Planck time (approximately 5.4 × 10⁻⁴⁴ seconds) is the unit of time defined by combining the three fundamental constants of physics — the speed of light c, the gravitational constant G, and the reduced Planck constant ℏ — into a quantity with dimensions of time. It represents the scale at which quantum gravitational effects are expected to become significant: shorter than the Planck time, our current theories of general relativity and quantum field theory both cease to be applicable.

The Planck time is not merely a very small number. It marks the edge of the describable. Events separated by less than the Planck time cannot, in principle, be ordered causally within any known physical framework. It is the temporal resolution limit of the universe as we understand it — below which the concept of "earlier" and "later" may lose meaning because spacetime itself is expected to become discrete, or foamy, or otherwise non-classical in ways no existing theory can specify. The origin of the universe occurred at the Planck time; what happened before is not merely unknown but possibly undescribable in the vocabulary of current physics.

Max Planck introduced these natural units in 1899, noting that they were defined entirely by the constants of nature and thus would be recognized by any civilization — they are not choices but discoveries. The Planck units represent the joints of reality as physics currently carves it. Whether nature itself is organized at those joints is the central open question of quantum gravity.