Concept
The Planck Scale
Pronounced: PLANK (rhymes with 'tank')
The almost inconceivably small scale of length and time at which both gravity and quantum mechanics matter at once, and where a theory of quantum gravity is needed.
Last updated 23 May 2026 · How we research
The Planck scale is the realm of the almost inconceivably small, where the familiar rules of physics are expected to break down. The Planck length is about a hundred million trillion times smaller than a proton, and the Planck time is the corresponding interval, the time light takes to cross that distance. They are built from the fundamental constants of nature and mark the scale at which the effects of gravity and quantum mechanics become equally important.
Why it mattered to Hawking
This is the crux of the great unsolved problem of physics. On everyday and astronomical scales, general relativity describes gravity perfectly, while quantum mechanics rules the world of particles. The two almost never need to be used together. But at the Planck scale, both apply at once, and they contradict each other. To describe nature there, physicists need a theory of quantum gravity that does not yet fully exist.
Hawking spent his career probing exactly the places where this matters: the singularity at the heart of a black hole and the first instant of the Big Bang, where conditions reach the Planck scale and a quantum theory of gravity becomes essential. His work on Hawking radiation and the no-boundary proposal were attempts to glimpse that missing theory from the outside.