  It is argued that the quadruple gravitational constant 4G can be seen as a
fundamental limit of nature. The limit holds across all gravitational systems
and distinguishes bound from unbound systems. Including the maximum force
c^4/4G allows extending the Bronshtein cube of physical theories to a cube of
limits. Every theory of physics refining Galilean physics - universal
gravitation, special relativity, general relativity, quantum theory and quantum
field theory - is defined by one fundamental limit. As a result, also
relativistic quantum gravity is defined by a limit: the minimum length in
nature. The minimum length is used to deduce the Planck-scale structure of
space. Numerous options are eliminated. Then, the minimum length is used to
deduce the main properties of the common constituents that make up space and
particles.
