  The idea of treating the horizon of a black hole as a stretched membrane with
surface tension has a long history. In this work, we discuss the microscopic
origin of the surface tension of the horizon in quantum pictures of spaces,
which are Bose-Einstein condensates of gravitons. The horizon is a phase
interface of gravitons, the surface tension of which is found to be a result of
the difference in the strength of the interaction between the gravitons on its
two sides. The gravitational source, such as a Schwarzschild black hole,
creates a transitional zone by changing the energy and distribution of its
surrounding gravitons. Archimedes' principle for gravity can be expressed as
follows: "the gravity on an object is equal to the weight of the gravitons that
it displaces."
