Comment by tsimionescu

> But where is entropy in this picture? How do you even define entropy for a system of 2 particles?

The answer is that this doesn't happen in a system with only 2 particles. The idea of gravity as an entropic phenomenon is that you introduce some other kind of particle that permeates spacetime, so there is no system that only contains 2 particles. You may use some idea like virtual particles from quantum field theory, or you may define "quanta of space time" as something that is not technically a particle but basically works like one in a handwavy sense.

But the basic point of these entropy based theories is to explain gravity, and typcilaly spacetime itself, as an emergent result of a collection of numerous objects of some kind. This necessarily means that they don't make sense if applied to idealized systems with very few objects - which is why they typically posit such isolated systems simply can't actually exist in reality.