Does Gravity Come from Black Holes? Insights from Loop Quantum Gravity and String Theory
Understanding gravity at the quantum level is one of the biggest challenges in physics. Two leading theoretical frameworks – Loop Quantum Gravity (LQG) and String Theory – offer different answers to how gravity works and how black holes fit into the picture. The question “Does gravity come from black holes?” is essentially asking whether gravity is caused by black holes or if it emerges from more fundamental ingredients. Below, we explore how each theory models gravity, how they describe black holes, and what they say about gravity’s origin (including roles of entropy, information, and spacetime structure).
Loop Quantum Gravity (LQG)
Overview: LQG is a quantum gravity theory that directly quantizes space and time themselves (following Einstein’s idea that gravity is the geometry of spacetime) . Instead of treating gravity as a force carried by a particle, LQG treats space as made of tiny discrete chunks or “atoms” of geometry. In simple terms, space is envisioned as a network of finite loops – called a spin network – that weave a very fine fabric of spacetime . Distances, areas, and volumes are quantized (coming in minimal units), with the scale set by the Planck length (~10^−35 m).
- Gravity at the Quantum Level: In LQG, gravity is the quantum geometry. The gravitational field corresponds to how these fundamental loops connect and spin. Because space itself has an “atomic” structure, gravity emerges as the collective effect of many quantum geometric states. There is no background stage on which physics happens – spacetime itself is built up from the quantum states of the gravitational field (LQG is background independent). At large scales, this network of quantized geometry behaves like smooth spacetime, reproducing Einstein’s general relativity. But at the Planck scale, space is granular, and the usual notion of a continuous spacetime breaks down . Gravity in LQG is thus not coming from matter per se or from black holes specifically; it comes from the geometry of spacetime itself being quantum.
- Black Holes in LQG: Black holes are extreme regions of curved spacetime, so they provide a testing ground for LQG. In LQG’s view, a black hole’s event horizon (the surface beyond which nothing can escape) has a finely quantized area. The horizon can be thought of as punctured by the “threads” of the spin network coming from the black hole’s interior . Each puncture carries a quantum of area, like a tiny patch on the horizon, so the total area is the sum of many discrete chunks. LQG researchers have shown that the number of ways these punctures (and the quantum states associated with them) can arrange to produce a given horizon area matches the black hole’s entropy. Black hole entropy in LQG comes out proportional to the horizon area, consistent with the Bekenstein–Hawking formula S = \frac{k_B A}{4 \ell_P^2} . In fact, LQG provides a geometric explanation for why black hole entropy is finite and proportional to area: the horizon’s quantum geometry has a finite number of microstates for a given area . These microstates are essentially the different ways the spin network threads (“polymer excitations”) can puncture the horizon, each puncture contributing a small quantized bit of area and curvature .