Path 7: Emergent/Entropic Gravity & Analogues

Gravity isn’t fundamental at all – it’s an emergent byproduct of other quantum phenomena, possibly related to entropy or condensed-matter-like degrees of freedom.

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Rationale: This diverse path asks: Could gravity be more akin to how elasticity or fluid pressure emerge from microscale physics, rather than a fundamental force? One line of thought (Sakharov’s induced gravity) is that maybe spacetime is like a medium and what we call Einstein’s equations are analogous to hydrodynamics – the continuum limit of some micro-theory. In 1995, Ted Jacobson showed that Einstein’s equation $R_{ab}-\tfrac{1}{2}Rg_{ab}=8\pi G T_{ab}$ can be derived from the simple assumption that entropy is proportional to horizon area together with the Clausius thermodynamic relation $\delta Q = T\,dS$ for all local Rindler horizons. This suggests gravity is essentially the thermodynamics of some underlying “atoms of spacetime.” More recently (2011), Erik Verlinde proposed that gravity is an entropic force: when material bodies carry entropy and information, the tendency to maximize entropy leads to an effective force that we perceive as gravity – in his simplistic model, Newton’s laws (and even a Newtonian form of Einstein’s law) emerged from entropy gradients. While Verlinde’s specific model is debated, it exemplifies the idea of gravity arising from information theory principles. Another emergent angle is analog gravity: various condensed matter systems (like flowing fluids, superfluids, Bose–Einstein condensates, or optical waveguides) have excitations that obey equations identical to fields in curved spacetime. For example, supersonic flow in a fluid can mimic a black hole horizon for sound waves (a “dumb hole”), complete with Hawking radiation. Indeed, laboratory analogs have observed Hawking-like thermal emission, giving confidence that aspects of quantum gravity (like Hawking’s prediction) are physically sound. Emergent gravity approaches aim to identify the micro-degrees of freedom whose collective behavior at large scales gives rise to spacetime and gravity. Some proposals for these micro-structures include: quantum entanglement patterns (e.g. the idea that spacetime geometry is built from entanglement – as seen in tensor networks or “ER=EPR” wormhole-entanglement duality), or a condensate of fundamental entities (like a superfluid vacuum theory where spacetime itself is a superfluid). The string-net condensation in certain quantum liquids can give rise to emergent gauge bosons and fermions; some research has even tried to get a massless spin-2 mode (graviton) emergent in such systems. While a fully emergent gravity model that reproduces Einstein’s laws at large scale remains elusive, this path offers a compelling vision: gravity as nature’s grand illusion, not fundamental but emergent from something deeper.

Prerequisites: Statistical mechanics and thermodynamics (since concepts like entropy and temperature of spacetime are central), quantum information (for ideas like entanglement = geometry), and familiarity with general relativity and field theory so one can recognize when a condensed matter equation maps to a gravity equation. Also, condensed matter physics background is needed to understand analog gravity experiments and models (e.g. know what a Bose–Einstein condensate is, or how effective field theories in materials work).

Dependencies: Emergent gravity can intersect with almost any other path. For instance, Path 6 (Holography) is an emergent viewpoint – gravity emerges from the boundary field theory. Path 1 (String) can be seen as emergent too: strings have gravitons as vibrations – i.e. gravity emerges from strings. But Path 7 usually implies something more radical: maybe no fundamental graviton at all, just collective behavior. It aligns with Path 11 (Experiments) because analog models provide experimental playgrounds: e.g. measuring Hawking radiation in a lab analog to bolster confidence in real black hole evaporation. Emergent ideas also influence Path 2 (LQG): the idea that spacetime atoms have entropy relates to loop gravity’s counting of microstates for horizons. Conversely, emergent gravity often uses simplified analogies rather than a precise theory, so it might depend on other paths to supply a rigorous foundation (for example, one might hope that whatever the true quantum gravity is, it has a thermodynamic interpretation – then emergent gravity arguments explain why that theory’s continuum limit is Einstein-like). The analog gravity subset doesn’t directly give quantum gravity, but it provides testing grounds and intuition.

Signs of Progress: A concrete emergent model that yields general relativity in a suitable limit would be a game-changer. For example, if someone identifies a specific quantum many-body system whose excitations at low energy include a massless spin-2 mode with Einstein-like interactions (and no unwanted extra modes), that would demonstrate emergent gravity in principle. So far, we have hints (like Sakharov’s induced gravity gets Einstein’s equations at one-loop, but with a huge cosmological constant unless fine-tuned; analog models get the kinematics but not the dynamics of gravity). Another sign of progress is experimental: the continued success of analog experiments. The 2016 observation of Hawking-radiation-like correlations in a BEC “black hole” was a milestone (it’s not direct proof of real Hawking radiation, but it shows the underlying mechanism works in a lab setting). If analog gravity could also simulate, say, cosmological particle production or analog “big bang” expansion, and those results matched theoretical predictions, it strengthens the case that our theories of quantum phenomena in GR (Hawking, Unruh, etc.) are on the right track. On the entanglement front, an important development was the Ryu–Takayanagi formula in holography which equates a geometric area to entanglement entropy – that’s evidence that “spacetime = entanglement” is more than poetry. If future studies manage to reconstruct nearly the entire geometry of a spacetime from entanglement properties of some quantum state (a program already underway using tensor networks and AdS/CFT), that would be a powerful confirmation that spacetime is emergent from quantum information. In summary, progress comes from either building a convincing toy model of emergent gravity (even in lower dimensions) or accumulating evidence that known gravity can be viewed as emergent from deeper principles (thermodynamics, entanglement), and that such principles can answer questions like “why is $\Lambda$ small?” or “what fixes the values of constants?” which fundamental approaches struggle with.

What to Upload Next

This path is explored primarily through the other paths and dedicated research papers. The detailed reading lists and stepping stones from Path 1 (Superstring/M-Theory), Path 2 (LQG), and Path 11 (Experiment) provide complementary resources for deeper exploration.