REVIEW 2 major objections 3 minor 25 references
The Hydrodynamic Approach to Quantum Gravity
T0 review · 2 major / 3 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read This essay argues that Einstein's equations are the long-wavelength hydrodynamic equations of a quantum system built from causal diamonds, each carrying a density matrix whose modular Hamiltonian has expectation and fluctuation both equal…
desk verdict A clear, honest restatement of the HST program with a genuinely new QPR conjecture, but the load-bearing step is unconstructed, so this is a research proposal, not a derivation. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing machinery is the Carlip-Solodukhin ansatz: the holographic screen of every causal diamond supports a $1+1$ dimensional conformal field theory (a two-dimensional quantum field theory with scale-invariant dynamics) whose central charge is proportional to $A_\diamond/4G_N$, so that the standard spectral-density formula for such a theory reproduces the area law and the fluctuation formula (2.2), with the spectral integral dominated by a saddle point at large central charge. The paper makes this concrete using the fact that the screen's Riemannian geometry is encoded in its Dirac operator: the screen fermions are written as sums of two-dimensional fields $\psi^a(t,z)$ times Dirac eigenspinors, with a UV cutoff chosen so the thermal ensemble sits in the regime where the spectral formula applies, and a quartic fermion interaction (3.3) that makes the system a fast scrambler, meaning information spreads across the screen on a Planck-scale timescale. The diamond's modular Hamiltonian is then essentially the Virasoro energy generator $L_0$ of this CFT, up to a redshift factor, and localized excitations are constrained states of the fermionic variables labeled by antisymmetric tensor indices, with an effectively conserved quantum number $n^{d-3}$ that mirrors black-hole mass formulas.
What would settle it
A concrete check: in a tensor-network model of a causal diamond in AdS/CFT, compute the modular fluctuation $\langle (K_\diamond-\langle K_\diamond\rangle)^2\rangle$ for a small generic diamond and compare it with $\langle K_\diamond\rangle=A_\diamond/4G_N$; a mismatch would falsify the empty-diamond-state postulate. Alternatively, compute the entanglement spectrum of the largest diamond in the overlap of two diamonds from each geodesic evolution; the Quantum Principle of Relativity predicts the two spectra coincide.
Extended reading notes
Core claim
The central claim is that a classical solution of Einstein's equations defines, for every causal diamond, a density matrix whose modular Hamiltonian satisfies $$\langle K_\diamond\rangle=\langle (K_\diamond-\langle K_\diamond\rangle)^2\rangle=\frac{A_\diamond}{4G_N},$$ where $A_\diamond$ is the maximal $(d-2)$-dimensional volume on the diamond's boundary. These conditions define the empty diamond state, the analog of the QFT vacuum adapted to the background. Starting from this assignment, half-sided modular flow between nested diamonds—time evolution generated by the ratio of their modular operators—becomes a sequence of unitary embeddings stepping along a geodesic in Planck-scale proper time steps; the paper conjectures that a Quantum Principle of Relativity—overlapping diamonds must have matched entanglement spectra in their common largest diamond—determines compatible unitaries on a Hilbert bundle over the space of time-like geodesics. If the conjecture holds, the successful predictions of QFT survive because experiments probe states near one trajectory and states that would form super-diamond black holes are excluded, while the classical appearance of particle trajectories follows from their being high-entropy 'exclusive gravitational jets' whose collective coordinates decohere. The paper also concludes that gravity is not background independent: every bona fide model is tied to a particular hydrodynamic background, and Euclidean gravitational path integrals should be read as hydrodynamic time averages, not as exact non-perturbative amplitudes.
Load-bearing premise
The load-bearing premise is the Carlip-Solodukhin ansatz: the holographic screen of every diamond hosts a two-dimensional conformal field theory with central charge proportional to $A_\diamond/4G_N$, together with a hand-selected cutoff and interaction that make it a fast scrambler; the paper adopts this ansatz from earlier work rather than deriving it.
Editorial extensions
If this is right
- Quantum field theory survives as an approximation: experiments along a near-geodesic trajectory probe only nearby states, and states that would form a black hole larger than the causal diamond are excluded, so the tested predictions of QFT are unaffected.
- Particle trajectories look classical because each particle is an 'exclusive Sterman-Weinberg jet'—a superposition of a hard particle with arbitrary soft gravitons—whose center-of-mass coordinate is shared by many qubits and therefore decoheres.
- Euclidean gravitational path integrals are reinterpreted as time-averaged hydrodynamic fluctuations rather than exact quantizations, which explains why amplitudes with non-trivial topology do not factorize.
- The Quantum Principle of Relativity, once it can be implemented, restricts which classical solutions of Einstein's equations admit a quantum-gravity completion and fixes the unitary dynamics on the Hilbert bundle.
- Gravity is not background independent: each consistent model is tied to a particular hydrodynamic background, and the maximal-entropy state of that background is left undisturbed.
Reading between the lines
- A consequence the paper leaves implicit: the overlap condition in the Quantum Principle of Relativity can be tested in tensor-network realizations of AdS/CFT before any continuum construction exists—compute the entanglement spectrum of the shared largest diamond from each geodesic evolution and check that the two spectra coincide.
- The constrained-fermion picture predicts an approximately conserved 'jet quantum number' $n^{d-3}$ for localized excitations on timescales of order the diamond size; a simulation of the fast-scrambling screen dynamics that fails to exhibit such approximate conservation would disfavor this specific fermionic realization even if the hydrodynamic picture survives.
- If gravitational path integrals are truly hydrodynamic time averages, then the spectral form factor of a gravitational system should follow the time-averaged hydrodynamic prediction rather than the quantized-phonon prediction wherever the two differ; this is a sharp, testable sign.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper argues that Einstein's equations should be viewed as the hydrodynamic equations of a quantum system in which each causal diamond of a classical background carries a density matrix e^{-K} with modular Hamiltonian K satisfying (2.1) and (2.2), i.e., ⟨K⟩ = ⟨(K-⟨K⟩)^2⟩ = A/(4G_N). The 'empty diamond state' is defined by these conditions. For nested diamonds along a geodesic, the author defines unitary embeddings analogous to half-sided modular flow and conjectures that a 'Quantum Principle of Relativity'—equality of entanglement spectra for tensor factors in overlapping diamonds—promotes these to a compatible Hilbert-bundle evolution over timelike geodesics. The paper then proposes a fermionic, fuzzy-sphere model for the screen CFT with a Thirring-like interaction (3.3) intended to produce fast scrambling, and sketches how localized excitations ('exclusive gravitational jets') and the emergence of QFT might follow. Later sections address consistency with QFT and the role of black-hole formation constraints.
Significance. The paper is a clear and well-organized statement of a research program. Its main strength is the explicit identification of the load-bearing assumptions: the covariant entropy principle (2.1)-(2.2), the Carlip-Solodukhin screen-CFT ansatz, and the QPR conjecture. The paper is admirably honest about the fact that the QPR is not implemented and that the microscopic model contains free choices, and it gives proper credit to the prior hydrodynamic derivations of Einstein's equations. If the conjectures are correct, the framework would offer a background-dependent, emergent-gravity picture with connections to black-hole entropy and cosmological horizons. However, the paper's new positive contribution is largely a proposal: the Hilbert-bundle construction and the microscopic model are not established, and the consistency of the QPR with any concrete model remains an open question.
major comments (2)
- [Section 3, QPR paragraph] The Quantum Principle of Relativity is the only proposed mechanism for promoting the per-diamond unitary embeddings to a compatible global evolution on the Hilbert bundle. The paper states that the QPR is 'easy to state, but hard to implement', that the determination of the unitaries is a conjecture, and that 'we only have a very hand waving understanding of the implications of the QPR'. No construction, existence proof, or example (including for the fermionic model of Section 3) is given for the overlap entanglement-spectrum condition. Because all later claims—emergent QFT, trajectories of localized objects, and the Hilbert bundle itself—depend on this compatibility, this is a load-bearing gap. The paper should either provide at least a toy model where the QPR condition is checked, or explicitly limit the abstract's claim to the nested-diamond construction and present the Hilbert-bundle promotion purely as a conjecture.
- [Section 3, Eq. (3.3)] The interaction L = N^{-1+2(d-3)} Tr[J(1) J(2)] is introduced as the fast-scrambling perturbation of the screen CFT, but no calculation demonstrates that this Thirring-like model is actually a fast scrambler on the fuzzy sphere, nor does the paper derive the claimed time scale. In particular, for d=4 the coefficient is N^{+1}, and the text's statement that this 'guarantees that the time scale for evolution is of order N' is not self-evident. Since the screen dynamics is the physical content of the model, the N-scaling argument should be made explicit and the choice of currents should be justified.
minor comments (3)
- [Section 2, Eq. (2.1)] Equation (2.1) is written as Tr(e^{-K}K) = A/(4G_N) without a partition function or normalization symbol, while the surrounding text uses normalized expectation values ⟨K⟩. Please clarify whether e^{-K} is assumed normalized or whether Z = Tr(e^{-K}) is implicit.
- [Section 2, Carlip-Solodukhin paragraph] The sentence 'The C-S ansatz implies that the central charge of the CFT is proportional to A/(4G_N)' could be misread as a derivation; the central charge is instead fixed by the assumed principle (2.1). The paper should state more explicitly that the area law is an input of the framework, not a derived consequence of the screen CFT alone.
- [Section 3, microscopic model discussion] The sentence 'The smallest central charge corresponds to the place where the C-S argument, which is based on the classical Einstein equations, breaks down' is vague; a quantitative estimate of this breakdown scale would help the reader assess the model's regime of validity.
Circularity Check
The 'area law from Cardy' is the covariant entropy principle restated: the screen CFT central charge is fixed to A/4G, so Cardy's formula returns the input area.
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self definitional
[Section 2, Eqs. (2.1)-(2.2); Section 3, 'The central charge is determined by the C-S principle.']
"These properties define the EMPTY DIAMOND STATE, the analog of the quantum field theory (QFT) vacuum, in the background geometry. ... The Covariant Entropy Principle Tr(e−K⋄K⋄) = A⋄/4GN, (2.1) ... Postulating that the theory on the holographic screen was indeed such a CFT, they rederived the area law from Cardy’s formula [10]. ... The central charge is determined by the C-S principle."
Eq. (2.1) is the definition of the empty diamond state and already contains the target A⋄/4GN. In Sec. 3 the central charge of the screen CFT is 'determined by the C-S principle', i.e. by the same fluctuation relation (2.2) whose value is A⋄/4GN. Since the variance of L0 in a large-c CFT is proportional to c, fixing c by A⋄/4GN makes Cardy's formula S ∝ c return S ∝ A⋄/4GN. The area law is therefore an input used to normalize the central charge (and the UV cutoff), not an output of the CFT. Calling this a rederivation of the area law is equivalent to the input by construction.
full rationale
The paper's derivation chain is: (2.1) defines the empty diamond state by ⟨K⋄⟩ = ⟨(K⋄ − ⟨K⋄⟩)^2⟩ = A⋄/4GN. The Carlip-Solodukhin ansatz places a 1+1 CFT on the holographic screen and the central charge is then 'determined by the C-S principle', which is the same already-area-normalized fluctuation relation. Cardy's formula therefore returns entropy proportional to the same A⋄/4GN that was put in to fix c; the advertised 'rederivation of the area law' is circular by construction, and the same normalization sets the UV cutoff and fuzziness scale. I do not score higher because other load-bearing pieces are not circular: the Quantum Principle of Relativity and the Hilbert-bundle promotion are explicitly labeled a conjecture and the 'biggest open question'; the N-scaling of interaction (3.3) is openly chosen to yield an order-N scrambling time rather than presented as a prediction; and Jacobson's hydrodynamic derivation of Einstein's equations is external and not a self-citation chain. The self-citation [11] supplies the C-S generalization, but independent checks [14-16] are cited for the fluctuation formula, so the score is driven by the definitional input/output match, not by self-citation alone. Overall: partial circularity, score 6.
Assumptions & free parameters
free parameters (3)
- central charge c of screen CFT =
c proportional to A/(4G_N)
- UV cutoff on transverse Dirac eigenspinors / fuzzy sphere rank N =
N set so Cardy's formula is valid and time scales are O(N)
- interaction coefficient N^{-1+2(d-3)} for Thirring-like term =
N^{-1+2(d-3)}
assumptions (6)
- domain assumption Covariant Entropy Principle: Tr(e^{-K} K) = A/4G_N for each diamond
- domain assumption Fluctuation of modular Hamiltonian equals its expectation: <K> = <(K - <K>)^2> in the empty diamond state
- ad hoc to paper Carlip-Solodukhin ansatz: screen of each diamond is a 1+1 CFT with central charge proportional to A/4G_N
- ad hoc to paper Quantum Principle of Relativity: largest diamond in overlap of two diamonds maps to tensor factors with identical entanglement spectra, determining all unitaries
- domain assumption Background solution of Einstein's equations provides the true hydrodynamic description; no background independent formulation exists
- standard math Cardy's formula gives the spectral density of the screen CFT in the relevant regime
invented entities (3)
-
Exclusive gravitational jets (constrained fermion states with energy n^{d-3})
-
Empty diamond state
-
Hilbert bundle over time-like geodesics with QPR connection
Cite this review
Pith. "Pith review of The Hydrodynamic Approach to Quantum Gravity." pith.science (2026). https://pith.science/paper/IPMJ436L
@misc{pith2026250515941,
author = {Pith},
title = {Pith review of: The Hydrodynamic Approach to Quantum Gravity},
year = {2026},
howpublished = {\url{https://pith.science/paper/IPMJ436L}},
note = {Machine review of arXiv:2505.15941}
}
read the original abstract
Several papers from the mid to late 1990s suggest that Einstein's equations should be thought of as the hydrodynamic equations of a special class of quantum systems. A classical solution defines subsystems by dividing space-time up into CAUSAL DIAMONDS and Einstein's equations are the hydrodynamics of a system that assigns a density matrix to each diamond whose modular Hamiltonian K has expectation value and fluctuation both given by A/4G. A is the maximal d-2 volume on the boundary of the diamond and G is Newton's constant. These properties define the EMPTY DIAMOND STATE, the analog of the quantum field theory (QFT) vacuum, in the background geometry. The assignment of density matrices to each diamond enables one to define the analog of half sided modular flow along geodesics in the background manifold, as a unitary embedding of the Hilbert space of a given diamond into the next one in a nesting with Planck scale time steps. We conjecture that this can be enhanced to a full set of compatible unitary evolutions on a Hilbert bundle of the space of time-like geodesics, using a QUANTUM PRINCIPLE OF EQUIVALENCE defined in the text. The compatibility of this formalism with the experimental success of QFT is discussed, as well as the theoretical mechanism by which QFT emerges from this version of quantum gravity. This is a slightly expanded version of an essay that won Honorable Mention in the Gravitation Research Essay Contest for 2025.
Figures
Reference graph
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