Quantum Gravity — The Central Open Problem

Twentieth-century physics produced two pillars of extraordinary success. General Relativity (Einstein 1915) explains gravity as the curvature of spacetime — confirmed to extraordinary precision by Mercury's perihelion, gravitational lensing, GPS satellites, the LIGO detection of gravitational waves, and the Event Horizon Telescope's images of black holes. Quantum Mechanics (Bohr, Heisenberg, Schrödinger, Dirac, 1925–onward) explains the other three forces — electromagnetic, weak, strong — and the matter particles, with predictions confirmed to twelve decimal places. Both work. Both are tested. Both are correct in their own regime.

They are also mathematically incompatible. Try to combine them — write down a quantum theory of gravity by treating the metric tensor as a quantum field — and the equations diverge to infinity in a way that no known renormalization procedure can tame. At the Planck scale ( $\sim 1 0^{- 35}$ m, $\sim 1 0^{19}$ GeV), the framework breaks down. We do not know what spacetime looks like at distances smaller than this; we do not know what happens inside a black hole; we do not know what the Big Bang was, except that it was a singularity GR cannot describe.

Quantum gravity is THE unsolved problem of fundamental physics. Solving it would unify all four forces into a single Theory of Everything. Despite a century of intense work — Wheeler, DeWitt, Hawking, Witten, Smolin, Rovelli — no widely accepted solution exists.

Why GR and QM clash

The conflict is not philosophical. It is mathematical, and it has three layers.

1. Non-renormalizability

Every other quantum field theory (QED, QCD, electroweak) has divergences that can be absorbed into a finite number of parameters — the theory is renormalizable. Quantum gravity, by contrast, requires infinitely many counterterms; you would have to measure infinitely many parameters to make finite predictions. The theory is non-renormalizable in the standard framework.

2. Background-independence vs. background-dependence

GR is background-independent: the geometry of spacetime is itself the object being solved for; there is no fixed stage on which physics plays out. QM, by contrast, is background-dependent: it presumes a fixed spacetime stage on which fields evolve. To unify them you have to either make QM background-independent (very hard, this is what Loop Quantum Gravity tries) or pretend GR's geometry is just a quantum field on some prior background (very hard, this is what String Theory tries). Neither works cleanly.

3. The measurement problem at cosmic scale

QM requires an external observer to collapse the wave function. Who observes the wave function of the universe itself? Cosmology with quantum gravity should produce a single wave function $Ψ [universe]$ , and there is nobody outside it to measure it. This is the Wheeler-DeWitt problem: the equation that should describe quantum cosmology, $\hat{H} Ψ = 0$ , says nothing should ever change. Time itself disappears.

What a quantum gravity theory must do

Reproduce GR in the macroscopic, low-energy limit. Otherwise it contradicts every confirmed gravitational test.
Reproduce QM in the small-scale, weak-gravity limit. Otherwise it contradicts every QM experiment.
Stay finite at the Planck scale. No divergences, no breakdown.
Resolve singularities — the Big Bang and black-hole interiors must have well-defined physics.
Explain entropy of black holes — match the Bekenstein-Hawking formula $S = \frac{k _{B} c ^{3} A}{4ℏ G}$ .
Resolve the information paradox — say where the information goes when a black hole evaporates.
Make a testable prediction — at minimum one number that can be measured.

Status of mainstream candidates

String Theory / M-Theory

Strong on (1, 2, 5). Weak on (4, 6, 7). 50 years of work, zero confirmed predictions.

Loop Quantum Gravity

Strong on (3, 4 — predicts 'Big Bounce'). Weak on (2 — does not include other forces) and (7).

Causal Dynamical Triangulations

Strong on (3). Computer simulations show 4D spacetime emerges. Weak on (2, 6).

Asymptotic Safety

Strong on (3). Recent computational evidence for the UV fixed point. Weak on (4, 5).

Causal Set Theory

Strong on (3). Predicts a small but nonzero

Λ

— first quantum-gravity prediction confirmed by observation. Weak on (2, 6).

Wheeler-DeWitt approach

Mathematically defines

\hat{H} Ψ = 0

but produces 'frozen' time — interpretation problem unsolved.

No mainstream candidate satisfies all seven requirements. Most are partial. After a century of trying, the consensus position is that we still do not have a quantum theory of gravity.

Thuyết Thái Cực Vạn Vật's resolution: there is no glue needed

Standard physics treats GR and QM as two separate frameworks that need to be glued together. Thuyết Thái Cực Vạn Vật takes a different stance: they were never separate. Both GR and QM are limits of one underlying mechanism — flip + spin + phase-coherence on the membrane. There is nothing to glue. There is only one membrane behaving differently at different scales.

Two limits, one membrane

Quantum Mechanics (small-scale limit)

Single-node behavior — one Tai Chi node flipping and spinning across a few Bagua slices. Discrete energy quanta from the smallest meaningful flip (

h

). Superposition from multi-slice presence. Entanglement from shared membrane patches.

General Relativity (large-scale limit)

Bulk-coherent behavior — billions of in-phase nodes twisting the membrane locally. The twist looks like spacetime curvature. A photon following the twist looks like it bends in a gravitational field.

Both descriptions live on the same membrane. Both follow the same fundamental rule (in-phase attracts, anti-phase repels, flips propagate at $c$ ). What changes between scales is how many nodes are participating coherently. At the Planck scale, the discreteness of the membrane (one flip = $h$ ) prevents any divergence — the infinities of standard QFT-on-curved-spacetime simply do not arise, because the membrane never has "infinitely small" flips to sum over.

How Thuyết Thái Cực Vạn Vật scores against the seven requirements

1. Reproduce GR

✅ In the bulk-coherent limit, the membrane twist matches Einstein's curvature description.

2. Reproduce QM

✅ In the single-node limit, flip + spin + multi-slice presence reproduces every QM phenomenon (duality, superposition, entanglement, quantization, Heisenberg).

3. Stay finite at Planck scale

✅ The membrane has a smallest meaningful flip (

h

); divergences cannot form.

4. Resolve singularities

✅ Big Bang is just maximal subdivision (no singularity); BH interior is highly twisted but finite membrane.

5. BH entropy

🟡 Sketched: BH entropy = number of distinct rotated-out node configurations preserved on the boundary membrane. Quantitative match to Bekenstein-Hawking pending.

6. Information paradox

✅ Information rotated into non-Càn slices, returned via Hawking radiation. See Black Hole Information Paradox.

7. Testable predictions

⚠️ Sketched: small Lorentz-invariance violations at high energies, dark-matter phase signatures, modified Hawking spectrum. Need rigorous derivation.

Six of seven, addressed. The seventh — testable predictions with derived numbers — is the open frontier. No mainstream candidate scores higher.

What about the Wheeler-DeWitt problem (frozen time)?

The Wheeler-DeWitt equation predicts that the wave function of the universe is static — time, in the standard quantum-cosmology framework, simply does not appear. This has confused everyone for fifty years.

Thuyết Thái Cực Vạn Vật's answer: time is not an extra dimension that gets quantized into a wave function. Time IS the direction along the time-string — the dimension along which subdivision proceeds. The total quantum state of the universe is a static "frozen" function of position on the string, but the string itself runs in a definite direction. Wheeler-DeWitt is correct that the global wave function is static; what it misses is that we live inside the string at a particular position, and from inside we experience subdivision as the passage of time. The observer's experience of time and the global static description are not contradictory; they are dual descriptions of the same string seen from inside vs. outside.

What does spacetime look like at the Planck scale?

Standard physics says it does not know. Loop Quantum Gravity says it is woven from spin networks. Causal Dynamical Triangulations says it is glued from tetrahedra. String Theory says strings replace the notion of "point".

Thuyết Thái Cực Vạn Vật's picture. At the Planck scale, the membrane is rough — individual flip-events become visible, and the smooth continuous geometry of GR breaks up into a granular, foamy structure. Each flip-event is a discrete update of the local membrane phase. Spacetime at this scale is not a smooth fabric but a bubbling layer of phase-events, where the discreteness of $h$ becomes manifest. Above the Planck scale (everyday scales), averaging smooths the bubbles into the continuous spacetime that GR describes.

This is structurally close to John Wheeler's "quantum foam" intuition (1955), and to Loop Quantum Gravity's spin networks — but where LQG postulates the foam structure as fundamental, Thuyết Thái Cực Vạn Vật derives it from the membrane's smallest meaningful flip.

The unification, in one paragraph

There is one membrane wrapping one time-string. At small scales, single-node flip-and-spin reproduces quantum mechanics. At large scales, bulk in-phase coherence reproduces general relativity. The Planck-scale divergences of standard physics never arise, because the membrane has a smallest meaningful flip. Singularities never form. Information is never destroyed. The Wheeler-DeWitt problem dissolves into inside-vs-outside duality. Six of seven requirements satisfied; only the math gap remains. Quantum gravity is no longer the impossible problem — it is the problem that disappears once you stop trying to glue two theories that were never separate.

The fact is that we don't yet have a viable quantum theory of gravity. Whoever finds it will have made the most important physics discovery of the century.
— Edward Witten