The century-long quest to reconcile quantum mechanics with gravity

Overview

For over a century, physics has rested on two pillars that refuse to fit together: quantum mechanics, which governs atoms and subatomic particles, and Einstein's general relativity, which describes gravity as the curvature of spacetime. Every attempt to merge them has produced either mathematical nonsense (infinities that can't be removed) or predictions too small to ever measure. A team at the Vienna University of Technology (TU Wien) may have changed that second problem. Their new "q-desic equation," published in Physical Review D, shows that when the cosmological constant—the term representing the universe's accelerating expansion—is included, quantum corrections to particle paths become large enough to potentially observe at cosmological distances.

In This Story

3 Videos

5 People

Quantum gravity corrections detected in galaxy rotation data

3 Context

The Wheeler-DeWitt equation (1967)

Key Indicators

10⁻³⁵ m

Quantum gravity deviation from gravity alone

Predicted deviation from classical paths considering only ordinary gravity—far too small to ever measure

10²¹ m

Scale where cosmological constant amplifies deviations

When the cosmological constant is included, quantum corrections become substantial at galaxy-cluster distances

~100 years

Duration of the unification problem

Physicists have sought to reconcile quantum mechanics and gravity since the 1920s, with no experimentally confirmed theory yet

Competing new frameworks in 2023–2026

At least three distinct approaches to quantum gravity have produced potentially testable predictions in the past three years

Interactive

Exploring all sides of a story is often best achieved with Play.

Ever wondered what historical figures would say about today's headlines?

Debate Arena

Two rounds, two personas, one winner. You set the crossfire.

People Involved

Benjamin Koch

Lead author of the q-desic equation paper

Ali Riahinia

Co-author of the q-desic equation paper

Angel Rincón

Co-author of the q-desic equation paper

Jonathan Oppenheim

Proponent of a competing "postquantum classical gravity" theory

Ginestra Bianconi

Author of entropy-based quantum gravity framework

Organizations Involved

Vienna University of Technology (TU Wien)

Research University

Published the q-desic equation framework

Austria's largest technical university, with a physics department that has produced significant work in quantum gravity and quantum field theory in curved spacetime.

Physical Review D

Scientific Journal

Published the q-desic equation paper and multiple competing quantum gravity frameworks

A leading peer-reviewed journal covering particle physics, field theory, gravitation, and cosmology, published by the American Physical Society.

Timeline

TU Wien publishes q-desic equation linking quantum mechanics and gravity
Theory

Benjamin Koch, Ali Riahinia, and Angel Rincón published the q-desic equation in Physical Review D, showing that quantum corrections to particle paths become large and potentially observable at cosmological scales when the cosmological constant is included.
March 9th, 2026
Warwick team builds first unified framework for detecting spacetime fluctuations
Experimental Framework

A University of Warwick-led team published a framework in Nature Communications categorizing spacetime fluctuations into three types and mapping each to measurable signatures in laser interferometers, from LIGO-scale to tabletop systems.
January 15th, 2026
Aalto University develops gravity theory compatible with Standard Model
Theory

Mikko Partanen and Jukka Tulkki published a quantum gravity framework using flat spacetime and gauge symmetries analogous to the other three fundamental forces, making gravity structurally compatible with the Standard Model of particle physics.
May 2nd, 2025
Queen Mary researcher derives gravity from quantum entropy
Theory

Ginestra Bianconi published a framework in Physical Review D treating the spacetime metric as a quantum operator, using quantum relative entropy to connect geometry and matter. It predicts a small, positive cosmological constant consistent with observations.
March 4th, 2025
UCL physicist proposes gravity may not be quantum at all
Theory

Jonathan Oppenheim published a "postquantum classical gravity" theory in Physical Review X, arguing spacetime remains classical while quantum mechanics is modified. He placed a 5,000-to-1 bet against quantum gravity proponents.
December 4th, 2023
Loop quantum gravity proposed
Theory

Abhay Ashtekar reformulated general relativity using new variables, laying the groundwork for loop quantum gravity—an approach that quantizes spacetime itself into discrete chunks without requiring extra dimensions.
January 1st, 1986
First superstring revolution begins
Theory

String theory emerged as a leading candidate for quantum gravity, proposing that fundamental particles are one-dimensional vibrating strings. It requires extra spatial dimensions and remains untested.
January 1st, 1984
Wheeler-DeWitt equation formulated
Theory

Bryce DeWitt published the first equation attempting to combine quantum mechanics with general relativity, applying canonical quantization to gravity. It remains foundational but is ill-defined in the general case.
January 1st, 1967

Scenarios

Quantum gravity corrections detected in galaxy rotation data

Discussed by: TU Wien researchers and cosmological observation groups analyzing galaxy-cluster-scale dynamics

If the q-desic equation's predicted deviations at 10²¹-meter scales are confirmed in galaxy rotation curve data or large-scale structure surveys, it would mark the first observational evidence for quantum gravity effects. This would validate the approach of quantizing the spacetime metric and could redirect the field away from string theory and loop quantum gravity toward operator-based methods. Astronomical surveys already in progress—such as those using the Vera C. Rubin Observatory or the European Space Agency's Euclid mission—could provide the data needed within the next decade.

Multiple frameworks converge on consistent, testable predictions

Discussed by: Physicists tracking the parallel development of the TU Wien, Bianconi, and Aalto frameworks, plus the Warwick detection methodology

The unusual situation of three distinct mathematical approaches (q-desic, entropy-based, and gauge-symmetric) all producing testable predictions within a three-year window could lead to a convergence. If independent frameworks agree on the same observable signature—such as specific deviations in cosmic microwave background data or gravitational wave signals—it would dramatically increase confidence that quantum gravity effects are real and point toward their correct mathematical description. The Warwick detection framework provides a shared methodology for comparing predictions across approaches.

Predictions fall below detection thresholds, field returns to theoretical stalemate

Discussed by: Skeptics within the quantum gravity community, particularly string theorists who note the long history of unfulfilled testability claims

Despite the mathematical elegance of the q-desic equation, its observable predictions may prove too subtle to distinguish from noise in astronomical data, or the specific approximations used (spherically symmetric, time-independent fields) may not generalize to the messier real universe. The Strings 2025 conference already struggled to find organizers for Strings 2026, suggesting institutional fatigue with untestable theories—and new frameworks that generate excitement but not data could deepen that fatigue rather than resolve it.

Tabletop experiments settle whether gravity is quantum

Discussed by: Experimental physicists working on nanoparticle interferometry and tabletop gravity experiments, including those designing QUEST and GQuEST interferometers

Rather than waiting for cosmological observations, laboratory-scale experiments using matter-wave interferometry with nanoparticles could determine whether gravity exhibits quantum behavior at all. If these experiments—some expected to produce results in the next few years—show gravity behaves classically, it would vindicate Oppenheim's postquantum approach and invalidate frameworks like the q-desic equation that assume gravity can be quantized. Jonathan Oppenheim's 5,000-to-1 bet with Carlo Rovelli and Geoff Penington would be settled.

Historical Context

The Wheeler-DeWitt equation (1967)

1967

What Happened

Bryce DeWitt applied the canonical quantization procedure—the standard recipe for turning a classical theory into a quantum one—to Einstein's general relativity. The result was an equation analogous to the Schrödinger equation but for the entire universe's geometry. John Archibald Wheeler championed the approach, and the equation bears both their names.

Outcome

Short Term

The equation proved mathematically ill-defined in the general case, producing intractable infinities. It could not be solved for realistic physical situations.

Long Term

Despite its technical problems, the Wheeler-DeWitt equation established the template for all subsequent quantum gravity attempts: take a classical description of spacetime, apply quantum rules, and see what changes. Every major approach since—string theory, loop quantum gravity, and now the q-desic equation—follows this basic logic.

Why It's Relevant Today

The TU Wien team's q-desic equation succeeds where the Wheeler-DeWitt equation struggled by narrowing the problem to a specific, solvable case (spherically symmetric, time-independent fields) and extracting concrete, potentially measurable predictions rather than attempting a general solution.

MOND and the galaxy rotation problem (1983)

1983

What Happened

Israeli physicist Mordehai Milgrom proposed Modified Newtonian Dynamics (MOND), an alternative to dark matter that modifies Newton's gravitational law at very low accelerations. He was responding to the same puzzle the TU Wien paper addresses: galaxies rotate faster than their visible matter can explain. MOND accurately predicted rotation curves for hundreds of galaxies without invoking invisible matter.

Outcome

Short Term

The mainstream physics community largely dismissed MOND in favor of dark matter, which fit better with cosmological observations at larger scales. MOND was seen as an ad hoc fix rather than a fundamental theory.

Long Term

MOND's predictive successes remain unexplained by dark matter models. The tension between MOND's accuracy at galactic scales and dark matter's success at cosmological scales persists after 40 years, suggesting that the correct theory of gravity at these scales may differ from both standard approaches.

Why It's Relevant Today

The q-desic equation offers a potential third path: quantum corrections to gravity that naturally produce deviations at galactic scales without requiring either invisible matter or ad hoc modifications to gravitational law. If confirmed, it could explain why both MOND and dark matter capture part of the truth.

The cosmological constant problem (1998)

1998

What Happened

Two teams of astronomers—led by Saul Perlmutter and by Brian Schmidt and Adam Riess—discovered that the universe's expansion is accelerating, confirming the existence of a cosmological constant (or "dark energy"). This value, when calculated from quantum field theory, comes out roughly 10¹²⁰ times larger than the observed value—the largest discrepancy between prediction and observation in all of physics.

Outcome

Short Term

Perlmutter, Schmidt, and Riess shared the 2011 Nobel Prize in Physics. The cosmological constant became central to cosmology but remained unexplained.

Long Term

The cosmological constant problem became a key benchmark for quantum gravity theories. Any successful unification must explain why this value is small but nonzero—a test that most approaches fail.

Why It's Relevant Today

The TU Wien result is especially significant because the cosmological constant is exactly the ingredient that amplifies quantum gravity effects from immeasurably small to potentially observable. Rather than being a nuisance, the cosmological constant becomes the lever that makes quantum gravity testable.