Can Gravity Be Quantum? Lab Bench Experiments on Quantum Gravity and the QGEM Proposal

Overview

PBS Space Time investigates whether gravity is fundamentally quantum or classical and outlines laboratory experiments that could reveal the quantum nature of spacetime without galaxy-sized apparatus. A centerpiece is the quantum gravity induced entanglement of masses (QGEM) idea, which seeks gravity-mediated entanglement between mesoscopic objects to imply gravitons and a quantum spacetime.

Key insights

• Two major paths to quantum gravity are quantizing gravity or gravitizing the quantum.
• Tabletop experiments aim to detect gravity's role in entangling quantum systems, signaling a quantum gravity regime.
• QGEM uses adjacent Stern-Gerlach interferometers with massive nanodiamonds to test gravity's quantum involvement.
• Even if gravity acts classically, tests of wavefunction collapse models (Penrose-Diosi, Oppenheimer post-quantum gravity) may reveal a fundamental diffusion in spacetime.

Introduction

The video tackles the century-old mystery of quantum gravity by contrasting the quantum world with the classical background of gravity. It explains why combining quantum mechanics with general relativity is notoriously hard and highlights the possibility that the solution might lie in testing gravity in the lab rather than in galactic-scale experiments.

Two Broad Paths to Quantum Gravity

The discussion splits into two routes: (1) quantize gravity so the gravitational field and spacetime geometry obey quantum rules, and (2) gravitize the quantum, treating gravity as classical and investigating how quantum matter could give rise to classical gravity. The video references historical ideas like string theory and loop quantum gravity, which seek a quantum theory of gravity, and it introduces alternative viewpoints like the Deosi Penrose model and Oppenheimer's post quantum gravity, which place gravity in a classical framework with quantum systems evolving within it.

Classical Gravity with Quantum Matter

Penrose's model suggests a wavefunction collapse driven by a mismatch between quantum superpositions of mass distributions and a single spacetime curvature. Oppenheimer's approach adds stochastic gravitational fluctuations that cause collapse. Both predict a kind of intrinsic randomness in gravity that could be tested in tabletop experiments. The video emphasizes that such models predict a gradual increase in collapse probability with object size and mass, potentially revealing a fundamental limit to quantum coherence for large systems.

Testing Gravity's Quantum Nature

The core tests are described from two angles: what happens if gravity is truly quantum, and what would indicate gravity is gravitizing the quantum. In the quantum-gravity scenario, gravity and spacetime exhibit superposition and entanglement, which could be evidenced by gravity-mediated entanglement between distant quantum systems. In the gravitized (classical gravity) scenario, collapse processes like Penrose-Diosi or gravitational diffusion would leave measurable traces on quantum states or on the gravitational field itself. The video notes that there are ongoing and planned tabletop experiments designed to constrain these ideas and potentially reveal new physics.

QGEM: A Landmark Laboratory Proposal

The most detailed experiment described is quantum gravity induced entanglement of masses (QGEM). It builds on the Stern-Gerlach interferometer and envisions entangling the gravitational fields of two nearby masses, such as nanodiamonds containing a spin defect. If gravity is quantum, the two interferometers would become entangled, producing correlations in the final spin states that would indicate gravitons mediate the interaction. A nanodiamond mass is highlighted as a practical candidate for creating sizable spatial superpositions with currently available technology.

Outlook

The video closes on an optimistic note: even though quantum gravity remains elusive in full theory, such experimentation could soon provide empirical insight into whether gravity is quantized or remains classical at the quantum level. It also mentions other experiments that seek evidence for or against gravity's quantum nature and hints at a broader research program combining theory, tabletop experiments, and advanced instrumentation.

Thanks to Brilliant for supporting PBS Space Time. The sponsor section is summarized at the end and describes interactive physics, math, programming, and AI learning resources available on brilliant.org.