Quantum gravity with a two-dimensional spacetime or worldsheet. In the string formulation, one sums over surface metrics and topologies with matter fields on each surface. This worldsheet theory is distinct from the target-space gravity described by the graviton in the closed-string spectrum.
A graviton is the massless spin-two quantum of the gravitational field. It occurs in the symmetric trace-free transverse sector of the first massless closed string level.
Semiclassical gravity couples a classical spacetime metric to quantum matter. In its simplest expectation-value form, the classical gravitational field is sourced by the quantum expectation of the matter stress-energy tensor.
The semiclassical Einstein equation isin units with . A single classical metric responds to the expectation value over all matter branches.
The Page–Geilker experiment used a quantum random decision to choose a macroscopic source-mass configuration and measured its field with a torsion balance. The observed field followed the configuration in the observed branch rather than the nearly cancelling expectation-value average predicted by the simplest no-collapse semiclassical Einstein equation.
When two masses occupy spatial superpositions, their branch-dependent Newtonian gravitational potential energy produces relative phases. If those phases cannot be separated into one phase for each mass, the interaction turns an initial product state into an entangled state.
The Bose--Marletto--Vedral experiment proposes placing two nearby masses in spatial superpositions and detecting the entanglement generated by their branch-dependent gravitational phases. Under locality assumptions and in the absence of other interactions, an entangling mediator must possess incompatible observables rather than behave as a single classical local variable.
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Quantum gravity is a field of theoretical physics that seeks to understand how the principles of quantum mechanics and general relativity can be reconciled into a single coherent framework. While general relativity describes gravity as the curvature of spacetime caused by mass and energy, quantum mechanics governs the behavior of the very small, such as atoms and subatomic particles. The challenge arises from the incompatibility between these two foundational theories.
