Beyond the S-matrix: observables and holography in asymptotically Minkowski quantum gravity

Determine whether quantum gravity in asymptotically Minkowski spacetime admits well-defined gauge-invariant observables beyond the S-matrix and develop a precise formulation of a flat-space holographic principle, analogous in spirit to Wilson-line observables in AdS/CFT.

Background

In asymptotically AdS settings, holography provides a non-perturbative definition of quantum gravity via boundary observables, while in Minkowski spacetime the S-matrix serves as a primary observable.

The paper highlights that it remains unclear whether additional gauge-invariant observables can be defined in asymptotically flat space or whether a precise holographic principle for flat space can be formulated, despite recent progress on celestial holography. Addressing this question impacts the broader interpretation of stochastic gravitational effects and their relation to fundamental versus effective collapse.

References

In asymptotically Minkowski space the S-matrix provides a sharp observable. However, it remains unclear whether it is possible to go beyond this by defining additional gauge-invariant observables, analogous to Wilson lines in AdS/CFT [Maldacena:1998im, Rey:1998ik], or by formulating the holographic principle [tHooft:1993dmi,Susskind:1994vu] in a precise way (for a recent approach see [PasterskiShaoStrominger2017,MeltonSharmaStromingerWang2024]).

— Wavefunction Collapse in String Theory  (2603.24429 - Itzhaki, 25 Mar 2026) in Section 7 (Discussion)