Reconstructing spacetime from the hologram, even in the classical limit, requires physics beyond the Planck scale

TL;DR

Reconstructing bulk spacetime from boundary data in AdS/CFT requires physics beyond the classical limit, as certain modes necessary for complete geometry are beyond the Planck scale, especially away from the vacuum.

Contribution

This paper demonstrates that classical gravity data alone is insufficient for bulk reconstruction in AdS/CFT, highlighting the need for quantum or Planck-scale physics.

Findings

Classical boundary data cannot fully determine bulk geometry.

Modes beyond the Planck scale are essential for complete reconstruction.

Near the vacuum, classical reconstruction is possible, but not generally.

Abstract

We argue in this essay that for classical configurations of gravity in the AdS/CFT setup, it is in general impossible to reconstruct the bulk geometry from the leading asymptotic behavior of the classical fields in gravity alone. This is possible sufficiently near the vacuum, but not more generally. We argue this by using a counter-example that utilizes the supersymmetric geometries constructed by Lin, Lunin, and Maldacena. In the dual quantum field theory, the additional data required to complete the geometry is encoded in modes that near the vacuum geometry lie beyond the Planck scale.