The expansion of the universe is like an inflating balloon: galaxies recede from each other, and space itself stretches. In such a picture, known as de Sitter space, there is a special point of view: an observer sees only a part of the world, bounded by a horizon beyond which light cannot penetrate.
Physicists have long suspected that everything happening inside this volume can be described by a 'flat' theory on the boundary, like a hologram on the surface of the balloon. In the new work, the authors construct such a holographic dual for a massive particle. They take a quantum field theory on the boundary and, using a mathematical trick, analytic continuation, obtain a description of the particle in the bulk. A special object emerges: a defect that preserves a certain symmetry (PT-invariance) and reproduces the correct quantum correlations — the so-called Bunch-Davies Wightman function.
But when the researchers tried to measure quantum entanglement via entropy, it turned out that entropy sees only half the picture — only the real part of an important quantity (the central charge). The imaginary part, responsible for more subtle effects, escapes. So the authors proposed a different tool: a single timelike geodesic — the path a massive particle could take in the bulk. By integrating correlations along it, one can recover the connection between an operator in the bulk and a combination of two blocks on the boundary (an OPE block and its 'Casimir' partner).
This trick not only gives the full picture of correlations but also allows one to derive the quantum anomaly, the breaking of classical symmetry at the quantum level, directly from the symmetries of the dS/CFT group. Thus, the geometry inside the volume and the physics on the boundary are tightly linked.
The holographic principle was first proposed for black holes: it turned out that all the information about what fell into the hole could be encoded on its surface, like on a film. Later, the idea was extended to the entire universe: perhaps our three-dimensional world is a projection from a distant two-dimensional boundary.
In Peter Watts's novel Blindsight, an alien ship uses holographic projections for communication, as if all the physics inside the volume is encoded on its shell.