Physics's holographic principle is not about display holograms

What you see

Nothing, because there is nothing to look at. The "holographic principle" in theoretical physics is a proposal about how information is encoded in a gravitational theory, not a display or a viewing apparatus. A reader who looks for overlap between this idea and the display holograms, volumetric screens, or Pepper's-ghost illusions covered elsewhere in this archive will find none in mechanism: the two uses of "holographic" share a word, not a phenomenon anyone can stand in front of.

How it works

The idea traces to a 1993 paper by Gerard 't Hooft, "Dimensional Reduction in Quantum Gravity," arguing from "unitarity, entropy and counting arguments" that the degrees of freedom describing a volume of space near a black hole can be represented on a lower-dimensional surface. Leonard Susskind developed the idea in a 1994 paper titled, directly, "The World as a Hologram," whose abstract proposes that three-dimensional reality can be treated as "an image of data that can be stored on a two dimensional projection much like a holographic image" — an explicit analogy to optical holography, borrowed because a hologram is a familiar surface encoding a three-dimensional appearance. Juan Maldacena gave the idea a concrete example in a 1997 paper, "The Large N Limit of Superconformal Field Theories and Supergravity," proposing that gravitational theories in anti-de Sitter space are equivalent to a conformal field theory defined only on that volume's boundary, the most-studied realization of the principle.

Viewing conditions and limits

There are no viewing conditions to state, because the papers describe a mathematical correspondence between two theory descriptions, not an apparatus producing an image under any lighting, angle, or distance. None of the three proposes, builds, or photographs a display; each remains, as the abstracts show, framed in degrees of freedom and field-theory duality rather than light, film, or lenses.

What it is not

An optical hologram, as covered throughout the rest of this archive, is a physical recording of an interference pattern between two beams of coherent light, later illuminated to reconstruct a light wavefront a viewer can see. Physics borrows "holographic" only because both ideas involve a lower-dimensional surface that fully encodes a higher-dimensional appearance or set of degrees of freedom — Susskind's own 1994 title makes the analogy explicit. But the physics principle is not built from recorded light, needs no laser or photographic medium, and is not supported or falsified by anything a person could see. Treating the two as one claim, or citing one to support marketing about the other, misreads both.

• Is a "holographic" claim about a physical display a viewer could stand in front of, or a theoretical correspondence in physics?
• Does the source use "holographic" to describe recorded light, or as an analogy for information encoded on a boundary?
• Would the claim still make sense if "holographic" were replaced with "boundary-encoded"?

Keeping this distinction sharp is worth it: theoretical physics found "holographic" a useful analogy for a real, actively studied idea about gravity and information, and that idea should be evaluated as physics, not mistaken for evidence about, or against, any optical display covered elsewhere in this archive.