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The holographic principle
1993· Theory
SpeculativeA serious idea with no experimental proof yet.
A proposal that all the information in a volume of space can be fully recorded on its boundary surface, like a hologram.
See it on the Science map →How it works
Normally you would expect storage capacity to grow with volume: double a warehouse's size in every direction and it holds eight times as much. Black holes break that rule. Their maximum information grows with surface area, about one bit for every few tiny Planck-sized squares on the horizon. Gerard 't Hooft (1993) and Leonard Susskind (1995) proposed that this is true of any region: the 3D world might be a full encoding of data on a 2D boundary, with one dimension effectively emerging.
Why it matters
If correct, space itself is not fundamental, and a theory of quantum gravity should be written in terms of boundary information. It reshaped how physicists attack black holes and spacetime.
Think of it like
Like a full building model reconstructed from the facade drawings alone: every room is encoded in the exterior plans, though nothing looks missing from inside.
How we know
Strong mathematical support: black-hole entropy and the exact AdS/CFT model (1997). No experiment has tested it for our universe; Fermilab's Holometer (2014-15) ruled out one specific version.
Still a mystery
Our universe is not shaped like the anti-de Sitter space where holography works precisely. Nobody knows how to write a holographic description of our expanding universe.
Connections (6)
▸Black-hole entropy: information on the surface inspired The holographic principleSpeculative
The relationship
A black hole is the most information-dense object possible, and its entropy grows with the area of its horizon, not its volume. So the most information any region can hold is set by its surface area. 't Hooft and Susskind took this limit literally: the region's full contents might be encoded on its boundary.
How the link was found
Jacob Bekenstein proposed black-hole entropy in 1972; Stephen Hawking fixed its exact value in 1974. Gerard 't Hooft drew the holographic conclusion in 1993, and Leonard Susskind extended it in 1995.
Is it proven?
Hawking's calculation and the 1996 Strominger-Vafa count of black-hole microstates in string theory agree on the area law. The leap from black holes to every region of space remains untested.
Open questions and mysteries
Hawking radiation has never been observed. And whether an area limit means the world is literally encoded on a boundary, or only bounded by one, is debated.
▸The Page curve and "islands": information escapes relies on The holographic principleSpeculative
The relationship
The Page curve tracks how much information leaks out of an evaporating black hole over time. The 2019 calculations that finally reproduced it used holographic tools: rules relating entanglement to surface areas, first worked out within AdS/CFT. They found hidden "islands" inside the black hole that count as part of the outside radiation.
How the link was found
Don Page described the curve in 1993. In 2019 Geoff Penington, and separately Almheiri, Engelhardt, Marolf and Maxfield, derived it using holographic entanglement methods.
Is it proven?
The calculations are consistent and were confirmed by several groups, but in simplified model universes, often lower-dimensional or anti-de Sitter. No observation tests them.
Open questions and mysteries
Whether the method works for realistic black holes in our universe, and what islands physically mean, are open.
▸Gerard 't Hooft proposed The holographic principleSpeculative
The relationship
't Hooft counted the number of independent quantum states a region of space could hold. Because packing in too much creates a black hole whose entropy scales with area, he concluded the true count tracks surface area. He proposed that physics inside could be described by roughly one bit per tiny Planck-sized patch of the boundary.
How the link was found
Gerard 't Hooft, in his 1993 paper "Dimensional reduction in quantum gravity," which compared the idea to a hologram.
Is it proven?
It follows logically from black-hole entropy. AdS/CFT (1997) later realized it precisely in model universes. It is untested in our universe.
Open questions and mysteries
't Hooft hoped it would point to a deterministic theory underneath; most others took it in different directions. Which reading is right is unknown.
▸Leonard Susskind developed The holographic principleSpeculative
The relationship
Susskind took 't Hooft's counting argument and showed how string theory could make it work, presenting the world as a hologram. His paper gave the idea its name and momentum, connecting it to his own black-hole complementarity and to string theory's way of storing information.
How the link was found
Leonard Susskind, in "The World as a Hologram" (1995). Willy Fischler and Susskind (1998) and Raphael Bousso (1999) extended it to expanding universes.
Is it proven?
AdS/CFT appeared two years later and matched it precisely, which greatly raised confidence. It remains unconfirmed by experiment.
Open questions and mysteries
Making holography work for an expanding universe like ours, rather than model universes, is still unsolved.
▸The holographic principle is made precise in AdS/CFT: a working hologram (in the math)Well supported
The relationship
The holographic principle was a general suspicion: a region's contents are encoded on its boundary. AdS/CFT is a fully specified example. Everything inside a 5D anti-de Sitter universe, gravity and black holes included, is exactly reproduced by a 4D quantum theory on its edge, with explicit rules for translating between the two.
How the link was found
Juan Maldacena (1997) gave the example. Edward Witten's 1998 paper "Anti de Sitter space and holography" explicitly linked it to 't Hooft and Susskind's principle.
Is it proven?
Boundary calculations reproduce black-hole entropy and other interior quantities. In 2006 Shinsei Ryu and Tadashi Takayanagi showed boundary entanglement equals interior surface areas, a sharp holographic match.
Open questions and mysteries
Our universe is not anti-de Sitter. Proposals for holography of expanding universes, such as dS/CFT (2001), are incomplete.
▸The holographic principle inspires talk of The simulation hypothesisPhilosophy
The relationship
The holographic principle says a 3D world could be fully encoded as data on a 2D surface. Popular accounts read that as "our world is a projection," which sounds like a rendered simulation. The resemblance is loose: holography is about how much information space can hold, and says nothing about a computer or a programmer.
How the link was found
Science writers and commentators made the link, especially after the 2000s. Physicists who developed holography have generally not endorsed it.
Is it proven?
No evidence connects them. Fermilab's Holometer (2014-15) searched for a predicted holographic "jitter" in space and found none, ruling out that specific model.
Open questions and mysteries
Whether information-based physics changes the philosophical odds of a simulation is debated, but not testable now.