## Is The Universe A Hologram?

Maybe.

The holographic principle is a property of quantum gravity and string theories which states that the description of a volume of space can be thought of as encoded on a boundary to the region—preferably a light-like boundary like a gravitational horizon. In other words, all of three-dimensional reality can be described as a two-dimensional sheet or surface of information that extends to the limits of the observable universe – what theoretical physicist and string theorist Raphael Bousso calls “a universal relation between geometry [surface area] and information” in space-time.

So what makes at least some theoretical physicists think that the universe may be a hologram, or at least be best described as a hologram.

It all starts with black hole physics. When an object becomes part of a black hole, two things happen. First, information about that object is lost. Second, the surface area of the black hole’s event horizon (the point at which the gravitational pull becomes so great as to make escape by both matter and energy impossible) grows. The first fact appears to violate the second law of thermodynamics, since one of the lost details was the object’s entropy, or the information describing its microscopic parts. But the second fact offered a way out: if entropy must always grow, and a black hole’s surface area must too, perhaps for the black hole they’re one and the same, and information is somehow stored on the horizon.

The beginning of the attempt to resolve the black hole information paradox within the framework of string theory, Charles Thorn in 1978 developed an approach to string theory based on the idea of string bits, observing that string theory admits a lower-dimensional description in which gravity emerges from it in what would now be called a holographic way. Then in 1993, Gerard t’Hooft proposed what is now known as the holographic principle. Quantum mechanics starts with the assumption that information is stored in every volume of space. But any patch of space can become a black hole, nature’s densest file cabinet, which stores information in bits of area. Perhaps, then, all that’s needed to describe a patch of space, black hole or no, is that area’s worth of information. t’Hooft argued exactly that – that the information contained within a region of space can be determined by the information at the surface that contains it. Mathematically, the space can be represented as a hologram of the surface that contains it.

In 1995, Leonard Susskind a Professor of Theoretical Physics at Stanford University, combined his ideas with previous ones of ‘t Hooft and Charles Thorn in a paper suggesting that the entire universe might be a hologram in which people are just seeing a projection of the real thing. Susskind suggested that the entire universe could be seen as a two-dimensional information structure “painted” on the cosmological horizon, such that the three dimensions we observe are only an effective description at macroscopic scales and low energies.

In 1997, theoretical physicist Juan Maldacena proposed a model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. Called the anti-de Sitter/conformal field theory correspondence, or AdS/CFT correspondence In this model, the mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity.

Maldacen’s model did two things: it solved apparent inconsistencies between quantum mechanics and general relativity, and it provided a way to translate back and forth between the two. Unfortunately, while Maldacena made a compelling argument, it was a conjecture, not a formal proof.

Now, two papers have come out demonstrating that the conjecture works for a particular theoretical case. In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In the first paper, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence. In the second, Hyakutake and his collaborators calculated the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match.

It’s important to note that the papers don’t suggest that our universe is a hologram. The computations describe a universe with ten dimensions in the realm of the black hole and a single dimension universe when calculating characteristics of a gravity free two-dimensional universe. It does, however, suggest that what can be calculated using different dimensional universes could one day be calculated for our own, and is one more step showing that the holographic principle could be useful in understanding the universe.

**References**

Aspect, A., Grangier, P., Roger, G. (1982), “Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell’s Inequalities”, Phys. Rev. Lett. 49 (2): 91–4.

Maldacena, J. M. *The Large N Limit of Superconformal Field Theories and Supergravity*. Adv. Theor. Math. Phys. 2, 231–252 (1998).

Hyakutake, Y. *Quantum Near Horizon Geometry of Black 0-Brane*. arXiv:1311.7526 (2013).

Hanada, M., Hyakutake, Y., Ishiki, G. & Nishimura, J. *Holographic description of quantum black hole on a computer*. (2013).

Maldacena, J. (1998). *The Large N Limit of Superconformal Field Theories and Supergravity*. Advances in Theoretical and Mathematical Physics 2: 231–252

Susskind, L. (1995). *The world as a hologram*. Journal of Mathematical Physics 36 (11): 6377–6371

t’Hooft, G. *Classical N-particle cosmology in 2+1 dimensions*. Class. Quantum Grav. 10 (1993) S79-S91.

t’Hooft, G. *Cosmology in 2+1 dimensions*. Nucl. Phys. B30 (Proc. Suppl.) (1993) 200-203.

t’Hooft, G. *The evolution of gravitating point particles in 2+1 dimensions*. Class. Quantum Grav. 10 (1993) 1023-1038.

t’Hooft, G. *Canonical quantization of gravitating point particles in 2+1 dimensions*. Class. Quantum Grav. 10 (1993) 1653-1664.

Thorn, Gordon. *String Representation for a Field Theory with Internal Symmetry*, (with Roscoe Giles and Larry McLerran), Phys. Rev. D17, 2058-2073 (1978)

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