Humans have conjectured answers about the nature of the cosmos, trying to make sense of its existence and our own. Thus, at least for practical purposes, we have managed to understand how the galaxies we see, the stars that adorn the sky, and the black holes were formed. We know the physics and mathematics necessary to design a spaceship that will fly through the solar system, and we can even put people on the moon. But the most fundamental questions remain unanswered: Is the universe infinite, or does it have an edge? Where does it end? Is our universe “all” that exists, or are there other universes with other physical laws? Is there a limited number of them, or are they infinite?

The information paradox

In the 1970s, the famous physicist Stephen Hawking was investigating the nature and properties of black holes when he theorized the existence of a radiation type — an effect that bears his name— that gradually evaporates black holes, giving rise to what is known as the information paradox. It establishes that the information of matter and radiation that falls into a black hole is lost once it has evaporated by the mechanism discovered by Hawking.

Although Hawking radiation is a theoretical phenomenon — as direct evidence has never been observed — it is feasible to assume that it happens. So, if we wait long enough — on the order of the universe’s age — we can see a black hole completely evaporate and, with it, all the information it once devoured. But the loss of information is prohibited by the laws of physics; it must necessarily be kept somewhere.

Countless investigations have tried to find the answer to this problem: the mechanism by which information is preserved when the black hole disappears. Where does it go?

The holographic principle

Physicists Gerard ’t Hooft and Leonard Susskind proposed the holographic principle in 1995. It’s a physical theory stating that the three-dimensional space we perceive as the world is a holographic projection. This theory was developed to resolve the paradox born from Hawking’s ideas and eventually expanded to other fields of theoretical physics, such as String Theory and cosmology.

Holograms are a three-dimensional representation of an object that materializes from the projection of reflecting light from a flat surface surrounding the stage where the represented object appears.

The DNA helix in the image appears three-dimensional. But in reality, it is the result of a projection from a two-dimensional surface that encloses it — the flat walls of the pyramid — where the information of the image we see is actually encoded.

Susskind and his colleague proposed that perhaps black holes are some kind of hologram. Its interior may appear three-dimensional, but it reflects all the information encoded on its edges: on its 2D surface. This way, the information does not disappear, as it evaporates with the black hole. Furthermore, it doesn’t have to “get out” of it — which is forbidden by the laws of physics — because it was never really inside. What was inside the black hole was the projection of that information, not it as such.

Maldacena’s duality

Juan Martín Maldacena is the author of the scientific article with the most citations in the history of physics. He has considered one of the most brilliant modern scientists regarding his revolutionary work, where he proposes a useful theoretical tool to shape the holographic principle. He currently occupies the same professorship that Einstein held at Princeton, so Maldacena could be said to be the Messi of physics — an even more pertinent comparison considering that he is also Argentine.

His article published in 1997 describes the AdS / CFT correspondence, also known as Maldacena duality. However, expanding on the formal meaning of these acronyms implies delving into very complex technicalities and concepts, so we will limit ourselves to using analogies.

Roughly speaking, the correspondence holds that the universe we see and experience as a 3D world is actually a hologram projected from encoded information onto the edge of a “vessel” infinitely far away from us, so we can’t see it. This implies that all the interactions and phenomena we observe are illusions. Everything that exists and happens in our world also initially exists and happens at its edges but in one less dimension. Same as with the DNA helix. In other words, Maldacena extrapolated to the entire universe the logic of the holographic principle applied to black holes by Susskind and company.

This is undoubtedly an outlandish idea that we are far from testing. Even so, Maldacena’s proposal is beneficial and is applied to solve theoretical problems in diverse fields, from cosmology and astrophysics to condensed matter and nuclear physics.

It is probably only a matter of time before we can decipher the true nature of this convoluted, mysterious, and fascinating universe.