A New Description of Black Holes

Describing black holes has always been a complex question: Are these mysterious cosmic objects spherical, smooth, and simple according to the theory of relativity? Or are they extremely complex and full of information as Stephen Hawking proposed? New research now proposes a surprising new possibility that incorporates these seemingly disparate descriptions.

According to new research by scientists at SISSA, ICTP, and INFN, black holes could be like holograms: all the information is amassed in a two-dimensional surface, which is then able to appear as a three-dimensional image. These cosmic bodies could be both incredibly complex and concentrate an enormous amount of information inside themselves in two dimensions, as well as aligning with Einstein’s theory of relativity, which describes black holes as three dimensional, simple, spherical, and smooth. In short, black holes appear as three dimensional, just like holograms. The new study, which unites these two discordant descriptions, has recently been published in Physical Review X.

Black holes present several quandaries, chief among them the great difficulties in putting together the principles of Einstein’s general theory of relativity with those of quantum physics when it comes to gravity. According to the first theory, black holes would be simple bodies not containing information. According to the other, as proposed by Jacob Bekenstein and Stephen Hawking, they would be incredibly complex systems characterized by enormous entropy, and consequently would have a lot of information inside them.

To study black holes, the two authors of the research, Francesco Benini (SISSA Professor, ICTP scientific consultant, and INFN researcher) and Paolo Milan (SISSA and INFN researcher), used an idea almost 30 years old, called the “holographic principle.” The researchers say: “This revolutionary and somewhat counterintuitive principle proposes that the behavior of gravity in a given region of space can alternatively be described in terms of a different system, which lives only along the edge of that region and therefore in a one less dimension. And, more importantly, in this alternative description (called holographic) gravity does not appear explicitly. In other words, the holographic principle allows us to describe gravity using a language that does not contain gravity, thus avoiding friction with quantum mechanics.”

What Benini and Milan have done “is apply the theory of the holographic principle to black holes. In this way, their mysterious thermodynamic properties have become more understandable: focusing on predicting that these bodies have a great entropy and observing them in terms of quantum mechanics, you can describe them just like a hologram: they have two dimensions, in which gravity disappears, but they reproduce an object in three dimensions.”

“This study,” explain the two scientists, “is only the first step towards a deeper understanding of these cosmic bodies and of the properties that characterize them when quantum mechanics crosses with general relativity. Everything is more important now, at a time when observations in astrophysics are experiencing an incredible development. Just think of the observation of gravitational waves from the fusion of black holes, a result of the collaboration between LIGO and Virgo or, indeed, that of the black hole made by the Event Horizon Telescope that produced this extraordinary image. In the near future, we may be able to test our theoretical predictions regarding quantum gravity, such as those made in this study, by observation. And this would be something absolutely exceptional.”