What happens in the event horizon

Black holes: event horizon as a hologram?

At the edge of the comprehensible: black holes could resemble a hologram, postulate physicists. As a result, only a two-dimensional “shadow” of what falls inside would remain on the event horizon. This idea, already postulated by Stephen Hawking, has now been concretized by physicists with the help of string theory. Your model could fix the discrepancies between quantum theory and Einstein's theory of relativity for these objects.

What happens to matter and radiation that falls into a black hole? According to the classical idea, this is a path of no return: All information is lost behind the event horizon because nothing can penetrate back to the outside. However, this contradicts the laws of quantum mechanics, according to which information in the cosmos can never be completely lost. It can only transform itself into other forms - even if it is into entropy.

Hologram as a solution

This “information paradox” in black holes has been causing headaches for physicists for decades. The British physicist Stephen Hawking also devoted a large part of his time to this problem - and proposed a possible solution as early as 2015. Accordingly, the information on the event horizon is converted into a "flattened" form reduced by one dimension. Physicists refer to this transformation as super translation.

In principle, this creates a kind of hologram of the information swallowed by the black hole at the event horizon. "The idea is that these supertranslations form a hologram of the incoming particles," explained Hawking. This could resolve the paradox because, although the information is retained as a holographic "shadow", its originator is irretrievably lost.

Dimension of gravity "shortened out"

Francesco Benini and Paolo Milan from the International Center for Advanced Studies (SISSA) in Trieste have now put this theory of a “hologram” on the black hole into concrete terms using string theory. According to this theory, all particles and all matter go back to different vibrational states of tiny, multidimensional threads. The processes on the event horizon can then be physically described by special states of the strings and the "hologram principle".

“The holographic principle allows us to describe gravity using a formula that does not contain gravity,” explain Benini and Milan. "This means that the behavior of gravity in a given part of space can be expressed using equations of a system that has one dimension less and only occurs at the edge of this region - like a black hole." Physicists describe this reduced variant of the universe as anti- de-sitter room.

Quantum theory and Einstein united

With the help of this principle, the physicists have developed an equation model that can describe the processes on the event horizon of rotating, electrically charged black holes. Their model meets the requirements of quantum mechanics, but can also be reconciled with Einstein's theory of relativity, as Benini and Milan explain. “Our method opens up a way to quantitatively research the quantum properties of black holes in this anti-de-sitter space,” the researchers said.

According to the physicists, their model demonstrates that it is in principle possible to transfer the holographic principle to black holes. "But this is only a first step towards a deeper understanding of these cosmic phenomena and their properties at the interface between quantum mechanics and the theory of relativity," state Benini and Milan.

Can that be proven?

In view of the great advances in astronomy, the researchers believe that their and other models for solving the information paradox may soon be tested in practice. Because the advances in the detection of gravitational waves and the first photo of a black hole demonstrate the rapid development of observation technologies.

“In the near future, we might be able to test our theoretical predictions through observations,” said Benini and Milan. “From a scientific point of view, that would be an absolutely exceptional event.” Until then, however, it will remain with models and speculations. (Physical Review X, 2020; doi: 10.1103 / PhysRevX.10.021037)

Source: Scuola Internazionale Superiore di Studi Avanzati, American Physical Society

5th June 2020

- Nadja Podbregar