Gravitational waves could demonstrate that black holes do not exist but are balls of strings

According to some calculations in superstring theory, black holes do not exist but instead objects with similar properties are formed called “ fuzzballs ”. These diffuse balls of superstrings would have particular signatures in the form of gravitational waves during collisions of stars that we believe, wrongly, to be real black holes.

Since the detection gravitational waves in 2015 and to a lesser degree thanks to the images of the collaboration Event Horizon Telescope , one might think that the existence of black holes is a fact of life. We can think so but we can not yet say it and a recent publication in the famous and renowned journal Physical Review D brings water to the mill of those who think that not only do black holes not exist but that it will soon be possible to demonstrate it thanks to gravitational astronomy and the progress of detectors such as Ligo and Virgo .

The article about this is from a team of physicists based at the University of Rome “La Sapienza”, the main Italian university, and it is freely accessible on arXiv . To understand what this is all about, let’s go back to the early 1980s, when what has been dubbed the golden age of black hole theory is coming to an end, and the theories of supergravity and to a lesser extent superstrings are booming.

Following his impressive work in astrophysics , Subrahmanyan Chandrasekhar receives the Nobel Prize from physical in 1983. As usual for the presentation of this prize, the winner gives a conference. At the end of that of the great

astrophysicist Indian, there are some fascinating remarks about the mathematical theory of black holes, which are roughly the following:

I do not know if the full scope of what I said is clear. Let me explain. Black holes are macroscopic objects with masses varying from a few solar masses to billions of solar masses. When they can be considered as stationary and isolated, they are all, each of them, exactly described by Kerr’s solution. This is the only known case where we have an exact description of a macroscopic object.

The macroscopic objects all around of us are governed by a variety of forces, described by various approximations of several physical theories.

In contrast, the only ones Building blocks of black holes are our basic concepts of space and time. They are thus, almost by definition, the most perfect macroscopic objects of the Universe . And since the theory of general relativity provides us with a family of solutions depending only on two parameters for their description, they are also the simplest objects in the Universe.

This simple remark is at the root of the famous paradox information arising from the discovery by Stephen Hawking of the famous quantum radiation from black holes.

Indeed, Chandrasekhar’s remarks concern the theory of black holes rigorously deduced from the theory of general relativity of Einstein . Like Hawking and another Nobel Prize in Physics, Roger Penrose will show it abundantly with their colleagues, within the framework of this theory we must consider that what defines a hole black is only the existence of a event horizon and absolutely not the existence of a space-time singularity. This event horizon is a closed surface constituting a border surrounding a region in which one can only enter and never leave, because for that it would be necessary to go beyond the

speed of light

. It is sometimes described as a membrane that can only be crossed in one direction and like all membranes it is in fact a dynamic object which can vibrate, deform, stretch but which would have the particularity of never

Entropy and paradoxical information theory of black holes

But according to Hawking’s calculations, by quantifying the behavior of light and matter around a black hole, these very compact objects would radiate like a heated body, more precisely what is called a black body . However, according to the theory of thermodynamics this radiation implies that a black hole has a quantity called the in tropia . In all known physical systems, a large entropy is associated with a very complex object in the sense that it is made up of very many particles described by a very large number of parameters and of which a large amount of information should be available for characterize them. When a gas falls into a black hole, this information is no longer available to an outside observer. Impossible also for the same reasons to communicate with a probe which would cross the horizon and to have precious information on what this probe would see since it could not send waves radio outside the black hole – moreover any information contained only in the memory of this probe would be irretrievably lost since it cannot be communicated outside the black hole.

In this video, Jean-Pierre Luminet tells us about the evaporation of black holes via Hawking’s radiation. This evaporation poses a conundrum known as the information paradox with the physics of black holes. © From the Big Bang to the living

In practice therefore, since the definition of information and entropy given from the work of Claude Shannon and John von Neumann , the loss of information generated by the event horizon results in entropy. Hawking, in particular, had shown before his discovery of black hole radiation that the surface of the event horizon had to grow when a black hole swallows something, and that in full agreement with the law of the growth of entropy of the thermodynamics if we identified the value of the entropy of a black hole to the product of the area of ​​its horizon by an appropriate constant of proportionality.

By having the declarations of Chandrasekhar in mind, we immediately understand that something is wrong. Black holes are strictly characterized by a small number of parameters, the mass, the angular momentum and the load, regardless of whether the object of a given mass that would fall into it is a block of iron or a book with much more information.

As a result, black holes should not be able to hold much information and some of it would not just be hidden but destroyed, or at least this is what one could naively deduce at first glance so that black holes should not be able to be endowed with a high entropy in a manner consistent with the fundamentals of known physics, unlike what are involved in the laws of thermodynamics and quantum mechanics applied to these objects, resulting from the theory of the has Einstein’s general relativity.

We are therefore faced with a paradox which is precisely that of information with black holes . There should be parameters hidden in very large numbers behind the small number of parameters describing a black hole and the solutions of Einstein’s theory would therefore only be artificially simplified descriptions of a physical system that could contain so many degrees of freedom. (positions and velocities of particles) than a gas as physicists say in their jargon. Black holes should therefore not be perfectly “smooth” and simple objects in the same way that the Earth is not a perfectly spherical sphere of matter and made of material simple and homogeneous.

For almost a decade, the study of this paradox led to new problems one of which was flushed out by Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully. It is known as the controversy of “ firewall ” ( firewall in English). Futura devoted the two previous articles, below, to his resolution so that we will not detail it and that it will be useful for the reader later to refer to it before continuing, but not necessarily on first reading.

It is enough to know that one One of the solutions to this paradox is to admit that black holes do behave in many astrophysical and physical situations as if they actually had an event horizon, but that in absolute terms this is not true. An event horizon would only be what is called in physics an effective and not fundamental concept, just as it is practical to consider that water or air are continuous fluids allowing calculations with the Navier-Stokes equations , when in reality we know well that they are formed from molecules .

A self-gravitating over-strings gas

It has often been argued that a quantum theory of gravitation and its coupling to matter would resolve all the questions left unanswered with black holes, in particular by removing the singularities at the heart of black hole solutions known in general relativity.

For years, physicists and in particular Samir Mathur of Ohio State University have argued that this must indeed be the case using superstring theory which would imply that beyond the stage of collapse of a neutron star becoming a black hole, the slightly more compact object formed would in fact be a ball of superstrings called “ fuzzballs ”in English, a theory that we owe precisely to Samir Mathur.

If we assume that the particles of matter and even all the fundamental quantum particles , gluons and bosons of Brout-Englert-Higgs included, are vibrating superstrings, so calculations show that the matter of a star collapsing without recourse under the event horizon a associated with the black hole it seems to become should not end up as a point of infinite density instead where space-time itself is annihilated according to Einstein’s non-quantum theory, at the heart of a black hole. Quantum superstrings would stretch and elongate to the point of forming a kind of quantum gas diffuse occupying all volume within an event horizon that would only be effective, a bit like what acts as a surface for the /sciences/definitions/univers-soleil-3727/”>Soleil.

But how to test such a theory?

Quasi-normal modes characteristic of black holes

The gravitational wave signal detected by Ligo and Virgo concerning black hole collisions was already a strong argument in favor of the existence of black holes from the theory of general relativity but it is not still to this day completely convincing. In fact, what we are really trying to highlight as these detectors, and others like Kagra, increase in sensitivity, is what we calls the quasi-normal modes of the black hole event horizon and we already have indices in this direction.

The quasi-normal modes are already known in classical physics with bells that are struck. A sound is produced which dampens over time and this sound can be broken down into several particular elementary frequencies and waves, analogous to the normal modes composing a vibrating string without noticeable damping. These quasi-normal modes constitute kinds of spectral identity cards of a black hole in curved space-time just like the spectrum bright elements for the composition of atmospheres stars.

Now, as we have said, an event horizon can behave like a vibrating membrane and it is therefore in theory possible to prove l ‘existence of this membrane with its own

vibration modes, the quasi-normal modes , by analyzing the gravitational signal of a black hole collision. It’s a bit like hearing the “sound” of the black hole and checking that it behaves as you would expect from a black hole.

In the article which has just been published, given the complexity of the calculations with the theory of fuzzballs , the researchers wanted to make a simpler test concerning the collision of two of these string balls and get an idea of ​​the gravitational signal produced.

They made numerical simulations concerning the behavior of a space- time described by a theory of supergravity in four dimensions called type N=2 and capturing the geometry of a solution of type fuzzball for what would be an effective externally disturbed black hole.

The calculations then show that initially the signal obtained is similar to that of the quasi-normal modes of a black hole newly formed by fusion of two colliding black holes, but after a certain time the signal no longer includes the modes describing a horizon and therefore betrays the fact that it is only ‘effective. As a bonus, the same calculations show that we could also highlight gravitational echoes, a phenomenon described in the previous article below, and would come here from gravitational waves propagating inside the ball of superstrings.

According to the researchers, Ligo and Virgo should one day be able to detect this signal.

Traces of Hawking’s quantum black holes in gravitational waves?

Article from Laurent Sacco

published on 01/24/2020

According to two physicists, a quantum processing of the horizon of black holes, which can help to understand the puzzles posed by the quantum black holes of Hawking, could lead to a signature of the quantum behavior of the horizon of the events. nts in gravitational waves. Ligo and Virgo could have already detected clues of this signature with the wave source GW170817.

We will probably never repeat it enough but black hole is defined neither by its density nor by the fact that it would have a singularity of space-time in its heart. The supermassive black holes have densities that may be that of water or air, and a quantum theory of gravity most likely suppresses the collapse of matter, light, and ultimately, space-time itself to the point of becoming a singularity, just as quantum laws stop l ‘collapse of electrons on the nuclei of atoms despite attraction electrostatic between these particles.

What defines a black hole rigorously – since in particular the work of Roger Penrose , Stephen Hawking , John Wheeler and other researchers of the 1960s – is the existence of a closed event horizon surrounding a region of space. There is a very precise formulation of the nature of this horizon but, roughly speaking, we can say that, within the framework of a classical theory of gravitation with a curved space-time (it is not necessary to assume that the equations Einstein’s equations are the right equations to describe the dynamics of this space-time completely), a black hole is a kind of bubble formed by a fictitious, effective membrane, which only allows matter and light to pass in one direction. Once in the bubble, they can no longer leave it because the gravitational field would require that a physical object, particle or wave, can sometimes propagate faster than light.

We know that Stephen Hawking, based in particular on the work of Jacob Bekenstein , Yakov Zel’dovich and Alexei Starobinsky, were led to discover that quantum mechanics implied that black holes, whether rotating or not, still had to evaporate while losing their masses and angular moments if they had any. A black hole must, in fact, emit hot radiation of the type of that of a black body with a temperature inversely proportional to its mass. Clearly, the smaller a black hole becomes, the hotter it becomes and the faster it evaporates.

The event horizon poses a problem with quantum information

The discovery of the Hawking radiation then led to the enigma of the information paradox , again a discovery of the deceased Stephen Hawking .

If quantum mechanics forces a black hole to radiate, its evaporation and the existence of an event horizon should lead to the destruction of information that was carried by objects falling into a black hole. All the information content of a book like Friedrich Hölderlin’s l ‘Hyperion should disappear forever in a black hole and only its mass can come out and only under the form of energy and particles of Hawking radiation.

Unfortunately, this destruction of information is prohibited by quantum mechanics.

There is an error but where?

However, at the beginning of the 2010s, while continuing their study of this enigma, the theorists came across even more serious contradictions which led to some of them to question the classic notion of the horizon of a black hole by invoking, for example, the existence of a “firewall” ( firewall in English).

Intense controversy ensued leading Stephen Hawking to arguably make his usual buzz a little mischievously by effectively suggesting that black holes, in the usual sense, did not exist and that the event horizon was only a convenient approximation. to understand certain phenomena but did not really exist.

Futura had devoted a previous article to this controversy that the reader can find below to learn much more.

Still, the idea that the classic event horizon

or not really there – because of quantum effects for example if we introduce the superstring theory to describe black holes as kinds of “balls of strings” called “ fuzzballs In English, a theory that we owe to theoretical physicist Samir Mathur of Ohio State University – is still very much in the minds of researchers. There are even more radical alternatives like the theory of gravastars which assumes that an entirely different object appears during the collapse of the stars, an object with a kind of shell solid almost in place of this horizon. But how to demonstrate or on the contrary refute it?

The rise of gravitational astronomy is perhaps changing the situation with the possible detection of quasi-abnormal modes black holes , as explained

Olivier Minazolli in Futura, and even the black hole images that can be provided via the Event Horizon Telescope as also explained to Futura Aurélien Barrau . Regret Pierre Binetruy was already talking a few years ago, as shown at the end of the video above, of being able to test quantum theories of black holes with gravitational waves.

Gravitational waves that bounce between two barriers

It is therefore with a certain interest that we read the article of a team of researchers composed of Jahed Abedi, postdoctoral researcher at Max Planck Institute for Gravitational Physics ( Albert Einstein Institute in Germany), and Niayesh Afshordi ( from the University of Waterloo and the Perimeter Institute for Theoretical Physics in Canada).

Available in free access at arXiv , it was published in the Journal of Cosmology and Astroparticle Physics and was even awarded with an award, the Buchalter Cosmology Prize .

The idea is as follows: We assume that the event horizon of a black hole is modified by quantum effects or a new physics. But in either case, they still make the exterior of the new object still behave in many ways like an ordinary rotating black hole. We can therefore always show that waves, particularly gravitational waves but also those emitted by the quantum evaporation of a black hole, will encounter a kind of wall , a barrier, produced by the structure of space-time but a little beyond the region where there would be a quantum horizon or what replaces the standard event horizon (for insiders see the Klein-Gordon equation at the bottom of the article).

If we take the analogy with a sound, then a part of the waves emitted by the black hole or what takes its place, will go through the wall and another will be reflected. The energy of the initial waves being distributed between these two parts. But if the location of the event horizon in fact also behaves like a partially reflecting wall, precisely because of quantum effects or new physics, then the waves will bounce between the two walls and a periodic signal – although weaker and weaker due to the sharing of energy in transmitted and reflected waves – will present itself as a kind of echo if we take the case of gravitational waves.

We can indeed apply this scenario to the case of the wave source called GW170817, resulting from the fusion of two neutron stars, and which was detected on August 17, 2017 by Ligo and Virgo. The black hole that must have formed during the fusion of these two stars compact must have produced waves for a while, like a bell whose vibrations dampen after a shock.

In their award-winning article, Jahed Abedi and Niayesh Afshordi announce ( although convincing evidence is not yet there) that, according to their still embryonic analyzes, one would begin to see clues to the presence of these gravitational echoes in the signal detected for GW170817.

Caution is required but the conclusions drawn by Niayesh Afshordi seem reasonable: “Our results are still provisional; there is still a small chance that what we are seeing is due to random noise in the detectors, but that will become less and less likely as and when we find more cases of this kind. Now that scientists know what we’re looking for, we can look out for more examples and get much stronger confirmation of these signals. Such confirmation would be the first direct probe of the quantum structure of space-time. ”

Black holes: does Stephen Hawking question their existence?

Article by Laurent Sacco published on 01/30/2014

Stephen Hawking had already caused a sensation 40 years ago by announcing that black holes do not trap energy forever and that they can evaporate.

It was a consequence of the laws of quantum mechanics. He again throws up trouble by suggesting that black holes don’t exist. But is this really what he says? The information having provoked reactions and sometimes fanciful affirmations, the answer deserves a fine analysis …

Stephen Hawking just made a new move media brilliance of which he has the secret.

We remember, for example, a few years ago the turmoil he caused with the boson of Browse -Englert-Higgs . His work on the theory of wormholes had led him to doubt the possibility of discovering this famous particle at LHC . He had therefore bet with physicist Gordon Kane that he would not be observed.

This year, two weeks after his birthday, Hawking deposited on arxiv an article short and unequaled in which he seems to claim that black holes do not exist.

In fact, the content of this article has already been exposed on Skype in August 2013, in front of his colleagues, during a conference of the Kavli Institute for Theoretical Physics , in Santa Barbara (California). It concerns a solution to an enigma discovered about two years ago by Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully (AMPS) while reflecting on the famous information paradox with black holes. This is therefore a new twist in the saga of the black body problem ), closely related to quantum mechanics, general relativity and thermodynamics.

Presented by Hubert Reeves and Jean-Pierre Luminet, Du Big Bang au alive is a multiplatform project that covers the most recent discoveries in the field of cosmology. Jean-Pierre Luminet explains here the history of the discoveries theory of black holes. © From the Big Bang to the Living

Hawking’s work on the theory of black holes , both from the point of view of classical general relativity and using the laws of quantum mechanics, are at the heart of the paradox discovered by AMPS. A few reminders on the classical and quantum theory of black holes are essential to understand what this paradox consists of. They also allow you to take a step back from recent statements by Stephen Hawking.

The gravitational collapse of stars

For a long time, the scientific community did not take seriously the existence of objects that are now called black holes, and which were predicted by Einstein’s equations of general relativity. Things started to change when in the early 1960s, a team in the United States (a trio of physicists who had been involved in the design of the US H-bomb) tackled a problem of numerical simulation. precise. Michael Mayn, Richard White, and Stirling Colgate used their skills in nuclear physics, fluid mechanics, and radiative transfer theory to simulate on computer star implosion realistically. It was a question of verifying the conclusions resulting from the simplified calculations carried out by Robert Oppenheimer and Hartland Snyder in the late 1930s.

Almost at At the same time, in the former USSR, one of the designers of the Soviet H-bomb, the great Yakov Zel’dovich, launches three of his colleagues on the same problem. Both teams achieved identical results. Above a certain mass, nothing can stop the gravitational contraction of a star, which ends up crossing a spherical surface whose size is given by the radius of Schwarzschild .

The final state of the material below this surface, however, remained problematic. The calculations carried out with general relativity seemed to imply that the curvature of space-time increased at the same time as the density of matter to end up reaching an infinite value: a singularity.

In 1965, the mathematician Roger Penrose demonstrated that this must always be the case within the framework of classical general relativity. It was enough to postulate very plausible conditions concerning the state of matter under the surface, which is now called the event horizon, that is, a region of space-time from which one cannot escape even at the speed of light.

Black hole and event horizon

It was already clear at that time, especially for the big one John Wheeler , that before reaching the infinite curvature predicted by the theorem of Penrose singularity, quantum mechanics had to be taken into account at the level of space-time itself. It probably had to remove the singularity, as it had stopped the collapse of electrons on the nucleus of the atom in Rutherford’s model.

From the point of view of general relativity , we could develop a precise theory of gravitational singularities, but no doubt that the final state of a star’s implosion would only be known when we had a quantum theory of gravity and even a unified theory forces and matter. In practice, the space-time of a collapsing star, or of any other sufficiently compressed mass of matter, evolved to a final state of equilibrium identical for an observer outside what is called the Schwarzschild solution describing a static and eternal black hole. The central singularity of this solution had to be an idealized and non-physical description of an extremely dense region dominated by quantum effects.

We could therefore be satisfied with developing for astrophysics a theory of completely gravitationally collapsed stars based on Schwarzschild’s idealized solution. We had therefore defined what is now called a black hole not by the fact that it would contain a real singularity of space-time and equations describing the behavior of matter, but by the existence of a event horizon . Stephen Hawking, in particular, has relied heavily on the surface properties of the event horizon to explore the physics of black holes. This surface defining a region from which no more information can emerge in classical physics, we can associate an entropy with it, since, in practice or in an absolute manner, it makes unavailable to an external observer the information contained in an object that has passed through the horizon.

Quantum black holes radiating

However, near the horizon, as everywhere in vacuum, pairs of particles appear and disappear due to the laws of quantum mechanics. The tidal forces exerted by the black hole can separate these particles, so that one sometimes falls into the black hole and the other escapes to the ‘infinite. The energy used to separate these pairs being taken to the black hole, its mass decreases and this loss is found associated with the energy carried by the particle radiated by the black hole. As Hawking was going to show during the 1970s, everything thus happens as if a black hole were starting to radiate like a black body evaporating. The thermal rays of the black body being very disordered, the evaporation of a black hole seemed to destroy information. The energy of a book thrown into a black hole would eventually come out, but the information it carried would be lost forever due to the existence of an event horizon.

For Leonard Susskind and Gerard ‘t Hooft, this must have been impossible, because it led to violate the laws of quantum mechanics. Hawking was well aware of this and he thought he had just discovered a key to going beyond these laws. But during the 1990s, the second revolution of the string theory , and in particular the famous AdS / CFT correspondence (also called conjecture from Maldacena) would change all that. Confirming ‘t Hooft and Susskind’s idea that a theory of quantum gravity had to reveal phenomena resembling those associated with holograms , this correspondence strongly implied that the laws of quantum mechanics were well respected by the evaporation of a black hole. In a way, the pairs of particles that caused a black hole to evaporate could be considered to be entangled. So that the information initially contained by it was, thanks to the quantum entanglement , all the same released and present in the radiation, although it appears very messy to an outside observer.

In this video, Jean-Pierre Luminet tells us about the evaporation of black holes via Hawking radiation. This evaporation poses a conundrum known as the information paradox with the physics of black holes. © From the Big Bang to the living

Now let’s come back to the paradox discovered by Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully. It has been known for about two years as the “firewall” controversy ( firewall in English). It is still the subject of many discussions between theoretical physicists, because it puzzles more than one who admits to being rather confused about it.

Maximally entangled particles

To understand it, you have to know that there are different degrees of quantum entanglement between physical systems.

There is in particular what is called maximum entanglement, which asserts that when two systems are maximally entangled, they can no longer be entangled with a third. Nothing prevents entanglement between more than two systems, but it is then no longer maximum. The quantum mechanics implies that if we wait long enough, a duration called Page time (in reference to physicist Donald Page), the past radiation emitted by a black hole before this time will be maximally entangled with the radiation emitted after this time.

If an object is thrown into the black hole after this Page time, it should also be entangled with the past and future radiation of the black hole contradicting the fact that they are already maximally entangled. If one refuses to modify the laws of quantum mechanics, it seems that the object should be prohibited from entering the black hole.

A firewall in contradiction with general relativity

AMPS came to the conclusion that just at the level of the horizon of a black hole, the free-falling object must meet an intense flow of energy, a firewall, tearing it apart and preventing it from entering the black hole. This scenario poses a problem: for fairly large black holes, such as a supermassive black hole containing billions of solar masses, there is no reason for a free-falling observer to notice anything near the horizon. The tidal forces are very weak and there is no quantum radiation for such a free-falling observer. What is more, such a black hole appears very cold to an outside observer even, in this situation, in Page’s time. It is actually a consequence of the principle of equivalence of general relativity, on which it relies heavily. The message seems clear: if we refuse to touch quantum theory, we need a firewall, but it contradicts general relativity …

There you go the place where we’re going to join Stephen Hawking … If we don’t change the principles of quantum mechanics and don’t touch those of general relativity, maybe we need to change the theory of black holes slightly. Hawking therefore proposes to question the absolute nature of the event horizon, which would make it possible to do without a firewall while maintaining the laws of quantum mechanics. In practice, the horizon would not define a region from which light could not escape, but a region where it would be trapped like matter for a long enough time.

Hawking therefore does not reject his work or black holes as a whole, he speaks of the existence of an apparent, effective horizon, as is the continuous description of a fluid by the Navier-Stokes equations . Hawking also proposes to reconsider a black hole as a kind of bound state of the gravitational field, turbulent and chaotic. The apparent loss of information giving rise to the existence of an entropy associated with the surface defined by the event horizon would therefore be analogous to that known in classical physics for a collection of particles, an artefact of a simplified macroscopic description .

Chaos, turbulence and black holes

Hawking’s article left his colleagues somewhat perplexed, and even doubtful, even if Hawking alludes to the match from Maldacena posing that at the border of a space-time AdS, a quantum fluid, resembling a plasma of quarks and gluons (in accordance with the holographic principle) reflect the behavior of a black hole in the process of s ‘evaporate. She suggests that the chaotic collapse of matter giving a black hole would be related to a turbulent state of this fluid. But this connection remains unclear in Hawking’s words.

When he speaks of a classic chaotic state of space-time and matter below the apparent horizon of ‘a black hole, one wonders if he does not have in mind a connection explored for a few years between the theory of black holes and that of turbulent fluids. It’s about the fluid-gravity correspondence , which uses AdS / CFT correspondence to translate problems of fluid dynamics in problems of general relativity.

What is certain is that Stephen Hawking compares the effective loss of information and predictability in a hole black with the inability to predict the weather long term. In principle, classical physics tells us, the fluid that constitutes the atmosphere behaves deterministically, but in practice, we quickly lose information about it and we can no longer make precise predictions. We could therefore, by analogy, conceive of a black hole as a very dense, chaotic and turbulent ball of fluid, but which ends up evaporating.

New black holes, balls of superstring?

Such a description is not radically new. As early as the 1950s and 1960s, John Wheeler used images from hydrodynamics to represent the physics of space-time. We can also think of the membrane paradigm developed during the 1970s and 1980s by Kip Thorne and Thibault Damour . Indeed, for the purposes of astrophysics, for example to study quasars , we can replace the description of a black hole with its horizon by that of a kind of viscous fluid bubble endowed with electrical and thermodynamic properties. We don’t have to worry about what’s inside this bubble, which therefore behaves like an apparent horizon.

We can finally ask ourselves if the solution Hawking proposes to the firewall paradox highlighted by AMPS has not already been given within the framework of string theory by theoretical physicist Samir Mathur of Ohio State University. He proposed ten years ago that black holes were kinds of “balls of strings” which he called “ fuzzballs ” in English. According to his calculations, taking into account the fact that the particles would actually be strings, once they entered a black hole, they would somehow begin to spread out until they took up the entire interior of the region below the horizon of a black hole. If the black hole is small, the image that emerges is that of a sort of equivalent of a very dense neutron star, but this time made up of quantum strings. Most of what the standard black hole theory contains would be retained, but the event horizon would be real, apparent, exactly as Hawking proposes. Mathur has recently published articles in which he asserts that if the description of black holes in terms of fuzzballs

is correct, we keep standard black hole thermodynamics and quantum theory while solving the information paradox and without having need a firewall.

One thing is certain: black holes are still far from having finished eliciting puzzles, and they are an extraordinary window on the most fundamental physics of the universe.

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