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.
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.
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 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.
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 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.
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
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.
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.
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?
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 .
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.
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 .
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.
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
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.
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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