Black holes and relativistic stars are among the most fascinating objects in the Universe and they attract the interest of both physicists and astrophysicists. The workshop will bring together theorists, observers, and experimentalists. The program will include invited talks, contributed talks, and time for free discussions.

# Archivi tag: event horizon

# Trying to fix the black hole information paradox

##### I fisici hanno discusso le affermazioni che sono state avanzate di recente da Stephen Hawking in merito ai buchi neri (post). Ormai sono decenni che si sta cercando di svelare il mistero che avvolge questi affascinanti “mostri del cielo” la cui estrema forza di attrazione gravitazionale è così intensa che nemmeno la luce riesce a sfuggire. Oggi il professor Chris Adami, della Michigan State University, ha deciso di buttarsi, per così dire, nella mischia per capirne di più e tentare di risolvere l’enigma.

The debate about the behavior of black holes, which has been ongoing since 1975, was reignited when Hawking posted a blog on Jan. 22, 2014, stating that event horizons, the invisible boundaries of black holes, do not exist. Hawking, considered to be the foremost expert on black holes, has over the years revised his theory and continues to work on understanding these cosmic puzzles. One of the many perplexities is a decades-old debate about what happens to information, matter or energy and their characteristics at the atomic and subatomic level, in black holes. “*In 1975, Hawking discovered that black holes aren’t all black. They actually radiate a featureless glow, now called Hawking radiation*”, Adami said. “*In his original theory, Hawking stated that the radiation slowly consumes the black hole and it eventually evaporates and disappears, concluding that information and anything that enters the black hole would be irretrievably lost*”.

But this theory created a fundamental problem, dubbed the information paradox. Now Adami believes he’s solved it.

“*According to the laws of quantum physics, information can’t disappear*”, Adami said. “*A loss of information would imply that the Universe itself would suddenly become unpredictable every time the black hole swallows a particle. That is just inconceivable. No law of physics that we know allows this to happen*”. So if the black hole sucks in information with its intense gravitational pull, then later disappears entirely, information and all, how can the laws of quantum physics be preserved?

The solution, Adami says, is that the information is contained in the stimulated emission of radiation, which must accompany the Hawking radiation, the glow that makes a black hole not so black. Stimulated emission makes the black hole glow in the information that it swallowed.

“*Stimulated emission is the physical process behind LASERS (Light Amplification by Stimulated Emission of Radiation). Basically, it works like a copy machine: you throw something into the machine, and two identical somethings come out*. *If you throw information at a black hole, just before it is swallowed, the black hole first makes a copy that is left outside. This copying mechanism was discovered by Albert Einstein in 1917, and without it, physics cannot be consistent*”, Adami said. Do others agree with Adami’s theory that stimulated emission is the missing piece that solves the information paradox? According to Paul Davies, cosmologist, astrobiologist and theoretical physicist at Arizona State University, “*In my view Chris Adami has correctly identified the solution to the so-called black hole information paradox. Ironically, it has been hiding in plain sight for years. Hawking’s famous black hole radiation is an example of so-called spontaneous emission of radiation, but it is only part of the story. There must also be the possibility of stimulated emission, the process that puts the S in LASER*”. With so many researchers trying to fix Hawking’s theory, why did it take so long if it was hiding in plain sight? “*While a few people did realize that the stimulated emission effect was missing in Hawking’s calculation, they could not resolve the paradox without a deep understanding of quantum communication theory*”, Adami said. Quantum communication theory was designed to understand how information interacts with quantum systems, and Adami was one of the pioneers of quantum information theory back in the ’90s. Trying to solve this information paradox has kept Adami awake many nights as demonstrated by his thick notebooks filled with 10 years of mathematical calculations. So where does this leave us, according to Adami? “*Stephen Hawking’s wonderful theory is now complete in my opinion. The hole in the black hole theory is plugged, and I can now sleep at night*”, he said. Adami may now sleep well at night, but his theory is sure to keep other physicists up trying to confirm whether he has actually solved the mystery.

Michigan State University: Plugging the hole in Hawking’s black hole theory

arXiv: Classical information transmission capacity of quantum black holes

MSU Professor Chris Adami believes he has found the solution to a long-standing problem with Stephen Hawking’s black hole theory in a groundbreaking new study recently published in the journal Classical and Quantum Gravity. Shot by G.L. Kohuth

# Hawking’s grey holes are made of an ‘apparent’ horizon

##### In questi giorni, i media si sono scatenati riportando la proposta provocativa di Stephen Hawking secondo la quale i buchi neri “non esistono” (post). Da qui, sono emersi tutta una serie di commenti che poi si sono trasformati in “discussioni satiriche” allo scopo di puntare il dito contro le affermazioni che fanno spesso gli scienziati famosi. La Scienza è, come viene spesso suggerito, un pò diversa dalla religione dove il clero è sempre in attesa della “grande notizia”. Dunque, qual è il significato fisico di questa affermazione fatta da uno dei giganti della fisica moderna? Dobbiamo riscrivere i libri di testo? Per rispondere alla domanda dobbiamo fare un passo indietro e capire bene il concetto di buchi neri in modo da arrivare al problema iniziale che ha portato lo scienziato inglese a ricredersi sulla natura dell’orizzonte degli eventi che circonda i buchi neri.

**A classical black hole**

In 1915, Einstein derived the equations of general relativity, revolutionising our view of gravity. While Einstein struggled with his equations, the German physicist Karl Schwarzschild was able to use them to determine the gravitational field outside of a spherical distribution of mass. But the conclusions of Schwarzschild were rather frightening, predicting that objects could completely collapse, with mass crashing down to a central “singularity”, surrounded by a gravitational field from which even light cannot escape. For any black hole, the delineation between light escaping and being trapped is a well-defined surface, the event horizon, separating our Universe from the mysteries close to the black hole. With this, the notion of the “classical” black hole was born, governed purely by the equations of general relativity. But while we know general relativity governs the force of gravity, the early 20th century saw a revolution in the understanding of the other fundamental forces, describing them in exquisite detail in terms of quantum mechanics.

**A quantum leap**

But the problem is that general relativity and quantum mechanics just don’t play well together. Simply put, the equations of quantum mechanics can’t describe gravity, whereas general relativity can only handle gravity. To talk about them both in situations where gravity is strong and quantum mechanics cannot be ignored, the best we can do at the moment is sticky-tape the equations together. Until we have a unified theory of gravity and the other forces, this is the best we can do. Stephen Hawking undertook one of the most famous attempts at this in the early 1970s. He wondered about what was happening at the event horizon in terms of quantum mechanics, where empty space is a seething mass of particles popping in and out of existence. At the horizon, this process separates particles, with some sucked into the central singularity, while their partners escape into space. What Hawking showed is, through a jerry-rigged version of gravity and quantum mechanics, black holes leak radiation into space, slowly sucking energy from their gravitational core, and that, given enough time, black holes evaporate completely into radiation. When quantum mechanics is thrown into the mix, the notion of a “classical black hole” is dead.

**Teapots and black holes**

There is, however, a bigger problem in including quantum mechanics into the study of gravity, and that problem is information. Quantum mechanics cares intensely about information, and worries about the detailed make-up of an object like a teapot: how many protons are there, and electrons, and where are they; they care about the fact that a teapot is a teapot, a particular arrangement of electrons and protons, which is different to something else, like a light beam or a sofa. When the teapot is thrown into a black hole, it is complete destroyed, first smashed into a million pieces, then atomised, and then the atoms ripped into their constituent parts, before being absorbed into central singularity. But the radiation that Hawking predicted being emitted from black holes doesn’t contain any information of what fell in; no matter how well you examine the radiation, you can’t tell if it was a teapot, a fridge or a small iguana called Colin that met their demise.

**Pushing boundaries**

It must be remembered that we are now pushing the boundaries of modern physics and, as we do not have a single mathematical framework where gravity and quantum mechanics play nicely together, we have to worry a little about how we have glued the two pieces together. In 2012, the problem was revisited by US physicist Joseph Polchinski. He examined the production of Hawking radiation near the event horizon of a black hole, watching how pairs of particles born from the quantum vacuum separate, with one lost irretrievably into the hole, while the other flies off into free space. With a little mathematical trickery, Polchinski asked the question: “*What if the information of the infalling particle is not lost into the hole, but is somehow imprinted on the escaping radiation?*” Like the breaking of atomic bonds, this reassignment of information proves to be very energetic, surrounding a black hole with a “firewall”, through which infalling particles have to pass. As the name suggests, such a firewall will roast Colin the iguana to a crisp. But at least information is not lost. While presenting a possible solution, many are bothered by its consequences of the existence of a firewall and that Colin will notice a rapid increase in temperature, he will know he is at the event horizon. This goes against one of the key tenets of general relativity, namely that an infalling observer should happily sail through the event horizon without noticing that it is there.

**Back to Hawking**

This is where Hawking’s recent paper comes in, suggesting that when you further stir the quantum mechanics into general relativity, the seething mass of the vacuum prevents the formation of a crisp, well-defined event horizon, replacing with a more ephemeral “apparent horizon”. This apparent horizon does the job of an event horizon, trapping matter and radiation within the black hole, but this trapping is only temporary, and eventually the matter and radiation are released carrying their stored information with them. As black holes no longer need to leak information back into space, but can now release it in a final burst when they have fully evaporated, there is no need to have a firewall and an infalling observer will again have a roast-free ride into the black hole.

**Are black holes no more?**

To astronomers, the mess of fundamental physics at the event horizon has little to do with the immense gravitational fields produced by these mass sinks at the cores of galaxies, powering some of the most energetic processes in the Universe. Astrophysical black holes still happily exist.

What Hawking is saying is that, with quantum mechanics included, the notion of a black hole as governed purely by the equations of general relativity, the “classical black hole”, does not exist, and the event horizon, the boundary between escape and no-escape, is more complex than we previously thought.

But we’ve had inklings of this for more than 40 years since his original work on the issue. In reality, the headlines should not be “black holes don’t exist” but “black holes are more complicated than we thought, but we are not going to really know how complicated until gravity and quantum mechanics try to get along”.

**After all, Hawking is just a man**

But one last vexing question: is Hawking right? Science is often compared to religion, with practitioners awaiting pronouncements from on high, all falling into line with the latest dogma. But that’s not the way Science works, and it is important to remember that, while Hawking is clearly very smart, to quote the immortal Tammy Wynette in Stand By Your Man, “after all, he’s just a man”, and just because he says something does not make it so. Hawking’s proposed solution is clever, but the debate on the true nature of black holes will continue to rage. They will continuously change their spots, and their properties will become more and more head-scratchingly weird, but this is the way that science works, and that’s what makes it wonderful.

The Conversation: Grey is the new black hole: is Stephen Hawking right?

# Information, weather forecast and black holes

##### In un recente articolo pubblicato su arXiv, Stephen Hawking si ricrede sui buchi neri. Si tratta, forse, di immaginazione collettiva? Probabilmente no, ma lo scienziato inglese porta alla ribalta un paradosso complesso della fisica che ha tenuto col fiato sospeso i teorici negli ultimi 18 mesi.

Most physicists foolhardy enough to write a paper claiming that “there are no black holes”, at least not in the sense we usually imagine, would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape. In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form. “*There is no escape from a black hole in classical theory*”, Hawking told *Nature*. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “*The correct treatment*”, Hawking says, “*remains a mystery*”. Hawking posted his paper on the arXiv preprint server on 22 January. He titled it, whimsically, ‘Information preservation and weather forecasting for black holes’, and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013 (watch video of the talk).

**Fire fighting**

Hawking’s new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues (see ‘Astrophysics: Fire in the hole!‘). In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein’s general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards, stretched out along the way, like spaghetti, and eventually crushed at the ‘singularity’, the black hole’s hypothetical infinitely dense core. But on analysing the situation in detail, Polchinski’s team came to the startling realization that the laws of quantum mechanics, which govern particles on small scales, change the situation completely. Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or ‘firewall’, that would burn the astronaut to a crisp. This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe, whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.

**Beyond the horizon**

Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.

In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended.

In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon. Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out ‘Hawking radiation‘. In that case, the event horizon would, in theory, become smaller than the apparent horizon.

Hawking’s new suggestion is that the apparent horizon is the real boundary.

“*The absence of event horizons means that there are no black holes, in the sense of regimes from which light can’t escape to infinity*”, Hawking writes. “*The picture Hawking gives sounds reasonable*”, says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “*You could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon*“. Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon. Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).

If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre.

Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were. “*It would be worse than trying to reconstruct a book that you burned from its ashes*”, says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy. Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “*In Einstein’s gravity, the black-hole horizon is not so different from any other part of space*”, says Polchinski. “*We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales*”. Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking’s, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “*The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls*”, he says. “*But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance*“.

Nature: Stephen Hawking: 'There are no black holes'

arXiv: Information Preservation and Weather Forecasting for Black Holes

see also: A Black Hole Mystery Wrapped in a Firewall Paradox

# Is our Universe a hologram?

##### E’ circolata di recente nei media la notizia pubblicata da *Nature *secondo la quale un gruppo di fisici giapponesi avrebbero formulato una teoria che “*potrebbe essere considerata l’evidenza più chiara sul fatto che il nostro Universo sarebbe una gigantesca proiezione*“. Nei loro articoli, Yoshifumi Hyakutake e colleghi della Ibaraki University in Giappone spiegano come la loro idea suggerisca che la realtà fisica, così come noi la concepiamo, potrebbe essere in definitiva un ologramma appartenente ad un altro spazio bidimensionale.

In 1997, theoretical physicist Juan Maldacena proposed that an audacious model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity. Maldacena’s idea thrilled physicists because it offered a way to put the popular but still unproven theory of strings on solid footing, and because it solved apparent inconsistencies between quantum physics and Einstein’s theory of gravity. It provided physicists with a mathematical “Rosetta stone”, a ‘duality’, that allowed them to translate back and forth between the two languages, and solve problems in one model that seemed intractable in the other and vice versa (see ‘Collaborative physics: String theory finds a bench mate‘). But although the validity of Maldacena’s ideas has pretty much been taken for granted ever since, a rigorous proof has been elusive.

In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In one paper, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence (see ‘Astrophysics: Fire in the Hole!‘). In the other, he and his collaborators calculate the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match. “*It seems to be a correct computation*”, says Maldacena, who is now at the Institute for Advanced Study in Princeton, New Jersey and who did not contribute to the team’s work.

The findings “

are an interesting way to test many ideas in quantum gravity and string theory”, Maldacena adds.

The two papers, he notes, are the culmination of a series of articles contributed by the Japanese team over the past few years. “*The whole sequence of papers is very nice because it tests the dual [nature of the universes] in regimes where there are no analytic tests. They have numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture, namely that the thermodynamics of certain black holes can be reproduced from a lower-dimensional Universe*”, says Leonard Susskind, a theoretical physicist at Stanford University in California who was among the first theoreticians to explore the idea of holographic universes. Neither of the model universes explored by the Japanese team resembles our own, Maldacena notes. The cosmos with a black hole has ten dimensions, with eight of them forming an eight-dimensional sphere. The lower-dimensional, gravity-free one has but a single dimension, and its menagerie of quantum particles resembles a group of idealized springs, or harmonic oscillators, attached to one another. Nevertheless, says Maldacena, the numerical proof that these two seemingly disparate worlds are actually identical gives hope that the gravitational properties of our Universe can one day be explained by a simpler cosmos purely in terms of quantum theory.

Nature: Simulations back up theory that Universe is a hologram arXiv: Quantum Near Horizon Geometry of Black 0-Brane arXiv: Holographic description of quantum black hole on a computer

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# A holographic origin for the Big Bang

##### Un gruppo di fisici teorici hanno pubblicato un articolo in cui propongono una nuova idea che spiegherebbe l’origine dell’Universo. Secondo gli scienziati, è possibile che lo spazio e il tempo vennero creati dal collasso quadridimensionale di una stella che spazzò i detriti nel cosmo per poi trasformarsi in un buco nero.

As it stands, the prevailing theory states the Universe was born from an infinitely dense singularity through some currently unknown mechanism. Actually, the entire big bang event itself is entirely unknown. Our equations have yet to be complete enough to describe the moment of creation, a revelation physicists think will follow the discovery of the theory of everything (which scientists might be one-step closer to doing). Until then, what happened “before the big bang,” the nature of the ‘singularity’ that caused the big bang, and the event itself will remain unknown without some major scientific breakthrough. At the moment, it’s anyone’s guess what happened. (Important side note: we have a lot of knowledge and experimental evidence talking about what happened immediately after the big bang, up to about 10^{-35 }or so seconds after the event, so our timeline for cosmology is still preserved.) The standard big bang theory has some limitations and some serious problems. It’s limitations are mostly summed up by our inability to mathematically or practically study the big bang singularity, as mentioned before. On the flip side, the big bang theory doesn’t really explain why the Universe has a nearly uniform temperature (that’s where inflation theory comes in, which suggests that the Universe went through a period of rapid, faster-than-light expansion in its early history).

This new model is based on the slightly older idea that our Universe is basically a three-dimensional membrane floating in a fourth-dimensional “bulk universe.” That’s the basic idea that’s supporting this new model.

The tenets for the new theory are as follows:

- The “bulk universe” has fourth-dimensional stars that go through the same life cycle that our three-dimensional stars go through.

- Just as with our stars, the stars in the bulk universe could go supernova and collapse into a black hole.

- This is where things start to get really cool. Just as our three-dimensional black holes have event horizons that appear two-dimensional, it’s plausible that the fourth-dimensional black holes have event horizons that appear three-dimensional.

- This three-dimensional event horizon is knows as a hypersphere. This is the region of space in which our Universe exists.

This new way of looking at the Universe has some strong points in its favor. The model is able to explain the expansion of the Universe and is able to describe the Universe’s nearly uniform temperature, with one (rather large) limitation. The model disagrees with observations made by the Planck telescope, which recently created the most detailed map we have of the cosmic microwave background (post). The hypersphere model has about a four percent discrepancy, which means the hypersphere needs to be refined before it’ll gain any credence.

This new model could go a long way to helping us understand the nature of inflation.

Currently, the only thing we really know about inflation is that “it’s happening.” We don’t know why or how, but the named mechanism for it is known as dark energy. The model proposes that inflation is caused by the Universe’s motion through higher dimensions of space. It’s important to note that the paper where this study was published does not state whether the paper has been submitted to peer review. So, whereas the hypersphere idea is fantastic and fun, it has a long way to go before we can considered a viable hypothesis.

From Quarks to Quasars: Revising the Big Bang? New Theory on Creation.

arXiv: Out of the White Hole: A Holographic Origin for the Big Bang