Archivi tag: black hole singularity

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

Black HoleIn 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