SINGULARITIES : IDENTIFY UNKNOWN
Quantum gravity takes singularity out of black holes
BY:- BALBEER SINGH DANU
Falling into a black hole may not be as final as it seems. Apply a
quantum theory of gravity to these bizarre objects and the all-crushing
singularity at their core disappears.
In its place is something that looks a lot like an entry point to another universe. Most immediately, that could help resolve the nagging information loss paradox that dogs black holes.
Though no human is likely to fall into a black hole anytime soon, imagining what would happen if they did is a great way to probe some of the biggest mysteries in the universe. Most recently this has led to something known as the black hole firewall paradox – but black holes have long been a source of cosmic puzzles.
According to Albert Einstein’s theory of general relativity, if a black hole swallows you, your chances of survival are nil. You’ll first be torn apart by the black hole’s tidal forces, a process whimsically named spaghettification.
Eventually, you’ll reach the singularity, where the gravitational field is infinitely strong. At that point, you’ll be crushed to an infinite density. Unfortunately, general relativity provides no basis for working out what happens next. “When you reach the singularity in general relativity, physics just stops, the equations break down,” says Abhay Ashtekar of Pennsylvania State University.
The same problem crops up when trying to explain the big bang, which is thought to have started with a singularity. So in 2006, Ashtekar and colleagues applied loop quantum gravity to the birth of the universe. LQG combines general relativity with quantum mechanics and defines space-time as a web of indivisible chunks of about 10-35 metres in size. The team found that as they rewound time in an LQG universe, they reached the big bang, but no singularity – instead they crossed a “quantum bridge” into another older universe. This is the basis for the “big bounce” theory of our universe’s origins.
In this new model, the gravitational field still increases as you near the black hole’s core. But unlike previous models, this doesn’t end in a singularity. Instead gravity eventually reduces, as if you’ve come out the other end of the black hole and landed either in another region of our universe, or another universe altogether. Despite only holding for a simple model of a black hole, the researchers – and Ashtekar – believe the theory may banish singularities from real black holes too.
That would mean that black holes can serve as portals to other universes. While other theories, not to mention some works of science fiction, have suggested this, the trouble was that nothing could pass through the portal because of the singularity. The removal of the singularity is unlikely to be of immediate practical use, but it could help with at least one of the paradoxes surrounding black holes, the information loss problem.
A black hole soaks up information along with the matter it swallows, but black holes are also supposed to evaporate over time. That would cause the information to disappear forever, defying quantum theory. But if a black hole has no singularity, then the information needn’t be lost – it may just tunnel its way through to another universe. “Information doesn’t disappear, it leaks out,” says Pullin.
SINGULARITIES
In the centre of a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time
curves infinitely, and where the laws of physics as we know them cease
to operate. As the eminent American physicist Kip Thorne describes it,
it is "the point where all laws of physics break down".
Current theory suggests that, as an object falls into a black hole and approaches the singularity at the centre, it will become stretched out or “spaghettified” due to the increasing differential in gravitational attraction on different parts of it, before presumably losing dimensionality completely and disappearing irrevocably into the singularity. An observer watching from a safe distance outside, though, would have a different view of the event. According to relativity theory, they would see the object moving slower and slower as it approaches the black hole until it comes to a complete halt at the event horizon, never actually falling into the black hole.
The existence of a singularity is often taken as proof that the theory of general relativity has broken down, which is perhaps not unexpected as it occurs in conditions where quantum effects should become important. It is conceivable that some future combined theory of quantum gravity (such as current research into superstrings) may be able to describe black holes without the need for singularities, but such a theory is still many years away.
According to the "cosmic censorship" hypothesis, a black hole's singularity remains hidden behind its event horizon, in that it is always surrounded by an area which does not allow light to escape, and therefore cannot be directly observed. The only exception the hypothesis allows (known as a “naked” singularity) is the initial Big Bang itself.
It seems likely, then, that, by its very nature, we will never be able to fully describe or even understand the singularity at the centre of a black hole. Although an observer can send signals into a black hole, nothing inside the black hole can ever communicate with anything outside it, so its secrets would seem to be safe forever.
BY:- BALBEER SINGH DANU
In its place is something that looks a lot like an entry point to another universe. Most immediately, that could help resolve the nagging information loss paradox that dogs black holes.
Though no human is likely to fall into a black hole anytime soon, imagining what would happen if they did is a great way to probe some of the biggest mysteries in the universe. Most recently this has led to something known as the black hole firewall paradox – but black holes have long been a source of cosmic puzzles.
Eventually, you’ll reach the singularity, where the gravitational field is infinitely strong. At that point, you’ll be crushed to an infinite density. Unfortunately, general relativity provides no basis for working out what happens next. “When you reach the singularity in general relativity, physics just stops, the equations break down,” says Abhay Ashtekar of Pennsylvania State University.
The same problem crops up when trying to explain the big bang, which is thought to have started with a singularity. So in 2006, Ashtekar and colleagues applied loop quantum gravity to the birth of the universe. LQG combines general relativity with quantum mechanics and defines space-time as a web of indivisible chunks of about 10-35 metres in size. The team found that as they rewound time in an LQG universe, they reached the big bang, but no singularity – instead they crossed a “quantum bridge” into another older universe. This is the basis for the “big bounce” theory of our universe’s origins.
Information paradox
Now Jorge Pullin at Louisiana State University and Rodolfo Gambini at the University of the Republic in Montevideo, Uruguay, have applied LQG on a much smaller scale – to an individual black hole – in the hope of removing that singularity too. To simplify things, the pair applied the equations of LQG to a model of a spherically symmetrical, non-rotating “Schwarzschild” black hole.In this new model, the gravitational field still increases as you near the black hole’s core. But unlike previous models, this doesn’t end in a singularity. Instead gravity eventually reduces, as if you’ve come out the other end of the black hole and landed either in another region of our universe, or another universe altogether. Despite only holding for a simple model of a black hole, the researchers – and Ashtekar – believe the theory may banish singularities from real black holes too.
That would mean that black holes can serve as portals to other universes. While other theories, not to mention some works of science fiction, have suggested this, the trouble was that nothing could pass through the portal because of the singularity. The removal of the singularity is unlikely to be of immediate practical use, but it could help with at least one of the paradoxes surrounding black holes, the information loss problem.
A black hole soaks up information along with the matter it swallows, but black holes are also supposed to evaporate over time. That would cause the information to disappear forever, defying quantum theory. But if a black hole has no singularity, then the information needn’t be lost – it may just tunnel its way through to another universe. “Information doesn’t disappear, it leaks out,” says Pullin.
SINGULARITIES
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Current theory suggests that, as an object falls into a black hole and approaches the singularity at the centre, it will become stretched out or “spaghettified” due to the increasing differential in gravitational attraction on different parts of it, before presumably losing dimensionality completely and disappearing irrevocably into the singularity. An observer watching from a safe distance outside, though, would have a different view of the event. According to relativity theory, they would see the object moving slower and slower as it approaches the black hole until it comes to a complete halt at the event horizon, never actually falling into the black hole.
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According to the "cosmic censorship" hypothesis, a black hole's singularity remains hidden behind its event horizon, in that it is always surrounded by an area which does not allow light to escape, and therefore cannot be directly observed. The only exception the hypothesis allows (known as a “naked” singularity) is the initial Big Bang itself.
It seems likely, then, that, by its very nature, we will never be able to fully describe or even understand the singularity at the centre of a black hole. Although an observer can send signals into a black hole, nothing inside the black hole can ever communicate with anything outside it, so its secrets would seem to be safe forever.
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