Loop Quantum Gravity: Black Hole Cores May Be Doorways

[Mindlusion]

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The cores of black holes may not hold points of infinite density as currently thought, but portals to elsewhere in the universe, theoretical physicists say.

A black hole possesses a gravitational field so powerful that not even light can escape. A black hole generally forms after a star dies in a titanic explosion known as a supernova, which crushes the remaining core into dense lumps.

A maddening enigma called a singularity – a region of infinite density – lies at the heart of each black hole, according to general relativity, the modern theory of gravity. The infinite nature of singularities means that space and time as we know them cease to exist there.

Scientists have long sought ways to avoid the complete breakdown of all the known laws of physics brought on by singularities. Now researchers suggest the centers of black holes may not hold singularities after all.

These new findings are based on loop quantum gravity, one of the leading theories seeking to unite quantum mechanics and general relativity into a single theory that can explain all the forces of the universe. In loop quantum gravity, the four dimensions of spacetime are composed of networks of intersecting loops — ripples of the gravitational field.

The researchers applied loop quantum gravity theory to the simplest model of black hole — a spherical, uncharged, non-rotating body known as a Schwarzschild black hole.

"We have been looking at various aspects of spherical models for several years," said researcher Jorge Pullin, a theoretical physicist at the Louisiana State Univ. in Baton Rouge. "We like them because they are at the frontier of what is possible in loop quantum gravity today — a bit more complicated than the cosmologies that have been studied over the last decade, but not so complicated as to become intractable. An 'aha' moment was when we realized we can carry out an important simplification of the equations of the model."

Instead of a singularity, they found the center of this black hole only held a region of highly curved spacetime.

"This is a clean treatment of what happens inside a black hole, using a quantum theory of gravity," said theoretical physicist Carlo Rovelli at Aix-Marseille Univ. in Marseille, France, who did not take part in this study. "It has long been expected that the singularities in the centers of black holes are cured by quantum gravity, and this is the conclusion that this work supports."

Theoretical physicists had previously shown that with loop quantum gravity, they could eliminate the singularity that past research suggested existed at the Big Bang. Instead of emerging from a point of infinite density, their work proposed the cosmos was born from a "Big Bounce," expanding outward after a prior universe collapsed.

"Perhaps in the future it can be shown that all singularities are removed by the theory," Pullin said.

Just as loop quantum gravity replaced the singularity at the Big Bang with a bridge to another universe, these new findings replace each singularity in black holes with "a bridge to another region in the future of our universe," Pullin said. Although prior studies also suggested black holes harbored such bridges, researchers had believed the singularities in black holes prevented any way of crossing those bridges.

"I think that this shows that loop quantum gravity is very vital and bubbling, and continues to produce exciting new results and new ideas," Rovelli said.

Pullin emphasized that they used a very simple model in this study, consisting of only highly curved spacetime without representing the actual matter found inside real black holes. The models for the study were also exactly spherically symmetrical, unlike many black holes, which spin and thus differ across their surfaces. Finally, in their model the black hole was there forever and will be there forever — in reality, black holes generally form after the collapse of stars and should one day evaporate away if they no longer have matter or energy to devour.

"Adding matter and having a black hole that evolves is what we are aiming for next," Pullin said.

Pullin and his colleague Rodolfo Gambini detailed their findings online in the journal Physical Review Letters.

 

[Nathanial.Dread]

Here's my big question: where did the mass of the singularity go?

The singularity, while infinitely dense, has a finite mass, which the the left-over material from the star that birthed the black hole, plus the material that's gotten sucked in.

If the singularity is no longer extant, then where did that matter go? Did it get shot out into some other part of the universe? Dropped into a new dimension? Turned into energy?

Unless they can account for the missing matter (and more importantly, the information the matter encodes), they're going to run up against the laws of conservation of information and matter/energy.

Sir Arthur Eddington said rather succinctly:

The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics, I can give you no hope; there is nothing for it but to collapse in deepest humiliation.

 

[hixidom]

I think that information actually is lost in a black hole.

A relevant(?) wiki link:

Black hole information paradox - Wikipedia

en.wikipedia.org

A Physicsforums discussion on the matter:

Is information lost in a black hole?

Ok, so we know that black holes emit Hawking radiation. Knowing this, it is safe to say that over time, black holes will decrease in size until they disappear totally. After decreasing, everything should go back to normal, you'd think. My question is, does the "data", that had been sucked in by...

www.physicsforums.com

A useful link from that discussion:

Black Hole Information Loss

I don't know anything about loop quantum gravity, but I would guess that it is a solution to this information-loss problem, rather than a cause. Perhaps it falls under the category of solutions that Wikipedia labels: "Information is stored in a baby universe that separates from our own universe".

Also, I don't think there is a law of conservation of information. Here is a quote that addresses that better than I could at the moment:

"Conservation of information", the Liouville's theorem, neccessarily implies determinism. QM is definitely not deterministic, as we understand it today. The unitary evolution of a quantum state is deterministic, but at the moment of collapse, new information appears.

Determinism alone is not enough to have Liouville's theorem, though. General relativity is deterministic, but information can be destroyed during creation of a black hole. The "no hair" theorem states that a black hole carries only few bits of information, forgetting about most properties of matter that had fallen into it.

The Liouville's theorem holds only if for each state there is exactly one future and exactly one past. QM violates the first, GR the latter. There are theories (like the Hawking radiation) that violate both.
Classical mechanics is perfectly deterministic and information-conserving.

The change of information capacity is an important indicator of the time arrow. It's the only microscopic process not symmetric under time reversal.

The important philosophical question: is the Liouville's theorem good or bad? Should we be happy that our current theories break information conservation, or should we seek for a better theory that preserves it?
Classical thermodynamics depends on Liouville's theorem and can not live without it.

There are theories that try to be information-preserving. I.e. there is a hypothesis that black holes actually hold some information (they have hair) that can be further extracted through the Hawking radiation.
Other theories do not have information preservation, but try to be time-reversible. The information loss in black holes is the same thing that the creation of information in quantum decays.

 

[nen888]

^..i thought the court was out in terms of whether the information is lost..

Stephen Hawkings (who held that position) changed his mind..

and how can information be 'lost' anyway?

also, from a higher dimensional p.o.v. (e.g. string theory etc) which can be external or 'above' time, no information can ever be lost..

 

[Nathanial.Dread]

If information can get lost, that opens some pretty knotty holes that would change a lot of our fundamental assumptions about how the universe works.

I'm not saying that that may not be the case, after all, once-upon-a-time, the sun orbited the earth, but so far, it makes some sense to believe in a theory of information conservation. If information can be lost, then we loose both the ability to know the past accurately, and deterministically predict the future.

Here's a lecture by Hawking himself on the Black Hole Information Paradox (which seem to remember hearing was solved, although I can't even begin to remember how).

http://www.hawking.org.uk/into-a-black-hole.html

 

[DeMenTed]

This is quite an interesting black hole story Black hole caught napping after meal

A black hole that's having a nap :O

 

[hixidom]

If information can get lost, that opens some pretty knotty holes that would change a lot of our fundamental assumptions about how the universe works.

The holes are already there; Our understanding of how the universe works is just a model. It is not 100% accurate; it is just accurate enough that no counter-evidence has been found YET. It was created based on systems that we have been able to experiment on thus far, so if our model happens to have knotty holes when we apply it to the most extreme and remote places in the universe, I think that is understandable. I don't think it would change any fundamental assumptions anyway. Like I said, "information" is not conserved (at least not that we can measure). Quantum mechanics is probabilistic rather than deterministic, the result being that, in cases where states at time A and time C need not be continuous at time B (i.e. any measurement of a quantum system), it cannot be said that usable "information" actually existed.

 

[joedirt]

If information can be lost, then we loose both the ability to know the past accurately, and deterministically predict the future.

Can anyone show you the past or accurately predict for you the future?

Seems as if information is indeed lost according to this definition no?

 

[nen888]

If information can be lost, then we loose both the ability to know the past accurately, and deterministically predict the future.

Can anyone show you the past or accurately predict for you the future?

Seems as if information is indeed lost according to this definition no?

..this raises the question: does the past 'exist'?

and also, what is the nature of time..and of course this is one of the big questions for physics to crack..
just because we can't reach another point in time from this one, in the particular dimensional 'slice' we perceive, doesn't mean that, in the larger picture, it doesn't exist..
the information may be lost to our point of view, but that's not the same as absolutely lost..
also, experiments have shown photons can be entangled with photons which no longer exist..the information is in the entanglement..

incidentally, as far as i understand it, the direction light rays travel is not exactly in space or time, but called 'null'..