Can Black Holes Really Die and What Are Their Secrets?

[Monsterboy]

If an Alcubierre warp drive could actually be built (which, since it requires exotic matter, it probably can't), it would allow you to, for example, travel to Alpha Centauri in much less than 4.3 years, even as seen by observers on Earth or Alpha Centauri. But the reason for this would be that the warp drive would drastically change the geometry of spacetime in between Earth and Alpha Centauri, such that the distance between them would be much less than 4.3 light years if you measured it along a path going through the warp bubble. So a light beam that passed through the warp bubble would get from Earth to Alpha Centauri even faster than the ship itself would.

(Btw, this description is heuristic only; it's the best I can do at translating what I understand of the math of the Alcubierre solution into ordinary English. There are a number of technical complications involved, so what I've said is certainly not a rigorous description of what the math actually says.)

Can Casimir effect be used for the Alcubierre drive if we can't find exotic matter ? Can low energy density vacuum allow "faster than light travel" ?

 

[Monsterboy]

Will super massive black holes eventually consume all the stars in their galaxy ?

 

[mfb]

All that do not escape from the galaxy (=most likely future of a star): probably. On a really, really long timescale.

 

[Drakkith]

Can Casimir effect be used for the Alcubierre drive if we can't find exotic matter ? Can low energy density vacuum allow "faster than light travel" ?

Not as far as we know.

Will super massive black holes eventually consume all the stars in their galaxy ?

No, many will eventually escape from their orbits due to gravitational interactions. There's also the possibility that the SMBH will evaporate before the majority of stars in the galaxy have time to decay in their orbits, but I don't know the timescales involved in either of these processes.

 

[mfb]

A black hole with a mass of 100 million solar masses emits about 10^-44 W of Hawking radiation.

A star with the mass of our sun, located 10000 lightyears away and orbiting this black hole, leads to the emission of many nanowatts of power as gravitational waves. The stars will get ejected or fall in long before the supermassive black holes evaporate.

Note that Hawking radiation scales with 1/M^2 while gravitational wave power scales with (approximately) M^3, so even with just 100 solar masses gravitational waves win by several orders of magnitude. In addition, the energy scale of the orbit is significantly smaller than the energy scale of the black hole.

 

[Monsterboy]

A black hole with a mass of 100 million solar masses emits about 10^-44 W of Hawking radiation...

can we detect such weak radiations ?
Are we sure that Hawking radiation occurs ? Is this based on conservation of information ?

Were micro black holes created in the LHC ? If yes, can we claim that it is because of hawking radiation that they disappeared ?

 

[Drakkith]

can we detect such weak radiations ?

Not a chance.

Are we sure that Hawking radiation occurs ? Is this based on conservation of information ?

We aren't sure, as we've never detected it. Per wiki: http://en.wikipedia.org/wiki/Hawking_radiation#Emission_process

Hawking radiation is required by the Unruh effect and the equivalence principle applied to black hole horizons. Close to the event horizon of a black hole, a local observer must accelerate to keep from falling in. An accelerating observer sees a thermal bath of particles that pop out of the local acceleration horizon, turn around, and free-fall back in. The condition of local thermal equilibrium implies that the consistent extension of this local thermal bath has a finite temperature at infinity, which implies that some of these particles emitted by the horizon are not reabsorbed and become outgoing Hawking radiation.

I'm afraid that if you want to know more details on this process, someone else will have to answer, as I have only a basic knowledge of black holes and hawking radiation.

Were micro black holes created in the LHC ? If yes, can we claim that it is because of hawking radiation that they disappeared ?

We don't know. None have been detected.

 

[mfb]

The approach described there to find Hawking radiation in similar systems had some success recently.

Were micro black holes created in the LHC ?

That would be a discovery you could not miss. It would get much more news coverage than the Higgs boson.

 

[Monsterboy]

That would be a discovery you could not miss. It would get much more news coverage than the Higgs boson.

Lol
Is it even possible for LHC to do that?

 

[Monsterboy]

Even if we manage to create a micro black hole in the LHC it will be impossible to detect hawking radiation right ? Because if if a SMBC with a mass of 100 million solar masses emits 10^-44 watts of hawking radiation then the radiation from a micro black hole should be even more undetectable right ?

 

[mfb]

Lol
Is it even possible for LHC to do that?

Do what?

Smaller black holes emit more radiation. A microscopic black hole would vanish within an incredibly short time, emitting many high-energetic particles. It is quite easy to look for that at the LHC experiments.

 

[ShayanJ]

Even if we manage to create a micro black hole in the LHC it will be impossible to detect hawking radiation right ? Because if if a SMBC with a mass of 100 million solar masses emits 10^-44 watts of hawking radiation then the radiation from a micro black hole should be even more undetectable right ?

If you mean in terms of the intensity, its exactly the opposite because the power of the radiation is proportional to Mass^{-2}. It will be very high for micro black holes but its so much high that the micro black hole evaporates in very very very small fraction of a second.

 

[Monsterboy]

Do what?

Is it possible for us to create micro black holes with the LHC ?

 

[Drakkith]

Is it possible for us to create micro black holes with the LHC ?

We don't know for sure, but most experts on the subject don't believe it's possible.

http://en.wikipedia.org/wiki/Safety_of_high-energy_particle_collision_experiments#Micro_black_holes

 

[Monsterboy]

Can black holes explode ? (Like a hypernova ,a really big one.)

 

[PeterDonis]

Can black holes explode ?

No.

 

[Monsterboy]

I

No.

I just read an article on it , so this loop theory not widely accepted ? http://www.nature.com/news/quantum-bounce-could-make-black-holes-explode-1.15573

 

[PeterDonis]

this loop theory not widely accepted ?

It's highly speculative at this point.

 

[Chalnoth]

I
I just read an article on it , so this loop theory not widely accepted ? http://www.nature.com/news/quantum-bounce-could-make-black-holes-explode-1.15573

That's a really terrible description of the picture. Such a bounce would not result in the black hole exploding in any real sense. What it would do is make the black hole generate an entirely new universe in its interior that would forever be disconnected from the universe outside the black hole.

But, as PeterDonis said, that's highly speculative.

What black holes do do, however, is evaporate. However, astrophysical black holes take a very, very long time for this to occur: a solar-mass black hole takes something like 10^{67} years to evaporate. But the evaporation does become faster the smaller it gets. Near the end of its lifetime, the black hole becomes unbelievably bright: when it has a year of life left, a black hole has a temperature of about a quadrillion Kelvin, meaning it's throwing out lots of high-energy particles. So the end of a black hole's lifetime would sort of look like a slow-motion explosion of highly energetic particles, getting brighter and more energetic as the end gets closer and closer.

 

[PeterDonis]

Such a bounce would not result in the black hole exploding in any real sense. What it would do is make the black hole generate an entirely new universe in its interior that would forever be disconnected from the universe outside the black hole.

This is one version of the "bounce" model (Hawking, among others, I believe, calls it the "baby universe" model), but it's not the only possible one, correct? Another possible model, I think, is that there actually is a "bounce" in the original universe, at some point when the spacetime curvature is still finite, that reverses the collapse and makes the collapsing matter expand back out into the original universe. In this kind of model, there is no actual black hole because there is no actual event horizon; there is only an apparent horizon, which appears when the collapse reaches a certain point, and then disappears again once the re-expansion has reached a certain point. This model is highly speculative, just like the "baby universe" model.

As I understand it, the article Monsterboy linked to is actually talking about the latter type of model.

 

[Chalnoth]

This is one version of the "bounce" model (Hawking, among others, I believe, calls it the "baby universe" model), but it's not the only possible one, correct?

There are other bounce models, certainly. But those models do not occur inside a black hole. Inside a black hole, it is impossible to do anything at all that impacts the world outside the horizon. To do so would be to violate General Relativity at such a fundamental level that it is highly unlikely to accurately describe reality. I also doubt it would be able to fit with current observations of black holes.

 

[PeterDonis]

There are other bounce models, certainly. But those models do not occur inside a black hole.

If there is no actual event horizon (as there is not in the second bounce model I described in my previous post), then there is no actual black hole. There is only an apparent horizon and an apparent black hole. There is also no actual singularity; there is just a region of spacetime inside an apparent horizon where the density of matter and energy gets very high for a while before the bounce spreads it out again.

There are plenty of physicists in the field who think that is how the black hole information paradox gets resolved: there's never an actual event horizon or an actual singularity, so there can be a global unitary quantum state everywhere. Again, as far as I can tell, that is the kind of model described in the article Monsterboy linked to.

current observations of black holes.

Current observations can only tell us about the presence, or highly likely presence, of apparent horizons--surfaces where, locally, radially outgoing light no longer moves outward. Observation alone cannot tell you whether there is an actual event horizon present; you have to know the entire future of the spacetime. If quantum gravity effects can significantly change your prediction of the entire future of the spacetime (which they most likely can), then you would need to have a well-tested theory of quantum gravity in order to know which of the apparent horizons you observe are actually event horizons (more precisely, which ones are closely associated with event horizons). We are obviously not in that position today.

 

[Chalnoth]

If there is no actual event horizon (as there is not in the second bounce model I described in my previous post), then there is no actual black hole. There is only an apparent horizon and an apparent black hole. There is also no actual singularity; there is just a region of spacetime inside an apparent horizon where the density of matter and energy gets very high for a while before the bounce spreads it out again.

There are plenty of physicists in the field who think that is how the black hole information paradox gets resolved: there's never an actual event horizon or an actual singularity, so there can be a global unitary quantum state everywhere. Again, as far as I can tell, that is the kind of model described in the article Monsterboy linked to.

These cosmological bounce models have nothing to do with astrophysical black holes. They have completely different dynamics, as they generally have a (fairly) uniform FRW-style universe collapsing in the pre-bounce phase. Such a universe doesn't have a black hole event horizon even in General Relativity, before any quantum considerations are taken.

Current observations can only tell us about the presence, or highly likely presence, of apparent horizons--surfaces where, locally, radially outgoing light no longer moves outward. Observation alone cannot tell you whether there is an actual event horizon present; you have to know the entire future of the spacetime. If quantum gravity effects can significantly change your prediction of the entire future of the spacetime (which they most likely can), then you would need to have a well-tested theory of quantum gravity in order to know which of the apparent horizons you observe are actually event horizons (more precisely, which ones are closely associated with event horizons). We are obviously not in that position today.

Quantum gravity isn't likely to change our idea of the horizon except to explain in more detail how the Hawking radiation is emitted.

 

[PeterDonis]

These cosmological bounce models have nothing to do with astrophysical black holes.

Cosmological bounce models don't, no. But the article Monsterboy linked to wasn't talking about a cosmological bounce model. The features of loop quantum gravity that are key to cosmological bounce models also allow other kinds of bounce models. Here's the paper by Rovelli et al. that the article was referring to; it's specifically about a bounce model for a black hole-like spacetime:

http://arxiv.org/abs/1407.0989

Quantum gravity isn't likely to change our idea of the horizon

It isn't likely to change our idea of the apparent horizon, no. But once again, the event horizon is a very different thing from the apparent horizon. The apparent horizon is local, so quantum gravity can only affect it if it affects local physics; and for a large enough black hole, curvature at the horizon is small enough that classical GR should be valid locally.

But the event horizon is global, so quantum gravity can affect it if it affects the global geometry of the spacetime. It can potentially do that if the curvature anywhere becomes large enough for the classical GR approximation to break down; and we know for sure that that happens in black hole models. So the question of whether quantum gravity effects are enough to make a spacetime not contain event horizons is an open question, even if we are pretty sure that quantum gravity effects can't stop apparent horizons from forming.

 

[Monsterboy]

http://www.iflscience.com/space/supermassive-black-hole-discovered-early-universe
Is this an example of a primordial black hole?

 

[Haelfix]

Here's the paper by Rovelli et al. that the article was referring to; it's specifically about a bounce model for a black hole-like spacetime:

http://arxiv.org/abs/1407.0989

It isn't likely to change our idea of the apparent horizon, no. But once again, the event horizon is a very different thing from the apparent horizon..

Rovellis Planck star proposal is a radical proposal that violates all sorts of laws of physics, including in particular GR, even at very long distance scales. The bounce event destroys the smooth horizon that GR predicts.

The problem doesn't really go away if you imagine that it is an apparent horizon either. Right now, in this room where I am typing, you have a very large amount of apparent horizons that form from the intersection of two outgoing light cones. It's only if you trace out the global history of the universe that you can determine if those rays loop around and stay trapped. So the question is, how does LOCAL unitary physics determine whether it is dealing with those fake apparent horizons, or the real thing? It is as if one must appeal to some sort of nonlocal mechanism to determine how things proceed.

Of course every solution of the black hole information problem involves some sort of really crazy proposals, so things are relative

 

[Quds Akbar]

1. Black holes are points of infinite density in spacetime (singularities), they are formed of collapsed stars
3. Black holes do die eventually: http://arxiv.org/pdf/1401.5761.pdf
4. They are points of zero volume, the circle is the event horizon
5.No, according to relativity, the more energy you gain the more mass you gain, so, theoretically, it would take infinite energy.
6. No

Those are the ones I am aware of, I am not sure of number two.

 

[mfb]

5.No, according to relativity, the more energy you gain the more mass you gain, so, theoretically, it would take infinite energy.

Not in the way "mass" is used in physics.

 

[Drakkith]

1. Black holes are points of infinite density in spacetime (singularities), they are formed of collapsed stars
3. Black holes do die eventually: http://arxiv.org/pdf/1401.5761.pdf
4. They are points of zero volume, the circle is the event horizon
5.No, according to relativity, the more energy you gain the more mass you gain, so, theoretically, it would take infinite energy.
6. No

1. A black hole is not a point of infinite density. That's the singularity.
4. Black holes have a non-zero volume.
5. You do not gain mass as your velocity increases.

 

[Chronos]

Relativistic mass has gone the way of the dinosaur, but, with prejudice ...