Ask a physicist anything. (6)

[chris4243]

Ok this has been touched on earlier, but if a black hole is in fact a "singularity," why would any matter at all ever need to be spewed back out? (And even if my intuition that the concept of singularity is false but is still 3D were correct, there is still the same problem)

Centrifugal force, electromagnetic fields, massive releases of energy... Nothing gets spewed out of the black hole, but what makes you think everything would get in in the first place?

 

[Chalnoth]

Ok this has been touched on earlier, but if a black hole is in fact a "singularity," why would any matter at all ever need to be spewed back out? (And even if my intuition that the concept of singularity is false but is still 3D were correct, there is still the same problem)

Well, a black hole isn't a singularity, but what's going on here is rather different and it gets a little bit complex. What's happening here is that when you have a rotating black hole, the rotation causes a sort of "bulge" to form outside the event horizon, called an ergosphere. When matter falls into this bulge, some fraction of the matter continues on over the event horizon into the black hole, while some of it gets propelled out of the black hole at the poles at extreme velocity. The extra energy this expelled matter picks up comes from the rotation of the black hole.

 

[Cactus Jack]

Do you think man will drill a hole to the Earth's mantle?

 

[mzungu]

Do you think man will drill a hole to the Earth's mantle?

Technological progress will allow us to do what is considered impossible today. There are almost no bounds to what we can do given the technology:
Scientists to Drill Earth's Mantle, Retrieve First Sample?

 

[Chalnoth]

Do you think man will drill a hole to the Earth's mantle?

Certainly not easily, but it looks like some want to try:
Scientists to Drill Earth's Mantle, Retrieve First Sample?

 

[Wiccan_Child]

Ok this has been touched on earlier, but if a black hole is in fact a "singularity," why would any matter at all ever need to be spewed back out? (And even if my intuition that the concept of singularity is false but is still 3D were correct, there is still the same problem)

Matter is spewed out not from the singularity itself, but from its effects on matter elsewhere. The event horizon is an imaginary boundary beyond which it's impossible (ish) to escape the black hole. However, just before the horizon, the gravitational pull is just enough to seperate pairs of particles. These particles pop into and out of existence all the time. At the event horizon (or thereabouts), these pairs are subjected to ever-so-slightly different gravitational pulls, separating them enough that they don't annihilate (as is usual). One particle gets sucked in, while the other zooms out into space. Thus, we should observe a shower of these particles from a black hole.

There's also the effect on normal hunks of matter that, instead of falling into the hole in a straight line, spiral into it. Before they get sucked in forever, the crazy torques and forces cause it to jettison out at extraordinary speeds - as observed here:

 

[Naraoia]

Matter is spewed out not from the singularity itself, but from its effects on matter elsewhere. The event horizon is an imaginary boundary beyond which it's impossible (ish) to escape the black hole.

"ish"?

 

[Chalnoth]

"ish"?

Well, in a classical black hole, that is with General Relativity and no quantum effects, it is utterly impossible to escape the event horizon.

Once you add in quantum effects, you find two things:
1. Black holes do not destroy information, meaning that everything that goes in must come out, if in a different form.
2. Hawking Radiation is produced at the horizon.

This says, to me, that when you take quantum mechanics into account, things that go into a black hole do come out, but the gravity is so strong that anything going in is so mangled that it leaves as almost perfectly-randomized (thermal) radiation.

 

[Naraoia]

Oh, I see.

 

[mzungu]

Well, in a classical black hole, that is with General Relativity and no quantum effects, it is utterly impossible to escape the event horizon.

Once you add in quantum effects, you find two things:
1. Black holes do not destroy information, meaning that everything that goes in must come out, if in a different form.
2. Hawking Radiation is produced at the horizon.

This says, to me, that when you take quantum mechanics into account, things that go into a black hole do come out, but the gravity is so strong that anything going in is so mangled that it leaves as almost perfectly-randomized (thermal) radiation.

Isn't it impossible to destroy information?

 

[Chalnoth]

Isn't it impossible to destroy information?

Probably. As long as the laws of physics are unitary, which so far as we know they are, then yes, it is impossible to destroy information. This means, basically, that if you knew absolutely everything about one particular slice of our universe in time, you could, in principle, infer the exact configuration at any other point in time: one slice in time has all of the information about our universe that ever was or ever will be.

 

[Wiccan_Child]

Probably. As long as the laws of physics are unitary, which so far as we know they are, then yes, it is impossible to destroy information. This means, basically, that if you knew absolutely everything about one particular slice of our universe in time, you could, in principle, infer the exact configuration at any other point in time: one slice in time has all of the information about our universe that ever was or ever will be.

This is something I've never quite got my head around.

Suppose something falls into a black hole. How does Hawking radiation contain 'garbled information' about that object's structure? What relationship does the stuff emitted by a black hole have with the mass that makes it up? Isn't it just a tiny hunk of amorphous matter? Aren't black holes hairless?

 

[Chalnoth]

This is something I've never quite got my head around.

Suppose something falls into a black hole. How does Hawking radiation contain 'garbled information' about that object's structure? What relationship does the stuff emitted by a black hole have with the mass that makes it up? Isn't it just a tiny hunk of amorphous matter? Aren't black holes hairless?

Well, we don't know. Clearly this is something that would have to be resolved by a theory of quantum gravity.

My guess is that it's something along the lines with how classical statistical mechanics work. In statistical mechanics, if you talk about the behavior of a gas, you know that there is a microscopic description of the physics that is exactly reversible, where you can take any one configuration and infer every other configuration that ever happens. For example, if I take a room, and start with all of the gas compressed into one cubic centimeter up in one corner of the room, then this gas will spread to fill the room, eventually reaching equilibrium, where it's spread evenly throughout the room. But if I could gain perfect knowledge about the positions and momenta of every atom in the gas, I could calculate that it all started up in one cubic centimeter up in the corner, and exactly when that happened.

I suspect that the relationship between the stuff that falls into a black hole and the radiation that comes out must be like that: it's so randomized that it is incomprehensible how we could calculate exactly what fell in. But if we had the correct theory of quantum gravity, and perfect knowledge of the Hawking radiation, then the physics tells us that we should, in principle, be able to determine everything that fell in as well.

 

[Wiccan_Child]

Well, we don't know. Clearly this is something that would have to be resolved by a theory of quantum gravity.

My guess is that it's something along the lines with how classical statistical mechanics work. In statistical mechanics, if you talk about the behavior of a gas, you know that there is a microscopic description of the physics that is exactly reversible, where you can take any one configuration and infer every other configuration that ever happens. For example, if I take a room, and start with all of the gas compressed into one cubic centimeter up in one corner of the room, then this gas will spread to fill the room, eventually reaching equilibrium, where it's spread evenly throughout the room. But if I could gain perfect knowledge about the positions and momenta of every atom in the gas, I could calculate that it all started up in one cubic centimeter up in the corner, and exactly when that happened.

I suspect that the relationship between the stuff that falls into a black hole and the radiation that comes out must be like that: it's so randomized that it is incomprehensible how we could calculate exactly what fell in. But if we had the correct theory of quantum gravity, and perfect knowledge of the Hawking radiation, then the physics tells us that we should, in principle, be able to determine everything that fell in as well.

But, why do we suppose that Hawking radiation has any relationship with the information 'destroyed' by a black hole? It's clear why the configuration of atoms of air in a room can tell you their state some time ago, since the atoms have just undergone mechanical collisions. But as far as I can tell, Hawking radiation is wholly unrelated to the matter that falls into a black hole.

Maybe I'll just have to wait for quantum gravity to be invented... bah

 

[Chalnoth]

But, why do we suppose that Hawking radiation has any relationship with the information 'destroyed' by a black hole? It's clear why the configuration of atoms of air in a room can tell you their state some time ago, since the atoms have just undergone mechanical collisions. But as far as I can tell, Hawking radiation is wholly unrelated to the matter that falls into a black hole.

Maybe I'll just have to wait for quantum gravity to be invented... bah

This is down to Hawking's solution to the information paradox:
Black hole information paradox - Wikipedia, the free encyclopedia

Here's a more detailed analysis:
Information 'not lost' in black holes - physicsworld.com

Anyway, this all depends upon the laws of physics being fundamentally unitary. Which it is conceivable that they may not be.

 

[chris4243]

But, why do we suppose that Hawking radiation has any relationship with the information 'destroyed' by a black hole? It's clear why the configuration of atoms of air in a room can tell you their state some time ago, since the atoms have just undergone mechanical collisions. But as far as I can tell, Hawking radiation is wholly unrelated to the matter that falls into a black hole.

I'm pretty sure it's just a guess. Hawking radiation is also just a guess. I'm not sure the existence of black holes is actually verified, but rather that we're fairly sure there's an upper limit to the size of neutron stars.

 

[pgp_protector]

I'm pretty sure it's just a guess. Hawking radiation is also just a guess. I'm not sure the existence of black holes is actually verified, but rather that we're fairly sure there's an upper limit to the size of neutron stars.

ESO - eso0846 - Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole

By watching the motions of 28 stars orbiting the Milky Way's most central region with admirable patience and amazing precision, astronomers have been able to study the supermassive black hole lurking there. It is known as "Sagittarius A*" (pronounced "Sagittarius A star"). The new research marks the first time that the orbits of so many of these central stars have been calculated precisely and reveals information about the enigmatic formation of these stars — and about the black hole to which they are bound.

 

[chris4243]

ESO - eso0846 - Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole

Yes, it's still potentially 3000 times bigger than the schwarzschild radius of the black hole. And since the supermassive black holes are of such low density, there's potentially plenty of alternative explanations for them.

 

[Chalnoth]

I'm pretty sure it's just a guess. Hawking radiation is also just a guess. I'm not sure the existence of black holes is actually verified, but rather that we're fairly sure there's an upper limit to the size of neutron stars.

Hardly. There is copious evidence for black holes, perhaps the strongest of which are the relativistic jets we see being ejected from many galaxies, including our own. And through the use of very long baseline interferometry, we may, potentially, be able to view the event horizon of Sagittarius A*. Anyway, see here for a rough overview of the evidence:
Black hole - Wikipedia, the free encyclopedia

As far as the Hawking Radiation is concerned, it has been verified empirically that when you form horizons, you get Hawking Radiation, by forming sound horizons in fluids. This is simply how horizons behave: they produce Hawking Radiation. And a black hole has a horizon, therefore it produces Hawking Radiation.

The proposed alternative explanations are vastly more speculative than black holes, and far less likely. I don't think any of them can explain all of the observational evidence we have, either.

 

[Wiccan_Child]

I'm pretty sure it's just a guess. Hawking radiation is also just a guess. I'm not sure the existence of black holes is actually verified, but rather that we're fairly sure there's an upper limit to the size of neutron stars.

Well, there's pretty strong evidence for both black holes and Hawking radiation - I just don't see the connection, either theoretical or empirical, between said radiation and the matter that falls into black holes.