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SollyLama
2003-Feb-10, 11:09 PM
Being the non-scientific sort, I'm kind of stumped on something:

Does Hawking Radiation take away from the mass of a black hole? Or are the particles created independant of the black hole?

If a black hole indeed radiates away mass, can it in time lose enough mass to revert to 'normal' matter? In other words, will it lose enough mass to become visible (like a brown dwarf)?
Or is the black hole effect irreversible? Once it crosses the threshold into black hole-ness, does it simply shrink as it loses mass, but retain a singularity beyond normal physics?

Also, do black holes spin, and how would we know? I know material sprials into them, suggesting rotation. The original star spun, but does a black hole have the physical properties (solid cohesion) needed to spin?

And for a bit of speculation- if you could ignore the physical effects of a black hole and stand within arm's reach of one, do you think it's be solid or not? Is the gravity too extreme for atoms to bind into complex forms, meaning the surface would lack solidity?

Enquiring minds want to know.

Zathras
2003-Feb-10, 11:29 PM
On 2003-02-10 18:09, SollyLama wrote:
Being the non-scientific sort, I'm kind of stumped on something:

Does Hawking Radiation take away from the mass of a black hole? Or are the particles created independant of the black hole?

The hawking radiation does take away mass from a black hole.


If a black hole indeed radiates away mass, can it in time lose enough mass to revert to 'normal' matter? In other words, will it lose enough mass to become visible (like a brown dwarf)?
Or is the black hole effect irreversible? Once it crosses the threshold into black hole-ness, does it simply shrink as it loses mass, but retain a singularity beyond normal physics?

A black hole will not change into any other kind of matter. There will eventually come a point, however, where all of the black hole's mass will radiate away as hawking radiation. This will take an extremely long time however.



Also, do black holes spin, and how would we know? I know material sprials into them, suggesting rotation. The original star spun, but does a black hole have the physical properties (solid cohesion) needed to spin?

Black holes do have angular momentum, which they keep from before they were black hole, as well as the addition of angular momentum from other mass. Whether it actually "spins," however, is a fuzzy question, as your comment about not being a rigid body in any sense. A black hole is nothing more than the remnant gravitational field caused by the original mass. Since the gravitational field is different when the mass is spinning than when it is not, then the remnant gravitational field will possess these same properties that will make it appear that the source mass has spin. But you are right; there is no-thing to "spin."



And for a bit of speculation- if you could ignore the physical effects of a black hole and stand within arm's reach of one, do you think it's be solid or not? Is the gravity too extreme for atoms to bind into complex forms, meaning the surface would lack solidity?

Enquiring minds want to know.


It would not be solid. The tidal forces will prevent complex structure to build. Beyond that, I'm not sure how to answer your question.

SollyLama
2003-Feb-10, 11:46 PM
For solidity, might the black hole not be solid from the density of the original material? A bunch of quantum particles jammed into a small space. Or (as is probably the case) does matter lose it's physical presence when reduced to quantum particles, regardless of how tightly they get packed together?

I'm fuzzy on the 'remnant gravity field' concept. I understood a black hole to be an object (even if it's not a solid one) made up of the quantum particles that make up atoms. The atoms cannot stay together, but the quantum material is still present.
I always thought of a black hole as nothing, yet potentially everything. It has no real matter (been torn apart in the gravity) but has all the quantum material to make anything at all- outside of the confines of the event horizon. Okay, I took alot of 'shrooms when I came up with that- made sense at the time.
Are you saying that a black hole destroys matter BEYOND the quantum level?

JS Princeton
2003-Feb-10, 11:52 PM
You might try the experiment yourself, Solly. Go past the event horizon and try to see what the stuff inside of it is. Unfortunately, you won't be able to report your results.

We don't know of any physics that should keep matter intact beyond the neutronium degeneracy. This means that many scientists expect that there is a singularity at the center of a black hole. However, we are not sure how the singularity looks or feels.

In short, the black hole is defined by its event horizon. It doesn't matter how the matter is distributed inside this event horizon, the Schwartchild Radius from the center defines the black hole. We expect that most things collapse to smaller sizes than Schwartzchild Radii (of course, no one has nor ever will be able to report on this), so the "surface" of the black hole isn't rigid at all. In fact, it's simply defined by the radius in space. In a sufficiently sized black hole, you could pass the event horizon and not even know it!

Russ
2003-Feb-11, 12:09 AM
On 2003-02-10 18:09, SollyLama wrote:
Being the non-scientific sort, I'm kind of stumped on something:

Does Hawking Radiation take away from the mass of a black hole? Or are the particles created independant of the black hole?

Yes, Hawking radiation does remove mass from a BH.


If a black hole indeed radiates away mass, can it in time lose enough mass to revert to 'normal' matter? In other words, will it lose enough mass to become visible (like a brown dwarf)?
Or is the black hole effect irreversible? Once it crosses the threshold into black hole-ness, does it simply shrink as it loses mass, but retain a singularity beyond normal physics?

The BH would retain its' qualities until all of its' mass was lost. The singularity would still be infinately dense, so it would just be a VERY small BH.


Also, do black holes spin, and how would we know? I know material sprials into them, suggesting rotation. The original star spun, but does a black hole have the physical properties (solid cohesion) needed to spin?

You can't tell for sure if BH's spin because you can't see beyond the event horizon. Intuition tells you that they should and there is some indrect indication they do. There is some data, new, that suggests "frame dragging" around suspected spinners. This is where the very "fabric" of space is grabbed by the BH and twisted by the spinning BH. Envision someone wringing out a wash rag. (I know, I know it's not a good analagy but it's the best I could come up with.)


And for a bit of speculation- if you could ignore the physical effects of a black hole and stand within arm's reach of one, do you think it's be solid or not? Is the gravity too extreme for atoms to bind into complex forms, meaning the surface would lack solidity?

It would not be solid. In fact, if you could negate the tidal forces, if you fell beyond the event horizon you would probably not see anything significantly different than what you saw outside the event horizon. The event horizon (EH) is just the point at which light can nolonger escape the BH.

Now about those tidal forces. In a stellar mass BH, we'll say 10 solar masses (SM), the difference in gravitational pull between your feet and head (assuming you're falling feet first) could be millions of G's (Earth surface @ sealevel = 1G). This would be enough to pull you apart an atom at a time. So you'd probably vaporize like someone in a Star Trek transporter. Never to return.


Enquiring minds want to know.


It is very good that you want to know. There are many people who just stumble through life, happy with their ignorance, and we wind up with movies like Dooms Day Rock or Armagedon.

Welcome to the BABB. My you post long and prosper! /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_lol.gif

Russ
2003-Feb-11, 12:20 AM
On 2003-02-10 18:46, SollyLama wrote:
For solidity, might the black hole not be solid from the density of the original material? A bunch of quantum particles jammed into a small space. Or (as is probably the case) does matter lose it's physical presence when reduced to quantum particles, regardless of how tightly they get packed together?

It is impossible to say. Your guess as to what the remnant mass is like is a good as anybody else's. I like to picture something about the size of a small marble with 10SM's of mass spinning at about 60 million RPM. Silly me!


I'm fuzzy on the 'remnant gravity field' concept. I understood a black hole to be an object (even if it's not a solid one) made up of the quantum particles that make up atoms. The atoms cannot stay together, but the quantum material is still present.
I always thought of a black hole as nothing, yet potentially everything. It has no real matter (been torn apart in the gravity) but has all the quantum material to make anything at all- outside of the confines of the event horizon. Okay, I took alot of 'shrooms when I came up with that- made sense at the time.
Are you saying that a black hole destroys matter BEYOND the quantum level?


As I said above, your guess is as good as any. Why not have it as you say? /phpBB/images/smiles/icon_biggrin.gif

Klausnh
2003-Feb-11, 02:20 AM
Discover Magazine had an article on spinning black holes:
http://www.discover.com/recent_issue/recent02.html
Scroll down to the July issue.

KLaus

DStahl
2003-Feb-11, 08:23 AM
Russ: "The BH would retain its' qualities until all of its' mass was lost. The singularity would still be infinately dense, so it would just be a VERY small BH."

The rate of production of the Hawking radiation increases as the circumference of the black hole event horizon decreases, as I recall. At some subatomic size the 'evaporating' black hole will be radiating furiously, white-hot; and I have read that as the event horizon shrinks toward the Planck size the singularity will disappear in a burst of energy, perhaps an explosion on the order of several megatons.

Here's a (probably inaccurate) way I think about the mass of black holes: general relativity says that spacetime curvature becomes infinite arbitrarily close to the singularity at the center of the black hole. Dumping more matter into the singularity doesn't make the spacetime curvature "more infinite" and I think that the singularity itself is unchanged. It doesn't get bigger, it doesn't get heavier, it's already gone as far as it can go.

What the infalling mass does do, I speculate, is increase the size of the region in which spacetime is critically curved--it increases the circumference of the event horizon. If you use the hoary old "spacetime is a 2-dimensional rubber sheet" analogy, then a black hole is an infinitely deep dimple in the rubber. Dumping more mass into the black hole doesn't make the dimple any deeper--it's already infinitely deep--but it does widen the dimple, make it larger around.

So, Hawking radiation sort of extracts the stored mass of this spacetime curvature and allows it to radiate away. Very slowly at first, then faster and faster, the dimple in the rubber sheet shrinks in circumference--but it remains infinitely deep right to the end.

Now, about that singularity: "singularity" as I understand it is math jargon for a condition in which an equation becomes singular, ie yields an infinite or a meaningless answer, like when one ends up dividing by zero. So relativity is really telling us that its equations give us an unreliable answer, a singularity, when applied to the center of a black hole. I think most physicists think that a complete theory of quantum gravity will tell us what a black hole singularity looks like; some have speculated that it will be something like a Planck-size knot of chaotic space where time has ceased to exist and the shape of space is indefinite and can only described by probabilities.

The "Planck size" I've been mentioning is the smallest size it's reasonable to talk about, 1.6 X 10<sup>-33</sup> centimeters. There's also a "Planck time" which is 1 X 10<sup>-44</sup> seconds. At these tiny sizes and incredibly short times I think that the laws of quantum mechanics rule supreme, and classical notions such as length and duration become indeterminate.

I think, anyway. Well, grist for discussion even if I've got some of it arsy-versy.



<font size=-1>[ This Message was edited by: DStahl on 2003-02-11 03:29 ]</font>