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dvb
2003-Dec-20, 01:20 AM
I read the black hole faq sticky, but it doesn't seem to answer my questions about black holes.

I'm curious to know where all the matter that gets sucked into a black hole goes. And also what exactly is a singularity? I've only heard of these things on star trek before I read the faq.

Any insight would be helpful thanks! :)

ToSeek
2003-Dec-20, 01:25 AM
A singularity is where God divides by zero. ;)

Seriously, the bottom line is that if you get enough matter scrunched up in one place, it will compact together under the force of gravity such that - according to our present understanding of physics - nothing will stop it from continuing to compact. So the assumption is that it contracts to a point of infinite density. But then all the equations break down, so there's no way of saying what really "happens" to the matter. We don't know. All we can say is that the gravitational force associated with the matter is still emanating from that point.

Some of the physicists on here will probably come up with a more accurate or comprehensive explanation, but that's the way I understand it right now.

dvb
2003-Dec-20, 01:42 AM
Thanks again ToSeek!

So you're saying that a singularity is pretty much just a name for the unknown?

Vermonter
2003-Dec-20, 03:17 AM
Yeah. The matter just stays there, but very, very compacted.

ToSeek
2003-Dec-20, 03:37 AM
Thanks again ToSeek!

So you're saying that a singularity is pretty much just a name for the unknown?

A very, very massive unknown. ;)

Well, I hate saying just "unknown," because that's a little too open. It's a large mass concentrated at a point that can theoretically exist based on general relativity. But I think it's fair to say that we really don't know what happens there or if any truly exist - perhaps there is some physics we don't know about that stops the compaction before we get to the "divide by zero" stage.

Again, I'm not a trained physicist, just an interested layperson, so take all this with a grain of salt.

tofu
2003-Dec-20, 04:37 AM
So you're saying that a singularity is pretty much just a name for the unknown?

A singularity is the name for something that has no volume. It's just a single point. Singular as in single.

Consider this, if you had a quantity of hydrogen gas it would expand as gases tend to do and you'd lose it. If you kept it in a balloon, it'd expand and push against the sides of the balloon. If you had a lot more H2, you'd just need a bigger balloon. Right? But if you had a incredibly tremendously huge about of hydrogen, you'd have Jupiter. There isn't much else in jupiter than plain old H2. There's just so much of it that it's held together under it's own gravity.

Jupiter is just Hydrogen, but it can't expand out into space because of it's own gravity. Gravity keeps the gas together, but what stops it from collapsing down to a much smaller volume? The answer is electromagnetism. The electromagnetic force in between individual atoms of hydrogen resists the crushing force of gravity. So the H atoms deep inside Jupiter are pressed together by gravity, but held apart by electromagnetism, and they support all the atoms above them - just like the columns of a building support the floors above.

What happens in a black hole is that something transpires that allows gravity to overcome electromagnetism. When that happens, it's like cutting out the support columns in a building. To use a particularly morbid example, think of the WTC in New York. The planes hit a certain floor and the fuel that spilled out eventually melted the supporting structure for that floor. When that happened, the floors above fell a distance of *just one floor* Say about five meters. That's all that happened. It would have stopped right there, but when those floors fell that short distance, they picked up enough momentum to break the support structure of the next floor, and they fell another 5 meters, and broke the support of that floor, etc etc.

So imagine something happened to the atoms at the center of Jupiter so that they could no longer support the atoms above them. They'd collapse together very tightly. All the atoms above would fall inward and repeat the process. Unlike the WTC, which collapsed into a pile of rubble, there's nothing to stop the collapse of a black hole. It just keeps falling in on itself forever, theoretically. The volume gets smaller and smaller and the density goes up.

That's a singularity. Zero volume and infinite density.

If you've managed to read this far, you probably wonder what happens to cause the support columns to fail in the first place. As far as I know, the only thing known to cause a black hole is the violent explosion of a very large star, an even known as a supernova.

A couple of other things: I know Jupiter has some kind of metallic core. Otherwise it wouldn't have a magnetic field. Also, I don't mean to suggest that Jupiter could collapse into a black hole. It doesn't have enough mass.

The question was asked, what happens to mass that falls into a black hole (please say falls in rather than sucked in). I've read that in most cases matter that falls in goes into orbit. It may be in orbit beneath the event horizon, so you'll never see it again, but it is still in orbit. But hey, I'm no expert so I'm sure someone will correct me if I'm wrong.

dvb
2003-Dec-20, 05:49 AM
Thank you very much everyone!

tofu, your explanation was excellent. It makes sense that all the matter could just be orbiting around inside the event horizon since the light of that matter will never escape, and we'd never know. So from what I understand now, a black hole really isn't a hole at all. It's just something we can't see unless matter is falling into it from outside the event horizon.

This makes me wonder now. You say that a black hole, which is a singularity has no volume to it. How can we measure that there's no volume to it when nothing can escape from it and we can't see past the event horizon? Maybe it just appears to have no volume because of the way it warps space-time in all directions, or maybe I'm wrong. lol

Any help would be appreciated. :)

tofu
2003-Dec-20, 06:14 AM
How can we measure that there's no volume to it when nothing can escape from it and we can't see past the event horizon?
We don't measure the volume.

Here's the thing. Well established and experimentally observed physics tells us that electromagnetism keeps things, pretty much anything including the Earth, Jupiter, the Sun, everything, from collapsing indefinitely under its own gravity. Ok?

So then someone asked what would happen if gravity was so strong that it overcame electromagnetism. The answer they came up with was that the object would then collapse into... nothing. If electromagnetism was overcome, there was no longer anything to stop the collapse. So logically, it collapses into zero volume.

The original name for such an object was “a gravitationally completely collapsed object.” And it was just a theory. Nothing more. Astronomers and physicists thought a lot about the theory and eventually realized that if such an object existed, there were ways they could detect it. So, using only the theory to guide them, they went looking. Low and behold they started finding them.

Nobody has ever measured the volume. They started with a theory that said “if it's possible for gravity to overcome electromagnetism, I predict the following” one of those predictions is that a singularity has no volume. The other predictions have been observed, so that's strong evidence that the theory is correct and therefore the other predictions are true as well. We can get to this conclusion without ever measuring the volume.

Grey
2003-Dec-20, 07:17 AM
So then someone asked what would happen if gravity was so strong that it overcame electromagnetism. The answer they came up with was that the object would then collapse into... nothing. If electromagnetism was overcome, there was no longer anything to stop the collapse. So logically, it collapses into zero volume.
I'm going to picky and make a small correction (hopefully, tofu won't mind :) ). Electromagnetic repulsion keeps the atoms of normal matter spaced as we see them. For a large amount of matter (say, a white dwarf star), this electric repulsion is already no longer sufficient. What keeps it from collapsing further is "electron degeneracy pressure" (essentially, the Pauli exclusion principle that no two electrons can be in the same energy state, so they can only get so close together). For a slightly larger amount of mass (above about 1.4 solar masses) the pressure is high enough that the electrons combine with the protons, forming neutrons, and it's now neutron degeneracy pressure that halts the collapse (this is a neutron star). If there's more mass than neutron degeneracy can support, then it collapses to a black hole as far as we know, because as far as we can tell there isn't anything else that could prevent further collapse.

dvb
2003-Dec-20, 07:50 AM
Wow, very deep. Thank you both!

informant
2003-Dec-20, 05:26 PM
Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?

Grey
2003-Dec-20, 06:27 PM
Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
No. That is, you are not mistaken, at least according to our current models. Got to be careful with those questions phrased in the negative. :)

Chip
2003-Dec-20, 08:11 PM
...Nobody has ever measured the volume. They started with a theory that said “if it's possible for gravity to overcome electromagnetism, I predict the following” one of those predictions is that a singularity has no volume. The other predictions have been observed, so that's strong evidence that the theory is correct and therefore the other predictions are true as well. We can get to this conclusion without ever measuring the volume.

The singularity may also have no location in space. The black hole and its event horizon certainly have a coordinate and location, but once we are at the singularity, where time and space end, perhaps there is a transformation.

The black hole's singularity could also be thought of as a mathematical precept within our physics and math. An "understudy" for what is actually there. Perhaps the singularity is analogous to a Mandelbrot set. A compression into a dual identity, at one and the same with a larger space that is the same location. Akin to the fractal images that shrink to reveal themselves to be part of the same image at it's largest, while shrinking to become again the same image. #-o

ToSeek
2003-Dec-20, 08:17 PM
The original name for such an object was “a gravitationally completely collapsed object.” And it was just a theory. Nothing more. Astronomers and physicists thought a lot about the theory and eventually realized that if such an object existed, there were ways they could detect it. So, using only the theory to guide them, they went looking. Low and behold they started finding them.

To nitpick a little, what astronomers have found are highly massive objects (i.e., sources of significant gravitational force) with what appears to be an event horizon. We have no way of knowing what goes on inside the event horizon: all we can say for sure is that there is x amount of mass inside there (where x is a big fat number). Theory tells us that most of the x is in a singularity (for the reasons that Grey explains), but there's no way of proving that observationally right now.

Cougar
2003-Dec-20, 08:30 PM
If there's more mass than neutron degeneracy can support, then it collapses to a black hole as far as we know, because as far as we can tell there isn't anything else that could prevent further collapse.
Grey's right. Of course, just because we don't know of any mechanism that could prevent further collapse doesn't mean there might not be such a mechanism. In fact, realistically this singularity idea presents us with a major breakdown of our ability to make sense of our surroundings. As Harvard GR prof. Rothman says,


...a singularity.... is a much more serious breakdown than a flat tire or a cracked engine block. It is, in fact, a physical impossibility -- a region where the laws of physics break down altogether and even spacetime comes to an end.

Problem is, we can't just look inside and see what the heck is going on!

But obviously the mass is still there because its gravity still affects its surroundings. So this idea of a black hole as a wormhole "transport" to another universe or another location within our universe would seem little more than science fiction.

Alex W.
2003-Dec-20, 09:01 PM
I've always had the problem with the singularity being a mass with zero volume... is this just a simplification for the equations? Because I can't imagine the mass reaching zero volume in anything less than infinite time. :-?

Hell, it's meant to be confusing, it's a black hole, it messes with relativity in crazy ways.

dvb
2003-Dec-20, 09:26 PM
Problem is, we can't just look inside and see what the heck is going on!

But obviously the mass is still there because its gravity still affects its surroundings. So this idea of a black hole as a wormhole "transport" to another universe or another location within our universe would seem little more than science fiction.

Maybe it leads into fluidic space where everything is matter. :D

Seriously though, it all seems hopeless to ever be able to explore a black hole. :( If we could perfect teleportation, we might be able to beam our bodies into the black hole, but we'd need to find some way to rematerialize on the other end.

eburacum45
2003-Dec-21, 01:57 AM
Or how about a wormhole that has one end inside a black hole's event horizon? I bet that would cause a few mathematical headaches.

Kaptain K
2003-Dec-21, 03:41 AM
Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
Yes and no! As Hawking said; "A black hole has no hair", meaning that all we know about a black hole is its mass, charge and spin. Anything that enters a black hole loses all individual identifuing characteristics. The particles of Hawking radiation are created at the event horizon from the gravitational energy of the black hole. So, while a black hole loses mass by Hawking radiation, technically, it is not the same matter that went in.

Espritch
2003-Dec-21, 06:52 PM
If there's more mass than neutron degeneracy can support, then it collapses to a black hole as far as we know, because as far as we can tell there isn't anything else that could prevent further collapse.

Well, there is also the possibility of quark degeneracy.

http://www.nature.com/nsu/020408/020408-8.html


Astronomers think they might have spotted a quark star, a mass of fundamental particles only a few kilometres across but weighing more than our Sun. If the star's nature is confirmed, it would be the first example of this state of matter.

I would like to note that that neutron and quark degeneracy relate to the quantum mechanical explanation for black holes. There is also a relativistic explanation for black holes that says, in essence, that when you move past the Schwartzchild radius, time and distance effectively flip so that movement forward in time effectively becomes movement inwards towards the singularity. So regardless of any other possible degeneracy forces that might exist, once matter (or energy) passes the event horizon, it moves inevitably towards the singularity.

For a better explanation of this than I can give, check the link from JREF and look towards the bottom for a post by Ziggurat.

http://www.randi.org/vbulletin/showthread.php?s=&threadid=29719&perpage=40&highli ght=black%20hole&pagenumber=2

ToSeek
2003-Dec-21, 08:24 PM
Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
Yes and no! As Hawking said; "A black hole has no hair", meaning that all we know about a black hole is its mass, charge and spin. Anything that enters a black hole loses all individual identifuing characteristics. The particles of Hawking radiation are created at the event horizon from the gravitational energy of the black hole. So, while a black hole loses mass by Hawking radiation, technically, it is not the same matter that went in.

Black holes also have a temperature, which was puzzling to physicists until Hawking figured out his eponymous radiation.

Tensor
2003-Dec-21, 09:37 PM
!

But obviously the mass is still there because its gravity still affects its surroundings. So this idea of a black hole as a wormhole "transport" to another universe or another location within our universe would seem little more than science fiction.

Cougar, I think you have this part wrong. It is my understanding that the a "white hole" is not prohibited. The huge gravitational energy generated (outside the event horizon) as the black hole forms, is able keeps the hole intact without the mass. In other words, once the event horizon is formed, the hole no longer need the mass (which makes sense if you think about it as nothing inside the hole can get out). If anyone (Grey, Tim, Kilopi, ) out there has more info, I would appreciate a correction.


edited for spelling

dvb
2003-Dec-21, 09:50 PM
The huge gravitational energy generated (outside the event horizon) as the black hole forms, is able keeps the hole intact without the mass. In other words, once the event horizon is formed, the hole no longer need the mass (which makes sense if you think about it as nothing inside the hole can get out).

I thought the event horizon was just a perimeter around the black hole where the point of no return is. I didn't realize it was something that was formed in some way or another.

Tensor
2003-Dec-21, 10:21 PM
The huge gravitational energy generated (outside the event horizon) as the black hole forms, is able keeps the hole intact without the mass. In other words, once the event horizon is formed, the hole no longer need the mass (which makes sense if you think about it as nothing inside the hole can get out).

I thought the event horizon was just a perimeter around the black hole where the point of no return is. I didn't realize it was something that was formed in some way or another.

dvb, very simplified, there was no event horizon around the star before and there is an event horizon around a black hole. Think of it this way, as the mass shrinks, space curves more and more until there is an event horizon. The growing curvature of spacetime is how the event horizon forms.

dvb
2003-Dec-21, 10:35 PM
Tensor

Thank you for clarifying. My feable mind is still learning. :)

Tensor
2003-Dec-21, 10:39 PM
Tensor

Thank you for clarifying. My feable mind is still learning. :)

Hardly feeble, you are asking good questions on things you don't understand and seem to grasp the answers, which puts you way ahead of some others that have posted here.

freddo
2003-Dec-21, 10:39 PM
Tensor

Thank you for clarifying. My feable mind is still learning. :)

Stick with it dvb - learning is all us feeble minds can do! Besides, your questions have been interesting.

dvb
2003-Dec-21, 10:47 PM
Thank you both!

I like to RTFM before asking questions so I don't annoy people. I know how it can be helping people with computers all the time. Learning is something I enjoy very much. :)

dvb
2003-Dec-21, 11:25 PM
Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
Yes and no! As Hawking said; "A black hole has no hair", meaning that all we know about a black hole is its mass, charge and spin. Anything that enters a black hole loses all individual identifuing characteristics. The particles of Hawking radiation are created at the event horizon from the gravitational energy of the black hole. So, while a black hole loses mass by Hawking radiation, technically, it is not the same matter that went in.

I read over all the posts again and seemed a little puzzled at this. If hawking radiation doesn't come from matter that fell into the black hole, then where could it have come from? Another dimension perhaps? And how can we prove that the matter isn't transformed into energy in the form of hawking radiation and expelled from the event horizon? Is there an imbalence of matter to energy that could prove why it's not?

Edit: sorry, I didn't read all of that correctly. The hawking radiation comes from gravitational energy. But you need matter to have gravity. So it would seem to me that the matter falling into the black hole creates a bigger gravitational field, which is then expelled in the form of hawking radiation. In an indirect way, it still seems that the matter going in still has is expelled in some way in the form of hawking radiation.

Tensor
2003-Dec-22, 12:20 AM
I read over all the posts again and seemed a little puzzled at this. If hawking radiation doesn't come from matter that fell into the black hole, then where could it have come from? Another dimension perhaps?

The radiation is created outside the black hole during virtual particle creation.



Edit: sorry, I didn't read all of that correctly. The hawking radiation comes from gravitational energy. But you need matter to have gravity.

You need energy to have gravity (remember mass-energy equivilance or E=Mcc). Normally, pairs (particles and antiparticles) of virtual particles borrow energy from space, become real particles, and almost immediately anihilate each other, returning the energy back. This is allowed by the Uncertainty Priciple of Quantum Mechanics. Near the black hole, when the particles are created, one can fall into the black hole and the other 'escapes' from the black hole. Since they can't anihilate each other as they normally would, they have to stay as real particles. But the escape of the real particle reduces the mass of the black hole (because the particle "borrowed" the energy from the black holes gravitational field).


So it would seem to me that the matter falling into the black hole creates a bigger gravitational field, which is then expelled in the form of hawking radiation. In an indirect way, it still seems that the matter going in still has is expelled in some way in the form of hawking radiation.

Well, in an inderect way, yes it is expelled. But it is no longer the same matter that fell into the black hole.

dvb
2003-Dec-22, 12:34 AM
You need energy to have gravity (remember mass-energy equivilance or E=Mcc). Normally, pairs (particles and antiparticles) of virtual particles borrow energy from space, become real particles, and almost immediately anihilate each other, returning the energy back. This is allowed by the Uncertainty Priciple of Quantum Mechanics. Near the black hole, when the particles are created, one can fall into the black hole and the other 'escapes' from the black hole. Since they can't anihilate each other as they normally would, they have to stay as real particles. But the escape of the real particle reduces the mass of the black hole (because the particle "borrowed" the energy from the black holes gravitational field).

Wow, that's very facinating stuff. I remember reading about virtual particles a couple days ago. But the concept that the event horizon could seperate the particles and antiparticles, and in turn reduce the mass of the black hole is so neat! This really helps my understanding of black holes now. Thanks Tensor!! :D

This must go against the laws of energy conservation to right?

Tensor
2003-Dec-22, 01:36 AM
Thanks Tensor!! :D

You are quite welcome.



This must go against the laws of energy conservation to right?

No, why? The emitted radiation is balanced by the loss of mass. Mass/energy is still conserved.

dvb
2003-Dec-22, 02:06 AM
Got it! :)

dvb
2003-Dec-22, 03:17 AM
Actually, hmm. I'm still a little bit stumped. I think I may have been refering to the first law of thermodynamics. I'm not sure if energy conservation is the same thing. At least in the first law of thermodynamics, it states that energy cannot be created or destroyed. This holds up under normal circumstances with virtual particles, but since both the particle and antiparticle don't anihilate each other in this instance, the borrowed energy remains to exist and isn't given back.

Maybe there's something else I'm not understanding here. #-o

Tensor
2003-Dec-22, 04:23 AM
At least in the first law of thermodynamics, it states that energy cannot be created or destroyed. This holds up under normal circumstances with virtual particles, but since both the particle and antiparticle don't anihilate each other in this instance, the borrowed energy remains to exist and isn't given back.

Maybe there's something else I'm not understanding here. #-o

Hmmmmm, ok try this. WARNING the numbers used here are not the actual numbers. They are extremely simplified to (I hope) make it clearer.

One particle does not fall into the BH

Starting Energy/mass
Particle Pair = 0
BH mass = 10
Total mass/energy = 10

Create particle pair by borrowing energy/mass from BH
Particle pair = 2
BH mass = 8
Total mass/energy = 10

Particle pair anihilates, giving energy/mass back to BH
Particle pair = 0
BH mass = 10
Total mass/energy = 10

One particle falls into BH

Starting Energy/mass
Particle Pair = 0
BH mass = 10
Total mass/energy = 10

Create particle pair by borrowing energy/mass from BH
Particle pair = 2
BH mass = 8
Total mass/energy = 10

One particle falls in, the other escapes
Particle pair = 1
BH mass = 9
Total mass/energy = 10

Notice that the total energy never changed, just the location. And also notice that the BH mass was reduced.

dvb
2003-Dec-22, 05:04 AM
Ok, I'm sorry, I was confused. lol

I understand that perfectly. I was under the impression you were talking about particles that just pop up at random in the universe. Energy that appears litterally out of no where and then anihilates itself. I read about this type of energy the other day, but I can't remember where. They say that because it only appears for a brief moment that the conservation law still holds true since it anihilates itself right afterwords.

I didn't realize this energy was borrowed from the black hole itself. Thanks once again for unconfusing me. lol ;)

Tensor
2003-Dec-22, 05:10 AM
Ok, I'm sorry, I was confused. lol


I didn't realize this energy was borrowed from the black hole itself. Thanks once again for unconfusing me. lol ;)

Remember, that this is an extremely simplified explanation to help you picture what is happening.

dvb
2003-Dec-22, 05:20 AM
So you're saying that the energy isn't borrowed from the black hole?

freddo
2003-Dec-22, 05:25 AM
So you're saying that the energy isn't borrowed from the black hole?

No-no... More like it is, but that's an extremely simple way to consider it.

dvb
2003-Dec-22, 05:32 AM
So you're saying that the energy isn't borrowed from the black hole?

No-no... More like it is, but that's an extremely simple way to consider it.

Thanks freddo and Tensor

I'll take your words for it. :)

semi-sentient
2003-Dec-22, 06:01 AM
Great read! More!!

I'd like to hear more about the possibilities of White Holes and their relation to Worm Holes. Are these just mathematical possibilities? I remember reading a while back that White Holes couldn't exist because the second they interacted with matter they were destroyed. Also, in theory, White Holes could only have existed early in the Universe. If memory serves, this was a possiblity introduced in "Brane Theory" because of the initial titanic collision of branes. Is this a fairly accurate interpretation or am I out in left field? 8-[

constible
2003-Dec-22, 06:39 AM
I have a question.

Why can't we calculate a volume for the black hole using the space confined by the event horizon?

This isn't to be confused with the volume of space contained by the matter (which goes to zero), but a volume with the event horizon distance as the diameter. As far as I know, we can roughly estimate the diameters of the black hole, couldn't we just calculate the volume using that?

Wally
2003-Dec-22, 03:37 PM
I've always had the problem with the singularity being a mass with zero volume... is this just a simplification for the equations? Because I can't imagine the mass reaching zero volume in anything less than infinite time. :-?

Hell, it's meant to be confusing, it's a black hole, it messes with relativity in crazy ways.

Infinite time is exactly what you may have at the "center" of a black hole!

wedgebert
2003-Dec-22, 05:04 PM
Great read! More!!

I'd like to hear more about the possibilities of White Holes and their relation to Worm Holes. Are these just mathematical possibilities? I remember reading a while back that White Holes couldn't exist because the second they interacted with matter they were destroyed. Also, in theory, White Holes could only have existed early in the Universe. If memory serves, this was a possiblity introduced in "Brane Theory" because of the initial titanic collision of branes. Is this a fairly accurate interpretation or am I out in left field? 8-[

One would think that if a White Hole is the exact opposite of a Black Hole (its repulstion is so strong, nothing can reach it), then the instant it forms it will "explode" an disappear.


I've always had the problem with the singularity being a mass with zero volume... is this just a simplification for the equations? Because I can't imagine the mass reaching zero volume in anything less than infinite time.

Hell, it's meant to be confusing, it's a black hole, it messes with relativity in crazy ways.

I don't think it's actually 0 volume. At some point there has to a form of matter (or something) that just cannot be broken down any smaller, at least not by the Black Hole's gravity. However, this level could be smaller than a Planck Length and thus for our purposes it has a volume of 0 since anything smaller than a Planck Length is meaningless.


have a question.

Why can't we calculate a volume for the black hole using the space confined by the event horizon?

This isn't to be confused with the volume of space contained by the matter (which goes to zero), but a volume with the event horizon distance as the diameter. As far as I know, we can roughly estimate the diameters of the black hole, couldn't we just calculate the volume using that?

You could do that, in fact it's fairly simple. You just have to calculate the distance where the force of gravity is equal to c and use tha as you diameter. For example, I believe if Sol collapsed into a black hole right now, its event horizion would extend out 3 miles.

However, this doesn't work for spinning black holes. If a black hole spins fast enough, the singularity starts to form a ring. This in turn starts to alter the shape of the event horizion. In fact, the faster the spinning, the closer you can get to the singularity without being sucked in.

What's even cooler is when the rotational energy of a black hole exceeds the mass energy. Spinning (Kerr) black holes actually have two event horizions. In addition to the normal EH that flips space and time, there is an inner EH that flips them back to normal. As a black hole spins, the outer EH stays the same, but the inner EH grows larger. When the rotational energy exceeds the mass energy, the event horizions actually vanish (according to theory) and you are left with a naked singularity.

Another weird theory is that due to strange space warping, nothing can fall into a spinning black hole unless it approaches along the plane of the spin. Otherwise the warpage of space actually repels you.

Alex W.
2003-Dec-22, 05:22 PM
I thought it had something to do with the Planck Length. :D

Swift
2003-Dec-22, 06:06 PM
When the rotational energy exceeds the mass energy, the event horizions actually vanish (according to theory) and you are left with a naked singularity.
And if you think singularities covered decently with an event horizon really mess with physics, naked singularities are really bad.

informant
2003-Dec-22, 07:17 PM
Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
Yes and no! As Hawking said; "A black hole has no hair", meaning that all we know about a black hole is its mass, charge and spin. Anything that enters a black hole loses all individual identifuing characteristics. The particles of Hawking radiation are created at the event horizon from the gravitational energy of the black hole. So, while a black hole loses mass by Hawking radiation, technically, it is not the same matter that went in.
(Emphasis mine.)
If everything that enters a black hole loses all identifying characteristics, how do you know that it isn't the same matter that gets out?

informant
2003-Dec-22, 07:38 PM
I've always had the problem with the singularity being a mass with zero volume... is this just a simplification for the equations? Because I can't imagine the mass reaching zero volume in anything less than infinite time. :-?
[layman speaking] About the infinite density/zero volume question, my guess is that that is a good model for what happens at "large" scales, but when the volume actually goes to zero quantum mechanics must step in at some point, and things may be a little different. (This is a bit like discussing the Big Bang, isn't it?) [/layman speaking]


Hell, it's meant to be confusing, it's a black hole, it messes with relativity in crazy ways.
As if relativity needed the craziness... :)


And if you think singularities covered decently with an event horizon really mess with physics, naked singularities are really bad.
Must… fight urge… to make… silly joke.

dvb
2003-Dec-22, 07:40 PM
What's even cooler is when the rotational energy of a black hole exceeds the mass energy. Spinning (Kerr) black holes actually have two event horizions. In addition to the normal EH that flips space and time, there is an inner EH that flips them back to normal. As a black hole spins, the outer EH stays the same, but the inner EH grows larger. When the rotational energy exceeds the mass energy, the event horizions actually vanish (according to theory) and you are left with a naked singularity.

Another weird theory is that due to strange space warping, nothing can fall into a spinning black hole unless it approaches along the plane of the spin. Otherwise the warpage of space actually repels you.

Facinating!!!

That's the first I've heard of a dual event horizon, or a naked singularity for that matter. Thank you for your knowledge!

*steals wedgebert's brain* :lol:

Edit: Woops spelling




Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
Yes and no! As Hawking said; "A black hole has no hair", meaning that all we know about a black hole is its mass, charge and spin. Anything that enters a black hole loses all individual identifuing characteristics. The particles of Hawking radiation are created at the event horizon from the gravitational energy of the black hole. So, while a black hole loses mass by Hawking radiation, technically, it is not the same matter that went in.
(Emphasis mine.)
If everything that enters a black hole loses all identifying characteristics, how do you know that it isn't the same matter that gets out?

That's pretty much the same question I asked in this thread earlier. lol[/quote]

informant
2003-Dec-22, 07:53 PM
With emphasis on same, this time. :)

Kaptain K
2003-Dec-23, 10:49 AM
The particles of the Hawking radiation are created outside of the event horizon, so they can hardly be the same particles that fell past the event horizon into the black hole.

Alex W.
2003-Dec-23, 04:41 PM
Normally, the particle and antiparticle of a pair which spontaneously form recollide immediately, so there's not net gain of matter. Near a black hole, one particle (the antiparticle, for example) falls into the black hole, and the other escapes, so there's going to be one extra particle, and the black hole will have absorbed an antiparticle, "losing" mass.

IIRC.

wedgebert
2003-Dec-23, 05:14 PM
Normally, the particle and antiparticle of a pair which spontaneously form recollide immediately, so there's not net gain of matter. Near a black hole, one particle (the antiparticle, for example) falls into the black hole, and the other escapes, so there's going to be one extra particle, and the black hole will have absorbed an antiparticle, "losing" mass.

IIRC.

The black hole doesn't lose mass because it absorbs an antiparticle. Sometimes the antiparticle is the one that escapes while the regular particle is caught, however there is still a net loss to the black hole.

What happens is that the energy to create the particles comes from the gravitational energy of the black hole. When one particle escapes, it means that the black hole lost 50% of the energy required to make the virtual particle pair and thus you have a net loss in energy, and since e=mc^2 is also m=e/(c^2), you have a net loss in mass as well.

Kaptain K
2003-Dec-23, 07:12 PM
Normally, the particle and antiparticle of a pair which spontaneously form recollide immediately, so there's not net gain of matter. Near a black hole, one particle (the antiparticle, for example) falls into the black hole, and the other escapes, so there's going to be one extra particle, and the black hole will have absorbed an antiparticle, "losing" mass.

IIRC.
Antimatter has positive mass.

informant
2003-Dec-23, 08:40 PM
I can understand that. But the original question was (http://www.badastronomy.com/phpBB/viewtopic.php?p=182471#182471):


Layman speaking: am I mistaken in saying that given enough time - a long, loooong time - the matter that falls into a black hole eventually gets out, in the form of Hawking radiation?
Not individual particles, but ‘matter’ in a broad, collective sense.

Alex W.
2003-Dec-23, 09:31 PM
Right, I think I've got it now.