View Full Version : How can a singularity be infinite?

luckyfrank

2010-Jan-05, 12:24 AM

How can a singularity be infinite ?

I dont understand this how can something have infinite density, so it's density goes on forever kinda sounds like a cop-out because physics cant come up with an answer to to explain it

Aswell black holes dont have mass or shape right ?

I imagine a massive star collapsing on itself to be ciruclar in shape almost like it's being squeezed from the outside in, so i ask it possible a black hole has mass and matter like that of a planet but it's gravity is so great that the light form this body cannot be seeing

WayneFrancis

2010-Jan-05, 01:49 AM

How can a singularity be infinite ?

I dont understand this how can something have infinite density, so it's density goes on forever kinda sounds like a cop-out because physics cant come up with an answer to to explain it

Ok simple answer. Any amount of mass that takes up zero space could be thought of as having infinite density.

As I squish stuff down the density of energy goes up and by energy I mean mass and energy.

Now no one knows what happens inside the Event Horizon, EH, of a black hole so no one knows what said energy and matter is doing there. We don't know if it actually becomes a singularity or not. It might have a finite size or it may not. We'll never know according to our current understanding of physics.

Aswell black holes dont have mass or shape right ?

Black holes most definitely have mass and as far as shape...spherical is a good estimate. There might be no surface but you could take the EH as its "shape" anything the touches that becomes part of the

Again inside the Black holed EH there are theories but right now our science breaks down.

I imagine a massive star collapsing on itself to be ciruclar in shape almost like it's being squeezed from the outside in, so i ask it possible a black hole has mass and matter like that of a planet but it's gravity is so great that the light form this body cannot be seeing

It very well could be that a black hole's EH is essentially its surface but even then SR and GR have definite things to say about said object.

The important thing to realise is the black hole does have mass. From our understanding it has to be at least 3-5 times that of our sun if it formed from star. Any less and it is just a neutron star which is another strange type of object but one we understand much better.

swampyankee

2010-Jan-05, 01:59 AM

Singularities occur all the time in mathematics, and in the mathematics used to describe physical phenomena. Gas dynamics predicts that shocks are infinitely thin surfaces of discontinuity, in theory, a resonance is infinite, etc.

The behavior of matter within black holes is completely unknown, and it's uncertain whether the equations of general relativity apply within them: the singularity is the result of the known laws of physics no longer working.

In other words, it's not a "cop-out"; it's an admission of ignorance.

WayneFrancis

2010-Jan-05, 02:04 AM

Well put swampy, I'll just note that the laws of physics works fine inside of a black hole's event horizon...it is just that our understanding of the laws of physics doesn't work :)

antoniseb

2010-Jan-05, 02:14 AM

How can a singularity be infinite ?

I dont understand this how can something have infinite density, so it's density goes on forever kinda sounds like a cop-out because physics cant come up with an answer to to explain it...

You got some good answers already.

Just as a short summary:

- the infinite density idea comes from the guess that the radius of the singularity is zero. So some finite amount of matter in zero space has infinite density.

- but the idea that inside the event horizon there is a single geometric point where all the mass is... that idea can't be demonstrated.

So, black holes aren't infinite, and probably aren't infinitely dense.

I dont understand this how can something have infinite density, so it's density goes on forever kinda sounds like a cop-out because physics cant come up with an answer to to explain it

That's basically what a singularity is... it's not a physical object, it means that our equations break down. IIRC dividing any number by zero generates a singularity, because we can't express what the solution is. Inside a black hole, we have a similar situation where our maths and physics just can't be made to work in those extreme conditions.

I suspect that if we have another revolution in mathematics or physics (comparable to say, Newton figuring out calculus, or Einstein figuring out relativity) then the new equations/system that we come up with may be able to resolve the singularities, and we'll be able to figure out what actually goes on there.

Centaur

2010-Jan-06, 05:09 AM

How can a singularity be infinite ?

I presume you meant infinitesimal (1 / ∞) in width and infinite in density. A related question may be how could an absolutely fundamental particle have any width? If its width were greater than infinitesimal (essentially equivalent to zero), then what would exist in the space beneath its surface? Would we imagine it to be some type of solid but indivisible matter? Our brains are programmed to think so, since at our macro level of existence many things appear to be solid and indivisible. But that is an illusion. Ultimately, all matter is likely composed of a collection of singularities (infinitesimally wide particles). Black holes are theorized to fall into that category of singularities.

north

2010-Jan-06, 06:08 AM

Originally Posted by luckyfrank

How can a singularity be infinite ?

it can't

it can't because energy , matter aren't and can't be compressed , infinitely

I presume you meant infinitesimal (1 / ∞) in width and infinite in density.

hmmm

A related question may be how could an absolutely fundamental particle have any width? If its width were greater than infinitesimal (essentially equivalent to zero), then what would exist in the space beneath its surface?

energy and/or matter

Would we imagine it to be some type of solid but indivisible matter?

yes

Our brains are programmed to think so, since at our macro level of existence many things appear to be solid and indivisible. But that is an illusion.

only to a point though

Ultimately, all matter is likely composed of a collection of singularities (infinitesimally wide particles). Black holes are theorized to fall into that category of singularities.

the new particle collider in Europe should prove interesting

I predict nothing much new , maybe a particle or two , but nothing much beyond this , if even this

north

2010-Jan-06, 06:34 AM

How can a singularity be infinite ?

I dont understand this how can something have infinite density, so it's density goes on forever kinda sounds like a cop-out because physics cant come up with an answer to to explain it

Aswell black holes dont have mass or shape right ?

actually black-holes are caused by mass , and the speed of that mass , and therefore causes an implosion or collapse inward towards the center of the mass of the object

I imagine a massive star collapsing on itself to be ciruclar in shape almost like it's being squeezed from the outside in, so i ask it possible a black hole has mass and matter like that of a planet but it's gravity is so great that the light form this body cannot be seeing

think hydraulics

you can only compress energy/matter to a point

and at that point certain point the matter kicks back , hence galactic jets of energy

luckyfrank

2010-Jan-06, 10:57 AM

We've all seeing the programmes on tv about black holes devouring stars and if i remember correctly they say that when this matter hits the singularity it is destoyed out of exsitence, i find this hard to comprehend does this matter just cease to exist or does the singularity retain this matter

WayneFrancis

2010-Jan-06, 02:36 PM

We've all seeing the programmes on tv about black holes devouring stars and if i remember correctly they say that when this matter hits the singularity it is destoyed out of exsitence, i find this hard to comprehend does this matter just cease to exist or does the singularity retain this matter

It is complicated. If a star is ripped apart by a black hole not all of the star will end up in the black hole. Much of the matter will not make it into the black hole if it happens to fast. Active galaxies are good examples of this. But yes anything that does make it in will be retained by the black hole. IE the mass of the star will add to the mass of the black hole making the event horizon of the black hole bigger.

The actual matter is destroyed as we know it but it isn't lost. Think of it as transformed into black hole material, what ever that is. To the rest of the universe what that matter was doesn't matter once it is in the black hole. There is a saying "A black hole has no hair". Unlike other objects where we can tell what makes the object up we have no clue with black holes. All we can tell you about a black hole is its mass, spin and charge. From the mass and spin we can tell you the size and shape of the event horizon but we can't tell you anything about what it consumed in the past by looking at it.

There is a process where the black hole will evaporate but for even stellar mass black holes you are talking many orders of magnitude longer then the current age of the universe for that to happen.

DrRocket

2010-Jan-06, 06:16 PM

How can a singularity be infinite ?

I dont understand this how can something have infinite density, so it's density goes on forever kinda sounds like a cop-out because physics cant come up with an answer to to explain it

Aswell black holes dont have mass or shape right ?

I imagine a massive star collapsing on itself to be ciruclar in shape almost like it's being squeezed from the outside in, so i ask it possible a black hole has mass and matter like that of a planet but it's gravity is so great that the light form this body cannot be seeing

A singularity is a point at which the mathematical model breaks down. Quite typically the value of some function grows without bound as one approaches the point of singularity. That is often stated as something, like density, is infinite at the singuarily, but that is really an abuse of language.

A simple singularity is the singularity of the function f(x)= 1/x at the point x=0. f is not defined for x=0 and it is incorrect to say that f(0) is infinity or negative infinity.

The singularities associated with black holes and the big bang are not necessarily physical objects, and in fact probably are not. They most often regarded as indications of the failure of general relativity to apply in the extreme circumstances that exist near the predicted singularity. If is commonly thought that the resolution will require a unified theory of quantum mechanics and general relativity. There could be other explanations, including alternatives to general relativity. There is a competing theory to general relativity, called Einstein-Cartan theory that does not predict a singularity. EC theory is not experimentally distinguishable from GR with current measurement technology and is a valid theory of gravitation, albeit a bit more complex mathematically, and a bit less restrictive in terms of initial assumptions.

So, in a sense you are right. Physics cannot explain a singularity. One objective of research is to resolve such issues. Physics is not a complete theory. We don't understand everything.

Neil Russell

2010-Jan-06, 09:42 PM

Yes you can pack matter into a BH infinitely. We know that matter taken into a BH is collapsed/compressed billions of times smaller, so no atoms etc as we know them ( hence there will be no photons, no light emitted) The weird thing is that matter as we know it is mostly space( that is the distance between things) so much so that neutrinos can pass right through the earth without hitting an atom. Hence the ability to infinitely collapse smaller and smaller.

tusenfem

2010-Jan-07, 07:46 AM

actually black-holes are caused by mass , and the speed of that mass , and therefore causes an implosion or collapse inward towards the center of the mass of the object

think hydraulics

you can only compress energy/matter to a point

and at that point certain point the matter kicks back , hence galactic jets of energy

This whole post does not make any sense.

north, if you answer a question in Q&A, please make sure that that anwer is also mainstream and correct.

black holes are cause by mass: yes

and the speed of that mass: NO (you are probably talking about the collapse of the progenitor star, which is NOT a black hole, but I cannot be sure)

and therefore causes an implosion...: NO, a black hole does not "cause an implosion, the black hole is the result of an implosion.

the matter kicks back, hence galactic jets: NO, the "matter will kick back" as it wants to stop the infall, which probably happens also inside a black hole, however, we don't know the physics, don't know the equation of state for matter at those conditions. However, the "galactic jets" are NOT caused by "matter kicking back". I assume that you mean the jets coming "out of a black hole" which are created by the matter in the accretion disk falling in and being accelerated by various processes, amongst others magnetic fields, away from the black hole along the rotational axis.

This whole answer of yours was not even wrong to quote a famous physicist.

cosmocrazy

2010-Jan-07, 03:13 PM

Yes you can pack matter into a BH infinitely. We know that matter taken into a BH is collapsed/compressed billions of times smaller, so no atoms etc as we know them ( hence there will be no photons, no light emitted) The weird thing is that matter as we know it is mostly space( that is the distance between things) so much so that neutrinos can pass right through the earth without hitting an atom. Hence the ability to infinitely collapse smaller and smaller.

IIRC you are basically describing a Neutron star rather than a black hole. Where all the subatomic particles are squashed together so that all the "empty" space inside the atoms no longer exists, but the matter itself is about as dense as it can get before it becomes something we have yet to observe or explain, occupying a very small volume of space. As far as I'm aware the matter in a black hole is calculated to become a singularity, a point where that matter is squashed further until it occupies no volume at all. As it has been pointed out though a "singularity" is a mathematical construct and I think it would be quite fair to say that in physical terms a singularity appears to be not logically possible.

What we can say is that a BH does exactly what it says on the tin - it becomes so dense that the gravitational field is so strong that the escape velocity is FTL. What lies at the centre of such, is currently un-defined in physical terms.

Jeff Root

2010-Jan-10, 11:14 PM

A black hole is a region of space in which the gravity is so intense that

light is not able to escape from it. Matter and energy at the center of

the black hole are the source of the gravity. If some of the matter is

moving in any direction other than radially, or if the matter is unevenly

distributed, the event horizon will not be perfectly spherical. However,

most of the time, the event horizon will be very close to spherical, for

reasons very similar to the reasons that the surfaces of planets are

usually very close to spherical.

The term "singularity" has different meanings in different contexts. When

experts in relativity such as Kip Thorne, Roger Penrose, or Stephen Hawking

use the term, it has a very particular meaning. When someone else uses it,

it may have another meaning.

In general, a singularity is a place, not a thing. The north and south poles

of the Earth are singularities: points in space where longitude is undefined.

The initial instant of the Big Bang is a singularity: the point in time at which

the cosmic expansion began.

When talking about the singularity at the center of a black hole, the term

is sometimes used to refer to the matter/energy of the black hole rather

than the point in space where that matter/energy is located.

The following is my own understanding of the singularity of a black hole.

As far as I have been able to tell, it is completely mainstream, but it is my

own interpretation, and may be incorrect.

Matter (of all kinds, including light) falling into a black hole increases the

mass of the black hole, which deepens the "gravity well". It stretches

spacetime in the time direction, increasing the spacetime distance from

the event horizon to the center. Also, increasing the density of the matter

at the black hole's center increases the depth of the gravity well. So as

the matter becomes denser and denser, the gravity well becomes deeper

and deeper. Since there is no force strong enough to prevent the matter

from increasing in density, the gravity well becomes deeper without limit,

and the matter keeps on falling, which causes it to become even denser,

also without limit. So the matter is forever falling deeper and deeper into

the gravity well, forever approaching infinite density more and more closely.

Quantum mechanics shows that the position and momentum of any matter

can only be detected simultaneously within certain limits. It implies that

the size of the matter at the center of a black hole can only be defined

within those limits. So according to quantum mechanics, the matter can

only make a finite body of a particular, calcuable size, which depends on

the total energy of the body. (That is, the mass plus the energy of its

motion and any other energy it may have.) This finite body is sometimes

referred to as the "singularity". However, it is not a "body" in any normal

sense. It is merely the limit of the theoretical ability to determine the

location of the matter and energy which generate the black hole. When

a star collapses to form a black hole, the matter very quickly falls within

this limit, and quantum mechanics cannot provide any more information

about what happens to the matter after that.

-- Jeff, in Minneapolis

DrRocket

2010-Jan-11, 12:49 AM

Quantum mechanics shows that the position and momentum of any matter

can only be detected simultaneously within certain limits. It implies that

the size of the matter at the center of a black hole can only be defined

within those limits. So according to quantum mechanics, the matter can

only make a finite body of a particular, calcuable size, which depends on

the total energy of the body. (That is, the mass plus the energy of its

motion and any other energy it may have.) This finite body is sometimes

referred to as the "singularity". However, it is not a "body" in any normal

sense. It is merely the limit of the theoretical ability to determine the

location of the matter and energy which generate the black hole. When

a star collapses to form a black hole, the matter very quickly falls within

this limit, and quantum mechanics cannot provide any more information

about what happens to the matter after that.

-- Jeff, in Minneapolis

The existence of black holes and the predicted sinularity are a consequence of the general theory of relativity and not of quantum mechanics. Quantum mechanics has nothing to do with the singularity, absolutely nothing.

The singularity represents a hypersurface on which the curvature tensor of GR fails to exist. It is not driven by quantum mechanics, and at our level of understanding cannot be, as that would require a unification of GR and QM>

QM may eventually provide a resolution for the predicted singularity. General relativity and QM are incompatible and there is some thought that once we have a unified theory that can simultaneously treat gravitation and quantum effects we will have a better understanding of what is going on inside a black hole.

RussT

2010-Jan-11, 01:18 AM

The existence of black holes and the predicted sinularity are a consequence of the general theory of relativity and not of quantum mechanics. Quantum mechanics has nothing to do with the singularity, absolutely nothing.

The singularity represents a hypersurface on which the curvature tensor of GR fails to exist. It is not driven by quantum mechanics, and at our level of understanding cannot be, as that would require a unification of GR and QM>

QM may eventually provide a resolution for the predicted singularity. General relativity and QM are incompatible and there is some thought that once we have a unified theory that can simultaneously treat gravitation and quantum effects we will have a better understanding of what is going on inside a black hole.

SO, you/mainstream "Know" you need to eliminate the 'singularities' IE: Big Bang Naked Singularity and the Black Hole Singularities...(and there is a 'singularity' built into SR that has been rationalized away, and yet another that no one even realizes.) and yet......

You/mainstream will "Defend" them with 'Glee and Furvor" to the death...;)

SO, how can they be eliminated when ya'll are so ardent in defending them???

DrRocket

2010-Jan-11, 01:23 AM

SO, you/mainstream "Know" you need to eliminate the 'singularities' IE: Big Bang Naked Singularity and the Black Hole Singularities...(and there is a 'singularity built into SR that has been rationalized away, and yet another that no one even realizes.) and yet......

You/mainstream will "Defend" them with 'Glee and Furvor" to the death...;)

SO, how can they be eliminated when ya'll are so ardent in defending them???

1. That is ridiculous, and misrepresents the mainstream understanding.

2. Q&A is not the place to debate ATM alternatives.

Jeff Root

2010-Jan-11, 02:04 AM

The existence of black holes and the predicted singularity are a

consequence of the general theory of relativity and not of quantum

mechanics. Quantum mechanics has nothing to do with the singularity,

absolutely nothing.

Yes, I agree. But as I said, sometimes the term "singularity" is used

to refer to the "body" rather than its location, and the only meaningful

definition of that "body" is provided by quantum mechanics.

The singularity represents a hypersurface on which the curvature

tensor of GR fails to exist. It is not driven by quantum mechanics,

and at our level of understanding cannot be, as that would require a

unification of GR and QM.

I agree that it has nothing to do with quantum mechanics. But could

you explain what you mean by a "hypersurface", and why the curvature

tensor of GR would fail there? Looking at it qualitatively, it seems to

me that GR should not fail at any particular place, but should instead

become less and less applicable as the point of infinite density is

approached more and more closely.

-- Jeff, in Minneapolis

DrRocket

2010-Jan-11, 05:38 AM

Yes, I agree. But as I said, sometimes the term "singularity" is used

to refer to the "body" rather than its location, and the only meaningful

definition of that "body" is provided by quantum mechanics.

I agree that it has nothing to do with quantum mechanics. But could

you explain what you mean by a "hypersurface", and why the curvature

tensor of GR would fail there? Looking at it qualitatively, it seems to

me that GR should not fail at any particular place, but should instead

become less and less applicable as the point of infinite density is

approached more and more closely.

-- Jeff, in Minneapolis

A hypersurface is a synonym for an embedded submanifold.

The failure of the curvature tensor to exist on thie hypersurface in question is simply a reflection of the fact that the hypersurface is defined as the set of points at which the curvature tensor fails to exist. It what you probably mean by "the point of infinite density", but it is not a point. It is more complicated than that.

For a discussion of the nature of singularities see the original papers of Penrose or the discussion in The large-scale structure of spacetime by Hawking and Ellis.

There is no "body" and the problem is not one of quantum mechanics, at least within what is known today and the theory that we have at this time.

steve_bnk

2010-Jan-13, 08:23 AM

We can create singlularities here on Earth witrh physical systems.

In the limit as x -> 0 1/x goes to infiunity, a singularity.

In electronic and electro-mechanical systems when a divde by zero occurs some variable will try to go to infinity, such as velocity or RPM in a motor.

The singularity at least on Earth can never go to infinity, that would require infinte energy and something always limits the singularity, such as available power or something breaks.

Black holes leak energy, do they not? Not knowing relativity in any detail, the singularity relating to black holes would appear to be infinite density and therfore infinite energy.

Mathematicaly a singularity is some parameter wanting to go to infinity or increasing without bound.

Jeff Root

2010-Jan-13, 01:47 PM

Infinite density does not imply infinite energy.

As I said above, my understanding is that the density of the matter forming

a black hole approaches infinite with time, but never reaches infinite because

the matter is always "deepening" the "gravity well". Spacetime is becoming

more and more stretched, without limit, but it would take forever to become

infinitely deep.

-- Jeff, in Minneapolis

luckyfrank

2010-Jan-13, 05:00 PM

Infinite density does not imply infinite energy.

As I said above, my understanding is that the density of the matter forming

a black hole approaches infinite with time, but never reaches infinite because

the matter is always "deepening" the "gravity well". Spacetime is becoming

more and more stretched, without limit, but it would take forever to become

infinitely deep.

-- Jeff, in Minneapolis

Do we know black holes actully have holes within them ?

And if so where is this hole in space can it be measured

noncryptic

2010-Jan-13, 08:39 PM

Do we know black holes actully have holes within them ?

And if so where is this hole in space can it be measured

We just call them black holes because stuff disappears in them.

They don't have holes, they are holes.

There's a super massive one at the center of our galaxy.

Its mass can and has been measured indirectly from the many stars that are in close and fast orbit around it. Given the orbit size and speed there's no way something other than a black hole fits in there.

Black Hole at the Galactic Core

http://www.youtube.com/watch?v=fytriKJ8xhE

Black hole evidence in the centre of our galaxy

http://www.youtube.com/watch?v=r3qSr5HmGkI

luckyfrank

2010-Jan-13, 10:28 PM

We just call them black holes because stuff disappears in them.

They don't have holes, they are holes.

There's a super massive one at the center of our galaxy.

Its mass can and has been measured indirectly from the many stars that are in close and fast orbit around it. Given the orbit size and speed there's no way something other than a black hole fits in there.

Black Hole at the Galactic Core

http://www.youtube.com/watch?v=fytriKJ8xhE

Black hole evidence in the centre of our galaxy

http://www.youtube.com/watch?v=r3qSr5HmGkI

Yes but where is this hole goin, if something is falling in it's travelling through space within the hole

Is this space that matter falls into the same space you an i and the rest of the universe is in or is within the black hole not 'normal space'

Hornblower

2010-Jan-13, 11:30 PM

Yes but where is this hole goin, if something is falling in it's travelling through space within the hole

Is this space that matter falls into the same space you an i and the rest of the universe is in or is within the black hole not 'normal space'

If something falls in I would expect it to settle in the center. It is here that what physicists call the spacetime continuum, or spacetime for short, is so strongly "warped" that our mathematical methods of physics break down and we have the mathematical predicament commonly called a singularity.

Our cutting-edge physicists are attempting to develop a unification of quantum mechanics and general relativity that just might resolve the singularity and predict the actual conditions at the center of a black hole.

cosmocrazy

2010-Jan-14, 12:18 AM

Yes but where is this hole goin, if something is falling in it's travelling through space within the hole

I'm not sure "hole" in the classical sense is the correct terminology for what is at the centre of a BH. It has been speculated that the actual fabric, as such, of space-time might well be warped so much it is torn. But again i'm not sure if this would be what mainstream physicists believe. I think the simple fact is that from appearance it would reassemble a bottom less hole since anything being pulled towards it would seem to be falling forever into a black abyss.

Is this space that matter falls into the same space you an i and the rest of the universe is in or is within the black hole not 'normal space'

One of the strange things to imagine about a BH is that it appears that the math predicts there to be a singularity at the centre of a BH. A point where spacetime doesn't exist as we know it. All the matter and energy going into the BH occupies a point of no volume. To us the only way we can imagine this, is that at this point it just blinks out of existence. But we know this cannot be true due to conservation laws and also that the BH radiates a gravitational field and has an event horizon radius, proportional to its mass.

steve_bnk

2010-Jan-14, 01:13 AM

Infinite density does not imply infinite energy.

As I said above, my understanding is that the density of the matter forming

a black hole approaches infinite with time, but never reaches infinite because

the matter is always "deepening" the "gravity well". Spacetime is becoming

more and more stretched, without limit, but it would take forever to become

infinitely deep.

-- Jeff, in Minneapolis

That is as I said, singularites in reality never become infinite. A true singularity in the form of a black hole would take infinte energy.

Jeff Root

2010-Jan-14, 05:06 AM

Do we know black holes actully have holes within them ?

As noncryptic said, a black hole is a hole; it doesn't have

a hole within it. It is a hole in the sense that stuff can go into the

hole and disappear from sight. John Wheeler thought for a long time

about what to call such an intense gravity well before he came up

with the name "black hole".

And if so where is this hole in space can it be measured

One of the most useful things to learn in science is how to ask a

good question. These are remakably poor questions. It isn't at all

clear what you are trying to ask.

Nothing inside a black hole can be measured from the outside.

However, a black hole has certain properties that are left over

from the matter which collapsed to form it, which can be measured

from the outside. Those properties include mass, angular moment,

electric charge, and magnetic moment. A black hole also has certain

properties of its own, which can also be measured from the outside,

including temperature.

If you fell into a black hole you would not get a much better view

of what is happening to it than you would from the outside. You

could directly measure the gravitational tidal forces inside the event

horizon, but the matter at the black hole's center would still be

invisible to you because light from it could not reach you.

Black holes were predicted over a century ago, from Newtonian

mechanics. Evidence for the existence of black holes did not become

clear until the last couple of decades, when supermassive black holes

having event horizons with diameters comparable to the diameter of

the Solar System have been discovered at the center of many large

galaxies.

-- Jeff, in Minneapolis

Jeff Root

2010-Jan-14, 05:18 AM

Infinite density does not imply infinite energy.

As I said above, my understanding is that the density of the matter forming

a black hole approaches infinite with time, but never reaches infinite because

the matter is always "deepening" the "gravity well". Spacetime is becoming

more and more stretched, without limit, but it would take forever to become

infinitely deep.

That is as I said, singularites in reality never become infinite. A true

singularity in the form of a black hole would take infinte energy.

No, it would just take infinite time. The amount of energy doesn't

determine the depth of the gravity well. The amount of time passed

since the black hole formed determines the depth of the gravity well.

-- Jeff, in Minneapolis

Jeff Root

2010-Jan-14, 05:31 AM

Yes but where is this hole going, if something is falling in it's travelling

through space within the hole

Spacetime is being stretched by the collapsing matter. If you insist

on having a "direction" in which the gravity well is deepening, it is

forward in time. The space containing the matter is being converted

to time.

Something similar happens with all matter, to a much lesser extent.

A tiny amount of the space that the Earth is in is converted to time

by the presence of the large quantity of Earth's matter. The result

is Earth's gravity. You feel your bottom being pressed against the

seat of your chair because Earth's matter distorts spacetime in such

a way as to convert a little bit of space into a little bit of time.

Is this space that matter falls into the same space you and i and the

rest of the universe is in or is within the black hole not 'normal space'

It is basically just more intensely distorted, but otherwise the same.

-- Jeff, in Minneapolis

filrabat

2010-Jan-14, 06:05 AM

If you ask me, black holes have a potentially infinite density, though not an actual infinite one. If black holes did have an actual density of infinity, then shouldn't the strength of its gravitational field likewise be infinitely strong in all locations in the universe - or at least the observable universe from the black hole's locational perspective? I mean, what is Infinity minus One but an imaginary number (for how can you subtract from infinity if you can't add to it?)

For that reason, I think a black hole as an immense but finite density, for everything else outside the event horizon experiences the black holes' gravitational force to a finite extent, otherwise we would be sucked into the black hole as well.

cjameshuff

2010-Jan-14, 06:32 AM

Cygnus X-1 is one example of some direct evidence of a black hole. It has an accretion disk and matter is constantly falling into the central mass, but there's no sign of it hitting any sort of surface. If it were a neutron star or similar high-density object, the matter would splash quite noisily against the surface, while a black hole has no solid surface to impact. This might answer some of luckyfrank's questions...I agree with Jeff that they are particularly poorly framed questions, though. The questions themselves assume things about black holes that are incorrect, and so are impossible to simply answer.

For that reason, I think a black hole as an immense but finite density, for everything else outside the event horizon experiences the black holes' gravitational force to a finite extent, otherwise we would be sucked into the black hole as well.

Gravity is dependent on mass, not density. The fact that we aren't pulled into the nearest black hole says nothing about the densities within that black hole. I agree that we'll probably eventually find it is finite and figure out better ways to describe the physics in the area of what is a singularity in current models, but your reasoning for why is faulty.

Jeff Root

2010-Jan-14, 06:33 AM

filrabat,

That is exactly what I was saying is NOT the case, when I said "Infinite

density does not imply infinite energy."

The strength of the gravitational field outside the body of matter does not

change as the matter collapses to a black hole. Suppose that a big star

suddenly collapsed, and all the matter fell to the center in a few seconds,

so that the matter was completely compressed to infinite density just a

few seconds after the collapse began, and no matter was thrown out.

Planets and spacecraft orbiting the star would continue to orbit exactly

as they had been. There would be no change at all in the gravitational

field outside the star. Only the space that had been inside the star gets

"deeper" as the star collapses.

Using the rubber sheet model, you have a large, heavy ball lying on the

rubber sheet, stretching it downward. The ball stays the same weight,

but shrinks in size, so it becomes more and more dense. The weight is

placed on a smaller and smaller area of the rubber sheet, causing that

part of the sheet to stretch downward more and more. But every part

of the sheet that the ball is not pressing down on stays stretched by the

same amount forever afterward, nomatter how much the ball continues

to shrink and increase in density, and nomatter how far the part of the

sheet under the ball is stretched down.

-- Jeff, in Minneapolis

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