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Old 10-13-2016, 02:38 AM
Zerc Zerc is offline
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Does a black hole have to have a point-like singularity at its core?

It seems to me that everything I read about black holes mentions that at the centre there is an infinitely dense one-dimensional point called a singularity.

As far as my basic understanding goes - when a suitable massive star reaches its end of life, gravity forces all its electrons, protons and neutrons to fuse together causing a neutron star. If the star was too massive to begin with these neutrons will then be forced together by gravity into a infinite density causing a singularity and a black hole.

What would happen if there was something type of matter that was denser than neutrons so that a star could form that had an escape velocity greater than the speed of light. Could we not have a black hole that had a solid surface somewhere inside the event horizon?

Is there any reason that such a dense form of matter could not theoretically exist? Is it because of the speed of light/space-time curvature/something else that anything inside the event horizon must end up in a singularity.

A possible alternative way of looking would be - what if the speed of light was only 10km/s wouldn't the earth be a black hole but with a solid surface inside the event horizon?

Thanks
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Old 10-13-2016, 07:50 AM
Chronos Chronos is offline
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There are postulated forms of matter even denser than neutron stars, resulting in even smaller objects called quark stars.

But for there to be anything non-point inside of a black hole would defy all that we know of physics. Simply put, matter fundamentally cannot be that strong. It's beyond even being a matter of strength: We're into comic-book-Superboy-changing-reality-by-punching-the-fabric-of-space territory, here.

Which is not to say that our understanding of physics can't be wrong. Most attempts at a theory of quantum gravity predict that there's something of very small (but nonzero) size in there. But we can't yet get any theory of quantum gravity to work, so that's all just speculation.
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Old 10-13-2016, 08:16 AM
Zerc Zerc is offline
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I'm not disagreeing with you when you say that matter cannot be that strong but my (simplistic) train of thought is that if there was something theoretically twice as strong/hard as neutrons then this star would be twice(?) as small as a neutron star. If we then had something twice as strong/hard as this new material then that star would be smaller and so on. It would then seem that at some stage something would be hard enough to form a star where the escape velocity is greater than c and then we have my black hole with a surface.

Where is my thinking incorrect? Is it the fact that the escape velocity is greater than c that means this matter cannot exist and if so, does that it would be theoretically possible we could have a sphere where the escape velocity is 0.99999999c just not one where it is c?

Last edited by Zerc; 10-13-2016 at 08:17 AM. Reason: Terrible spelling
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Old 10-13-2016, 08:37 AM
Zerc Zerc is offline
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Just thinking a bit about this - is this because the entire volume of the star inside the black hole is going to end up in the singularity because of the switching of space and time inside a black hole?
The same way the singularity is in the future inside a black hole just as Tuesday is in my future and also just as inevitable?
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Old 10-13-2016, 03:17 PM
Chronos Chronos is offline
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Just thinking a bit about this - is this because the entire volume of the star inside the black hole is going to end up in the singularity because of the switching of space and time inside a black hole?
The same way the singularity is in the future inside a black hole just as Tuesday is in my future and also just as inevitable?
Yup, that's it exactly.

To your other question, there's no fundamental reason why you couldn't have a substance or object that's just barely shy of becoming a black hole-- In fact there has been some serious scientific speculation about just such objects, usually called "gravatars". Most mainstream physicists consider the idea absurd, because it just requires far too much fine-tuning and hand-waving, but it's not completely in the realm of crackpottery.
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Old 10-13-2016, 04:20 PM
Pantastic Pantastic is offline
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Also, there's a significant number of physicists who think that a black hole doesn't actually collapse to a singularity, that a quantum theory of gravity will show that some other effect intervenes. The problem is that we don't have a working quantum theory of gravity yet, so this is pretty much just speculation. For example Loop Quantum Gravity, one proposed theory, predicts that a star would start to collapse to a black hole, then quantum gravity effects would cause it to stop and bounce back, but because of time dilation the bounce would take too long for us to observe directly. http://www.nature.com/news/quantum-b...xplode-1.15573
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Old 10-14-2016, 12:36 AM
Zerc Zerc is offline
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I must say that a lot of things to do with black holes and relativity seem very weird/counter-intuitive to me but that's obviously my problem. The one thing that really seems impossible to me is how can the singularity be infinitely dense? It just seems "obvious" () to me that would have to be a certain finite density. Loop quantum gravity mentioned in the article by Pantastic seems like at least a nice alternative

Still very interesting to think about and try to understand even if I realize my understanding of these topics is only skin deep/

Thanks for the answers.
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Old 10-14-2016, 01:59 AM
Mangetout Mangetout is offline
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Originally Posted by Zerc View Post
The one thing that really seems impossible to me is how can the singularity be infinitely dense? It just seems "obvious" () to me that would have to be a certain finite density.
Density is a function of mass and volume - if the object has zero dimensions, and any mass, the density has to be... well... incalculable - it's a divide-by-zero problem, I think.

What's less intuitive to me personally is how two of them can have different masses when their density is infinite and their size is the same - but I guess that's probably because 'infinite' itself isn't an intuitive concept, and 'the same' probably doesn't apply here.
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Old 10-14-2016, 02:25 AM
Derleth Derleth is offline
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Density is a function of mass and volume - if the object has zero dimensions, and any mass, the density has to be... well... incalculable - it's a divide-by-zero problem, I think.
It's exactly a divide-by-zero problem. That's why it's called a singularity, after all: In mathematics, a singularity is a point where a function is either undefined or "blows up" or becomes ill-behaved in some way. As a trivial example, 1/x is singular for x = 0.

So a singularity in a black hole is a point where our functions to describe reality blow up: Finite mass in zero volume is a divide-by-zero moment. In a deep sense, the fact we have a singularity is more of a reflection of our ignorance than any physical reality, because the physical world makes sense. The physical world doesn't have any nonsensical parts, just parts we haven't learned how to make sense of yet.

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What's less intuitive to me personally is how two of them can have different masses when their density is infinite and their size is the same - but I guess that's probably because 'infinite' itself isn't an intuitive concept, and 'the same' probably doesn't apply here.
This, actually, makes perfect sense: Fundamental conservation laws must hold, so as long as the black hole didn't emit a huge burst of energy or get a massive influx of matter or something, it has to have approximately the same mass as the collapsing degenerate matter which formed it. Anything else would do truly nasty things to the logical consistency of our theories.
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Old 10-14-2016, 02:31 AM
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How in the world would infinite curvature and a singularity actually look in 4D space? The singularity would be at what time? Time = infinite?

How does that reconcile with the fact that black holes seem to radiate? If a black hole radiates x amount of mass-energy at t = y wouldn't that imply that the mass distribution would reflect that?

At the instant right before y the black hole is mass M+x and at the instant y the black hole is now mass M how can the components of the black hole be chilling at the singularity?

Plus how can you argue with Stephen Hawking? http://www.nature.com/news/stephen-h...-holes-1.14583
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Old 10-14-2016, 03:58 AM
Mangetout Mangetout is offline
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This, actually, makes perfect sense: Fundamental conservation laws must hold, so as long as the black hole didn't emit a huge burst of energy or get a massive influx of matter or something, it has to have approximately the same mass as the collapsing degenerate matter which formed it. Anything else would do truly nasty things to the logical consistency of our theories.
Sure - the counterintuitive part is how a massive singularity and a less massive singularity can have different masses, but be the same size, and both be infinite density - i.e. 'the same' density (I know there are different sizes of infinity, but I don't think these are those, although this could well be a Hilbert case of infinity+1 still being infinite, and the same size of infinite), but it goes back to the collapse of function - they're only 'the same' because the result of any divide-by-zero is 'the same'.
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Old 10-14-2016, 08:55 AM
Chronos Chronos is offline
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Quoth octopus:

How in the world would infinite curvature and a singularity actually look in 4D space? The singularity would be at what time? Time = infinite?
That depends on what reference frame you're measuring time in. To anyone inside of a black hole, the singularity is the future, and arrives after all too short a time.

The usual way of representing black holes in 3+1 dimensions is via Penrose diagrams.
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Old 10-14-2016, 12:21 PM
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Maybe someone could explain this to me. From the POV of an observer falling into a black hole, because of time dilation, the rest of the universe around you would speed up so much that the entirety of the universe's life would play out while you're still falling. Doesn't that mean that from our POV outside the black hole, that the things falling in never finish falling in?


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Also, there's a significant number of physicists who think that a black hole doesn't actually collapse to a singularity, that a quantum theory of gravity will show that some other effect intervenes. The problem is that we don't have a working quantum theory of gravity yet, so this is pretty much just speculation.
I've always favored the idea that there will be no relativistic singularity but will be some weird quantum state. Sure, it's speculation, but so is the explanation of the singularity!
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Old 10-14-2016, 02:43 PM
Chronos Chronos is offline
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Doesn't that mean that from our POV outside the black hole, that the things falling in never finish falling in?
Yes, but the point is moot, because you'll very quickly receive the last photon from the poor sod falling in.
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Old 10-14-2016, 02:55 PM
Asympotically fat Asympotically fat is offline
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Penrose proved that, in general relativity with the assumption of non-negative mass density, there is always a singularity where there is a black hole. However there is nothing to say that a singularity must be "point-like" or "ring-like" (as in the case of a Kerr black hole) or any designation that suggests it is somewhat similar to a spatial location/shape.
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Old 10-14-2016, 03:00 PM
Asympotically fat Asympotically fat is offline
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Maybe someone could explain this to me. From the POV of an observer falling into a black hole, because of time dilation, the rest of the universe around you would speed up so much that the entirety of the universe's life would play out while you're still falling.
This is not true. If you take Schwarzschild example: for a given observer crossing the event horizon at a certain time there's a very definite and finite limit as to how far in the future of the black hole exterior they will be able to see before they hit the singualrity.

Last edited by Asympotically fat; 10-14-2016 at 03:00 PM.
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Old 10-14-2016, 03:07 PM
Asympotically fat Asympotically fat is offline
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This is not true. If you take Schwarzschild example: for a given observer crossing the event horizon at a certain time there's a very definite and finite limit as to how far in the future of the black hole exterior they will be able to see before they hit the singualrity.
Actually that's not strictly true as if, once they hit the event horizon, the observer travels arbitrarily close to the speed of light radially outwards from the black hole they can observe arbitrarily far in the future. However at the moment an observer hits the singularity they will not be witnessing the infinite future of the exterior.
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Old 10-14-2016, 03:44 PM
wolfpup wolfpup is online now
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This is not true. If you take Schwarzschild example: for a given observer crossing the event horizon at a certain time there's a very definite and finite limit as to how far in the future of the black hole exterior they will be able to see before they hit the singualrity.
What the poster is struggling with, as am I, is some kind of intuitive explanation for the apparent paradox between the "frozen in time" consequence of infinite time dilation and the fact that according to everything we know both empirically and theoretically, matter does fall into black holes in a pretty straightforward Newtonian fashion, creating powerful accretion disks in the process, and black holes become more massive accordingly.

My attempts to gain an intuitive non-mathematical insight into this apparent paradox may or may not be sound, so I invite appropriate criticism. They have centered around the idea that this apparent freezing due to infinite time dilation is merely an artifact that arises from the use of inappropriate coordinates and implicit but incorrect assumptions about simultaneity. The Schwarzschild solution to the Einstein field equations for a non-rotating black hole (and the equivalent Kerr solution for a rotating one) are valid as one approaches the event horizon and can be used to show the aforementioned switching of the time dimension with the radial spatial dimension beyond the EH, but it's ill-behaved at the EH itself, where it blows up and appears to show the time dilation factor tending to infinity as the distance to the EH tends to zero. However under the appropriate geometries and coordinate systems the spacetime curvature at the EH can be shown to be well-behaved and finite.

There seem to be multiple different ways of trying to grasp this intuitively. One way is that from the perspective of an external observer, time dilation really does freeze the infalling object -- in space. But the Schwarzchild coordinate system that leads to this conclusion also leads to the conclusion that space itself is rapidly flowing into the black hole, exceeding the speed of light beyond the EH (which space is allowed to do, as distinct from objects in it), where time and the spatial dimension pointing to the singularity have correspondingly switched places and the singularity becomes the object's future.

At no point would you actually see the intrepid astronaut frozen forever at the EH (he'd be red-shifted out of detectability anyway), but more importantly, I'm suggesting that the idea that the astronaut would forever be at rest at the EH as seen by an external observer seems to stem from a misapplication of the Schwarzchild solution.
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Old 10-14-2016, 03:45 PM
markn+ markn+ is online now
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Doesn't that mean that from our POV outside the black hole, that the things falling in never finish falling in?
Yes, exactly. In fact from outside the hole, we never even see the hole actually form in the first place, because the collapsing matter seems slow down and never reach the event horizon. That's why the Russian name for "black hole" translates as "frozen star". (Also because "black hole" is a moderately obscene phrase in Russian.)

But as Chronos says, this is theoretical. As a practical matter, the light from the infalling matter dims to invisibility in a finite amount of time.

--Mark
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Old 10-14-2016, 06:03 PM
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Yes, exactly. In fact from outside the hole, we never even see the hole actually form in the first place, because the collapsing matter seems slow down and never reach the event horizon. That's why the Russian name for "black hole" translates as "frozen star".
And yet obviously black holes form, since we see evidence of lots of them around, just like we have lots of indirect evidence and theoretical descriptions of matter falling into them, in perfect conformance with Newtonian physics. A star that remains much above the Chandrasekhar limit (and, more specifically, above the Landau-Oppenheimer-Volkoff limit for one that has collapsed into a neutron star) will collapse into a black hole. Few would argue otherwise.

I would argue with your premise, however. As I tried to suggest above, despite the Russian name and the illusion of slowdown to a complete halt of an infalling object, the "freezing" really is an illusion. What we see as the wavelength gets increasingly red-shifted and the photon arrival increasingly longer does not reflect what is happening locally.
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Old 10-15-2016, 12:08 PM
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Yes, of course in the frame of the infalling matter, it reaches the event horizon and indeed reaches the singularity in a finite time. I'm not fully understanding your objection -- are you just unable to accept that different observers experience different proper times between two events? If a person holding a clock falls into a black hole, he will of course see his clock ticking normally. According to General Relativity, an external observer would see the clock slow down more and more as it approaches the horizon. Do you accept that?

--Mark
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Old 10-15-2016, 07:45 PM
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Yes, of course in the frame of the infalling matter, it reaches the event horizon and indeed reaches the singularity in a finite time. I'm not fully understanding your objection -- are you just unable to accept that different observers experience different proper times between two events? If a person holding a clock falls into a black hole, he will of course see his clock ticking normally. According to General Relativity, an external observer would see the clock slow down more and more as it approaches the horizon. Do you accept that?

--Mark
Of course. I'm not arguing against the different frames of reference. I'm arguing against what I interpreted -- perhaps incorrectly -- to be your implication that matter can never actually fall into a black hole. If that were true, singularities and black holes as we understand them could never form in the first place. But certainly the predominant scientific consensus is that it can, and they do.

This is as good an article as I've found trying to explain this apparent paradox. The key quote:
If we program a space probe to fall freely until reaching some randomly selected point outside the horizon and then accelerate back out along a symmetrical outward path, there is no finite limit on how far into the future the probe might return. This sometimes strikes people as paradoxical, because it implies that the in-falling probe must, in some sense, pass through all of external time before crossing the horizon, and in fact it does, if by "time" we mean the extrapolated surfaces of simultaneity for an external observer. However, those surfaces are not well-behaved in the vicinity of a black hole. It's helpful to look at a drawing like this.

This illustrates schematically how the analytically continued surfaces of simultaneity for external observers are arranged outside the event horizon of a black hole, and how the in-falling object's worldline crosses (intersects with) every timeslice of the outside world prior to entering a region beyond the last outside timeslice. The timeslices have the form
tj − T = 2m ln(r/2m − 1)
where T is the (inward) Eddington-Finkelstein time coordinate. We just repeat this same shape, shifted vertically, up to infinity. Notice that all of these infinitely many time slices curve down and approach the same asymptote on the left. To get to the "last timeslice" an object must go infinitely far in the vertical direction, but only finitely far in the horizontal (leftward) direction.

The key point is that if an object goes to the left, it crosses every single one of the analytically continued timeslice of the outside observers, all the way to their future infinity. Hence those distant observers can always regard the object as not quite reaching the event horizon (the vertical boundary on the left side of this schematic). At any one of those slices the object could, in principle, reverse course and climb back out to the outside observers, which it would reach some time between now and future infinity. However, this doesn't mean that the object can never cross the event horizon. It simply means that its worldline is present in every one of the outside timeslices. In the direction it is traveling, those time slices are compressed infinitely close together, so the in-falling object can get through them all in finite proper time (i.e., its own local time along the worldline falling to the left in the above schematic).
Another quite interesting intuitive explanation from the same site is found here. Interpreted strictly in terms of Schwarzchild coordinates, one can regard an infalling object as taking infinitely long to pass the event horizon, and then (recalling what happens to the time dimension beyond the EH in the Schwarzchild solution) traveling back in time to the present in Schwarzchild coordinate time as it progresses from the EH to the singularity, yielding a net transit time to the inevitable singularity as seen by an external observer to be not much different from its own proper time. I have no idea if this interpretation makes physical sense, but not much beyond the event horizon really does. If we launch a massive object into a black hole and very soon observe that the radius of its event horizon has grown, then something like this must be true.
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Old 10-15-2016, 09:35 PM
billfish678 billfish678 is offline
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Only somewhat related...but apparently some folks actually saw a star turn into a black hole...sorta...

http://www.mnn.com/earth-matters/spa...ole-first-time
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Old 10-15-2016, 10:08 PM
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That's not just tangential, it's actually extremely relevant to the above conversation. Perhaps more knowledgeable posters than I will want to comment, but ISTM that if the interpretation is correct and this is what they think it is -- namely that a dying star well beyond the Chandrasekhar limit has suddenly disappeared -- then it's proof positive that matter can be observed to fall into a black hole in a short time from a frame of reference at an effectively infinite distance outside the gravity well. The only way a massive collapsing star can suddenly disappear is if it is engulfed by its own event horizon.
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Old 10-16-2016, 08:15 AM
Asympotically fat Asympotically fat is offline
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Originally Posted by wolfpup View Post
What the poster is struggling with, as am I, is some kind of intuitive explanation for the apparent paradox between the "frozen in time" consequence of infinite time dilation and the fact that according to everything we know both empirically and theoretically, matter does fall into black holes in a pretty straightforward Newtonian fashion, creating powerful accretion disks in the process, and black holes become more massive accordingly.

My attempts to gain an intuitive non-mathematical insight into this apparent paradox may or may not be sound, so I invite appropriate criticism. They have centered around the idea that this apparent freezing due to infinite time dilation is merely an artifact that arises from the use of inappropriate coordinates and implicit but incorrect assumptions about simultaneity. The Schwarzschild solution to the Einstein field equations for a non-rotating black hole (and the equivalent Kerr solution for a rotating one) are valid as one approaches the event horizon and can be used to show the aforementioned switching of the time dimension with the radial spatial dimension beyond the EH, but it's ill-behaved at the EH itself, where it blows up and appears to show the time dilation factor tending to infinity as the distance to the EH tends to zero. However under the appropriate geometries and coordinate systems the spacetime curvature at the EH can be shown to be well-behaved and finite.

There seem to be multiple different ways of trying to grasp this intuitively. One way is that from the perspective of an external observer, time dilation really does freeze the infalling object -- in space. But the Schwarzchild coordinate system that leads to this conclusion also leads to the conclusion that space itself is rapidly flowing into the black hole, exceeding the speed of light beyond the EH (which space is allowed to do, as distinct from objects in it), where time and the spatial dimension pointing to the singularity have correspondingly switched places and the singularity becomes the object's future.

At no point would you actually see the intrepid astronaut frozen forever at the EH (he'd be red-shifted out of detectability anyway), but more importantly, I'm suggesting that the idea that the astronaut would forever be at rest at the EH as seen by an external observer seems to stem from a misapplication of the Schwarzchild solution.
The easiest way to see is from a Penrose diagram like the below:

https://pbs.twimg.com/media/BcxA5rGCQAA-98_.png

We can ignore the bottom (white hole interior) and left (parallel universe) regions of the diagram(s) as they are features of the extended solution.

In a Penrose diagram light always has worldlines at 45 degree angles to the vertical axis, and the worldlines of objects moving at less than the speed of light are always at an angle that is less than 45 degrees to the vertical axis.

It should be easy to see that nowhere on the BH singularity (upper zig-zag) is causally connected to the whole of the BH exterior region (right region), which means an observer hitting the singularity cannot see the entire future of the Universe played out before they hit the singularity.

The misapprehension that you are able see the entire future of the Universe before you hit the singularity comes from taking Schwarzschild coordinates (the dotted lines in the BH exterior region. NB as illustrated the lines do not mark out equal amounts of coordinate time or distance) too literally. Schwarzschild coordinates map all the events that lie on the event horizon to the same time coordinate* as timelike future infinity (i+), to which all events in BH exterior region are causally connected and in the past (therefore if you make it to i+ you are able to see every event that happened in the BH exterior region).

*Strictly speaking they don't because the Schwarzschild time coordinate goes to infinity at both the event horizon and at i+, so the problem is more from making faulty assumptions by incorrectly taking limits when the coordinates are singular.
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Old 10-16-2016, 06:08 PM
Chief Pedant Chief Pedant is offline
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Is there a way to easily explain the difference between the singularity at the center of a very massive black hole versus the singularity at the center of a small black hole? If both are considered infinitely dense, how is there a difference between them?
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Old 10-16-2016, 07:03 PM
Chronos Chronos is offline
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They have different masses. No, really, that's it.

Well, they could also have different electric or magnetic charges and/or different angular momentum, but that's independent of size.
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Old 10-16-2016, 09:44 PM
Enola Straight Enola Straight is offline
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Originally Posted by billfish678 View Post
Only somewhat related...but apparently some folks actually saw a star turn into a black hole...sorta...

http://www.mnn.com/earth-matters/spa...ole-first-time
It occurred to me; a giant star with a diameter of dozens of AUs...several lighthours...due to causality, the center of the star's core collapsing to singularity would not cause any immediate noticeable effect on the star's surface. I can imagine as the effect reaches the surface, the star would indeed wink out...unless the star is rotating, creating an accretion disk.


BTW...instead of an infinitesimal geometric point possesing infinite density, a singularity of one planck mass per cubic planck distance would create a non-"divide by zero" situation.

It also occurred to me, instead of a 3D tiny object, it could be a 4D object as per General Relativity, 11D as per M-Theory, 26D as per Bosonic String theory, or even a 248D as per Exceptionally Simple theory of Everything.
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Old 10-17-2016, 03:37 PM
markn+ markn+ is online now
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Perhaps more knowledgeable posters than I will want to comment, but ISTM that if the interpretation is correct and this is what they think it is -- namely that a dying star well beyond the Chandrasekhar limit has suddenly disappeared -- then it's proof positive that matter can be observed to fall into a black hole in a short time from a frame of reference at an effectively infinite distance outside the gravity well.
I don't think that really proves your point. As the matter falls into the hole, its time slows down from our perspective. It emits (more and more red-shifted) photons at a slower and slower rate, until the rate of photon emission becomes so slow that it effectively stops radiating, from an external perspective. The matter seems to have disappeared, but it's still emitting photons at a rate too slow to be easily detectible.

--Mark

Last edited by markn+; 10-17-2016 at 03:41 PM.
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Old 10-17-2016, 04:19 PM
Chronos Chronos is offline
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That's a peculiar definition of "still emitting photons". At a very short time after the object crosses the event horizon, the last photon from it will be received. No other photons, no matter how redshifted, will ever come out.
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Old 10-17-2016, 06:40 PM
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But I thought that nothing ever does cross the event horizon, from an external viewpoint. Infalling objects just asymptotically approach it. No?

--Mark
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Old 10-17-2016, 06:58 PM
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Originally Posted by Chronos View Post
That's a peculiar definition of "still emitting photons". At a very short time after the object crosses the event horizon, the last photon from it will be received. No other photons, no matter how redshifted, will ever come out.
Well, no photons or information specifically tied to that particular object that fell in. There's still predicted to be Hawking radiation, though as I understand the mechanism that starts off as particles, even though they annihilate and become photons nearly immediately. Photons are predicted to be emitted from the area of the black hole, at any rate.
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Old 10-17-2016, 09:41 PM
wolfpup wolfpup is online now
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Originally Posted by markn+ View Post
But I thought that nothing ever does cross the event horizon, from an external viewpoint. Infalling objects just asymptotically approach it. No?
Except that as per several different takes at intuitive explanations that were provided upthread, this is due to an incomplete interpretation of infalling objects only as seen in the Schwarzchild coordinates in which the event horizon appears to be a singularity, but it's only a coordinate singularity and not a physical one. If this was a complete interpretation, then as already noted no objects could ever fall into a black hole and black holes could never form.
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Originally Posted by Some Call Me... Tim View Post
Well, no photons or information specifically tied to that particular object that fell in. There's still predicted to be Hawking radiation, though as I understand the mechanism that starts off as particles, even though they annihilate and become photons nearly immediately. Photons are predicted to be emitted from the area of the black hole, at any rate.
Hawking radiation is unrelated to infalling ordinary matter, which AFAIK is believed to typically produce X-rays and gamma rays due to the extreme forces of tidal effects. Hawking radiation is usually described as being related to the black hole's interaction with the quantum vacuum -- with the absorption of one of the partners of the virtual particle pairs constantly appearing in the vacuum, and removing it from the universe before it can annihilate its partner.
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Old 10-18-2016, 12:41 AM
Some Call Me... Tim Some Call Me... Tim is offline
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Quote:
Originally Posted by wolfpup View Post
Hawking radiation is unrelated to infalling ordinary matter, which AFAIK is believed to typically produce X-rays and gamma rays due to the extreme forces of tidal effects. Hawking radiation is usually described as being related to the black hole's interaction with the quantum vacuum -- with the absorption of one of the partners of the virtual particle pairs constantly appearing in the vacuum, and removing it from the universe before it can annihilate its partner.
To continue my quibble, Hawking radiation is related to the mass of the black hole, which in turn is related to the amount of mass that has fallen in. As you add mass the amount of energy released per unit time will decrease, though of course over the truly long term the energy released before the hole evaporates will go up.

Last edited by Some Call Me... Tim; 10-18-2016 at 12:46 AM.
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Old 10-18-2016, 07:27 AM
Chronos Chronos is offline
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Quote:
Quoth wolfpup:

Hawking radiation is unrelated to infalling ordinary matter, which AFAIK is believed to typically produce X-rays and gamma rays due to the extreme forces of tidal effects.
It's not due to tidal effects. Very large black holes, such as those found in the centers of galaxies, have very small tidal effects at the horizon, and yet still often cause infalling matter to release prodigious amounts of energy. Mostly, it's just due to the fact that the vicinity of a black hole is often quite crowded.
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Old 10-18-2016, 08:42 PM
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Yes, that's more or less true, and thank you for the correction as it was wrong for me to imply that tidal effects are the sole cause of what are actually very complex phenomena.

However I would ask whether you don't agree it's true that tidal effects (i.e.- the amount of gravitational gradient near the EH), black hole rotation, and mass accretion rate are among the factors governing the behavior of the visible phenomena produced by infalling objects, like accretion disks and jets of ejected matter and energetic radiation.

For example, even for supermassive black holes, tidal effects are significant if the object in question is large enough, like a nearby star, leading to tidal disruption events that shred the star into an accretion disk. It's been suggested that a star falling into a sufficiently large ultramassive black hole might disappear with much less visible effect. By the same token, a much smaller object that was small relative to the gravitational gradient of the BH might fall through the EH more or less intact. I think perhaps the key here is that in such cases, most of the energy of the object's gravitational potential would vanish beyond the event horizon and would never be seen.

Perhaps somewhat nitpicky points, but I'm just suggesting that the gravitational gradient isn't by any means irrelevant, though you're absolutely right that it's not usually the primary cause of X-ray or gamma ray emission.
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Old 10-19-2016, 09:02 AM
Chronos Chronos is offline
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Yeah, tidal effects can shred stars into an accretion disk, but there are a lot of other effects that can give you an accretion disk, too. I suspect that, for a supermassive black hole, dynamical friction is the dominant one, though I'm not certain on that.
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Old 10-19-2016, 01:40 PM
Asympotically fat Asympotically fat is offline
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Originally Posted by Chronos View Post
Yeah, tidal effects can shred stars into an accretion disk, but there are a lot of other effects that can give you an accretion disk, too. I suspect that, for a supermassive black hole, dynamical friction is the dominant one, though I'm not certain on that.
TBH I don't think it is entirely known what causes the emissions from quasars, though it is undoubtedly related to accretion. It has been suggested a significant portion of the emission may be related to the Penrose effect, which is the classical limit of Hawking radiation.
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Old 10-19-2016, 02:20 PM
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You mean, that the accretion is spinning down the black hole? I've never heard that one. Wouldn't that require that the hole and its accretion disk somehow form with opposing angular momentum?
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Old 10-20-2016, 12:36 PM
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Originally Posted by Chronos View Post
You mean, that the accretion is spinning down the black hole? I've never heard that one. Wouldn't that require that the hole and its accretion disk somehow form with opposing angular momentum?
You're right in that the bog-standard kinematic Penrose effect is not really a feasible way (I believe) for a black hole to actually be radiating energy, the proposed method is the Blandford–Znajek process.
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Old 06-12-2017, 02:01 PM
Okrahoma Okrahoma is offline
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Originally Posted by markn+ View Post
That's why the Russian name for "black hole" translates as "frozen star". (Also because "black hole" is a moderately obscene phrase in Russian.)
Just saw this - that's incorrect. "Black hole" is translated exactly same into Russian.

See https://ru.wikipedia.org/wiki/%D0%A7...8B%D1%80%D0%B0
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