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The strange fate of a person falling into a black hole

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Re: The strange fate of a person falling into a black hole

#111

Post by CookieMon »

monocasa wrote: post #108
CookieMon wrote: post #107

Earlier quoted context omitted.

I wouldn't think it could matter to an outside observer whether the mass inside an event horizon had converged to a point of infinite density or not, there just needs to be enough mass collapsed to an area small enough to create an event horizon. If the outside observer were to cross the event horizon to check whether there was a singularity in the middle, they'd approach the same frame of reference, no longer be aff…

Sure, it doesn't matter really to an outside observer, I just threw that in to hit home the time dilation aspect. You seem to be assuming a preexisting singularity. My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line?

No I'm not assuming a preexisting singularity, I'm assuming (possibly incorrectly) that you needn't reach infinite density for an event horizon. i.e. a singularity doesn't need to have formed, and you couldn't tell whether one had anyway.

(My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wouldn't change despite density not being infinite. However that thought experiment is newtonian thinking, not GR, so perhaps flawed)

Edit: I assume "singularities" are a red herring here and you're also thinking that as the mass approaches whatever density is required for an event horizon, time slows to zero for the outside observer so no event horizon happens either, but that would leave mass on and around the smaller-or-proto-event-horizon so you get the situation from the link guard-of-terra posted "He also said that the black-hole-with-an-essentially-undetectable-object-just-outside-its-event-horizon is a very good approximation to a black hole of a slightly larger mass.". So perhaps for outside observers, black holes are just very good approximations of black holes - unless you go in for a closer look, in which case you find they are real black holes.

Re: The strange fate of a person falling into a black hole

#112

Post by PuffinBlue »

Thrymr wrote: post #109
PuffinBlue wrote: post #43

I always assumed the differing 'views' of what would happen crossing into a black hole was a product of the interplay of the equivalence principle and 'relativity of simultaneity' that Special/General relativity already accounts for. I had thought that upon reaching the event horizon the outside observer sees time 'stand still' because no light from your continued motion the other side of the event horizon can reach…

> Is this line of reasoning incorrect - I'm assuming it is - why? You're trying to reason about black holes with special relativity, rather than general relativity. Special relativity explicitly ignores acceleration (and equivalently, gravitation). You mention the equivalence principle, which is relevant, but "spacetime curtavure of velocity c" makes no sense, because the curvature leads to acceleration, not velocity…

Well that was sort of the point really, I was seeing how special relativity's relativity of simultaneity would work in an accelerated reference frame from the point of view of both observers.

Intuitively (dangerous I know) it seemed that just because you're accelerating, that doesn't mean relativity of simultaneity wouldn't apply.

I assumed the equivalence principle would be relevant because thinking about simulaneity of relativity in an accelerated reference frame (like an accelerating train at high c) would mean any outcome of the thought experiment would likely be transferable to relativity of simultaneity in a gravitational field, seeing as path through curved spacetime and acceleration is 'equivalent' (with caveats).

Spacetime curtavure of velocity c - this is clumsy language but c behaves differently right? It's a constant, so unlike a helicopter hovering above the earth, spacetime's curvature must 'equal' c at the event horizon i.e. cause it to travel on a curved path back towards the singularity, or at least cause it to orbit on the event horizon.

In this sense I was thinking about spacetime itself as the thing that was accelerating and the light was stationary at the event horizon, which is just a mental analogue really and likely unhelpful.

Re: The strange fate of a person falling into a black hole

#114

Post by digsmahler »

btilly wrote: post #97
digsmahler wrote: post #89

Earlier quoted context omitted.

Are there such coordinate systems that "describe stuff falling bast the event horizon in finite time"? How does that not violate general relativity?

To the first, yes. For example http://en.wikipedia.org/wiki/Lema%C3%AEtre_coordinates is one. To the second, when we talk about "coordinates" in GR, we're talking about parametrizing a volume of space-time. Given one parametrization, it is possible to create another parametrization that blows up at any particular spot - not because space-time is weird there - but because you picked a bad coordinate system. The existe…

Thank you! This will require some study.

Re: The strange fate of a person falling into a black hole

#115

Post by dragonwriter »

monocasa wrote: post #108
CookieMon wrote: post #107

Earlier quoted context omitted.

I wouldn't think it could matter to an outside observer whether the mass inside an event horizon had converged to a point of infinite density or not, there just needs to be enough mass collapsed to an area small enough to create an event horizon. If the outside observer were to cross the event horizon to check whether there was a singularity in the middle, they'd approach the same frame of reference, no longer be aff…

Sure, it doesn't matter really to an outside observer, I just threw that in to hit home the time dilation aspect. You seem to be assuming a preexisting singularity. My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line?

> My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line?

Isn't it just in the future of every particle within the event horizon (perhaps arbitrarily far from the particle's perspective, and necessarily infinitely far from any outside observer's perspective.)

Re: The strange fate of a person falling into a black hole

#116

Post by monocasa »

CookieMon wrote: post #111
monocasa wrote: post #108

Earlier quoted context omitted.

Sure, it doesn't matter really to an outside observer, I just threw that in to hit home the time dilation aspect. You seem to be assuming a preexisting singularity. My question is how does that singularity get there if the compaction itself that would form the singularity continually pushes the goal post down the line?

No I'm not assuming a preexisting singularity, I'm assuming (possibly incorrectly) that you needn't reach infinite density for an event horizon. i.e. a singularity doesn't need to have formed, and you couldn't tell whether one had anyway. (My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wo…

No, singularities aren't a red herring here, they are very muchthe core of what I'm asking. I don't really care that much about the event horizon (at least for this specific question : ) ).

Imagine two neutron stars colliding and forming a black hole. Now imagine the particle inside one of these that's closest to where the singularity's point would be upon formation. How does that singularity form given that the actual compaction that creates it also continually keeps pushing the goal post farther away due to gravitational time dilation.

Re: The strange fate of a person falling into a black hole

#117

Post by repsilat »

dragonwriter wrote: post #100
logfromblammo wrote: post #78

Earlier quoted context omitted.

Reality only splits in two in the sense that the observer-dependent reality of the person falling into the black hole and the observer-dependent reality of the person remaining outside of it can no longer exchange information anywhere except inside of the black hole's event horizon. The half of your light cone that extends into the future remains entirely within that event horizon. The outside observer's cone can rem…

> There might even be a sort of Zeno's Paradox inside, where once you cross the event horizon, there's another event horizon beyond that, such that anything that much closer to the singularity than you are can never communicate with you, in the same way that you can never communicate with the universe outside the first event horizon. Isn't every step in from the event horizon effectively an event horizon? If all the…

I don't think that's quite what they meant by that.

In silly "outside universe coordinates", inside the black hole you can define a "minimum inwards speed" as a function of distance from the singularity. This is how fast a photon will go towards the singularity if fired directly away from it. It's zero at the event horizon, and it increases as you go in.

Now, what (I think) logfromblammo was saying was, "If I'm at the event horizon, can there be someone close enough to the singularity that I can never catch up?"

Communication isn't much of a problem for two people falling into a black hole one after another if they're close enough. The messages from the person lower down don't "go up", they just go down more slowly than the person higher up. If they're too far apart, though, this might not work.

Re: The strange fate of a person falling into a black hole

#118

Post by CookieMon »

monocasa wrote: post #116
CookieMon wrote: post #111

Earlier quoted context omitted.

No I'm not assuming a preexisting singularity, I'm assuming (possibly incorrectly) that you needn't reach infinite density for an event horizon. i.e. a singularity doesn't need to have formed, and you couldn't tell whether one had anyway. (My thinking is that if the mass of the singularity was redistributed evenly into a small sphere in the center of the black hole, the gravitational strength around the black hole wo…

No, singularities aren't a red herring here, they are very muchthe core of what I'm asking. I don't really care that much about the event horizon (at least for this specific question : ) ). Imagine two neutron stars colliding and forming a black hole. Now imagine the particle inside one of these that's closest to where the singularity's point would be upon formation. How does that singularity form given that the actu…

Is there any time dilation in the same reference frame as where the singularity is forming?

(I'm running on rules of thumb rather than a genuine understanding of GR: I would normally answer "no", but "frame of reference" might not be a useful concept where a singularity is forming)

It is kinda neat that the only time where a singularity has formed is contained within the place where the singluarity formed. That isn't even yo dawg.

Re: The strange fate of a person falling into a black hole

#119

Post by maxerickson »

ars wrote: post #91
monocasa wrote: post #77

So I've had a question about stellar mass black holes that I can't seem to be able to get answered (the question was deleted for /r/askscience). How does a stellar mass black hole actually form a singularity given the fact that there's gravitational time dilation? Wouldn't the heart of a black hole slow down relative to an outside observer as mass is added to it so that you never actually reach infinite density? It s…

I reached the same conclusion years ago, and the answer is very simple (and controversial): Black holes do not exist. If they existed they could exist, but it's impossible for them to form. I've asked many many physicists this very same question, and none could answer me (or I've gotten hand-wavy non answers). It's like no one wants to be the first to declare this well accepted thing as non-existing. The black holes…

Astrophysicists aren't insisting that black holes have singularities on the insides (just speculating that there are objects that are well described as a gravity well with an event horizon).

Re: The strange fate of a person falling into a black hole

#120

Post by 4ad »

monocasa wrote: post #77

So I've had a question about stellar mass black holes that I can't seem to be able to get answered (the question was deleted for /r/askscience). How does a stellar mass black hole actually form a singularity given the fact that there's gravitational time dilation? Wouldn't the heart of a black hole slow down relative to an outside observer as mass is added to it so that you never actually reach infinite density? It s…

This is a simple explanation: http://motls.blogspot.co.at/2008/11/why-can-anything-ever-fa...
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