Is there an inside to a Black Hole?

[stevil]

TL;DR Summary

If time slows down as you approach the Event Horizon does it get so slow that time stops?

Disclaimer: I'm not a scientist, please excuse my ignorance if the following seems stupid.

Since time slows down as an object gets nearer to the EH, would an outside observer, living and observing for an almost infinite amount of time ever see an object/particle cross the event horizon threshold?

If we are to view the shape of the universe (SpaceTime) by observing the path of light we clearly see that the Universe is non euclidean, therefore it is a mistake to consider a 3D Euclidian coordinate system.

Energy/matter tends to spiral around a black hole rather than fall directly towards it.

Question: Is it possible that there is no inside? That SpaceTime doesn't exist and that what we perceive from the outside as spherical object isn't an object at all, but instead is the consequence on SpaceTime of all the energy and matter that is outside the EH, the accretion disk that is orbiting?

I guess I have considered this alternative because I don't like the idea of a point of singularity with infinite density (which seems absurd).
I understand in a closed universe that there is no outside of the universe, but that if you travel in a straight line you may end up back where you started (as if the universe were a sphere). So applying this concept of non Euclidean shape is it possible that this also applies to the black hole or specifically "what is inside the EH"?

I've searched around on the internet but have not found people to have explored this. Is it because this idea is too absurd and shows my naivety in physics and cosmology?

 

[phinds]

Summary:: If time slows down as you approach the Event Horizon

Time does NOT slow down as you approach the EH. That IS what is seen by a remote observer but it has no effect on the reality of the infalling person, who sees time progress at exactly the same pace (one second per second) that it does whether he's near a BH or not.

 

[stevil]

Time does NOT slow down as you approach the EH. That IS what is seen by a remote observer but it has no effect on the reality of the infalling person, who sees time progress at exactly the same pace (one second per second) that it does whether he's near a BH or not.

Yes, I understand this, that is why I was asking about the outside observer. Would they ever see this "infalling person" cross the threshold or would this infalling person appear to be stuck in time?

 

[phinds]

Yes, I understand this, that is why I was asking about the outside observer. Would they ever see this "infalling person" cross the threshold or would this infalling person appear to be stuck in time?

The outside observer can only see the infaller actually fall in if they wait around until the BH evaporates. Since this would take and amount of time that would make the current age of the universe effectively zero (a rounding error at about the 50th or 60th decimal place) it's not likely, no.

 

[stevil]

The outside observer can only see the infaller actually fall in if they wait around until the BH evaporates. Since this would take and amount of time that would make the current age of the universe effectively zero (a rounding error at about the 50th or 60th decimal place) it's not likely, no.

So from the observer's perspective if they can never see anything ever fall beyond the BH's EH, not even given an almost infinite amount of time, then how are we expected to accept that anything can ever go inside?
Why do we speculate about what is on the inside if it seems impossible for anything to go inside? (Impossible because there isn't enough time for this ever to happen).

Is it possible that there is no inside and that the black hole is just a consequence of all the stuff on the outside?

 

[DaveC426913]

Would they ever see this "infalling person" cross the threshold or would this infalling person appear to be stuck in time?

The infalling victim would appear to slow down more and more as they approach the horizon, but it would get hard to tell because they would also get dimmer and more red-shifted until - for practical purposes - they could no longer be seen. Out at your observation post, you'd get fewer and fewer photons per unit time, and their wavelengths would get greatly attenuated.

 

[phinds]

... not even given an almost infinite amount of time ...

I cannot imagine how you got that from what I said, which was most emphatically NOT an "infinite" amount of time, just an incredibly long time.

OOPS: I missed the "almost"

 

[stevil]

I cannot imagine how you got that from what I said, which was most emphatically NOT an "infinite" amount of time, just an incredibly long time.

OOPS: I missed the "almost"

I was trying to keep things simple, rather than introduce lots more terms. (sorry, my bad)

You said "until the BH evaporates" but even that would mean that stuff can't be observed to fall inside the EH until the BH basically dies, which seems to me to be too late for the BH to grow larger due to new material falling inside it. So if we can't observe new material fall inside it and consequently making it bigger, then how does it get bigger?

 

[stevil]

The infalling victim would appear to slow down more and more as they approach the horizon, but it would get hard to tell because they would also get dimmer and more red-shifted until - for practical purposes - they could no longer be seen. Out at your observation post, you'd get fewer and fewer photons per unit time, and their wavelengths would get greatly attenuated.

Thanks Dave, this is an interesting aspect.

I guess what I am trying to do is somewhat of a thought experiment rather than worry about the practicalities of taking measurements. But what you have said is very interesting and I am wiser for it. Thanks.

I am trying to work out from an outsider's perspective if it is possible from a time perspective for stuff to travel inside a black hole passing through the Event Horizon. (It's possible I'm not crafting my questions very well, sorry)

 

[phinds]

I was trying to keep things simple, rather than introduce lots more terms. (sorry, my bad)

You said "until the BH evaporates" but even that would mean that stuff can't be observed to fall inside the EH until the BH basically dies, which seems to me to be too late for the BH to grow larger due to new material falling inside it. So if we can't observe new material fall inside it and consequently making it bigger, then how does it get bigger?

The fact that a remote observer can't see things fall in has nothing to do with whether or not they ACTUALLY fall in (they do). The BH effectively gets bigger (has a larger gravitational effect), as far as the remove observer is concerned, as soon as added mass passes the orbit of the observer.

 

[stevil]

The fact that a remote observer can't see things fall in has nothing to do with whether or not they ACTUALLY fall in (they do). The BH effectively gets bigger (has a larger gravitational effect), as far as the remove observer is concerned, as soon as added mass passes the orbit of the observer.

I'm not doubting that. I'm fully accepting of the fact that a BH can get bigger.

My question is whether stuff can travel from outside the even horizon into the inside. I am wondering if the BH could be a consequence of all the stuff on the outside rather than a consequence of stuff on the inside.

I'm not trying to propose a new theory, but I don't have a great grasp on current theory and I have struggled to find anywhere on the internet information about this, that is why I am posting a few questions on a Physics forum.

Is the current theory that time slows down as you get closer to the EH?
At what point (in relation to the EH) is it proposed that the delta of time approaches zero?
Has anyone in Cosmology considered whether the BH is a consequence of the stuff outside it rather than stuff inside it?
I'm interested to know the current understanding of things, rather than proposing wacky theories.

 

[PeterDonis]

Hi @stevil

I suggest starting out by reading this Insights article of mine:

https://www.physicsforums.com/insights/black-holes-really-exist/

It covers the basics of why and to what extent physicists believe that black holes, or more precisely their interiors, "exist".

You might also try this series of articles of mine on the Schwarzschild spacetime geometry (the link is to the first of four articles):

https://www.physicsforums.com/insights/schwarzschild-geometry-part-1/

 

[russ_watters]

So from the observer's perspective if they can never see anything ever fall beyond the BH's EH, not even given an almost infinite amount of time, then how are we expected to accept that anything can ever go inside?

It's pretty much the same situation and math viewed/calculated from opposite directions so I can't imagine why we'd accept one side and reject the other.

Why do we speculate...

It's not speculation, it's theory/math.

...if it seems impossible for anything to go inside? (Impossible because there isn't enough time for this ever to happen).

That's not what happens/what the models say happens. This isn't much different from a perspective illusion where you can re-construct other views of the situation.

My question is whether stuff can travel from outside the even horizon into the inside.

Put some thought into that. Why an object moving at high speed toward a black hole stop at the event horizon? It makes no sense.

Is the current theory that time slows down as you get closer to the EH?

You've already been told no. The person/object crossing the event horizon experiences nothing unusual. Why would it?

 

[stevil]

Put some thought into that. Why an object moving at high speed toward a black hole stop at the event horizon? It makes no sense.

I was wondering whether it ends up spiralling around the black hole rather than plowing straight into it. Whether SpaceTime near the black hole has an angular direction around the black hole.
I don't view the universe as Euclidean. I don't assume the universe always behaves according to my intuitive thoughts.

You've already been told no. The person/object crossing the event horizon experiences nothing unusual. Why would it?

I already addressed this too. Perhaps I'm not using the right words or terms, but as far as I understand it, an observer will always consider time to be constant from their own perspective, but if an observer is near a large mass, i.e. an observer on the sun as opposed to an observer on the Earth, time will pass at a slower rate for the one near the larger mass. If you brought the two clocks back together you would fine the that the Earth clock would have advanced further than the Sun clock.

 

[stevil]

Hi @stevil

I suggest starting out by reading this Insights article of mine:

https://www.physicsforums.com/insights/black-holes-really-exist/

It covers the basics of why and to what extent physicists believe that black holes, or more precisely their interiors, "exist".

It's funny or sad (if you are me) to read where it says
"it won't be for any of the simplistic reasons associated with common misconceptions i.e. it won't be because they take an infinite of time to form".

I don't quite understand the reasoning. It talks about using the falling object's clock rather than the observer's clock. But I exist here on Earth and there are black holes which can be "seen" from Earth. i.e. in the middle of the Milky Way. For that black hole to exist from the perspective of Earth or elsewhere in our universe, then an almost infinite amount of time needs to have passed, but our universe is less than 14 billion years old.

Then there was the argument which says "a black hole exists regardless whether there is some coordinate chart in which it takes an infinite of time for something to fall into it."
Well at least it identifies this issue, it does seem to me to be just handwaving the issue off rather than addressing it head on.
It references Oppenhiemer-Snyder model as proof of the existence of spacetime not in causal past of future null infinity.

I don't have the knowledge or skills to be able to understand that, but at least it is addressed. I was wondering if there was no SpaceTime at all within the event horizon. I don't know if there is a difference between "no SpaceTime at all" vs "spacetime not in causal past of future null infinity"

LOL, please excuse my insane ignorant ramblings.

 

[Dale]

My question is whether stuff can travel from outside the even horizon into the inside.

Yes. To the best of our current knowledge, stuff can travel from outside to inside the horizon. Just like stuff can travel from past to future but we cannot get any information back from the future, similarly with the horizon.

I am wondering if the BH could be a consequence of all the stuff on the outside rather than a consequence of stuff on the inside.

I would say that a BH, like anything else, is a consequence of the stuff in the past.

 

[stevil]

I would say that a BH, like anything else, is a consequence of the stuff in the past.

Yes of course.

But, if we have a massive hollowed out sphere. With a huge amount of mass, what impact does that have on SpaceTime inside that sphere?
Is it possible for that hollowed out sphere to be so massive that SpaceTime itself cannot exist inside it, due to the curvature that the massive hollow sphere puts onto SpaceTime? and so without SpaceTime on the inside, the sphere cannot collapse on itself into a point of singularity at the centre of the sphere.

Anyway, sorry, just thinking out loud.

 

[Dale]

Yes of course.

But, if we have a massive hollowed out sphere. With a huge amount of mass, what impact does that have on SpaceTime inside that sphere?
Is it possible for that hollowed out sphere to be so massive that SpaceTime itself cannot exist inside it, due to the curvature that the massive hollow sphere puts onto SpaceTime? and so without SpaceTime on the inside, the sphere cannot collapse on itself into a point of singularity at the centre of the sphere.

Anyway, sorry, just thinking out loud.

Inside a hollow spherical shell spacetime is flat. However, there is a limit to how massive a sphere can be before it collapses. If that happens then the spacetime inside the shell would only remain flat for as long as it takes for the spherical shell to collapse. Once the shell collapses then the spacetime is no longer flat. Nothing about the flat spacetime inside the shell would oppose collapse.

 

[PeterDonis]

I was wondering whether it ends up spiralling around the black hole rather than plowing straight into it.

This can and does happen yes. Look up "accretion disc".

Whether SpaceTime near the black hole has an angular direction around the black hole.

Of course it does. Spacetime is 4 dimensional everywhere. Its geometry is not flat everywhere, but "not flat" is very, very different from "doesn't have an angular direction".

For that black hole to exist from the perspective of Earth or elsewhere in our universe, then an almost infinite amount of time needs to have passed

Wrong. This is one of the misconceptions I address in the articles I linked to.

The mistake you appear to be making is thinking that "time" has some absolute meaning. It doesn't. There is no "universal time" that is the same for all objects. There is only proper time along particular objects' worldline, i.e., "time" according to each object's own clock. At least, that's the only invariant kind of time.

It is possible to adopt a convention that assigns particular "times" by some object's clock to events that are not on that object's worldline--for example, to assign "times" to events on the worldline of an object falling into a black hole, according to an observer far away. But any such assignment is, as I have just said, a convention; it has no physical meaning. It's just a way of organizing the data.

it does seem to me to be just handwaving the issue off

No, it isn't; you simply haven't read carefully enough. The definition of a black hole "existing" is given explicitly in the article, and obviously is independent of any choice of coordinates. That means you can't possibly either prove or disprove whether a black hole exists or not by looking at coordinates. You have to look at invariants. (In fact, this is not just true of questions about whether a black hole exists; it's true of any question of physics. That is one of the important things that General Relativity tells us.)

I was wondering if there was no SpaceTime at all within the event horizon.

There is.

I don't know if there is a difference between "no SpaceTime at all" vs "spacetime not in causal past of future null infinity"

Yes, there is a huge difference. The first is just wrong. The second is true for any spacetime that contains a black hole.

if we have a massive hollowed out sphere. With a huge amount of mass, what impact does that have on SpaceTime inside that sphere?

The geometry of spacetime in a vacuum region inside a hollow sphere is flat. This is a simple example of what is called the "shell theorem", which says that a spherically symmetric distribution of matter outside some radius has no effect on the spacetime geometry inside that radius.

Is it possible for that hollowed out sphere to be so massive that SpaceTime itself cannot exist inside it, due to the curvature that the massive hollow sphere puts onto SpaceTime? and so without SpaceTime on the inside, the sphere cannot collapse on itself into a point of singularity at the centre of the sphere.

No.

 

[PeterDonis]

sorry, just thinking out loud.

You should stick to your earlier intent from post #11:

I'm interested to know the current understanding of things, rather than proposing wacky theories.

Your "just thinking out loud" from post #17 is much more like proposing wacky theories than asking about the current understanding of things.

 

[stevil]

Wrong. This is one of the misconceptions I address in the articles I linked to.

Yes, you did. Great article by the way, well written too. A bit over my head. But I enjoyed reading it.

The mistake you appear to be making is thinking that "time" has some absolute meaning. It doesn't. There is no "universal time" that is the same for all objects. There is only proper time along particular objects' worldline, i.e., "time" according to each object's own clock. At least, that's the only invariant kind of time.

I do understand that time isn't constant. I understand that time runs constant for a participant, but for a distant observer they may consider the participant's time to be fast or slow.
I understand that in the presence of a large amount of mass that time dilutes but the person walking on the large mass won't know that.

It is possible to adopt a convention that assigns particular "times" by some object's clock to events that are not on that object's worldline--for example, to assign "times" to events on the worldline of an object falling into a black hole, according to an observer far away. But any such assignment is, as I have just said, a convention; it has no physical meaning. It's just a way of organizing the data.

The problem is probably just me, not currently being able to grasp how I personally can observe a black hole getting bigger even though from my point of view time stops a the EH and nothing appears to be able to fall in.

I understand that the object falling in has no concept that time has slowed for them, if they were able to see outwards they would see the universe grow old and disburse in the blink of an eye, but their own watch would tick by normally.

The geometry of spacetime in a vacuum region inside a hollow sphere is flat. This is a simple example of what is called the "shell theorem", which says that a spherically symmetric distribution of matter outside some radius has no effect on the spacetime geometry inside that radius.

Aha, this answers my question perfectly. Thanks Peter.

 

[stevil]

Your "just thinking out loud" from post #17 is much more like proposing wacky theories than asking about the current understanding of things.

Fair enough, but it was useful for me because in the previous post you gave me the answer I was looking for. (not the answer I was hoping for, but the answer I was looking for none the less.) Thanks.

 

[PeterDonis]

for a distant observer they may consider the participant's time to be fast or slow.

Any such consideration requires choosing a simultaneity convention. There is no invariant fact of the matter about how fast or slow a distant object's clock runs relative to yours. Many treatments of Special Relativity, unfortunately, obscure this fact by making use of the obvious simultaneity convention of global inertial frames (which are only possible in flat spacetime) and treating them as though they had some sort of absolute meaning.

how I personally can observe a black hole getting bigger

By orbiting the black hole and observing how your orbital parameters change as things fall into it and its mass increases.

from my point of view time stops a the EH and nothing appears to be able to fall in.

This "point of view" is a particular choice of simultaneity convention. It has no invariant physical meaning. The only invariant fact is that you cannot see anything that happens at or below the event horizon.

if they were able to see outwards they would see the universe grow old and disburse in the blink of an eye

No, they would not. The outside universe would actually appear more and more redshifted ("appear" here means what the infalling observer would actually see in the light signals reaching them), and the infalling observer would only be able to see a very little of its history before hitting the singularity and being destroyed.

An observer "hovering" just outside the horizon of a black hole would see the universe grow old while only a little time passed by his own clock. But this observer is in a very different state from one that is freely falling into the hole. Another unfortunate consequence of the common use of the term "time dilation" to describe the "hovering" observer's experience is that it makes it seem like any observer near or inside the hole would have the same experience, which is, as I have just described, not the case.

 

[stevil]

An observer "hovering" just outside the horizon of a black hole would see the universe grow old while only a little time passed by his own clock. But this observer is in a very different state from one that is freely falling into the hole. Another unfortunate consequence of the common use of the term "time dilation" to describe the "hovering" observer's experience is that it makes it seem like any observer near or inside the hole would have the same experience, which is, as I have just described, not the case.

I was meaning the observer before they fell into and beyond the EH.
Thanks for your patient explanations though. You know a lot of stuff.

 

[stevil]

By orbiting the black hole and observing how your orbital parameters change as things fall into it and its mass increases.

As far as I understand it, there would be no difference on me and my orbit, whether the material in the black hole's accretion disk fell into the black hole or remained in the accretion disk. Whether it is in the accretion disk or in the black hole it still has the same gravity effect on me.

 

[PeterDonis]

I was meaning the observer before they fell into and beyond the EH.

An observer free-falling into a black hole sees the rest of the universe as redshifted even before he crosses the horizon.

As far as I understand it, there would be no difference on me and my orbit, whether the material in the black hole's accretion disk fell into the black hole or remained in the accretion disk.

As long as the disk is closer to the hole than you are, that is the case, yes. However, we are talking about material that falls into the hole. That material will disappear from your view (because light coming from it, well before it actually crosses the horizon, will become so redshifted that you can't detect it any more), but its gravitational influence will still be detectable by you as part of the overall mass of the hole.

A key point in this regard is that there are no stable orbits around a black hole inside a radius three times the horizon radius, and there are no closed orbits at all, even unstable ones, inside a radius 1 and 1/2 times the horizon radius. So any material you see that is closer to the hole than that must be falling into the hole. The only way to be closer to the hole than 1 and 1/2 times its horizon radius and not fall in is to have a rocket engine or some other means of generating thrust. Material falling in from an accretion disk does not have that. So the idea that the material somehow has not fallen into the hole does not work.

 

[stevil]

Inside a hollow spherical shell spacetime is flat. However, there is a limit to how massive a sphere can be before it collapses. If that happens then the spacetime inside the shell would only remain flat for as long as it takes for the spherical shell to collapse. Once the shell collapses then the spacetime is no longer flat. Nothing about the flat spacetime inside the shell would oppose collapse.

Yeah, I figured it would collapse in on itself before it got to a point where SpaceTime at the centre would be curved. (Not that I am saying that SpaceTime would be curved inside the shell or sphere)
I don't know this stuff, you people do, so I'm just learning here.

Better for me to learn about physics here than for me to continue having faulty thoughts.

 

[PeroK]

Better for me to learn about physics here than for me to continue having faulty thoughts.

That's certainly true. Understanding GR and black holes in particular requires tackling some mathematical and conceptual hurdles. Popular science sources do their best to convey an impression of what the physics says, but that's really all you get. If you start speculating and hand waving on the basis of popular science sources, then there is no way to confirm or invalidate these speculations.

The Insights on this forum are probably the best bridge from popular science to the real subject. Ultimately, however, if you really want to understand Relativity, you need to put in the hard yards (like every physics undergraduate student must) and study it as an academic subject, fully supported by its mathematical basis.

 

[phinds]

if an observer is near a large mass, i.e. an observer on the sun as opposed to an observer on the Earth, time will pass at a slower rate for the one near the larger mass.

No, it will not. It will pass at exactly the same rate for each of them.

If you brought the two clocks back together you would fine the that the Earth clock would have advanced further than the Sun clock.

Yes, but that's not because time passes at different rates for them, it's because they took different paths through space-time. Look up "differential aging".

It's exactly the same as when two cars start at one point and end up and another point, each traveling at exactly 60 mph for the whole trip and yet they end up having traveled different distances because they took different routes, NOT because they were traveling at different speeds.

 

[timmdeeg]

No, it will not.

Comparing sun versus Earth I think @stevil intended to say that gravitational time dilation increases with increasing mass. If one of two synchronized clocks far away from a mass M is brought close to M, hovers there for some time it will show less elapsed time depending on M (sun versus earth) compared to the far away clock after being brought back.

 

[DrDu]

A particle falling into a black hole appears to us to fall the slower the nearer it gets to the event horizon. When it's de Broglie wavelength becomes comparable to the radius of the event horizon, we can't say for sure whether it has not already fallen into the black hole?

 

[phinds]

If one of two synchronized clocks far away from a mass M is brought close to M, hovers there for some time it will show less elapsed time depending on M (sun versus earth) compared to the far away clock after being brought back.

Which is exactly what I pointed out but it is not correct to say that time passes at different rates, as I also pointed out.

 

[russ_watters]

Energy/matter tends to spiral around a black hole rather than fall directly towards it.

[separate post]

I was wondering whether it ends up spiralling around the black hole rather than plowing straight into it.

This can and does happen yes. Look up "accretion disc".
[emphasis added]

Just to be clear, whether something spirals-in or falls straight in depends on the initial trajectory. There's a lot of spinning in the universe because of matter distributions not being uniform, and any indirect trajectory is going to involve a rotational/orbital component. So it's common but not required.

As long as the disk is closer to the hole than you are, that is the case, yes. However, we are talking about material that falls into the hole. That material will disappear from your view (because light coming from it, well before it actually crosses the horizon, will become so redshifted that you can't detect it any more), but its gravitational influence will still be detectable by you as part of the overall mass of the hole.

I would assume this sort of thing has been modeled/simulated? E.G., as a star spirals into a black hole it could disappear from view, yet we would still detect gravitational waves as it continues to spiral towards the event horizon? And once behind the event horizon are there still gravitational wave emanations as it continues to spiral toward the singularity or at that point does the field settle down and become static again?

 

[timmdeeg]

Yes, but that's not because time passes at different rates for them, it's because they took different paths through space-time. Look up "differential aging".

I know "differential aging" from the SR/twin paradox. Does it include gravitational time dilation too concerning the scenario of post #30?

 

[PeroK]

I know "differential aging" from the SR/twin paradox. Does it include gravitational time dilation too concerning the scenario of post #30?

Yes, differential ageing is just the difference in spacetime length of different timelike paths between two points in spacetime.