# I Where is the matter in a black hole?

1. Oct 8, 2015

### Green dwarf

I understand that, as matter approaches the event horizon of a black hole, according to the time frame of someone outside the black hole, it would slow down and, after an infinite time, stop completely at the event horizon. So, if we could observe it, all this matter would be accumulating just outside the event horizon rather than passing through it. This might suggest that the matter that forms a black hole is in a thin shell near the event horizon rather than at a singularity in the centre. Is this the case or have I missed something?
As more matter arrives, the black hole would presumably grow and the event horizon would move outwards, thus engulfing the matter rather than the matter 'falling' in. Would the matter then proceed to the singularity? I suspect maybe not: I have heard it suggested that time is reversed inside the event horizon. If that is the case, might the matter inside then move back outwards towards the event horizon, thus still preserving the thin shell?

2. Oct 8, 2015

### BvU

Hello Green one, welcome to PF !

Is there a basis for your 'understanding' ?

And your suspicion (other than hearsay) ?

3. Oct 8, 2015

### newjerseyrunner

From an outside observer, things would take an infinite amount of time to crush down into a singularity, but from an internal reference frame, that's not true. The event horizon is not a physical place, it's a mathematical location where the escape velocity >= c. That location is significantly above the singularity, and the curve of spacetime is not infinite at the event horizon.

4. Oct 8, 2015

### DanC

But if all of the matter that has ever fallen into a black hole appears to stop near the event horizon, why do we not see that congestion of matter?

5. Oct 8, 2015

### newjerseyrunner

Because it's redshift is also shifted infinitely. In order for us to see it, it has to emit some sort of radiation, as things get closer and closer to the event horizon, the wavelengths get shifted towards the red, if you shift it too far, it's not observable anymore. Matter falling into a black hole doesn't APPEAR to slow down, it DOES slow down from our reference, it's not an illusion, it's actually happening. That means that the wavelengths of light coming off of it become stretched.

6. Oct 8, 2015

### phinds

Yes, it slows down in our reference frame but we know that it isn't "actually happening" to the matter itself, just to our perception of it. There is zero affect on the matter itself, which just falls to the singularity without ever noticing that there is something called an event horizon.

7. Oct 8, 2015

### DanC

Wow. Cool, thanks!

8. Oct 8, 2015

### Chronos

The event horizon is a coordinate surface that has no physicality. Putting aside the fact nothing is actually there, an EH is, by definition, the boundary beyond which no event can ever be observed. Stuff falling into a black hole has no clue it reaches or passes the EH. That does not spare it the indignity of being atomically deconstructed upon wandering too close to the black hole center of gravity [singulariy].

9. Oct 8, 2015

### Green dwarf

Thanks for your replies, particularly newjerseyrunner who seems to agree with my proposition.

I understand that from the perspective of the matter falling in (let's say it's a cow), the cow should just pass though the event horizon without incident and then proceed to the singularity to get squashed. But, in the fraction of a second it takes the cow to get through the EH (by the cow's clock), a very large amount of time will have passed by the clock of an observer outside the black hole. In fact, if the amount of time that passes isn't infinite, then it should be very large, maybe billions or trillions of years, especially if no more cows are following the first cow to enlarge the black hole and so move the EH outwards past it.

Now that raises another point. My understanding is that over such long time periods, black holes can evaporate by Hawking radiation. This would mean that the EH would then recede inwards, eventually leaving the cow stranded outside of the EH again. From the perspective of the outside observer, the cow never reached the singularity, but instead just bounced off the event horizon (albeit very slowly).

This suggests that no matter will ever actually reach the centre of a black hole and that there will be no point-singularity. In fact, it would seem that it is impossible to create a point-singularity. As the mathematics and physics of singularities seem to be problematic, could this be a way around the problems? As in some ways, the Big Bang can be thought of as a singularity in reverse, could it have some relevance to that as well?

Please excuse my use of 'understandings' and 'suspicions', but I'm not an astronomer or physicist - just a school teacher with an interest in astronomy and a bit of a fascination for this sort of stuff.

10. Oct 8, 2015

### phinds

The cow would not seem to bounce, since the photons being emitted just outside the EH would continue to be just outside the EH as it shrank, although by the time Hawking Radiation has had time to do anything significant to the size of the BH, the cow would have long since red-shifted WAY beyond any ability to detect it.

11. Oct 8, 2015

### Green dwarf

Just another thought following on from my last post. In the picture I painted, from the perspective of the outside observer, the cow will never reach the centre of the black hole to become part of a singularity; and yet from the perspective of the cow, it does. This seems to be a contradiction. This is probably because there's a flaw in my logic. But is it possible that the space inside the event horizon becomes a different universe which can have a different (and seemingly contradictory) history?

12. Oct 8, 2015

### phinds

This happens because our perception, as remote observers, is basically an "optical illusion". We KNOW what happens to the cow, just as we know what our perception is. That doesn't mean there's any need to be confused because the two are different.

13. Oct 8, 2015

### Chronos

Your scenario only work in the context of some kind of absolute reference frame - which is a show stopper under GR.

14. Oct 9, 2015

### newjerseyrunner

Not quite. I agree with the proposition that to an outside observer, the cow would take an infinite amount of time time fall into the singularity. What you seem to be misunderstanding is that the event horizon is not the singularity, it's well above it.

For example, take the black hole at the center of our galaxy. It's event horizon is gigantic (I think it's like the radius of the orbit of Jupiter, but it's singularity is just a point at the very center of it. The event horizon is where the force of gravity is so high that escape velocity is greater than c, it simply means that we can't observe anything happening on the other side of it, that does not mean that things aren't happening inside of it. The mathematics works exactly the same way until you hit the singularity itself.

So the warping of spacetime at the event horizon is enough to make the escape velocity greater than c, but that's not an infinite warp yet. From an outside observer, an object falling into the hole's time frame would slow down, but it wouldn't be stopped. An object has to be moving at the speed of light (which is impossible) for time dilation to go to the extreme that it stops all together, or be in an infinite gravity well (which is the singularity itself, not the event horizon.)

The event horizon is not a thing, it's just a location.

Here's an analogy: If you are flying to an isolated region of earth, lots of times, a helicopter can't carry enough fuel to get all the way there and all the way back, it refuels once it gets there. Therefore, logically, there is a point during it's travel where what's left in the tank is less than what's required to turn around and go back. It's called the point of no return, you're either making it to your destination or crashing. It's not a physical location, neither is the event horizon.

15. Oct 9, 2015

### BvU

Is the green one now enlightened well enough to 'understand' what he has to bring up to contradict the time reversal suggesters ?

16. Oct 9, 2015

### Green dwarf

I don't think I'm misunderstanding that. I see the event horizon as a spherical shell of finite radius and the singularity as the point at the centre of the shell. So the event horizon will be some distance away from the singularity.

I get the impression that you are saying that time doesn't stop at the event horizon, but rather at the singularity. This would solve my problem. But I did a bit of a google search and the majority of things I found stated or assumed that time does stop at the EH. One source (http://physics.stackexchange.com/questions/123874/why-does-time-stop-in-black-holes) gave some maths behind the conclusion, though my mathematical knowledge isn't up to following it.

If time does stop at the EH, then my problem still remains in that it still seems that, from the point of view of an observer distant from the black hole (let's call him Eric), the cow still slows down asymptotically as it approaches the EH and can never quite reach it, let alone pass through it (except if the black hole grows to engulf the cow). The same would apply to everything else that ever fell into the black hole. So, in Eric's universe, there is a black hole consisting of a shell of matter with little or nothing inside it and no singularity at its centre.

Now, admittedly Eric can't see into the black hole to know if it is a shell or a point mass, so to some extent the conclusion is purely theoretical, but it does to me seem that the idea that a singularity cannot form would have some implication for something.

The one thing that I can think of that might solve the problem is the idea that the the black hole grows as more stuff falls into it, so the EH moves outwards past the stuff rather than the stuff having to move inwards past the EH. The important thing then is what happens to time inside the black hole. In my initial post, I assumed it reversed so that stuff fell outwards towards the EH rather than in towards the centre, but I imagine it's a lot more complicated than that. People seem to be talking about geodesics and time becoming space-like and vice versa, though I must admit, that's all a bit beyond me.

17. Oct 9, 2015

### Green dwarf

I tend to see the observer's perception not as an optical illusion, but rather as what happens in the observer's frame of reference or in the observer's universe. To me it seems just as real and valid a perception as that of the cow.

18. Oct 9, 2015

### phinds

That's fine, but the infalling person/object doesn't care.

19. Oct 9, 2015

### Staff: Mentor

That's not what happens. The cow did cross the event horizon and light was reflected from the cow as that happened; when the black hole evaporates that light is able to escape and make it to our eyes so we finally get to see the cow going through the horizon. We still don't get to see the cow below the event horizon and reaching the central singularity - below the horizon all the light from the cow, no matter the direction it is emitted, falls into the central singularity.

To understand the relationship between time for an observer outside the event horizon and for something falling through the event horizon, you'll want to learn to interpret a Kruskal diagram. There are also a bunch of threads in the relativity forum here.

20. Oct 10, 2015

### Green dwarf

Thanks Nugatory.

I think I'm trying to understand something that can't be understood without a lot more background knowledge than I've got. I've tried coming to grips with general relativity, but it's hard. Maybe I should just accept the limits to my understanding and spend my time on pursuits that have a better chance of success.

21. Oct 12, 2015

### newjerseyrunner

This might help:
https://qph.is.quoracdn.net/main-qimg-dc24b4d176580645a59601343e075c5e?convert_to_webp=true [Broken]

See how the event horizon is high above the singularity? In order for time to "stop" you have to be at a point in space with infinite curvature. That exists only at the singularity, at the event horizon, time and space are deeply stretched, but not infinitely.

Objects appear to stop moving at the event horizon because the light can no longer get to the outside, but they don't actually stop. Time may be slowing very much, but it's not stopped yet. Let's go to the math of an object falling into a black hole from observer (got my formula from: https://en.wikipedia.org/wiki/Gravitational_time_dilation)

t0 = time you would see as the observer
tf = time of the object
G = gravitational constant
M = mass of black hole
r = distance from black hole to object
c = speed of light

t0 = tf √(1 - (2GM/rc^2))

Notice that no matter how close you get to the black hole (which can be well inside of the event horizon) time never reaches 0. The only way to have time completely stop is to make r equal 0.

Last edited by a moderator: May 7, 2017
22. Oct 12, 2015

### eltodesukane

Where is the matter in a black hole?
-- There is no matter in a black hole.
From our point of view as an observer, it takes an infinite amount of time for a particle falling on a black hole to cross the event horizon. Since the big bang, not a single particle has ever cross the event horizon of any black hole in the whole universe. Matter just get closer and closer to the event horizon, and freezes there, as time slows down to a still at the event horizon (which is why black holes are also called frozen stars).
All the black hole matter is outside the black hole, in a frozen shell surrounding it.
So there is no matter inside a black hole, since nothing can get there in the whole lifetime of the universe.

"..the matter that forms a black hole is in a thin shell near the event horizon.."
-- Yes.

23. Oct 12, 2015

### newjerseyrunner

No.

It takes an infinite amount of time to fall into the SINGULARITY. Space is not infinitely curved at the event horizon, nor is it special in any way other than being the radius at which escape velocity >= c. Escape velocity has nothing to do with time dilation.

You are confused because people say you can't see someone fall into it. That has nothing to do with time dilation, that has to do with redshift, which is related to escape velocity. You can't OBSERVE what's going, but it's not because of time stopping.

24. Oct 12, 2015

### BvU

Sigh ....
If physics were a democracy, we'd all still be afraid to fall off the earth at its edge. In PF we're somewhere between a meritocracy and a democracy (which does make it fun, so let's keep that!). I thought we had the green one back on earth with feet on ground (post #20) and head out of clouds and now elto starts all over again.

Is the distinction between what happens to straying cows and what is observed at least something that is now all clear, acceptable and understandable ?

I think green dwarf (post #20) is a little too shy: everyone has an upper limit to his/her abstraction level beyond which fantasy takes over (kind of like an analogon to the event horizon ). The worst we can do is to stop being curious and not keep wondering what's beyond !

My mental toolbox isn't adequate to deal with the GR goodies either, but I sure love to keep an open mind and venture over this horizon from time to time !

25. Oct 12, 2015

### BertMorrien

Since there are by definition no obervations of what's behind the event horizon, a black hole can only be described in terms of phenomena that are observed.
Maybe matter that falls in a black hole experience only a gravity gradient and maybe this gradient can become strong enough to tear protons and even quarks apart, we simply cannot know for sure.
IMO a black hole is completely described by its mass and angular momentum. If somethings falls into it, it's mass and angular momentum is affected.