# Black Hole Inconsistencies

1. Sep 2, 2004

### FSC729

I have some conceptual questions regarding black holes:

1. According to Stephen Hawking black holes radiate or rather extract energy from the void to make it appear as if black holes radiate, hence black holes will eventually fade away as time goes on. Basically black holes theoretically have a finite lifetimes.

2. If an astronaut where to fall into a black hole and rush past the event horizon, according to all accounts I've heard on physics shows, he/she will fall for a while, be stretched out, and either be stetched out so much he/she will die, or will hit the singularity.

I can see that one can be stretched out and die, but if the black hole has a finite lifetime, and according an observer outside the black hole it will take forever for you to hit the singularity, then how can you possibly hit the singularity if black holes have a finite lifetime.

If an outside observer lived long enough he/she would see the black hole fade away before you hit the singularity. Hence when you get to the singularity it would no longer be there and you would most likely be caught in black hole explosion, or something else.

Am I missing something???

John G.

2. Sep 2, 2004

### pervect

Staff Emeritus
I haven't been able to actually work through the math on this problem, but usually reliable sources such as Ted Bunn's "Black Hole faq"

http://cosmology.berkeley.edu/Education/BHfaq.html#q9

say this doesn't happen. Note that the observer does see you cross the horizon when the BH evaporates, the infinite time dilations occur only when the BH does not evaporate.

Also note that the inside of even a simple Schwarzschild non-evaporating black hole is a dynamic environment, not a static one. This is a consequence of the fact that the 'r' and 't' coordinates of the Schwarzschild metric interchange roles - inside the event horizon, r becomes a time coordinate, and t becomes a space coordinate. The 'r' coordinate is obviously not symmetrical, so the black hole is changing as you move into it, it's not like a "static" singularity.

Unfortunately, it's likely that the Schwarzschild metric doesn't work inside a black hole, even when it's not evaporating - you need something else, called a BKL singularity because the Schwarzschild solution probably isn't stable. I've seen references to the BKL singularity, but not the actual metric itself. Given the complexities involved, I'm mostly reduced to taking the word of people like Tedd Bunn on faith, that the black hole doesn't evaporate before one hits the singularity.

3. Sep 4, 2004

### chronon

I have to say that I'm with you on this one, and I'm not convinced by the standard explanation of why it doesn't happen. Note that if nothing can fall into a black hole then it seems unlikely that black holes can form in the first place (although black holes are strange things, you never know). I've written a page about it on my website (www.chronon.org/articles/blackholes.html)
It may be that there is a deeper mathematical explanation of why it doesn't happen, but I've never found it. My impression is that it is actually something no-one is sure about. My feeling is that Hawking's recent results on black holes conserving information push the result towards not being able to fall into a black hole before it evaporates.

4. Sep 4, 2004

### RingoKid

I would have black holes as balance mechanisms to keep the universe on an even keel around a fixed inflation point...

...essentially sucking in matter and depositing it at the "edges/frontiers" of the expanding universe

the radiation would be the information coming back regarding the process of creation/destruction taking place at the "edges/frontiers"...

I would also have the singularities inside a BH to be infinitely long and thus heavy "strings" only a planck length in diameter and accounting for the missing dark matter.

Black holes would fade away as the shifting mass of the universe moves on it's spherical fulcrum (think of a planetary pole shift and apply it to the universe)

So new black holes will always be created to redistribute matter around and old ones will fade away by collapsing the event horizon to planck size and getting sucked to the edges of the expanding spacetime bubble...

...of course this is all speculation on my part and I have no idea of the maths involved or whether it is possible being the uneducated layman that i am.

some feedback would be nice, so far I have always been met with deafening silence probably due to the fact that it is so far off the extreme that no one really knows or that it is so ludicrous that it is not worth feeding back on.

Not that i'm worried as it makes perfect sense to me and if i hold it as a personal truth then I don't actually find it any less a possible reality than heaven and helll of which there are a great many more number of people who adhere to that speculation without proof either.

As it is, the speculations came from extrapolating a thought when asking myself why do massive objects collapse to form black holes exactly where they do???... not how ??? but why ???and then searching for disproof but not finding any...

then again maybe I've found it and am too stupid to realise cos the maths is way over my head

whatever, I certainly have/had fun visualising it, does that make sense to anybody else ???

not that that matters either so cheers for the rant

peace

5. Sep 4, 2004

### pervect

Staff Emeritus
Well, one of these days, I want to get around to calculating, or finding, whether or not the causaul disconnection that occurs when an object falls through the event horizon can be reversed by jumping in after it.

Basically, the questions are, if someone jumps through the event horizon, and you "wait a while", then jump in after him, whether or not you can see him and whether or not he can see you.

6. Sep 4, 2004

### pervect

Staff Emeritus
OK, I have found a reference which seems to suggest that when you fall through the event horizon of a black hole, you can see the people and objects who fell through before you, and they can see you.

It's also got some interesting visuals. It originated as a show for a planetarium, so it seems to have reasonably good credentials.

So, given this encouragment, here's the way I think things happen. I could still be wrong.

When you fall into a black hole, I think that in some sense it hasn't finished forming yet. You can see at least some of the stuff that's fallen in before you, possibly even all of it. This ties in with the fact that the mathematical description of the black hole is not static in time afater you cross the event horizon.

You get to experience the formation first hand, as the singularity forms and spacetime collapses around you. Even though the singularity hasn't formed yet, there is no way to prevent it from forming - except for the laws of quantum gravity, perhaps. While quantum gravity may prevent the singularity from being truly pointlike, the difference won't matter much, you're still going to get crushed quite flat. Actually, the experience is going to be more like being pulled apart by a taffy machine, if Kip Thorne's description in "Black Holes & Time Warps" is right . (See the section on the BKL singularity).

The amount of time you'll have to do anything at all is limited. The best case would be with a very massive black hole - with a 10 billion solar mass black hole you'd have on the order of 20 hours left before you became part of the singularity.

I don't think the eventual evaporation of the BH (which is going to take an EXTREMELY long time for a 10 billion solar mass BH) is going to affect your fate much. You'll have 20 hours of proper time left before reaching the singularity, the stuff falling in isn't going to magically disappear in that time.

7. Sep 4, 2004

### FSC729

Thanks for all your help guys.

8. Sep 4, 2004

### pervect

Staff Emeritus
Thanks for an interesting question. I've spent some time staring at a Kruskal diagram, and I'm beginning to think that I need to re-examine some of my ideas as expressed in my last post. I've been looking around the web for a URL with a Kruskal diagram like the one in my textbook to post, but so far haven't had any luck.