Black holes question... Communicating with the outside

[Ziggurat]

Isn't it a more important issue, though?

What's the difference, to someone outside the black hole, between saying "a falling object never crosses the event horizon" and saying "the object does cross the event horizon, but its crossing of it can't ever affect me in any way"?

I think the difference is whether or not there is some observer which can observe the object crossing, not whether or not any particular observer can observe it crossing. Picking the former criteria becomes more obviously meaningful if you consider, for example, the event horizon created by adopting a uniformly accelerating reference frame. The accelerating observer will now observe an event horizon, and they will observe that objects never cross it - but us non-accelerating obervers can see that objects quite clearly do, and so would the accelerating observer if he ever stopped accelerating.

 

[JJR]

Once you're past the event horizon, the singularity is no longer simply located at some different location from you: the sigularity is actually your future, and the event horizon is your past. To send a signal out of the event horizon, you need to be able to time travel. Going faster than the speed of light is equivalent to time travel, BTW, but thinking in these terms I think makes it a little more clear why it's an absolute impossibility.

Wow, so if I were a black hole and someone drew my blood, the needle would be in teh future, and my epidermis would be the needle's past, and the nurse would be living in the past. Just to antropormorphize an quantum astronomical object . . . ha! I turned a schwartz shield horizon into flesh!

EDIT: Heavy dude!

 

[Ziggurat]

Wow, so if I were a black hole and someone drew my blood, the needle would be in teh future, and my epidermis would be the needle's past, and the nurse would be living in the past. Just to antropormorphize an quantum astronomical object . . . ha! I turned a schwartz shield horizon into flesh!

EDIT: Heavy dude!

Hey, no drinking on Sunday!

 

[JJR]

Hey, no drinking on Sunday!

Moi? I can think of quite a few things I'd rather do than drink so . . . got a point there, pard.

What I said was correct, though . . . if I were, in fact, a black hole. It's interesting . .. when you think about Cause and Effect . . . what CAUSED the universe to first exist? The first matter, that is.

If the Big Bang was a black hole exploding, then the universe CAUSED itself, since a Black Hole is nothing, so therefore, "nothing" was created from nothing. Also, if the Big Crunch ever wins out over the mysterious Dark Energy . . . then the universe as we know it is something from which light and even time can not escape from . . . perhaps just a big, puffed out Black hole inside of a larger universe . . . complete with an event horizon somewhere beyond the stars . . . this will be very interesting.

If this were the case, then not only is matter not what we think of it as . .. since, of course it's mostly empty space and it's really electricity that keeps my hand from easily passing through my desk like a ghost . . . then our perception of life is all wrong, too. We ARE ghosts, since life can not exist inside of a Black Hole.

Wow man.

"Luminous beings are we . . . not this crude matter." - Yoda

 

[Ziggurat]

We ARE ghosts, since life can not exist inside of a Black Hole.

What ever made you think that?

Life can't survive the impact with the sigularity (and will get ripped apart by tidal forces shortly before reaching it anyways), but there's nothing that says that life can't survive (though for how long depends on the size of the black hole) inside the event horizon.

 

[Yllanes]

From reading this thread, I've come to the conclusion that I know less than I thought I did about black holes. Can anybody reccomend a decent book on the subject?

Sure, what kind of background do you have? (high school/undergraduate/graduate physics...) There are many interesting books and resources at all levels.

 

[Oualawouzou]

From reading this thread, I've come to the conclusion that I know less than I thought I did about black holes. Can anybody reccomend a decent book on the subject?

You tell me. Every time I ask a question about black holes, I think I just need this simple answer to more or less complete my layman's understanding of the phenomenon.

I'm a fool.

 

[JQH]

Sure, what kind of background do you have? (high school/undergraduate/graduate physics...) There are many interesting books and resources at all levels.

Undergraduate physics. So I won't get scared off by books with some maths content!

 

[Cuddles]

Of course all this raises the question - if someone is stupid enough to fall into a black hole, do you really want to communicate with them?

 

[Yllanes]

Undergraduate physics. So I won't get scared off by books with some maths content!

OK, then you have many good options. A book I particularly recommend is B. F. Schutz, A First Course in General Relativity. The author is an international expert in General Relativity and his books are always very clearly written. This one is no exception. A very good account at an undergraduate level of General Relativity and Cosmology. It includes a chapter on black holes that more than covers all we have talked about in this thread. The preface talks about prerequisites:

The student is assumed already to have studied: special relativity, including the Lorentz transformation and relativistic mechanics; Euclidean vector calculus; ordinary and simple partial differential equations; thermodynamics and hydrostatics; Newtonian gravity (simple stellar structure would be useful but not essential); and enough elementary quantum mechanics to know what a photon is.

Another options, perhaps simpler, are K. Thorne, Black Holes and Time Warps and E. Taylor and J. A. Wheeler Exploring Black Holes: Introduction to General Relativity. I have not personally read these ones, but I have heard good things about them and I know other books by the authors, so I'm sure they won't disappoint.

If this is too easy, the next step is Sean Carroll's Spacetime and Geometry, more advanced but very readable and extremely up to date. What's even better, a slighty reduced version of the book is available for free as lecture notes for an MIT graduate course on GR.

The next step would be the really professional books, but I don't think you are looking for that.

 

[JQH]

Thanks, Yllanes. I'll search them out.

 

[JJR]

What ever made you think that?

Life can't survive the impact with the sigularity (and will get ripped apart by tidal forces shortly before reaching it anyways), but there's nothing that says that life can't survive (though for how long depends on the size of the black hole) inside the event horizon.

So we're not dead; we're flat . . . like beings from an area of space that has only 2 dimensions without a third . .. like height, for example. No y on the graph.

We're flat, dood.

 

[Ziggurat]

So we're not dead; we're flat . . . like beings from an area of space that has only 2 dimensions without a third . .. like height, for example. No y on the graph.

We're flat, dood.

Flat usually just means squished in one direction. I think we need a new word for what happens when you're squished in all three directions to a point.

 

[Yllanes]

Flat usually just means squished in one direction. I think we need a new word for what happens when you're squished in all three directions to a point.

In the words of Misner, Thorne and Wheeler (Gravitation, W. H. Freeman, 1973)

Consider the plight of an experimental astrophysicist who stands on the surface of a freely falling star as it collapses to R=0.

As the collapse proceeds toward R = 0, the various parts of the astrophysicist's body experience different gravitational forces. His feet, which are on the surface of the star, are attracted toward the star's center by an infinitely mounting gravitational force; while his head, which is farther away, is accelerated downward by a somewhat smaller, though ever rising force. The difference between the two accelerations (tidal force) mounts higher and higher as the collapse proceeds, finally becoming infinite as R reaches zero. The astrophysicist's body, which cannot withstand such extreme forces, suffers unlimited stretching between head and foot as R drops to zero.

But this is not all. Simultaneous with this head-to-foot stretching, the astrophysicist is pulled by the gravitational field into regions of spacetime with ever-decreasing circumferential area 4 pi r². In order to accomplish this, tidal gravitational forces must compress the astrophysicist on all sides as they stretch him from head to foot. The circumferential compression is actually more extreme than the longitudinal stretching; so the astrophysicist, in the limit R -> 0, is crushed to zero volume and indefinitely extended length.