[dlorde]

Aha, found it - on Wikipedia (Falling Into A Black Hole):

...From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon: and it appears to become redder and dimmer, because of the extreme gravitational red shift caused by the gravity of the black hole. Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon. All of this is a consequence of time dilation: the object's movement is one of the processes that appear to run slower and slower, and the time dilation effect is more significant than the acceleration due to gravity; the frequency of light from the object appears to decrease, making it look redder, because the light appears to complete fewer cycles per "tick" of the observer's clock; lower-frequency light has less energy and therefore appears dimmer, as well as redder.

[DoctorBeaver]

I read it in a book by John Gribben, the author of many science books. I borrowed it from a library about 4 years ago.

Looking through his list of books I found "Unveiling the Edge of Time: Black Holes, White Holes, Worm Holes ". It could have been that 1.

[yor_on]

DB, here is the link to that those 'new' BH I was referring too. As I read it over a year ago and also found that it then was quite supported by a lot of physicists, it made me wonder where it had gone. http://www.physorg.com/news101560368.html "what is controversial about the new finding is that "from an external viewer's point it takes an infinite amount of time to form an event horizon and that the clock for the objects falling into the black hole appears to slow down to zero," said Krauss, director of Case's Center for Education and Research in Cosmology."

[dlorde]

I read it in a book by John Gribben, the author of many science books. I borrowed it from a library about 4 years ago.

Oh yes, Gribben - I've got some of his - 'In Search Of Schroedinger's Cat', 'Schroedinger's Kittens', 'the Matter Myth'. He's usually pretty good.

[yor_on]

The evidence for Black Holes seems rather overwhelming
But when it comes to those spinning at almost 'c' I can't help wondering over how this framedragging expresses itself. First you have the sheer mass in itself, in a non rotating black hole (Schwarzschild geometry) you will have a point of no return at the EV. And with a spinning BH (Kerr metric)it seems to me that this 'point of no return' should be seen even further out, before reaching any EV (event horizon)? Shouldn't this move the EV? And also 'collect' the matter falling in into a concentrated 'density' dragging it around that BH as all matter has an 'inertia'?

[DoctorBeaver]

I think I've remembered what that Blue Curtain thing is all about. It's from the perspective of an observer falling into the EH. Does that sound better?

[yor_on]

If we have light emitted from a object falling into a spinning BH, that light will have to make its way back up the 'gravity well' to the observer, will it then do that 'both' ways, as seen thinking of that spin? I mean, being reflected going against that spin will give it a extreme redshift, but when reflected as light when 'spinning' towards you shouldn't it get a blueshift and because of that have a 'easier' way out from that BH, still being redshifted due to the gravity well but not as much as the light reflected seen from that other angle? But then again it will only have one way to go and that is searching the easiest path out, which will be a spiral, ah, forget it, the light can only go one way
And 'red blue shift' will be 'equalized' as it moves around I suppose?
Or, will it? Depending on where its finally comes from meeting your observer??

[yor_on]

This Site is nice too:)
http://chandra.harvard.edu/resources/flash/bh_truth.html

[Vern]

And 'red blue shift' will be 'equalized' as it moves around I suppose?
Or, will it? Depending on where its finally comes from meeting your observer??

It seems that a spinning black hole should appear as both red shifted and blue shifted depending upon the circumference observed. My guess is a galactic black hole would be spinning in the plane of galaxy in the centre of a huge accretion disk. I doubt that black holes exist as a singularity for the same reasons cited in yor_on's link. You can never get completely there because of the relativistic behaviour of light and matter.

[LeeE]

I think I've remembered what that Blue Curtain thing is all about. It's from the perspective of an observer falling into the EH. Does that sound better?

That sounds reasonable.

[dlorde]

I think I've remembered what that Blue Curtain thing is all about. It's from the perspective of an observer falling into the EH. Does that sound better?

I don't see it myself []

If you're falling towards the EH, you're accelerating. You'll be accelerating away from everything further out, so if you look back, it will all be red-shifted. Everything in front of you (toward the EH) is accelerating away from you into the BH, so that should be red-shifted too. So where does the blue light come in? Maybe if you were stationary at the EH, you'd see stuff coming in toward you as blue-shifted...

[DoctorBeaver]

dlorde - I don't remember the details of it. It was at least 4 years ago and I didn't know much about physics in those days (I still don't, but I knew even less then). It didn't really sink in.

[dlorde]

dlorde - I don't remember the details of it. It was at least 4 years ago and I didn't know much about physics in those days (I still don't, but I knew even less then). It didn't really sink in.

OK, I was just curious.

[itisus]

"That's a hundred thousand tredagrams, and you'll never get the chance to use that word in relation to anything else."

It's a lot less than a googleplex though, and there is one of those in Mountain View.  I've been in Mountain View a few times; it's no big deal.

It's late, but my scratch paper gives about 10^(-8) grams /m^3.  Air at sea level is on the order of kg/m^3.

I have wondered whether the universe should be considered a black hole.  Nothing escapes unless it is evaporating, its size is unknown but much larger than 50 billion suns, and the usual Schwarzschild interior solution is dubious.

[LeeE]

I think I've remembered what that Blue Curtain thing is all about. It's from the perspective of an observer falling into the EH. Does that sound better?

I don't see it myself []

If you're falling towards the EH, you're accelerating. You'll be accelerating away from everything further out, so if you look back, it will all be red-shifted. Everything in front of you (toward the EH) is accelerating away from you into the BH, so that should be red-shifted too. So where does the blue light come in? Maybe if you were stationary at the EH, you'd see stuff coming in toward you as blue-shifted...

You've got to remember the time-dilation effects too.

[Vern]

The Event Horizon is a region around a black hole that marks the boundary where light closer in can never exit the black hole. Why does that fact give the boundary special properties?  The natural laws should still apply. A steel rod part way past the EH should still allow its internal construct to follow a force on the outside part that pulled it out of the EH.

I think we globally describe the event horizon then in our minds, give it special properties that the General theory of Relativity does not give it.

We like to say that, once past the event horizon, nothing can escape. Maybe that should be modified to say that nothing operating under its own momentum can escape.

[dlorde]

You've got to remember the time-dilation effects too.

OK, please explain, I don't see how time-dilation would cause an observer falling into the BH to see blue-shifted light.

[dlorde]

The Event Horizon is a region around a black hole that marks the boundary where light closer in can never exit the black hole. Why does that fact give the boundary special properties?  The natural laws should still apply. A steel rod part way past the EH should still allow its internal construct to follow a force on the outside part that pulled it out of the EH.

I think we globally describe the event horizon then in our minds, give it special properties that the General theory of Relativity does not give it.

We like to say that, once past the event horizon, nothing can escape. Maybe that should be modified to say that nothing operating under its own momentum can escape.

Yes, there is no special property at the EH, it's simply the point at which the escape velocity is > c. An observer falling through the EH wouldn't notice anything - assuming the BH is large enough that tidal forces don't rip him apart  []

[Vern]

That was my thinking exactly. However, we seem to be thinking of the EH as the singularity itself. It seems to me that the EH would be some distance from the singularity.


Wiki article explaning the Schwarzschild radius

In 1916, Karl Schwarzschild obtained an exact solution[1][2] to Einstein's field equations for the gravitational field outside a non-rotating, spherically symmetric body (see Schwarzschild metric). The solution contained a term of the form 1 / (2M − r); the value of r making this term singular has come to be known as the Schwarzschild radius. The physical significance of this singularity, and whether this singularity could ever occur in nature, was debated for many decades; a general acceptance of the possibility of a black hole did not occur until the second half of the 20th century.

[DoctorBeaver]

Apparently you don't actually need a singularity for an event horizon to form. If the mass is spread over a large enough area you could still get an EH. I've seen this theory proposed for the universe. It was referred to as a brown hole (sounds a bit dodgy, that).