Ten things you don’t know about black holes

[Disinfo Agent]

Thought I'd post a link to this here, since so many people ask about black holes.


Allow me to be your tour guide to infinity. Or the inverse of it, I suppose. Since it’s Halloween this seems appropriate… and my book Death from the Skies! just came out, and there’s lots of ways a black hole can destroy the Earth. Mwuhahahaha.

Bad Astronomy Blog (http://blogs.discovermagazine.com/badastronomy/2008/10/30/ten-things-you-dont-know-about-black-holes/)

[Solid Bismuth]

Yeah, I read that article.

Black holes are so amazing and mysterious!

[Jeff Root]

It looks to me like Phil took several of his ideas from my posts in the days
just before Halloween...

-- Jeff, in Minneapolis

[Disinfo Agent]

I liked the article, and learned from it, but I have one little quibble about section 2) They’re not infinitely small.

What the BA writes here is plausible but speculative, right? He says he's basing himself on the math, but the truth is that theoretical physics hasn't been able to unify the math of general relativity with the math of quantum mechanics yet.

[Jeff Root]

I don't *completely* agree with what Phil wrote in #2, but exactly what
about it do you consider to be "speculative"?

Actually, I just re-read it more carefully, and with all the weasel-stuff he
put in, I *do* agree with #2 completely. I might place the emphasis just
slightly differently, but only slightly, and no reservation at all about his
weasel-limited statements of fact. :D

-- Jeff, in Minneapolis

[Seeka]

In that utube video he said the sun cant become a black hole, why not?

[John Jaksich]

There are couple of reasons that I can think of off the top of my head:

1. Our Sun's mass is too small----its final destination is a white dwarf ---> black dwarf

2. Given our Sun's small mass---it would need an accompanying Star (normally termed a binary system) to become a supernova


I have made this post a little simple but it (in a nutshell) is two primary reasons.

Anyone else care to add...

[Neverfly]

There are couple of reasons that I can think of off the top of my head:

1. Our Sun's mass is too small----its final destination is a white dwarf ---> black dwarf

2. Given our Sun's small mass---it would need an accompanying Star (normally termed a binary system) to become a supernova


I have made this post a little simple but it (in a nutshell) is two primary reasons.

Anyone else care to add...

Not enough mass to cause internal gravitational collapse. In other words- It doesn't weigh enough to crush itself under its own weight. A very large star that has the potential to become a black hole is held in balance- the outward pressure from the nuclear reactions pushing against the strong inward pull of gravity.
When the fusion sputters and the fuel is exhausted, the star can no longer push outward with enough force to overcome gravity and the mass collapses inward.
Again, as above- a simplistic description.

[John Jaksich]

Not enough mass to cause internal gravitational collapse. In other words- It doesn't weigh enough to crush itself under its own weight. A very large star that has the potential to become a black hole is held in balance- the outward pressure from the nuclear reactions pushing against the strong inward pull of gravity.
When the fusion sputters and the fuel is exhausted, the star can no longer push outward with enough force to overcome gravity and the mass collapses inward.
Again, as above- a simplistic description.

Thank you for your correction...but for most people...we should be aware of learning curves since the message may overwhelm the reader...

**************************

"It is not so much what you know...but how you present yourself and your knowledge..." --anonymous

[Neverfly]

Thank you for your correction...but for most people...we should be aware of learning curves since the message may overwhelm the reader...

I was expanding upon, not correcting;)

[adi11235]

I'm not sure if this is ever mentioned in black-hole related discussions.

A black hole may never actually form completely, from an outsider's point of view. As matter collapses toward the event horizon, time moves slower and slower. We can never see something actually fall beyond the event horizon. That's why it makes little sense to talk about a "size" of a black hole other than the event horizon itself (and the outside vicinity).

Or... do we have any theories around where a BH can actually form in finite time?

[Jeff Root]

I'm not sure if this is ever mentioned in black-hole related discussions.
It is mentioned fairly often, and I'm usually annoyed by it. :)



A black hole may never actually form completely, from an outsider's
point of view. As matter collapses toward the event horizon, time
moves slower and slower. We can never see something actually fall
beyond the event horizon. That's why it makes little sense to talk
about a "size" of a black hole other than the event horizon itself
(and the outside vicinity).

Or... do we have any theories around where a BH can actually form
in finite time?
There is some truth in everything you said. However, some parts
are exactly right, while others are misleading to the point of being
essentially wrong.

A black hole definitely forms completely, and does so very rapidly.

Matter falling into a black hole reaches the center (the "singularity")
in a short period of its own proper time.

It is exactly right that we can never see anything fall beyond the
event horizon, but what we would see is an object accelerating
downward, away from us, and redshifting to invisibility in a fraction
of a second. One moment it would be falling away from us with a
moderate but rapidly-increasing redshift, and a second later it would
be gone.

Gravitational lensing does mean that during that last fraction of a
second, the object would stop shrinking in angular size as it moves
away from us, but it would continue to get dimmer just like it would
if it were accelerating away from us in normal space. Its time would
appear to us to slow to a stop, but that is due as much to light
travel time as to time dilation, and it only lasts for a fraction of a
second before the object redshifts to invisibility.

The matter that falls into a black hole definitely continues to exist
after it falls past our event horizon-- at least for a while, if not
forever-- and it is not outside the bounds of science to try to
puzzle out what is happening to that matter.

My personal view is that, because the matter causing the black
hole becomes denser and denser without limit as time passes,
the gravity well becomes deeper and deeper without limit, so the
distance from the event horizon to the center increases forever,
without limit. Anything falling to the center of a black hole has an
infinite distance to fall, relative to the outside Universe. However,
it will reach the center in finite time by its own measure because
of the way it experiences its journey through spacetime, and it will,
in some sense, reach the center in finite time for us because of the
limitations of uncertainty in the object's position and momentum.
Once a particle has reached a position some tiny distance from the
center, we can't say that it hasn't reached the center yet, because
uncertainty doesn't allow us to pin down the position precisely.

-- Jeff, in Minneapolis

[adi11235]

Thank you for your answers, Jeff.

I don't mean that if we can't see something cross the EH therefore it never crosses. Rather that from our time frame, nothing does. Even if we can imagine timelines relative the object falling in, we exist in separate timelines. So matter would accrete around the EH, never falling in.

Now, if I understand you correctly, if a particle gets to within Planck distance of the EH, it could cross through (with a given probability) and be actually absorbed by the BH in finite (outside) observer time. Is that correct?

Also, what do you think of this:
http://www.newscientist.com/article/dn12089


Adi

[mfumbesi]

Finally I can access the BA's blog, for some reasons I couldn't access it last year.
(blocked at work....)

[BigDon]

Thanks DA and Jeff.

You guys do this for a living?

[Disinfo Agent]

I'm not a physicist, if that's what you're asking. :)

[Buttercup]

Black holes seem like "anchors" to me, and also as "seeders" [of energy]; they provide necessary features to the universe. Maintainers. I'm not a scientist by a long shot, but I have a hunch the universe couldn't exist without black holes and that life [here and wherever else] is dependent upon them in some indirect but real way.

Those comments made based on what I've read/assimilated over the years. Could be completely wrong. And no, I can't write 1/2 page of theory backing it up. ;-P