Hey BABBers-

As part of my day job, I am possibly going to write a black hole FAQ. There already is one on the web, but I am looking to be more up-to-date, make it my style, cover other topics, etc.

So (seriously here), what kind of frequently asked questions would you like to see covered in a FAQ? I have my own list, but I want to see what others have. Check out the other one first and see if it didn't answer your questions.

Thanks!

The BA

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Phil Plait
The Bad Astronomer
http://www.badastronomy.com
badastro@badastronomy.com

Does your style mean the trademark bad/good format?

Lets see:
Bad: Sol will collapse into a black hole.
Bad: When it does, Earth will be sucked in.

No, this will be like a real astronomer wrote it. :wink: It'll be a straightforward FAQ, with a question-and-answer format.

I am interested in knowing how far the gravity from a black hole extends, what the suppossed size of the one in the center of the Milky way is, and how different types of stars become black holes.
Thanks.

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We've got a five dollar fine, for whining: Chris Ledoux

I have only the most rudimentory understanding of how black holes are theorized to work... so any FAQ info will be great from my point of view, as I find them fascinating. The question that has always vexed me... and forgive me if I display my ignorance of current theories on black holes... what happens to matter once it hits the singularity? I was always taught that matter can neither be created nor distroyed... it simply changes states. So what happens to the matter contained within, say, a star that crosses the event horizon and eventually hits the singularity? Is it compressed into the same infintessimally small space as the singularity itself?

1) Is a black hole really black or is it that we just can't see what color it is and it might as well be black?

2) If all of the energy emitted by the accretion disk around a black hole is highly red shifted, shouldn't black holes look red?

3) Could the elusive "dark matter", that is supposed to be most of the mass in the universe, be small, stellar/planet mass, black holes?

4) If a black hole is spinning, wouldn't the space around it be dragged around too, like someone rolling themself up in a blanket? If this is true, does space itself get dragged into the black hole?

These are all questions that I've been asked and to which I wish I'd had a better answer.

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It's just one of those damn things of which there are many few. -- Dan Blocker

My first question is, "How do they know black holes really exist?" So I read this:
And then my next question is, "How do they do that?" So then I read this:

Which doesn't help.

Still wanna know "howdeydodat?"



As in, "Measure it with what? And howzitwork?" Pretend I have no clue what "very-long-baseline interferometry" is...

I'd like a good, clear explanation of the "negative energy" that's involved in Hawking radiation. I think I have a handle on it, but I feel like I'm on pretty shaky ground.

The simple explanation is that the BH radiates from the event horizon because "negative energy" virtual particles fall into the hole, leaving the "positive energy" particles outside to become Hawking radiation. Somehow the gravitational gradient at the EH makes the inflowing particles seem to be lower than zero energy. That seems pretty, uh, funky...

I would like to see a discussion of the relationship between black holes and galaxy formation. I recall reading an article about a pretty well theorized and researched relationship between black hole mass and galaxy mass, but recently I read something on the web (at space.com, or someplace similar) that new evidence weakens this theory.

Thanks in advance.

Aporetic
www.polisci.wisc.edu/~rdparrish

Yes, Grendel took the words right out of my keyboard.

Paint us a picture of how all this sucked-in matter can exist (?) as a point (or whatever it is.) If I could somehow tour around this point as an imaginary point myself- what would life be like there? This would be one of those exercises like living with 2-D entities.

Cheers,
RBG

Ooh! Ooh! Mista Kotta!

I'd like to see this FAQ delve into the nits of black hole formation. Specifically:

• What's the upper limit for the mass of a neutron star before it would collapse into a black hole?
• Could a neutron star become a black hole by accreting more material in a mass-exchange binary? (I.e. like the way a white dwarf can become a neutron star by exceeding 1.4 solar masses?)
• If so, is there any cataclysmic event associated with this collapse of a neutron star into a black hole, like the way a collapsing white dwarf becomes a Type 1a Supernova?
• When a big star collapses into a black hole at the end of its life, can it go supernova? Hypernova? Gamma ray burster?
• In the case of a white dwarf or neutron star, not all of the progenitor star's mass gets bound up in the white dwarf/neturon star. A star in the process of becoming a white dwarf can shed up to half its mass as a planetary nebula, and a star that becomes a neutron star blasts a lot of itself into deep space as a Type II Supernova. When a star becomes a black hole, does all of its mass get sucked into its event horizon, or will a good deal of it escape into deep space? If the latter, how much will escape?

I don't care what you say so long as you start it off properly:

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Everything I need to know I learned through Googling.

Would a spinning star increase it’s rate of spin as it collapses into a black hole? Could the centrifugal force of the spinning, collapsing stars prevent a collapse into a singularity?

A discussion of the time effects for a person falling into a black hole vs. a person observing someone falling into a black hole.

Well, looking through the list of questions, I do not see:

"Where do black holes come from?" / "How are black holes created (or formed)?"

Then there is the question about the different types of black holes (i.e. supermassive black holes, other massed black holes). Just what are the different ones, are their properties different, location, ...?

There needs to specifically something about the black holes at the centers of the galaxies.

I also wonder about the "picture" we have of black holes: the accretion disk and two jets. Why is it like that? If gravity pulls in from all sides, why is matter coming in from only the disk - or is it only that? Why the disk? How does the disk form? What about those jets? (Not sure how to put all of this into one neat little question, though.)

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"As I lay beneath the Southern Cross, the stars tell more than I could" . . . David Meece

I'd like to see an update on the latest sky surveys as to where and how many black holes or potential back holes appear to be out there.

Massive black holes seem to be in the center of many galaxies, so do we know yet if that is the norm? And have exceptions been found, and are those galaxies different? Is it a time/distance phenomonon? Or not?

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You could add something about the possibility of using black holes as the entry for wormholes. In particular, under what conditions will this be possible. This was a question in some threads about a month ago and despite the SF connotations has been the subject of some recent articles in PhysRevLett. In any case, it's a common question.

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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." - William Thompson, 1st Baron Lord Kelvin

"If it was so, it might be, and if it were so, it would be, but as it isn't, it ain't. That's logic!" - Tweedledee

This isn't right. This isn't even wrong. - Wolfgang Pauli

I think a question about the formation of black holes (hypernovae, collapsing neutron stars) and the mechanisms behind them would be really good as well as the size of black holes and stuff (Schwarzchild radius).

BTW, what website will this FAQ be posted? (ducking and running) :wink:

Difference between Kerr black hole, Kerr-Newman black hole, Reissner-Nordström black hole and Schwarzschild black hole

Once a singlarity has formed is there any way in which it can made to be not a singlarity?
If a tiny blackhole evaporates, would it evenually disappear? What's the smallest mass, that black hole could have?

How about dealing with the publics conceptions/misconceptions of what a blackhole would look like. Is a swirling whirlpool of light accurate?
Differences between rotating and non-rotating BH.

I'd like to see the similarities and differences between a black-hole singularity and the beginning of time, pre-Big Bang singularity.

What is the difference (okay, besides the obvious) between rotating and non-rotating black holes? Ditto for charged vs. uncharged black holes. I never figured out how a singularity could have any spare electrons, which means I probably missed the whole point of the concept.

I seem to recall reading a discussion of the above, though it was a long time ago, and honestly I couldn't follow much of it.

Another good question might be this -- How common are black holes? How many are you likely to find in a mythical 'average' block of space, say, ten light years on each side?

Too, is it true that every black hole is really the doorway to fantastic adventures (hey, that's what the Sci-Fi channel implies...)

Hehhehheh... Phil, I think you may have opened Pandora's box here. Black holes seem to attract as many questions as they do particles!

This may have already been asked but here goes:

Are Black Holes really circular "drain-type" where there is a flat face toward which everything is being pulled or is it a point at the very center neither above or below the material disk?

I can explain more if no one understands, I don't think I would. -Colt

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Be not afraid of any man no matter what his size; when danger threatens, call on me, and I will equalize.

Hi Phil,

Here are a couple of (loaded) questions:

Do astronomers and physicists think of the singularity within a black hole as a representation for a zone where theory breaks down, or as an actual location of infinite mass?

Does the current understanding of black holes allow for alternative theories about the singularity, and if so, what are they? :wink:

Chip

Difference between space and space-time.

I would love to see more people educated about black holes and time/space warping such as:

Q: If time slows to zero as one reaches the event horizon of a black hole, how can one say the things fall all the way in?

Q: If time goes "backwards" within the EH of a BH,(Yes, using abbreviations like EH, BH, GR, BABB, BA, etc... makes me feel smart ) don't things "fall" back to the EH and get stuck there?

Q: What is a worm hole, and what does it have to do with BH's?

Q: What is Hawking Radiation?

Q: What does "Black holes have no hair" mean?

Q: What is the difference between a white dwarf, a neutron star, and a BH?

Q: Will the sun ever turn into a black hole?

etc... etc... etc...

I'm most interested in astro-physics, posmology and particle physics. You may be wanting stuff that requires a little less math, but these are some questions that interest me.

I'd be interested in the state of play as to actual black hole candidates. The puzzle seems to be that we have many observational candidates in the sense of black holes at galactic centres (particularly spiral galaxies, I think), but very few candidates outside the centres - in our Galaxy, as far as I know, Cygnus X-1 and a mere handful of others.

It seems to say something about the likelihood of black hole formation - i.e. perhaps, outside a galactic centre, this cannot happen as result of a hypernova or supernova, and requires some unusual process (for example a neutron star accreting matter from a companion star). I guess therefore it falls under the FAQ section for 'How do black holes form?'...

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Fin
Skep-ti-cult® member #488-28303-790

Q. What are the relationships between the density and mass of an object as it collapses to form a black hole?