View Full Version : Black holes

2003-Jul-16, 11:50 PM
I have always wondered how black holes function, look inside and how old can they get. They are one of the universes most incredible creations, but still I haven't got a clue of how they function or look like inside or why there are even out there.

Here are some questions I've asked, hope to get a better understanding of them.
I will aprecciate very much any replies.

How does a black hole occure?
How's the inside of a black hole?
Would anyone survive the trip to the center of a black hole?
Why do black holes create disks of gas around them when aproaching a star,
instead of just swallowing it?
I've heard that one day black holes will overheat and start to everporate. How come?
Are black holes new doors to other universes or dimensions?

2003-Jul-17, 06:41 AM
I'm an 18 year old jazz piano player, not an astronomer, but I think I can answer most of your questions based on theory that I have read. Somebody feel free to correct me if I have made a mistake.

- How does a black hole occur?
When a sufficiently massive star's nuclear fuel is exhausted, the energy it produces is nolonger great enough to counteract the gravitational forces pulling the matter in the star to the centre of gravity. The star contracts, and becomes dense until the matter is so dense, and the radius small enough so that the escape velocity (the velocity needed to escape the object's gravitational pull) is greater than the speed of light (300000 km/sec). Hence no light (or anything else) can escape (according to classical physics anyway). There is a certain distance from a black hole dependant on its size and mass beyond which no light can escape. This is known as the event horrizon.

- How's the inside of a black hole?
Think of an incredibly dense collapsed star. Not very pleasant.

- Would anyone survive the trip to the center of a black hole?
Highly unlikely. Black holes are small and dense enough so that the gravitational force acting on your head and your feat could be different enough to rip your body in two. Keep in mind that some black holes can be very very small and highly dense (think the mass of mountain in something the size of a peanut). Even if you did make it to the center, you'd never get out again, except as some types of radiation (read below).

- Why do black holes create disks of gas around them when aproaching a star,
instead of just swallowing it?
This is one specific scenario which may not always be the case. I'm guessing if they are not approaching directly, then gas from star could spiral in towards the black hole due to its gravity (in the same way that if Earth was travelling a little slower, we could gradually spiral in towards the sun).

- I've heard that one day black holes will overheat and start to everporate. How come?
When quantum mechanics and the uncertainty principle (which states that the more accurately we can measure an object's velocity, the less accurately we can measure its position and visa versa) are taken into consideration, there is a certain probability that radition could, for a very short period of time, effectively travel faster than light and thus escape a black hole (I have oversimplified this situation in the extreme...I suggest reading some of Stephen Hawking's literature, as it was he who originally put forward the idea that black holes actually emit radiation and act in thermal equilibrium with the universe). In theory once black holes emit enough radiation they will lose all of their mass.

- Are black holes new doors to other universes or dimensions?
There are some theories suggesting that spacetime as defined by Einstein's General Relativity might be curved enough around a point of infinite density (a singularity) to branch off and lead to with other parts of the universe or indeed other "baby universes". Imagine a sheet on your bed where objects like a marble ball cause depressions in the sheets (this is the way Einstein explained gravity). Your sheets could be curved enough to wrap around on themselves and creat pockets of spacetime separate from the rest of the universe. You'd never be able to travel to them though as you'd be turned into mush before you made it anywhere near that far. I'm really bad at explaining things so I won't try any harder than I already have. Hawking's literature is fantastic at explaining general relativity and all kinds of black hole related ideas. You should read a brief history of time, the universe in a nutshell, and 'black holes and baby universes' for a more accurate picture.

Hope I was of some assistance.


2003-Jul-17, 07:32 AM
Nicely done. :-)

2003-Jul-17, 10:31 AM
What an outstanding list of answers! - I had to mention though that you missed an opportunity to use a rather fantastic word when explaining the difference in gravitational pull between your feet and your head as you cross the event horizon.....
Spaghettification !!!!

A brilliant word, shame to miss it out when faced with such a great chance to use it :)

Of course your answer was brilliant even without this humdinger of a word.


2003-Jul-17, 05:48 PM
Well, I'm not a scientist, but I'd like to try to clarify the part about the "inside" of a black hole. As I understand it, there is no inside. The singularity at the heart of a black hole has no dimension at all (including time), only mass. It's more like an infinitely small mathematical point than something small and squished. All of what we normally think of as space and time is totally collapsed into this one point, which has only position and mass, although that mass can be enormous. If you were to somehow get within sight of a black hole that wasn't completely hidden by the superheated matter falling into it, what you would see (or not see, since it's black) would be the event horizon, which is the point in space where the escape velocity from the singularity equals the speed of light. Thus the event horizon would appear as a black sphere which surrounds the singularity. Light coming from the other side of the event horizon won't reach your eyes, since it fails to escape the gravitational pull of the singularity and falls back in to the center. This absence of light getting beyond the event horizon is what makes a black hole black.

2003-Jul-18, 04:39 PM
Black holes rotate, and I believe this has some effect on how matter tends to form a disk around it. It all depends on the angle and direction that the matter takes as it approaches the black hole.

2003-Jul-21, 08:43 AM
How does a black hole's rotation (or that of any large object for that matter) affect the motion of matter surrounding it? Is this to do with the "twisting effect" it has on the fabric of spacetime?


2003-Jul-28, 06:44 PM
Here is a good webpage with info on this exact topic...

How Black Holes Work (http://www.howstuffworks.com/black-hole.htm)

. ..-={A}=-.. .

2003-Jul-28, 09:57 PM
Thanks everyone for your replies, I'm learning alot from them. I apricciate them very much.

And Arramon that webpage was a big help.

2003-Jul-28, 10:02 PM
You're welcome =)

There's definitely a plentiful amount of info out there for us to learn from....
And finding the worthwhile sites are great!!
Others i find, i will share also =)

. ..-={A}=-.. .

2003-Aug-02, 03:22 AM
When a massive star shrinks to form a black hole, its mass doesn't change, just its density. So how come its gravitational field increases? Gravity is a result of mass, isn't it, not density? :huh:

2003-Aug-02, 02:51 PM
The gravity you feel on the surface of an object--such as the Earth--depends on both the object's mass (the more the mass, the greater the surface gravity) AND on the distance to the object's center (the less the distance, the greater the surface gravity). When a star collapses, its mass doesn't increase, but the distance from the surface to the center decreases, thereby boosting the surface gravity.

For more on black holes, see the (heavy) book by Kip Thorne, Black Holes and Time Warps (http://www.amazon.com/exec/obidos/ASIN/0393312763).

2003-Aug-05, 03:04 AM
There are a lot of very good anwsers here.
I've only quikly scanned through but a lot of good responses.

In short a black hole is a hole lot of nothing. ;)

2003-Aug-05, 08:04 AM
Originally posted by Planetwatcher@Aug 5 2003, 03:04 AM
There are a lot of very good anwsers here.
I've only quikly scanned through but a lot of good responses.

In short a black hole is a hole lot of nothing. ;)
Quite the opposite...a black hole is not really a hole but a pouch and it contains more matter than our sun but in an itty bitty singularity.

2003-Aug-05, 11:29 AM
I once read somewhere that none of the laws of science don't work once you're in a black hole, and that Einstein's theory of Relativity and the um.......! something else is combined together to see what happens or something like that :blink:
Bah! I just forgot the other part, never mind me :ph34r:

Oh and yeah, there are three types of black holes, Supermassive black holes, located at the centre's of Galaxy's, Stellar Blackholes, which are created when a massive star collapses and mini black holes created at the big bang.

2003-Aug-05, 02:37 PM
charged..uncharged... spinning...not spinning.... charged and spinning.. not charged and not spinning... there are different kinds of black holes sitten around out there...

The hole could be so small that you wouldn't know it was there until you were stuck in it... unless you could detect the gravitational pull of it before hand...

as for time and space(mass/matter) within... they are reversed, so it is said, in relation to one another before the singularity was breeched... 3 parts matter(space) becomes one part time, one part time becomes 3 parts matter(space), so you'd have 3 parts time and one part matter(space).... thats an iffy one to me... thats like saying you'd have past present future awareness, and that your mass/matter would be contained in all...

yeah... the spice extends life... it expands consciousness...

..it just may be that the pull/push within is so great, you are dispersed and used, like any other object being drawn in... matter drawn in, energy released.. both seem to be functioning at the same input/output levels... stars move in, jets of plasma are released... you move in, and something of you is released...

i do believe that time wouldn't be a factor within a black hole... unless you could seperate yourself from the force around you, become a seperate void, but the integrity of that void must be so strong, that it could withstand the singularity.
An anti-grav bubble may help you to get close, but going beyond the event horizon may require additional counter-active components... a temporal shielding of some kind... and if we could figure a way to be as the expelled elements are, we may find a way to exit the hole... just as long as we weren't something the hole needed to digest... mmmmmmmmmmmm..... food...

now i'm hungry... :P

. ..-={A}=-.. .

2003-Aug-06, 07:50 PM
Phil Plait over at Bad Astronomy gets so many questions about Black Holes that he's building a special FAQ about it. The guide is still in development on his discussion forum, but you can see the kind of discussions going on here:


2003-Aug-07, 10:49 PM
:o Gosh and I thought a black hole was a politicians expense account

2003-Aug-07, 11:49 PM
When a massive star shrinks to form a black hole, its mass doesn't change, just its density. So how come its gravitational field increases? Gravity is a result of mass, isn't it, not density? (Faulkner Posted on Aug 2 2003)

My understanding is that the "gravitational field" doesn't strengthen, as the object is indeed of the same mass. For example, if the sun was to hypothetically shrink and become a black hole (which couldn't really happen, but will assume that its matter becomes sufficiently dense for it to do so), the planetary orbits wouldn't change as the same gravitational force is exerted.

The only thing that changes therefore is the object's radius and therefore its density. This means that you can get closer to the centre of gravity of that object without being inside that object, and thus the gravitational force 2 metres away from the centre is going to be stronger than 2000 km away. For example the acceleration due to gravity on Earth at sea level is 9.8 ms^-2. This is, in other words, at a distance that is one radius length (about 6400 km I think) away from the centre of gravity (assuming it is perfect sphere). If Earth was to shrink and become more dense, the acceleration due to gravity at a distance of 6400 km would be the same, however if you were to stand on the 'new surface' of Earth, you would be much closer to the centre of mass and thus the gravitational force and therefore the acceleration due to gravity would increase.

It is theoretically possible to have blackholes with a diameter of less than a millimetre with the mass of say a small mountain. Because of their extreme density, light will not be able to escape but only when it's very very very close to the centre of mass (thus the event horizon will also be close to the centre of mass). These blackholes would have very little impact on their surroundings as far as gravity is concerned.


2003-Aug-08, 02:59 AM
Someone just mentioned that if you ever did enter a black hole, you would never be able to get out, except perhaps as radiation. I have also read many times that a black hole has such gravity that nothing can escape, not even light. And I have read that astronomers have detected a black hole by seeing the xrays that are emitted from it. Here's my question: Aren't radiation and xrays just particular "flavors" of light? If "light" can't escape, how is it that "xrays and radiation" can?

2003-Aug-08, 02:08 PM
A black holes strong gravity actually has two factors involved.

1 A great and ever increasing mass density from the origional star and everything
which has been sucked in. As more material is brought into the sigularity, the mass continues to increase, but never grows in size because it's density increases with it compacting it more and more. But since the mass continues to increase so does it's associated gravity.

2 A great vaccum of the space which the star origionally occupied now must be filled. Everything being sucked in to fill that space becomes super compacted into the singularity which still leaves a void to be filled. Even though it's always being fed it never gets filled up. Sort of like my son. ;)

So the area of space called the Swartscoff Radius (I may have misspealed the name) between the singularity and the event horizon really is the empty space here. That's where the objects being sucked in streches beyond imagonation.

As others have stated what goes in dont come out. Even with special shielding, the energy and elements which make it up will just become part of the sinularity and fail to function.