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tommac
2008-Dec-23, 04:07 AM
Can a black hole be produced ( temporarily/permenently ) from a series ( at least 2 ) of objects that were near the gravitationalrequirement of a black hole?

What is the minimum density needed to form a BH? ( this is a real question as I dont know )

Lets say we named this as dmin. Lets say we had two masses that were just slightly above dmin. Now we brought these two very close to each other. Wouldnt the extra gravitational pull from the other dmin object provide the extra gravitational potential to push it into BH status?


BTW I think I asked this question before but cant find where I asked it nor have I ever seen the question answered. If this has already been answered can you point me to the OP?

astromark
2008-Dec-23, 04:18 AM
briefly, no. The two or more objects would need to actually merge and become a bigger than minimum mass. For a black hole to be born. A quasar, or other exotic object could be created., but not a black hole.

tusenfem
2008-Dec-23, 08:23 AM
You know, there ARE books that you might read before asking all these uncountable questions.

There is no minimum density, there is a minimal mass requirement. Try to go to the library and look in an astrophysics book and find Chandrasekhar mass. That will give you an indication about how and what. I would advise Bowers & Deeming, Astrophysics I and II, which are excellent introductory books.

sabianq
2008-Dec-23, 01:08 PM
Hey Tommac,
Why do get the feeling that you are working on something?

Hmmmm........

mugaliens
2008-Dec-23, 04:58 PM
I will help you find the answer, as when I do so, I learn things.

You need to start looking up your own information, as you're too smart not to be doing this work yourself. Resources that directly answer your question, such as "Calculating Density of Black Holes (http://astrophysics.suite101.com/article.cfm/calculating_density_of_black_holes)," at Suite101.com, are readily available. I found this one by Googling, "density of a black hole" then reading through the first seven, and choosing the first one that best fit to what I already know to be true.

Cosmic, huh?

It's work, tommac. It involves effort, something that you continually appear to be loathe to do. Judging by the comments of others herein, I am not alone in my assessment.

Now if you'll excuse me, my patience has reached an end.

captain swoop
2008-Dec-23, 05:03 PM
Getting quick answers to specific questions is ok but it doesn't give you an overall picture. It doesn't give you the reasons for the answer or the background theory.

As Mugaliens says, work is required.

Solfe
2008-Dec-24, 06:17 AM
I had a similar thought here:
http://www.bautforum.com/astronomy/10543-odd-black-hole-question.html

solfe

tommac
2008-Dec-24, 04:27 PM
After some thought I think I am able to answer my OP.

Basically I think the max gravitational well is at the center of mass. In this case if there was a mass that was very near being a black hole then the max of that would be at a point in the center.

Thus the distance always causes diminished gravity. It would not be possible to cause a black hole from two or more objects without one of those objects already containing a black hole or being a black hole.

Does that sound right? If you can understand what I am saying.

mugaliens
2008-Dec-24, 11:04 PM
You're right in saying that the max depth of the gravitational well is at the center, but that's not the location of the max gravitational attraction, which is at the surface for a homogenously-distributed mass.

For most masses in space, however, lower levels are more compressed than upper ones, so the max gravitational attraction actually is found somewhat below the surface.

tommac
2008-Dec-24, 11:49 PM
You're right in saying that the max depth of the gravitational well is at the center, but that's not the location of the max gravitational attraction, which is at the surface for a homogenously-distributed mass.

For most masses in space, however, lower levels are more compressed than upper ones, so the max gravitational attraction actually is found somewhat below the surface.


Well even the earth for example. The earths core has the greatest gravitation potential more than its surface.

I was almost going to write an ATM that proposed that at the center of ALL masses was a black hole, but then I realized that the rate of increase of gravitational potential diminished more quickly that r^2 and in fact diminished at r^3. ( Well maybe not as fact but as I understand it ).

Back to the BH issue. If there were two masses where the surface was close to a BH then you could have two of these masses close to each other as to create a BH .... but you would have a black hole somewhere under the surface already.

Tensor
2008-Dec-25, 04:02 AM
.... but you would have a black hole somewhere under the surface already.

That happens during a normal collapse into a black hole. Check out the difference between the apparent and absolute Horizons (http://archive.ncsa.uiuc.edu/Cyberia/NumRel/BlackHoleAnat.html#ApparentVsEvent).

mugaliens
2008-Dec-25, 10:11 AM
Well even the earth for example. The earths core has the greatest gravitation potential more than its surface.

Technically speaking, the greater the height, the greater the gravitation potential, as the term derives from "potential energy," which is equal to 1/2 m*G*h (kg-m). It's a bit more complicated in a varying gravitational gradient, but...

Thus:

Center: Lowest gravitational potential (0), lowest gravitational attraction (force due to gravity).

Slightly below the crust: Low gravitational potential, but highest gravitational attraction (about 1.09).

On the surface: Medium gravitational potential, and normal gravitational attraction (1.0).

In orbit: Higher gravitational potential, lower gravitational attraction (<1, and varies depending on altitude of the orbit).

Deep space: Highest gravitational potential, near-zero gravitational attraction.

Does this make sense?


I was almost going to write an ATM that proposed that at the center of ALL masses was a black hole...[quote]

I'm glad you didn't do that! It would have been a massacre... :whistle:

[quote]Back to the BH issue. If there were two masses where the surface was close to a BH then you could have two of these masses close to each other as to create a BH .... but you would have a black hole somewhere under the surface already.

If there's a black hole under the surface, the masses will not remain as they are for very long, as you're essentially asking it to creat a vaulted (spherical, actually) arch over essential empty space. It would cave in, created a larger void, greater concentration of mass and resultant gravitational pull, causing more cave-ins...

Plus, a black hole at the center of our planet wouldn't stay there! If you'll recall from other threads, an object inside a spherical radius where the matter is uniformly distributed outside that radius experiences no gravitational attraction at all, as the vectors from each ith unit of mass net to zero. As a result, any tidal forces from an external source would be free to act on that black hole, causing it to become an internal wrecking ball, moving one way, then the next, gobbling up matter from the inside out, until everything collapsed and was sucked into the black hole.

Another consideration is that fact that a black hole with the mass of the Earth would occupy about a tablespoon in size, and would go BANG in a microsecond, obliterating the Earth, if it were inside of it. In fact, a black hole about a meter across would evaporate in about one second, with enough resultant energy to vaporize our entire planet.

Finally, there's the matter of how it would get there. If it were to fall into the planet, it would tunnel through the planet like it wasn't even there, but ripping the planet to shreds as it did so, oscillating back and forth. However, we've already shown that it wouldn't last but a second, unless it was much, much larger, thereby gobbling the planet as a whole, en masse.

Thus, no black holes inside of planets, please... :) The concept just doesn't hold up under scrutiny.