# Black Holes .

Discussion in 'Astronomy, Exobiology, & Cosmology' started by river, Aug 11, 2019.

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1. ### DaveC426913Valued Senior Member

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They are symmetrical.

3. ### DaveC426913Valued Senior Member

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And you can tell that by looking at a picture for 15 seconds can you? When 10 minutes ago, you didn't know what a black hole was made of?

5. ### river

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What is they that are symmetrical ?

7. ### DaveC426913Valued Senior Member

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A BH with its accretion disc is shaped kind of like Saturn with its rings.

Saturn looks the same from all sides - it's symmetrical about its rotational axis.
If you fly around its equator, you'll see the same thing from every angle.
Same with a BH. If you fly around to the opposite side, it looks the same.

But if you fly to Saturn's pole, you will be looking down on the plane of its rings.
Same with a BH. If you fly to its pole (without getting crisped by its jets), you will be looking down on its accretion disc.

Saturn:

BH:

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I see .

9. ### river

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There is no symmetry in a singularity .

10. ### DaveC426913Valued Senior Member

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That's great but we're not talking about a singularity. We are talking about what we can see outside the event horizon.

11. ### river

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So the event horizon and the singularity are not related to each other ?

So you can have an event horizon without a singularity .

12. ### DaveC426913Valued Senior Member

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They are related.
The matter outside the event horizon orbits, just like any other body, so it forms a disc with an axis.

You cannot have an event horizon without a singularity.
An event horizon forms when the mass of the BH is so high that light cannot escape.
The event horizon is not a thing; it is simply the radius at which the escape velocity of the BH exceeds the speed of light.

13. ### river

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What form of mass is this BH ( black hole ) made ?

14. ### DaveC426913Valued Senior Member

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Regular matter.
Black holes are formed when stars run out of fuel and collapse.

15. ### river

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But black holes precede galaxies .

All galaxies are thought to have a black-hole at their core .

Last edited: Aug 11, 2019
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16. ### DaveC426913Valued Senior Member

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No.
Black holes are formed at the centre of galaxies, from infalling matter.

17. ### river

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Why ?

What form of matter has such high speeds to do this ?

18. ### DaveC426913Valued Senior Member

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Not speed. Just gravity.

If you have the mass of a large star in one place, and there is no radiative pressure from fusion, the matter will naturally collapse. There is so much matter that gravity overwhelms the atomic forces holding atoms apart and they get crushed into a singularity.

19. ### RainbowSingularityValued Senior Member

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...
i would quite like to listen to some physicists discuss this aspect.

is nothing also related to infinity ?
can a theoretical mathematical end point be found for a black hole ... etc...

20. ### RainbowSingularityValued Senior Member

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you have a problem with my symmetry ?

21. ### DaveC426913Valued Senior Member

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I am simplifying.

'Singularity' simply means our current models break down. We will need some new models to explain what happens there.

We don't know what occurs at the centre of a black hole, but its mass still causes a dent in space time just as if it were there.

22. ### RainbowSingularityValued Senior Member

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still exits as energy ?

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Not a physicist but anyway.....Adding to Dave's excellent rundowns....
[1] When a star exhausts all its available fuel, we have three possible results, depending on the mass of the star...[a] Stars around our own Sun's size, end up as White Dwarfs...incredibly dense degenerate matter held up from further collapse by EDP [Electron Degeneracy Pressure] Larger mass stars are able to overcome EDP but are held up by further collapse by NDP [Neutron Degeneracy Pressure] [c] The largest of the stars at the end of their lives, are able to overcome even the NDP and form BH's.

[2] When the collapse reaches the Schwarzchild radius of any given mass, further collapse is compulsory, and so the mass collapses to a singularity.

[3] GR though fails us at the quantum/Planck level, and so we cannot reasonably speculate on the state of the matter at that region.

[4] Most physicists today, reject the singularity as defined by infinite spacetime curvature and density [or the mythical point singularity, rather they simply accept a singularity as defined where GR and the laws of physics desert us, that is the quantum/Planck level.

[5] If that is correct, and if the singularity of infinite qualities does not exist, then its reasonable to assume a surface of sorts, at or below the quantum/Planck level.

[6] We in actual fact can never extract or observe anything below the EH [where the escape velocity equals "c"] and are only able to come to the reasonable assumptions above, if GR is viable and valid, and we have no reason to expect it isn't.