## Black Holes and Gravity

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Belial
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### Re: Black Holes and Gravity

ekzrated wrote:
EricH wrote:Actually, the gravitational pull decreases, as you move deeper into the planet; what keeps increasing is the pressure, from all the material that's now above you

Okay. First, let's call the center of the earth "down", because normally you're pulled directly toward the earth's center. The reason the earth pulls you toward it is because of the mass "below" you, pulling you "down".

Now, as you progress deeper and deeper into the planet, there is less and less mass "below" you pulling you "down", and more mass "above" you. So you're not pulled as strongly toward the center, because there's less mass below you to pull on you.

However, all of that mass "above" you is also being pulled "down", so it exerts pressure on you, which acts a bit like gravity sometimes. Except it also pushes in on your sides.
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Anpheus
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### Re: Black Holes and Gravity

GMontag wrote:
Anpheus wrote:The straightest lines of all come from photons, which travel very very fast, and the more energetic they are, the less they will stray from what we consider a straight line.

I'm pretty sure this is wrong. All photons will follow a geodesic regardless of their energy.

I thought I may have been wrong, but I was under the impression that higher energy photons would follow a straighter path (following from the properties of inertia.) Thanks for the correction.

EricH wrote:That the internal heat of Earth comes from Hawking radiation of a microscopic black hole? There's too much of it. A black hole big enough to radiate that kind of heat would have swallowed the planet a long time ago.

I was under the impression that the heaviest black holes emit Hawking radiation the most slowly, and with the lowest temperature, and that the smallest black holes would have the highest perceived temperature? This is why scientists are not afraid of "micro-black holes" being created in laboratories: the evaporation time would be nearly instantaneous.
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william
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### Re: Black Holes and Gravity

Anpheus wrote:
EricH wrote:That the internal heat of Earth comes from Hawking radiation of a microscopic black hole? There's too much of it. A black hole big enough to radiate that kind of heat would have swallowed the planet a long time ago.

I was under the impression that the heaviest black holes emit Hawking radiation the most slowly, and with the lowest temperature, and that the smallest black holes would have the highest perceived temperature? This is why scientists are not afraid of "micro-black holes" being created in laboratories: the evaporation time would be nearly instantaneous.

You're right. Interestingly, if a black hole's "temperature" is lower than that of its surroundings it gets bigger and then colder.
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Banksy
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### Re: Black Holes and Gravity

Yeah, most space is only empty on average.
There's particles and antiparticles being created and obliterating each other all the time, pretty much everywhere we might call empty.

Something I wanted clear up; are black holes so infinite?
As in; can they, in theory, take in all the matter in the universe?
Or is there a limit to how much they can take in?
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### Re: Black Holes and Gravity

Banksy wrote:Yeah, most space is only empty on average.
There's particles and antiparticles being created and obliterating each other all the time, pretty much everywhere we might call empty.

Something I wanted clear up; are black holes so infinite?
As in; can they, in theory, take in all the matter in the universe?
Or is there a limit to how much they can take in?

Not quite true, they're called virtual particles because unlike regular antiparticles, they don't annihilate and give off the combined energy of the two particles. Don't confuse virtual particles with regular particles and their anti-counterparts.
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ekzrated
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### Re: Black Holes and Gravity

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Anpheus
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### Re: Black Holes and Gravity

Which idea? The idea that there is a black hole at the center of major astral bodies? That is very far fetched, as an extremely hot black hole would be so tiny as to disappear (it would radiate away its energy through a complex process so quickly it couldn't pull in nearby matter fast enough to stay) or it would be so massive as to, well, suck in the rest of the surrounding matter.
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ekzrated
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### Re: Black Holes and Gravity

So then it can only be too small, or too big? So we still think there is no limit to how much mass a black hole could absorb?
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Anpheus
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### Re: Black Holes and Gravity

The one we are all but absolutely certain exists at the center of our galaxy, Sagittarius A*, is extremely massive and therefore very stable. I couldn't give you an estimate on how long it would take for that thing to evaporate, but I'd suppose that supermassive black holes will be the last thing to evaporate when the universe dies a heat death.
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ekzrated
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### Re: Black Holes and Gravity

Thanks. Again, being uneducated, it's hard to find the right answers to the questions I have at times.
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Anpheus
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### Re: Black Holes and Gravity

A lot of information can be found at Wikipedia, be wary of being too trusting and always look for sourced statements, but it's otherwise really quite good these days.
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ekzrated
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### Re: Black Holes and Gravity

Anpheus wrote:A lot of information can be found at Wikipedia, be wary of being too trusting and always look for sourced statements, but it's otherwise really quite good these days.

Och... Haven't you noticed by now that I'm far too lazy to do all that work myself?

But seriously, thanks. This has all been very informative.
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### Re: Black Holes and Gravity

Anpheus wrote:Which idea? The idea that there is a black hole at the center of major astral bodies? That is very far fetched, as an extremely hot black hole would be so tiny as to disappear (it would radiate away its energy through a complex process so quickly it couldn't pull in nearby matter fast enough to stay) or it would be so massive as to, well, suck in the rest of the surrounding matter.

There is basically one size where the black hole would be in thermal equilibrium, and it's not a stable equilibrium.
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### Re: Black Holes and Gravity

Right, it would either have to be so small that it couldn't possibly affect the area around it and would be gravitationally insignificant, but would then evaporate quickly, or it would be so large that it would have the impact suggested, but that would allow it to suck in the rest of the planet over a rather short period of time.
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mrbaggins
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### Re: Black Holes and Gravity

Regarding black holes sucking in indefinitely no, not as such.

It is a widely believed fact (Love that line) that a black hole is an entity in space, much like a planet or a star. It's just that either its size or density is so great that it has a huge gravity force on its surroundings, and as such can 'suck' in everything, including light nearby.

Another but less followed 'fact' is that a black hole grows whenever something less dense than it gets sucked in. The amount that it 'grows' is directly proportional to the relative difference in mass of the black hole to its new victim. IE: If a bird flew into a black hole, it will change size less than if Earth were to fall in.

It is even less likely believed that should a black hole get large enough, it would cease to be a black hole. However, before this point, it is expected that it would inwardly collapse, the results of which could vary widely. More than likely, it would turn into a black hole again, but denser (And smaller, and hotter) than before.

Whilst it certainly makes an interesting image that there is a 'black hole' inside everything, the flaw rests in the fact that there basically is. A black hole is nothing more than a large collection of dense material. At the center of Earth is expected to be a large deposit of (if memory serves from reading this as a kid) molten metal. The only difference between these two is that the large enough to collapse...

Something you may not be aware of, but need to be for this, is that Pressure, temperature and size are all related. Hot material takes up more space (or is under greater pressure) than cold. Large items are either under less pressure or hotter than small items.

And so, basically, the earth is small enough that its net heat effect is low (We can live on its surface at around 300 kelvin) and at its center, its quite hot and under a lot of pressure.

A black holes surface isn't the area where light cannot escape, its on a surface much like the earths, just nothing would survive there. It would be insanely hot and the pressure, even at "Ground zero" would literally tear apart a planet under its own weight.

So... er... in summation? The earth and a black hole are pretty much one and the same, its just that one is so much larger/denser/hotter than the other, in a manner that means it can somewhat sustain itself.
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antonfire
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### Re: Black Holes and Gravity

mrbaggins wrote:Regarding black holes sucking in indefinitely no, not as such.

It is a widely believed fact (Love that line) that a black hole is an entity in space, much like a planet or a star. It's just that either its size or density is so great that it has a huge gravity force on its surroundings, and as such can 'suck' in everything, including light nearby.
Not quite. Black holes are qualitatively different. A black hole is a singularity.

mrbaggins wrote:Another but less followed 'fact' is that a black hole grows whenever something less dense than it gets sucked in. The amount that it 'grows' is directly proportional to the relative difference in mass of the black hole to its new victim. IE: If a bird flew into a black hole, it will change size less than if Earth were to fall in.
Yea, this is pretty well established.

mrbaggins wrote:It is even less likely believed that should a black hole get large enough, it would cease to be a black hole. However, before this point, it is expected that it would inwardly collapse, the results of which could vary widely. More than likely, it would turn into a black hole again, but denser (And smaller, and hotter) than before.
No. Nobody who knows a decent amount about the physics of black holes believes that.

mrbaggins wrote:Whilst it certainly makes an interesting image that there is a 'black hole' inside everything, the flaw rests in the fact that there basically is. A black hole is nothing more than a large collection of dense material. At the center of Earth is expected to be a large deposit of (if memory serves from reading this as a kid) molten metal. The only difference between these two is that the large enough to collapse...
Kind of.

mrbaggins wrote:Something you may not be aware of, but need to be for this, is that Pressure, temperature and size are all related. Hot material takes up more space (or is under greater pressure) than cold. Large items are either under less pressure or hotter than small items.
Or.. made of a different material.

mrbaggins wrote:And so, basically, the earth is small enough that its net heat effect is low (We can live on its surface at around 300 kelvin) and at its center, its quite hot and under a lot of pressure.

A black holes surface isn't the area where light cannot escape, its on a surface much like the earths, just nothing would survive there. It would be insanely hot and the pressure, even at "Ground zero" would literally tear apart a planet under its own weight.
Fuck no! A black hole has no "surface" like the Earth.

mrbaggins wrote:So... er... in summation? The earth and a black hole are pretty much one and the same, its just that one is so much larger/denser/hotter than the other, in a manner that means it can somewhat sustain itself.
Nope. Again, a black hole is a singularity, the Earth is not. Nobody really knows what goes on inside a black hole, and in a sense it's not even a physical question, because (in theory) we can't get any information from beyond the event horizon.

You might be thinking of neutron stars.

But, seriously, where did you get your information? Either you're confused or whoever you got it from is confused.
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### Re: Black Holes and Gravity

So, what's the general consensus? Am I fucking crazy or is my idea possible?
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EricH
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### Re: Black Holes and Gravity

I'm going to disregard most of mrbaggins' post, because almost everything in it is either false, misleading, or pointless, as far as I can tell. The one point that seems to be relevant is:
mrbaggins wrote:a black hole grows whenever something less dense than it gets sucked in. The amount that it 'grows' is directly proportional to the relative difference in mass of the black hole

(that's actually not quite accurate either; if something equally dense, like another black hole, is absorbed, the black hole still grows.)
To put that differently--the size of a black hole is directly proportional to its total mass. When anything falls in, its mass gets added, and the black hole increases in size accordingly.

ekzrated wrote:So we still think there is no limit to how much mass a black hole could absorb?

Microscopically small ones might evaporate before they have a chance to grow; if not, a black hole will keep absorbing whatever matter or energy comes its way, and the more it absorbs, the faster it can absorb more, because material flows faster, through a larger hole.
There's a point where it evaporates exactly as fast as it absorbs new material, but the material would have to keep flowing at a constant rate, to keep the black hole at that size, otherwise it will either shrink to nothing, or grow to absorb all the matter available. The rate matter would be falling into a black hole is much different, in the core of a planet, than it would be in a planetary nebula, during the formation of a solar system. Having a black hole remain at the equilibrium point throughout the process is an unlikelihood akin to rolling a marble along a wire. For a mile. The idea of finding black holes at the center of all celestial bodies is so mind-bogglingly improbable as to exceed my knowledge of mathematical notation.

ekzrated wrote:So, what's the general consensus? Am I fucking crazy or is my idea possible?

The two options are not exclusive--it's _possible_ to flip a coin and get heads six billion times in a row, but to claim that the current coin flip, the only one observed, is the six billionth consecutive flip that's come up heads, is fucking crazy.
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### Re: Black Holes and Gravity

Thank you. That clears things up a bit.
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### Re: Black Holes and Gravity

Hrm... yeah... I just read what I wrote the other night... I have no idea what I was confusing it with... must have been neutron stars.

Note to self, dont post at that time in the morning.
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