Black holes

[LetThereBe]

As much as I hate to parade my ignorance, I wondered if someone could answer two things I've never understood.

I have heard black holes defined as having infinite density, or infinite mass. If that were the case wouldn't the force of gravity be infinitely strong, regardless of distance? Wouldn't the entire universe have been swallowed up as soon as a black hole began to exist?

If the speed of light is constant, how can it be effected by gravity?

[SteveA]

Quote by: LetThereBe As much as I hate to parade my ignorance, I wondered if someone could answer two things I've never understood. I have heard black holes defined as having infinite density, or infinite mass. If that were the case wouldn't the force of gravity be infinitely strong, regardless of distance? Wouldn't the entire universe have been swallowed up as soon as a black hole began to exist? If the speed of light is constant, how can it be effected by gravity?

They don't have infinite mass but this limited amount of mass is assumed to be compressed into a an infinitely small point (though there are debates about whether this is true - a black hole might be as large as the event horizon - we can't 'see' it to verify this).

Anyway, density is calculated as the amount of matter in some fixed amount of space. So something that's dense is typically heavy per unit volume of it. (A cup of air isn't very dense compared to a cup of lead)

So a black hole would be infinitely dense because you have many stars fit into a point smaller than the tip of a needle, and trying to gather a cup full of black holes would be impossible so that's why they say black holes are infinitely dense, but the mass (total matter) is still limited. You're correct in assuming that if a black hole had infinite mass we'd all be moving at lightspeed into a giant black hole right now. (A wierd paradox in this would be that we'd have infinite mass as well - matter traveling close to the speed of light gains mass, so we'd become black holes moving into a giant black hole ... wierd)

[PatrickHenry]

Quote by: LetThereBe I have heard black holes defined as having infinite density, or infinite mass. If that were the case wouldn't the force of gravity be infinitely strong, regardless of distance? Wouldn't the entire universe have been swallowed up as soon as a black hole began to exist?

A black hole has infinite density, not infinite mass. Its mass is the exact mass that has gone into the making of the BH. This is a finite number, not infinite. The reason for the infinite density is that a black hole is a singularity, having no dimension, only location. A black hole is a POINT.

The density formula is: mass/0=infinity

The gravitation is based upon the mass, though. A finite mass=finite gravity

Light is affected by the nature of the Space/Time continuum. The fabric of S/T is distorted by gravitation, and light follows the curved pathway... Sort of like a marble rolling inside a curved funnel, the only bathway is round and round and eventually down in, never to emerge...

Try this: http://en.wikipedia.org/wiki/Black_hole

[LetThereBe]

Very interesting. Thank you for the answers!

[Plasma Snake[D]]

Uh Huh. All this speculation makes me want to vomit.

[5010]

If somehow you travelled to the center of the Earth, would you be weightless? The net forces cancel out.

If everything in a black hole occupies the center, then the attraction between everything nets to 0, meaning no gravity.

I don't see how it can occupy a single point.

[PatrickHenry]

Quote by: 5010 I don't see how it can occupy a single point.

Gravitational collapse: http://en.wikipedia.org/wiki/Gravitational_collapse

[Sonart]

Quote by: Plasma Snake Uh Huh. All this speculation makes me want to vomit.

Then it's probably good for your personal comfort, and the benefit of the rest of us, that you haven't taken up a career in science. :rolleyes:

.

[5010]

Quote by: PatrickHenry Gravitational collapse: http://en.wikipedia.org/wiki/Gravitational_collapse

Excerpt from your link:
"This is a physical singularity, a problem unsolved in present day physics"

So I guess I'm not the only one who wonders how everything in the body can be pulled into a center point by gravity when everything at the center point cancels out gravity.

[PatrickHenry]

It is a logical hurdle all right, 5010.

[Nono]

Somebody in the 1920s called Einstein "the right man in the right place at the right time with the right theory that nobody really understands". Even Einstein fretted that "maybe the Lord is leading me down the garden path".
Pretty counter-intuitve stuff.

[Protostar]

I'm glad to see I'm not the only one who finds black holes fascinating. I think it's amazing that something has the gravitational force so strong that not even light can escape it. I think it's amazing (and frightening at the same time) that once you go beyond a black holes event horizon you can never go back. Everything about black holes is amazing to me. I just wish I could understand the math used to analyze them too but I struggle with calculus so thats not going to happen.

[Plasma Snake[D]]

Damn its just that the main source for peoples ideas for speculation comes from hollywood. people throw words like 'event horizon' around like they really know what they are talking about... all I'm saying is that if you cant even see black holes(only the effects of one), then how do we know for certain how it is.

[LetThereBe]

I heard somewhere that larger black holes (like those expected at the center of galaxies) would be the least dangerous kind, because the tidal forces would be much weaker.

[Protostar]

Quote by: LetThereBe I heard somewhere that larger black holes (like those expected at the center of galaxies) would be the least dangerous kind, because the tidal forces would be much weaker.

Actually you're wrong. It's not that the tidal forces are much weaker, it's just that because it's so big (super massive black holes are what we are talking about here) it takes longer to get to the point where you begin to feel the tidal forces. In fact, it may take days or weeks after you past the event horizon before you even begin to feel the tidal forces begin to act on you. Of course after you past the event horizon its too late. Since the black hole is so big it would take alot long for you to be spagettified. Which means you would be in excruciating pain for a very long time.

[Sonart]

Quote by: Plasma Snake Damn its just that the main source for peoples ideas for speculation comes from hollywood. people throw words like 'event horizon' around like they really know what they are talking about... all I'm saying is that if you cant even see black holes(only the effects of one), then how do we know for certain how it is

Now see there, children? This is what happens when you drop out of school and don't go to college. So you kids stay in school, study hard, and you too can one day comprehend complex conceptual ideas that the movies you watch are sometimes made from.

.

[RickSp]

Quote by: Sonart Now see there, children? This is what happens when you drop out of school and don't go to college. So you kids stay in school, study hard, and you too can one day comprehend complex conceptual ideas that the movies you watch are sometimes made from. .

Nah, an education is a waste when watching TV and most movies.

Last night I watched "NUMB3RS" for the first time. It is a cop show where a genius mathematician solves mysteries. Three minutes into the show the "genius" makes a really dumb comment about paper airplanes and Reynolds numbers. Ruined his already limited credibility with me. If I had no idea what he was talking about it, might not have bothered me. I got bored turned off the TV and went back to reading a book.

[SteveA]

Black holes slowly evaporate from what I've heard. Small black holes do this very rapidly. So energy does escape. This is probably a good thing or a small black hole might eventually 'eat up' an entire planet.

Also, the event horizon is just a mathematical construction. It's the distance away at which it would be impossible to escape, though the closer you got to a black hole, the further 'inside' you could see. Of course getting an up close view would be a one way ticket though

Some people have wondered whether we might be inside a black hole. This could explain the 'missing'/dark matter that it would take for the universe to eventually collapse. If we were inside a black hole, maybe there's some matter outside it that hasn't been 'eatten' yet (like another black hole). I'm not too familiar with the math behind it all but consider if the initial period when the universe supposely expanded rapidly immediately after the Big Bang, that we were just a newly formed black hole with little matter in the universe, then if some other matter had entered our universe (we ate some nearby matter) that could be seen as an increase in the energy/matter in the universe which might have slowed the expansion from the increased gravity. It could be that in the future we'd encounter another large mass and further increase the matter in the universe and slow or reverse the expansion. So maybe our universe could be seen as one of a few or many massive bodies orbitting and collapsing into each other.

There could be the argument that we have no proof of an increase in energy in the universe, but consider that such events would be unpredictable and not something you could easily reproduce and prove. Also it could be that if the increase in energy is uniform, it might simply be that whatever is used to sense it is also more massive so relative to the sensor the quantity sensed still appears the same - but possibly the gravitational constant changes. Which again could explain the initial period of fast expansion to the universe.

[SteveA]

http://en.wikipedia.org/wiki/Gravitational_constant

"The accuracy of the measured value of {G} \ has increased only modestly since the original experiment of Cavendish. {G} \ is quite difficult to measure, as gravity is much weaker than other fundamental forces, and an experimental apparatus cannot be separated from the gravitational influence of other bodies. Furthermore, gravity has no established relation to other fundamental forces, so it does not appear possible to measure it indirectly. A recent review (Gillies, 1997) shows that published values of {G} \ have varied rather broadly, and some recent measurements of high precision are, in fact, mutually exclusive."

Ah ha! The gravitational constant might not be a constant after all. I bet it has a generally increasing trend. Though another thing that would be interesting ... what if we were inside a black hole that evaporated instead? Would we appear as energy in a different universe?

[Plasma Snake[D]]

Im just saying before anyone goes down these roads to be careful cause you could be COMPLETELY missing the target.