Couple of questions about black holes

[Tuko246]

Howdy! First time poster here, hope my first questions aren't too dumb. I've always figured that there must be a black hole at the center of every galaxy to keep the galaxy together with it's gravity, but have read that only some galaxies have black holes at their center. I was hoping somebody could help clear that up for me. Also, I have always wondered if there is a connection between black holes and white holes, specifically could the matter that is sucked into black holes be spewed out of white holes? Sorry if the questions are a bit novice.

[Folkert]

I don't know about white holes, but definitively matter that disappears in black holes stays there (not counting the very slow Hawking radiation process).

with respect to keeping Galaxies together: it is actually dark matter. Even the super massive black holes are not wavy enough by far to account for te rotation curves of galaxies.

Still most galaxies indeed have a (massive) black hole in their center, possibly the remnant of early universe star aging processes, that were heavier, more active, and closer together.

Folkert

[tom33pr]

1) I would be careful with the following statement: 'every galaxy has a black hole in the center'... We don't know that.

2) What happens to the matter after it has been acquired by a black hole has not been scientifically established either. In fact it has been one of the most famous conflicts between the theoretical physicists over the last 20 years or so... So of them will tell you (in basic terms) the matter is lost forever, the others - no, this violates the law of conservation of matter, and it will be brought (scrambled though) back by the Hawking radiation process. There are a lot of those who believe in matter escaping to other, parallel universes etc etc... We just don't know that..

There's an interesting book by Leonard Susskind called: The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum.

I'd recommend it if you are interested in this subject.

Regards

.

[acey]

I've always figured that there must be a black hole at the center of every galaxy to keep the galaxy together with it's gravity, but have read that only some galaxies have black holes at their center. I was hoping somebody could help clear that up for me. Also, I have always wondered if there is a connection between black holes and white holes, specifically could the matter that is sucked into black holes be spewed out of white holes? Sorry if the questions are a bit novice.

Super-massive black holes don't hold galaxies together - you could have a collection of stars, dust or gas of the same density at the centre and everything would proceed just the same. The black holes are assumed to have formed from the gravitational collapse of such central condensations.

White holes are hypothetical objects for which no physical evidence exists, but the hypothesis is indeed that matter falling into black holes would emerge from white ones.

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

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

[Tuko246]

1) I would be careful with the following statement: 'every galaxy has a black hole in the center'... We don't know that.

2) What happens to the matter after it has been acquired by a black hole has not been scientifically established either. In fact it has been one of the most famous conflicts between the theoretical physicists over the last 20 years or so... So of them will tell you (in basic terms) the matter is lost forever, the others - no, this violates the law of conservation of matter, and it will be brought (scrambled though) back by the Hawking radiation process. There are a lot of those who believe in matter escaping to other, parallel universes etc etc... We just don't know that..

There's an interesting book by Leonard Susskind called: The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum.

I'd recommend it if you are interested in this subject.

Regards

.

Sorry for the small font, I reposted this here from another subforum as I felt if fit better here, and the copying process made it quite tiny. Thanks for the info, I think I will look into that book... The subject has always intrigued me. Perhaps black holes could be a way of travelling between parallel universes.

[Tuko246]

Super-massive black holes don't hold galaxies together - you could have a collection of stars, dust or gas of the same density at the centre and everything would proceed just the same. The black holes are assumed to have formed from the gravitational collapse of such central condensations.

White holes are hypothetical objects for which no physical evidence exists, but the hypothesis is indeed that matter falling into black holes would emerge from white ones.

http://en.wikipedia....sive_black_hole

http://en.wikipedia....wiki/White_hole

I thought I read that a black holes have infinite density, and that is what made them black holes, and I may be mistaken, but I figured that for a bunch of matter to have the same density it would have to be in the form of a black hole. Thanks for the links.

[acey]

I thought I read that a black holes have infinite density, and that is what made them black holes, and I may be mistaken, but I figured that for a bunch of matter to have the same density it would have to be in the form of a black hole. Thanks for the links.

The "density" of a black hole depends on how you define its size. The singularity (at the centre) has no size and therefore infinite density, but if you take the event horizon as the size (i.e. the "sphere of no return" surrounding the singularity) then the black hole has finite density (i.e. a finite mass divided by a non-zero volume). In fact the larger the black hole (as measured by its event horizon) the lower the density: supermassive black holes have a density comparable with that of water. Or to put it another way, if you could get enough water to make a sphere as big as the event horizon of a supermassive black hole, then the water would collapse under its own gravity to form a black hole whose event horizon would extend out to where all the water had been.

[Tuko246]

The "density" of a black hole depends on how you define its size. The singularity (at the centre) has no size and therefore infinite density, but if you take the event horizon as the size (i.e. the "sphere of no return" surrounding the singularity) then the black hole has finite density (i.e. a finite mass divided by a non-zero volume). In fact the larger the black hole (as measured by its event horizon) the lower the density: supermassive black holes have a density comparable with that of water. Or to put it another way, if you could get enough water to make a sphere as big as the event horizon of a supermassive black hole, then the water would collapse under its own gravity to form a black hole whose event horizon would extend out to where all the water had been.

Does the singularity then contain all of a black hole's mass?

[Ronnie67]

not all theories explain a black hole as a singularity. There are some exotic ideas out there, but in some cases the black hole is simply extremely dense, but not necessarily infinitely dense (but still dense enough to have an event horizon). The singularity arises from relativity. The singularity is a point where volume goes to zero, not where mass goes to infinity.

The reason that the volume is zero rather than the mass is infinite is easy to see in an intuitive sense from the creation of a black hole. You might think of a volume of space with some mass which is compressed due to gravity. Normal matter is no longer compressible at a certain point due to Coulomb repulsion between atoms, but if the gravity is strong enough, you might get past that. You can continue compressing it infinitely (though you'll probably have to overcome some other force barriers along the way) - until it has zero volume. But it still contains mass! The mass can't just disappear through this process. The density is infinite, but the mass is still finite.