Celestia Forums

black hole theory seems very odd. There are a couple of points that I would like clarified.

1) the difference between a black hole and the center of gravity of a system of supergiant stars. The galaxy must have a center of gravity and science assumes that it is a black hole which sucks in matter. The direction of gravitational force of a system would be toward the center of mass, which may or my not be an object.

2) how a star could collapse into a black hole. i.e. if a black hole has such a strong gravitational field that even light cannot escape, and if gravity is caused by matter, then a black hole would have a large amount of matter ... how then could a star collapse into a black hole after a supernova explosion if most of the matter had been ejected in the blast ? surely the star would have had enough matter anyway to cause a strong enough gravitational field that would prevent light from escaping?

3) If stars could have strong enough gravitational fields to stop light escaping, then why doesn't physics talk about super-large stars becoming black holes without collapsing but by acreting enough matter ?

4) If the universe started as a big bang and all the matter in the universe was exploded from a point then surely if gravity is proportional to mass then the universe would have been a black hole at the beginning, and no matter would escape because the speed of light would not be suffiecient, so either the theory of a black hole is wrong or matter can travel at greater than light speeds and therefore special relativity is wrong.
( this does not account for matter spontaneously appearing from nothing after the big bang)

Number 1... that's right.

Number 2, as a star undergoes nuclear fusion. This force keeps the star from collapsing in on itself.

Number 3, mostly the same as number 2. Superlarge stars still have fusion to keep them up.

Number 4, perhaps tr00fi could explain this, lol. I read vaguely somewhere that gravity was a repulsive force in the early universe, or something like that. I'm probably far off (tr00fi, help?).

The works of Stephen Hawking and Kip Thorne offer insight into black holes. Plus I'm sure there a number of technically accurate websites that do the same.

Black holes were solutions to some equations derived from relativity theory. The theory predicted black holes. Then black hole candidates were found, and then black hole existence confirmed.

ok ....

I've found loads of info about black holes online. It seems that most discussions of black holes describe only stars that have collapsed rather than center of mass point, which would be the orbital center of a system of large stars.

Apparently the black hole has to be smaller than the Shwarzchild radius or something before it traps light in the event horizon. This can only happen in old stars that have burned up all of their hydrogen, helium and other gases, since without them stars can no longer sustain fusion. After the gas supply has been exhausted and a star has been through the red giant phase it starts becoming smaller and smaller, collapsing in on its own gravity, causing a greatly condensed core - which can either explode or turn into a black hole theoretically. The reason that the fusion keeps it puffed up is that it causes an outward pressure.

The gravitational force of a star at the surface is lower when it has a large radius, since the equation F = G*M1*M2/R^2 is divided by the radius, and this number is larger for a smaller radius. This explains why the larger stars do allow light to excape.

I still don't understand about the big bang theory though...

If I can recall physics correctly the answer to your question has to deal with time. When the universe was formed, the four fundamental forces were not in place. (Gravitational Force, Strong Force, Weak Force, Electromagnetic Force) Basically during the first pico-microseconds of the big bang, nothing in the universe existed to stop it from expanding. It was not until a little bit later that the other forces started to take effect but by that time all the matter was blasted away at near the speed of light. Or that?€™s how the theory goes.

I would take a look at wikipedia http://en.wikipedia.org/wiki/Big_Bang

cyber_space_doc wrote:Apparently the black hole has to be smaller than the Shwarzchild radius or something before it traps light in the event horizon.

This means that the matter in a black hole is not necessarily a singularity. As long as the radius of matter is smaller than the Shwarzchild radius, it would still not let light back out. It could well be a ball of quarks or something - more likely for stellar-mass black holes - but as long as the radius is small enough it doesn't really matter what's inside as we cannot observe it.

One thing I don't understand is why singularities are referred to as points with infinite density when that would violate the uncertainty principle and thus is likely to be incorrect. If you plonk the value for the mass of a black hole into the appropriate equations, one gets very small values for the radius (and correspondingly large values for the density), but one does not get a zero radius and infinite density. So why are singularities always referred to as infinite-density points?

Correct, there maybe a physical lower limit to how far stuff can be squeezed into. In a nutshell the smallest units of measurements are though to be Planks Measurements. In your case the smallest that a black hole can be is equal to 1 Planks length in radius. Any smaller and the distance is so small that is it incapable of holding the quantum of energy inside of the Schwarzschild radius. One can argue that the singularity of the back hole is equal to Planks length, any smaller the singularity should not exist, and would wink out of the universe. Unfortunately our understanding of quantum mechanics is incomplete, so no one knows for sure at this time. It may be that quantum physics has its own version of Chandrasekhar limit, in which case stuff becomes so dense or so small that quantum mechanics, fails to explain what happens.

Point is that we do not know, but easier to say infinite density then to say something like near infinite density, but that in it self would be a contradiction. I am sure as our understanding improves we will come up with the terminology to differentiate between the concept of infinity and googolplextic density, and no that is not a word either but perhaps I can coin it. :lol:

Is it possible for black holes to transform back into stars if they accrete enough matter?

The gravitational force is proportional to acceleration (F=ma), so what is the maximum acceleration in the universe?

What if the black hole could enter a state of perpetual outward force equalling its graviational force? i.e. a slow expansion to outside the schwarzchild radius, due to massive outward nuclear forces. Perhaps the nuclear forces could suddenly ignite if the black hole accreted the right substances - after all stars apparently die if they have burned all their fuel.

what if the matter in the black hole eventually dies enough due to particle lifetime to make the black hole visible again?

apparently the maximum acceleration is the plank acceleration which can be found on:

http://planck.com/technical1.htm

there is also a planck limit to the size of a black hole, which puts limits on other physical quantities.



-I was thinking that perhaps a black hole could cause a type I supernova if it came into contact with a massive star.

-perhaps the center of a black hole crushes matter into liquid quarks, where they can reform back into hydrogen again?

It's interesting to speculate as to what could or could not occur inside a black hole, otr to what it might contain - but once a star is within it's Schwarzchild radius then nothing that happens inside can ever be observed by someone on the outside Universe and nothing inside the hole can ever re-emerge through the event horizon and into the outside Universe. If I remember rightly, even Hawking radiation emanates from the area just above the event horizon.

As for the Big Bang, I seem to remember reading that in the very early Universe, an inflationary force caused by false-vacuum pressure (I think) was far more than enough to counter gravity and caused the Universe to expand at an expotential rate.

if the center of a black hole achieves a high enough density then surely it could produce an effect similar to a big bang? With the forces all unified into one big explosive force?

String theory would suggest that tachyon particles or massless particles could be emitted at faster than the speed of light anyway ... however it would also suggest that the center of the black hole might be so compressed that the multi-dimensional world of the superstrings becomes embroiled into one great large hole into a parallel universe. I would think that the hole would lose its gravity as matter escaped into the parallel universe.

Nope. No big bang.

Remember, the gravitational force that results in a black hole is too great for anything to escape.
(I'm ignoring the Hawkings effect, which is too small to make any difference for large black holes.)

You will find quite a few people able and willing to explain the details on the BAUT forum at
http://www.bautforum.com/

cyber_space_doc wrote:...
String theory would suggest that tachyon particles or massless particles could be emitted at faster than the speed of light anyway ... however it would also suggest that the center of the black hole might be so compressed that the multi-dimensional world of the superstrings becomes embroiled into one great large hole into a parallel universe. I would think that the hole would lose its gravity as matter escaped into the parallel universe.

Who told you what String theory would suggest??

I would suggest that people who have no idea about String theory should better renounce using it for the sake of argumentation...


Bye Fridger

t00fri wrote:

cyber_space_doc wrote:...
String theory would suggest that tachyon particles or massless particles could be emitted at faster than the speed of light anyway ... however it would also suggest that the center of the black hole might be so compressed that the multi-dimensional world of the superstrings becomes embroiled into one great large hole into a parallel universe. I would think that the hole would lose its gravity as matter escaped into the parallel universe.

Who told you what String theory would suggest??

I would suggest that people who have no idea about String theory should better renounce using it for the sake of argumentation...


Bye Fridger

don't worry I was only joking ...

if you take what they say during the television shows on this page:-

http://www.pbs.org/wgbh/nova/elegant/

and jumble up some random statements then you might be forgiven for saying the statement about tachyon particles travelling at greater than light speeds etc.


I'll have to watch through the programs again to make sure I understood properly. I am sure they said that the electromagnetic force is billions of times stronger than the gravitational force. This would suggest to me that it could be stronger than the black hole gravitational field, however I haven't done the math yet and I am not sure about the electro-weak force.

cyber_space_doc wrote:

t00fri wrote:

cyber_space_doc wrote:...
String theory would suggest that tachyon particles or massless particles could be emitted at faster than the speed of light anyway ... however it would also suggest that the center of the black hole might be so compressed that the multi-dimensional world of the superstrings becomes embroiled into one great large hole into a parallel universe. I would think that the hole would lose its gravity as matter escaped into the parallel universe.

Who told you what String theory would suggest??

I would suggest that people who have no idea about String theory should better renounce using it for the sake of argumentation...


Bye Fridger

don't worry I was only joking ...

if you take what they say during the television shows on this page:-

http://www.pbs.org/wgbh/nova/elegant/

and jumble up some random statements then you might be forgiven for saying the statement about tachyon particles travelling at greater than light speeds etc.

The problem with string theory is that it takes years of study by a fully qualified theoretical particle physicist to REALLY understand "what string theory suggests". The same is actually true for black-hole theory.

Sorry for reacting kind of "allergic" if I read people making obviously "undigested" statements about these VERY sophisticated issues. More and more people only transfer the "undigested" science slogans that originated from clicking some WEB page by an anonymous and usually unqualified author to other forums...and so on...That kind of "small talk" is "contageous" and most importantly misleading for others who might read it.

Experienced theoretical particle physicists like myself can immediately tell whether a person's writing reflects "insight" or not

If there is really interest to understand some of these fundamental issues, the only sensible and fruitful option is to pick a book by a professional author with a well-known reputation and study it in leisure. TV broadcasts can never provide real information about these things, since everything is presented much too fast, people's backgrounds vary tremendously and there is barely time to think about what was said.

Bye Fridger

the tv show is actually online, so it can be viewed at leisure:

http://www.pbs.org/wgbh/nova/elegant/program.html

Although I am aware that this is a popular science program not a technical program. The author Brian Greene actually wrote the book entitled "The Elegant Universe". The renderings of 3d space are quite worth watching.

**** this is in no way intended as an authoritve discourse ****

I posted this because I wanted to express my disbelief in black hole theory, however I have changed my mind.
I can appreciate that nothing can escape from a black hole by definition, however I do have logical problems with the black hole theory. After reading descriptions on wikkipedia and other websites I found that I did not believe a lot of what I was hearing, for instance the ideas about what is known as a Schwarzschild wormhole did not seem to make sense. Thinking as above the previous post in terms of the popular science concepts of string theory suggest some possibilities that could be explored by a qualified mathematical physicist, and since it is a completely fresh way of thinking about space and time it is almost a relief. The problem is of course that there is no evidence of the existence of superstrings and other dimensions.

My main problem with the mathematical work is that there are too many singularities - real singularities can easily be modelled in electronics however the actual result is never an infinate amplitude or whatever is being modelled, but this may because of bad components.

****

cyber_space_doc wrote:
**** this is in no way intended as an authoritve discourse ****

I posted this because I wanted to express my disbelief in black hole theory, however I have changed my mind.
I can appreciate that nothing can escape from a black hole by definition, however I do have logical problems with the black hole theory. After reading descriptions on wikkipedia and other websites I found that I did not believe a lot of what I was hearing, for instance the ideas about what is known as a Schwarzschild wormhole did not seem to make sense. Thinking as above the previous post in terms of the popular science concepts of string theory suggest some possibilities that could be explored by a qualified mathematical physicist, and since it is a completely fresh way of thinking about space and time it is almost a relief. The problem is of course that there is no evidence of the existence of superstrings and other dimensions.

My main problem with the mathematical work is that there are too many singularities - real singularities can easily be modelled in electronics however the actual result is never an infinate amplitude or whatever is being modelled, but this may because of bad components.

****

I am afraid all this (unqualified) chatting has VERY little to do with the subject of this thread "Physics and Astronomy".

Bye Fridger

its quite easy to mistake string theory for "comic book" physics

lol

cyber_space_doc wrote::D

its quite easy to mistake string theory for "comic book" physics

lol

Discussions about String theory make little sense with backgrounds arising from clicking WEB pages and watching TV shows.

Among the world's brightest theoretical physicists are working in string theory, but this difficult field ABSOLUTELY REQUIRES a speciific theoretical physics background that takes time and lots of efforts to acquire.

A simple example relating to what you wrote above:

The handling of singulariities are "bread and butter" tasks in theoretical physics and mathematics. Yet only a thorough understanding can provide the information about when singularities are merely unphysical artefacts that can be "regularized" and when there is a true problem due to their existence. In quantum field theory that is a most successful framework, there are always singularities appearing initially. It is a standard (technical) procedure to /rearrange/ the theory in a manner that all unphysical singularities manifestly cancel and the physical result is entirely free of singularities (so-called Renormalization).

Bye Fridger

this is *slightly* off subject, however it is a relevant point to overall astronomy that involves black holes...

I just noticed that I don't have a theory for why black holes and other gravitating objects form an accretion *disk* rather than a *sphere*.

http://www.celestiamotherlode.net/catal ... __Cham.jpg

there are also jets that fire upwards perpendicular to the plane of the disk.

The shape and position of the jets looks more like a magnetic field than the basic gravtational field. Does the disk form due to a coriolis force?