Black Holes-mathematical singularity or physical reality?

[djmike]

Black Holes-mathematical singularity or physical reality?

Hi, I posted maybe a week ago and got some excellent feedback on experimental eveidence for GR. this post is intended to start debate, not as a question demanding an answer...


after reading around for some time and speaking to my lecturer on GR i have found out that Einstein didnt believe black holes existed as a physical phenomenna. from what i can make out he said something like them being a inconsistancy with the mathematics and not a physcial reality. If this is the case why are we seeing 'evidence' for black holes everywhere?

how possible is it that 100% of the missing dark matter is black holes?

how reliable are these claims that black holes have been detected? could they not be explained by other means? how can we even detect a black hole if all electromagentic radiation is lost within them and gravity waves have also not been detected yet? it all seems a little bizzare, especially when it does boils down to a mathematical singularity which has attempted to be explained by incorporating a physical phenomenna? and if Einstein didnt believe it, why should i?

 

[shrumeo]

Can we tell the difference between a neutron star and a black hole observationally?

 

[matt.o]

black holes are detected indirectly through their gravitational interactions with the surrounding media (ie gas, stars, light etc.), although I have seen talks on radiation emitted at a black hole's horizon.


It is about .001% possible that 100% of dark matter is made up of black holes (if that!).

 

[Garth]

Basically if you have a Kepler mass (binary object, i.e.- star + ?X) of around 3M_Sun, or so, in a small volume (i.e. significant fluctuations in the X-ray emission from X over a time scale of seconds) then the gravitational forces on that unknown object X will be so strong that no known forces would be able to withstand them. If indeed there are no stronger forces in the physical world then a situation of collapse to a singularity will be inevitable. An event horizon would form as the object collapsed down within a radius of 2MG/rc² at that radius from the singularity.

matt.o microlensing surveys of stars in the Magellanic Clouds have detected MACHOs - invisible massive objects that could be either black holes or very dim red dwarfs - that might explain about 10% of the galactic/intergalactic DM, but no more.

Garth

 

[matt.o]

thats right. as i said it is about .001% possible!

 

[JesseM]

Cosmologists seem to think that models involving "cold, collisionless dark matter" are the best fit for observations, but this paper discusses some possible alternatives--see the list starting on p. 8, which among others includes "strongly self-interacting dark matter", "repulsive dark matter", "self-annihilating dark matter", and:

7. Massive Black Holes (BH): If the bulk of the dark matter in galactic halos were in the form of massive black holes with mass of about one million solar masses, then several dynamical mysteries concerning the properties of our galaxy could be better understood (53). In normal galaxies dynamical friction between the massive black holes and the ordinary matter would cause those in the central few kiloparsecs to spiral into the center, depleting those regions of dark matter and providing the ubiquitous central massive black holes seen in normal galaxies.

The paper goes on to discuss the predictions each model would make, and how these predictions compare with observations.

 

[Garth]

If the bulk of the dark matter in galactic halos were in the form of massive black holes with mass of about one million solar masses, then several dynamical mysteries concerning the properties of our galaxy could be better understood (53).

Now that is an interesting observation.

But where did the mass for these BHs come from?

If they formed out of non-baryonic matter then we would be looking for a new species, hardly collisionless!

If they formed of primordial baryonic matter then the BBN baryon fraction of 4% max would be compromised.

However one advantage of massive black holes as DM candidates is that they have been detected and seem already quite ubiquitous.

Garth

 

[eNathan]

Well, I dent see (mathematical) why we can't believe in black holes. Of course, you could believe that some unknown, physical phenomenon prevents mass from concentrating itself to the point where you must travel faster than c to escape its gravitational force. I highly doubt that, we are left to believe (as of now) that black holes exist.

 

[Ich]

They recently have found the missing baryonic matter: http://physicsweb.org/articles/news/9/2/2/1
I don´t think non-baryonic DM could form black holes.

 

[selfAdjoint]

They recently have found the missing baryonic matter: http://physicsweb.org/articles/news/9/2/2/1
I don´t think non-baryonic DM could form black holes.

Why not? It gravitates, and a black hole "has no hair", meaning that other forces don't affect its physics. If the non-baryonic DM is "hot" then maybe it can't settle down in a chunk, but AFAIK there are still viable theories of "cold" DM which could.

 

[Ich]

I just thought that you need some sort of friction to increase the density beond some critical point. I imagine a chunk of WIMPS to behave like a globular cluster, which would rather evaporate than concentrate.

 

[blue_sky]

Well, I dent see (mathematical) why we can't believe in black holes. Of course, you could believe that some unknown, physical phenomenon prevents mass from concentrating itself to the point where you must travel faster than c to escape its gravitational force. I highly doubt that, we are left to believe (as of now) that black holes exist.

Well I guess you right from a math. point of view. In my opinion the physics is something different from math., i.e., math. is the "tool" we use to describe the universe through laws (physics).
Personally I do not believe that BH exist also if they are a math. possibility.
There are many reasons for that, some of them philosophical; the 1 I would like to mention is that if you consider our universe the fact that now is expanding means that in the past it should have been an huge BH and this is against our present universe, as far as I can imagine.
I have an idea on which I'm working that seems to indicate that BH do exist only in the limit of infinite mass and so they do not exist in our universe.
Where can I publish an article on this subject?

Thanks

blue

 

[RoboSapien]

black holes are detected indirectly through their gravitational interactions with the surrounding media (ie gas, stars, light etc.), although I have seen talks on radiation emitted at a black hole's horizon.


It is about .001% possible that 100% of dark matter is made up of black holes (if that!).

Keep it up Matt.

Even Jupiter and our moon is detectable in that way.

Whats bloggin me is : How is that when a star starts to collapse instead of exploding due to Nuclear Fusion of higher atoms, it becomes a black hole ?

 

[SpaceTiger]

Ah, good that Robo revived this thread, as I didn't notice it before. Right down my alley...

If this is the case why are we seeing 'evidence' for black holes everywhere?

The case for black holes is, at this point, largely dynamical. What we've done is show that there are cases where a large amount of mass exists in a very small space (too small for all other reasonable possibilities) by looking at the velocities of nearby objects. In practice, it's extremely difficult to observationally demonstrate many of the more interesting properties of a black hole (like the event horizon), so we're partially just trusting GR on this one.

how possible is it that 100% of the missing dark matter is black holes?

Well, we know that there are multiple components to the dark matter, but I'll assume that you mean most of it. It is entirely possible, but becoming less and less likely with time. We've already ruled out black holes as the dominant form of dark matter if they are mostly PBHs of mass less than [tex]10^{14}~g[/tex] (Hawking evaporation) and more than [tex]10^4 M_{sun}[/tex] (they would disrupt globular clusters, alter the power spectrum, etc.). Also, microlensing results have ruled out stellar mass black holes as the dominant form.

A friend of mine and Jerry Ostriker are currently working on a paper that might make these limits much more strict, so stay tuned.

how reliable are these claims that black holes have been detected?

If you trust GR on those scales, then very reliable. If you don't, then all we can say is that we've detected objects are either black holes or can't be explained by GR.

could they not be explained by other means?

The dominant evidence is from supermassive black holes like the one at the center of our galaxy. People have tried very hard to explain the dynamical measurements in other ways (including extremely compact clusters), but anything else we can think of that might account for the dynamical masses quickly collapses into a black hole anyway. If they're not black holes, we need new theory.

how can we even detect a black hole if all electromagentic radiation is lost within them and gravity waves have also not been detected yet?

Well, the primary evidence, as I said, is dynamical, but it's not true that they don't create electromagnetic radiation. As matter falls into them (presumably in an accretion disk), it loses a lot of energy via radiation, as long as it's outside the event horizon. This is the current theory for where quasars come from.

and if Einstein didnt believe it, why should i?

That, I'm afraid, is a question you'll have to answer for yourself.

 

[DrChinese]

...if Einstein didnt believe it, why should i?

I would like to think that in the 50 years since Einstein died (April 1955), we have learned a few facts about the universe that Einstein did not have access to. In the past 10 years, it has become evident that most galaxies have black holes at their center. I think Einstein would have had no difficulty accepting the current evidence, especially since it is a natural outgrowth of GR in the first place and was conceptualized by Schwartzschild as early as 1916.

...Or you can choose to selectively ignore new information. The choice is yours. By the way, do you watch color TV, listen to CDs or talk on cell phones? Einstein never did.

 

[RandallB]

. . . a little bizzare, especially . . . a mathematical singularity . . . a physical phenomenna?
and if Einstein didnt believe it, why should i?

DJ
I think Einstein had a way of wanting to put things into perspective and I believe the "singularity" issue is the part he had trouble getting a perspective on - as do I.
So when I think of “Black Holes” I preferred to think of them a FROZEN STARS, a term used by the Russians which better respects the SR and mostly GR effects that must be taken into account when considering a Frozen Star. They were recognizing that as you view the light from an object getting closer and closer to the event horizon two thing happen. It becomes dimmer as more of the light from it going out at an angle get bent away and even into obit or a spiral down into the massive mass of the star. And we will see it moving slower and slower as per GR our time will be running much faster than the time with the object as it gets closer to the horizon.
Now for the object to actually completely cross the horizon and even the material inside the horizon stretched out from edge to edge of the horizon to reach “singularity” should only take a few seconds at most! However with just a bit of GR mass we must recognize that small amount of time in there is going to mean quite a bit of time out here where we are! Something like multiples of the amount of time since the BigB started! So, I see no chance that even the earliest “Black Hole” formed shortly after the BB is anywhere near becoming a “singularity” in the sense of a western view of a Black Hole.

Now add to that mix, the idea of Hawking Radiation – very slow by all accounts - but considering the great amount of time available before a Frozen Star can really become a “singularity” there is a real likelihood that it would evaporate before it got there.

I suspect Einstein would be very pleased with that idea.
Who’s right? What will happen if a true singularity is ever reached!?
What will it look like when a “Black Hole” finally evaporates!?
I’d love to stick around to see – but I’m not making any plans.

RB

 

[pervect]

The information never gets back outside, but an observer falling into a non-rotating black hole will reach its center in a finite amount of his time (proper time). So I don't think the "frozen star" analogy is really all that good. It used to be popular before black holes were understood as well as they are now, but I don't think it is very useful nowadays.

In order to avoid getting too tied up in some unending philosophical debate and to be reasonably clear, I think it's important to talk about actual predictions (if any) the "frozen star" model makes.

If one follows the collapse of an highly idealized, uniform, presureless sphere, one finds that the observer on the sphere doesn't see light from the whole universe before he reaches the singularity. Another way of puting it - there is a cutoff time beyond which you cannot send a message to someone who has made a (one-way) trip into a black hole if one expects the message to reach him before he is destroyed by the tidal forces near the singularity. This is a considerably different prediction than that which the "frozen star" model seems to make, the "frozen" idea (if it says anything at all) would seem to imply that the observer would see the entire universe before he reached the singularity, as he is "frozen", but the universe continues on. This isn't what happens at all.

There are some worthwhile pictures of the collapse of a unfiorm pressureless spher in a couple of coordinate systmes (Finklestein, Kruskal) at http://casa.colorado.edu/~ajsh/collapse.html

though they are a bit hard to make out.

As I understand it, it's wrong to say that black holes could evaporate before one reached them. See Tedd Bunn's black hole FAQ

http://cosmology.berkeley.edu/Education/BHfaq.html#q9

 

[Entropy]

The information never gets back outside

Not necessarily. There are theories that information can escape via Hawking radiation.

 

[RandallB]

The information never gets back outside, but an observer falling into a non-rotating black hole will reach its center in a finite amount of his time (proper time).

Didn't say that an observer falling in that appeared "frozen" was frozen. My concern is with the finite time the observer ( I assume his time is what you mean by “proper time”) takes to go from the horizon to the “singularity”. Just how long in “our” time will that “proper” time take to pass??

As I understand it, it's wrong to say that black holes could evaporate before one reached them. See Tedd Bunn's black hole FAQ

Even Ted in your reference talks about reaching the Black Hole by crossing the Horizon and an outside observer may well see the Hawking radiation that takes that observer out of the black hole at nearly the same time as crossing in. Meaning the falling observer may be radiated away before getting a chance to get all the way to the “singularity”. So if Hawking radiation is correct I have doubts that the “singularity” ever need actually exist. That could be why no one can actually define what a “singularity” is; QM nor GR.

 

[russ_watters]

Whats bloggin me is : How is that when a star starts to collapse instead of exploding due to Nuclear Fusion of higher atoms, it becomes a black hole ?

It does explode due to fusion of higher atoms. That's what a supernova is. The black hole is what's left.

 

[marlon]

My concern is with the finite time the observer ( I assume his time is what you mean by “proper time”) takes to go from the horizon to the “singularity”. Just how long in “our” time will that “proper” time take to pass??

I think this must be finite, though i am not too sure. Besides, did you even wonder why the time of this observer is finite. this has to do with the fact that inside the black hole, spacetime is THAT much curved that this observers time coordinate is interchanged with his position coordinate. This really means that his position coordinate is timelike. Just like, here on earth, you cannot prevent next sunday from happening, this observer cannot prevent from evolving towards the singularity...

marlon

 

[RandallB]

I think this must be finite, ...not too sure. . . . this observer cannot prevent from evolving towards the singularity...

OK ??
I think the question is - of that finite time how much time passes outside or here on earth? And then from outside how long would Hawking Radiation take to evaporate the black hole? More or less than it takes to reach a "true" singularity (whatever that may be).

 

[ohwilleke]

how possible is it that 100% of the missing dark matter is black holes?

Not likely. Black holes are strong point sources of gravity. Dark matter theory requires dark matter to be distributed halo like around galaxies, providing a very diffuse source of gravity. If gillions of black holes were distributed in that manner around galaxies we would expect to see both a honeycomb of point sources of the radiation associated with black hole fringes around galaxies where dark matter theory predicts lots of dark matter, and kaleoscope like lensing patterns around such galaxies. Neither are observered.

 

[pervect]

Didn't say that an observer falling in that appeared "frozen" was frozen. My concern is with the finite time the observer ( I assume his time is what you mean by “proper time”) takes to go from the horizon to the “singularity”. Just how long in “our” time will that “proper” time take to pass??

Your understanding of proper time is correct. Speaking in very lose popular terms, I often refer to proper time as "wristwatch time", the time that someone wearing a hypothetical wristwatch would see.

Unfortunately, I don't think there's a way to answer the question of how ling it takes in "our time". The standard way of comparing times at different spatial locations is to exchange light signals, and to assume that light travels at a constant velocity of 'c'. This is what allows us to compare times (our watches).

But we can't exchange light signals in this case. So I don't think there is any meaningful concept of simulataneity between the observer falling into the black hole and the outside observer. To assume that there is is in my opinion a throwback to pre-relativistic thinking, the idea that there is some sort of "absolute time".

Even Ted in your reference talks about reaching the Black Hole by crossing the Horizon and an outside observer may well see the Hawking radiation that takes that observer out of the black hole at nearly the same time as crossing in. Meaning the falling observer may be radiated away before getting a chance to get all the way to the “singularity”. So if Hawking radiation is correct I have doubts that the “singularity” ever need actually exist. That could be why no one can actually define what a “singularity” is; QM nor GR.

We can't see the singuarity because it is hidden from us by the event horizon.

The physics of the event horizon are well understood (at least in terms of theory) and not singular, though. The Schwarzschild coordinates have a coordinate singularity there, but there is no physical singularity at the event horizion.

We can't tell anything about the actual singularity beyond the event horizon from looing at it from outside - the only way we can tell is to actually take the one-way trip into the black hole.

So in some sense we've almost left the realm of the experimental, though not quite. There is a definite prediction to be made on what a person who actually carried out the experiment would see. The main problem is that there is no way even in principle (with standard theory) that he could communicate his results back to us, safely outside the event horizion.

But there is still a definite prediction to be made as to the fate of the observer, even if there isn't anyway for said observer to carry out thye experiment and inform us of the results. Standard theory as I understand it (see the quote from Ted Bunn) does predicts that said observer will, in fact, be torn apart by tidal forces, and does not predict that the black hole will evaporate before he reaches the singularity.

 

[SpaceTiger]

Not likely. Black holes are strong point sources of gravity. Dark matter theory requires dark matter to be distributed halo like around galaxies, providing a very diffuse source of gravity.

That's only known very crudely. Nothing about our observations of dark matter require it to be more diffuse than one would get from 10^4 solar mass black holes.

If gillions of black holes were distributed in that manner around galaxies we would expect to see both a honeycomb of point sources of the radiation associated with black hole fringes around galaxies where dark matter theory predicts lots of dark matter

Nah, space is mostly empty, so we wouldn't expect them to be accreting very much material.

and kaleoscope like lensing patterns around such galaxies.

I think you overestimate the lensing power of black holes. In actuality, they would produce microlensing events, and current surveys only place limits on objects of ~1 solar mass. Even the supermassive ones at the centers of galaxies have a negligible effect on light passing near the galaxy.

Despite all these things, however, I agree that primordial black holes are an unlikely candidate for dark matter.

 

[RandallB]

. . . predicts that said observer will, in fact, be torn apart by tidal forces, and does not predict that the black hole will evaporate before he reaches the singularity.

I’m sure the falling observer can get ripped up pretty good even before crossing the horizon. In fact the explosive heat and force coming from outside the horizon of the forming massive Black Hole at the center of EVERY (my opinion) Galaxy was a key part of the setting the orbital path of the limbs and every thing around it, maybe even some of the base elements that make us up.

I mainly wanted djmike (hope your still looking in DJ) not to give up on the idea of Black Holes just because he didn’t like the singularity part. I think Black Holes are important part of how it was all formed, and I don’t need to consider what a singularity might look like as I'm guessing it may well never actually occur. Thus I also don’t worry much about Worm Holes, White Holes, or Warp Speed and how they get around in Star Trek. Chances are unlikely of getting any real good data into clear the issue from inside the horizon anyway.

 

[pervect]

It turns out that in Schwarzschild coordinates the tidal forces (as measured in a local units) are 2GM/r^3, the same formula as for Newtonian gravity. Since the Schwarzschild radius R=2GM/c^2, the result of the tidal force at the Schwarzschild radius (the event horizon) is inversly proportional to M^2.

Thus for a large enough value of M (a sufficiently massive black hole) it's quite possible for an observer to cross the event horizon without being pulled apart by tidal forces. An accretion disk could also ruin your whole (hole?) day - but one could avoid most of that problem by chosing the path/plane in which the drop into and beyond the event horizon was made to avoid the accretion disk.

 

[RoboSapien]

It does explode due to fusion of higher atoms. That's what a supernova is. The black hole is what's left.

Noooo, I am an amature but that's not what I meant to ask. Please try to understand. I mean when the star is about to explode the pressure at the center must be immense and hence fusion will start there first as the fusion starts that will add more pressure in the center and hence star should begin to explode from the center leaving nothing to form a black hole.

I know I am wrong but how ?

 

[russ_watters]

Noooo, I am an amature but that's not what I meant to ask. Please try to understand. I mean when the star is about to explode the pressure at the center must be immense and hence fusion will start there first as the fusion starts that will add more pressure in the center and hence star should begin to explode from the center leaving nothing to form a black hole.

I know I am wrong but how ?

An explosion that pushes outwards also pushes inwards. And for matter at the very center, how is it going to explode and push its way through the star - there is a star in the way!? Also, the star is collapsing rapidly, so before it explodes it first has to build up enough pressure to halt the collapse. So the reaction gets more and more energetic, the pressure gets higher and higher, and...

 

[Chronos]

When a star initially forms, it's core is mostly hydrogen. It ignites at a very modest density and is much too hot to permit further compactification. So a collapsing primordial gas cloud, regardless of size, cannot skip past the star phase and directly form a black hole. For that to happen, you need iron. Elements lighter than iron 'burn' hot enough to support their own weight, iron does not. The only place iron exists in sufficient quantities for this to happen is in the core of extremely massive stars that have used up their supply of fusionable [lighter than iron] fuel in their core. At that point the core rapidly begins to cool until it can no longer support the weight of its iron coffin. The layers surrounding the core, which still contain burnable fuel, overheat and detonate upon collapse. Those layers, not the core are the source of the explosion. Some of the core is, however, lost because the explosion is not perfectly symmetrical. The amount that survives the blast determines whether a neutron star or a black hole is formed.

 

[RoboSapien]

...Elements lighter than iron 'burn' hot enough to support their own weight, iron does not. The only place iron exists in sufficient quantities for this to happen is in the core of extremely massive stars that have used up their supply of fusionable [lighter than iron] fuel in their core. At that point the core rapidly begins to cool until it can no longer support the weight of its iron coffin...

If iron is unable to support its own weight then what is going to stop the fusion of thoes atoms ?

And from where did all other atoms heavier than atoms came from ? Is it fusion ?

And why will the fusion stop at any of the heavy atoms ?

 

[selfAdjoint]

If iron is unable to support its own weight then what is going to stop the fusion of thoes atoms ?

What stops it is that as an element iron is at the bottom of the curve of binding energy. It takes input energy to either fuse an atom of iron to something heavier or to fission it into something lighter. With the elements lighter than iron you get a little more energy out of fusion than you put in, so the fusion can spread, but fusing iron is like rolling a rock uphill.

 

[matt.o]

And from where did all other atoms heavier than atoms came from ? Is it fusion ?

The elements heavier than iron arre formed by fusion in supernova.

 

[SpaceTiger]

The elements heavier than iron arre formed by fusion in supernova.

If you define fusion to be an exothermic reaction, then this isn't quite true. Actually, elements heavier than iron are formed in two ways, the r-process and s-process. The first process does occur in supernova remnants, but it has to do with the bombardment of elements with neutrons. This process is endothermic and requires the extreme temperatures from the supernova remnant to occur. Many elements formed in this way will decay to more stable states, but a lot of neutron-rich isotopes of heavy elements are formed. The s-process also involves neutron bombardment, but it occurs very slowly in massive stars.

 

[Billy T]

Ohwilleke, in post 23, expressed his reasons for thinking that black holes were not dark matter. SpaceTiger noted in post 25 that Ohwilleke argument needed black holes larger than 10^4 solar mass. I like to think most black holes are only a few SMs - cores of long dead stars. I also tend to think there may be more of them now than all the stars currently emitting light. (Many generations of relatively large stars died and left BHs behind long before sun was born etc. Also because many meteors are chunks of iron, and iron not being able to exothermically fuse being the reason for black hole formation by large stellar cores, it seem clear to me that the instablity of the typical collapse is far form symetric. That is, for me it is possible that very big chuncks of iron could separate also during the collapse of a supernova core - perhaps big enough back when allmost all stars were very big compared to the sun, that a few separate, but gravitationally bound BHs could have been created by each supernova - the iron meteors even got away. Admittely not very standard theory (this year any way )

Any comments would be welcome, and BTW, what about the original question Ohwilleke was responding to: Can most of the DM be BHs, especially the few solar mass ones I think must be very common?