# Black Holes-mathematical singularity or physical reality?

1. Feb 3, 2005

### 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????

2. Feb 3, 2005

### shrumeo

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

3. Feb 3, 2005

### 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!).

4. Feb 3, 2005

### Garth

Basically if you have a Kepler mass (binary object, i.e.- star + ?X) of around 3MSun, 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

5. Feb 3, 2005

### matt.o

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

6. Feb 3, 2005

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

7. Feb 4, 2005

### Garth

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

Last edited: Feb 4, 2005
8. Feb 7, 2005

### eNathan

Well, I dent see (mathematical) why we cant belive in black holes. Of course, you could belive 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 belive (as of now) that black holes exist.:surprised

9. Feb 9, 2005

### Ich

10. Feb 9, 2005

Staff Emeritus
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.

11. Feb 9, 2005

### 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.

12. Feb 9, 2005

### blue_sky

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

Last edited: Feb 9, 2005
13. Mar 12, 2005

### RoboSapien

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 ?

14. Mar 12, 2005

### SpaceTiger

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

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.

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 $$10^{14}~g$$ (Hawking evaporation) and more than $$10^4 M_{sun}$$ (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.

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.

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.

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.

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

15. Mar 14, 2005

### DrChinese

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.

16. Mar 14, 2005

### RandallB

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

17. Mar 14, 2005

### pervect

Staff Emeritus
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

18. Mar 15, 2005

### Entropy

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

19. Mar 15, 2005

### RandallB

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??
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.

20. Mar 15, 2005

### Staff: Mentor

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