Can there be black holes in a universe of finite age?

[Rolando]

I was editing my previous post, addressing this while

 

 

Even if we ignore the flaws in that description, all you are saying is that we cannot observe objects at the event horizon, not that the event horizon doesn't exist.

 

But then nothing is at the event horizon for very long, so that hardly matters.

 

It takes more time for light to reach us from a gravitational well, the closer it comes to a black hole in its properties. An object turns into a black hole only when it has contracted to its event horizon. At that point, it takes an eternity for light to reach us from there. If the Universe has only existed for a finite time, it cannot contain such black holes and event horizons.

 

And, in our view, everything that is at the event horizon remains there for an eternity.

Edited January 24, 2015 by Rolando

[MigL]

Because any signal we receive from a distant astronomical object is in the form of EM radiation, we need only consider that case.

Any material we would 'see' collapsing or falling into a BH would be sending this information to us in the form of EM radiation. Now EMR can only move at c , it cannot accelerate. What it does is lengthen its wavelength as it climbs out of the steep gravitational well on its way to us. If the gravitational well is infinitely steep ( there, happy now ? ), then it is red-shifted to an infinitely long wavelength, or zero frequency, and therefore zero energy. In effect it disappears, or goes 'black', hence the name black holes.

 

That is what we would expect to see, NOT stars 'frozen' at the event horizon, no longer collapsing and shining forever.

 

And you do know that they have been indirectly observed/confirmed by their x-ray fingerprint and gravitational effects that are too large for neutron stars to explain.

[Strange]

An object turns into a black hole only when it has contracted to its event horizon.

 

That is not how black holes, and event horizons, form.

 

At that point, it takes an eternity for light to reach us from there.

 

Not quite.

 

If the Universe has only existed for a finite time, it cannot contain such black holes and event horizons.

 

Again, you are confusing what we can observe with what exists.

 

And, in our view, everything that is at the event horizon remains there for an eternity.

 

Not true.

[Rolando]

 

That is not how black holes, and event horizons, form.

 

 

Not quite.

 

 

Again, you are confusing what we can observe with what exists.

 

 

Not true.

 

I would be grateful for more convincing comments. And let us keep the discussion to what we can observe.

[Strange]

 

I would be grateful for more convincing comments.

 

You have been given more detailed explanations, which you chose to ignore. I didn't see much benefit in repeating them.

 

 

And let us keep the discussion to what we can observe.

 

I thought the thread was about the existence of black holes. If you want to change the subject ...

 

We can observe things that have all the characteristics of black holes.

 

And we could, in principle, observe something fall to the event horizon and disappear. (I was tempted to say "fall through the event horizon" but that would imply we could see it after it had passed the horizon, which obviously isn't the case.)

Edited January 25, 2015 by Strange

[Rolando]

I said: ” An object turns into a black hole only when it has contracted to its event horizon.” This was not a good formulation. I am aware that black holes are assumed to form in various ways, and this is not very relevant here. I would then say that ”An object turns into a black hole only when it has reached a state at which its extension coincides with its event horizon.” Is this acceptable?

 

If it is untrue that it takes an eternity for light to reach us from the event horizon of a black hole, then I would like to know how to calculate how long it takes.

 

According to my, admittedly somewhat outdated, knowledge of black holes, anything that is exactly at the event horizon would appear as frozen to an outside observer if it was observable at all, which it is not really, because of infinite time dilation and redshift.

 

I thought the thread was about the existence of black holes. If you want to change the subject ...

I did not want to change the subject. I had expressed myself inappropriately.

[Mordred]

No problem, As this is on topic, here is related posts. I would love to get this guy on this forum.

 

https://briankoberlein.com/2015/01/21/lies-teacher-told/

[Strange]

According to my, admittedly somewhat outdated, knowledge of black holes, anything that is exactly at the event horizon would appear as frozen to an outside observer if it was observable at all, which it is not really, because of infinite time dilation and redshift.

 

Even ignoring red-shift, the object would rapidly disappear. It will emit a finite number of photons before it falls through the event horizon and will therefore disappear from view. If it appeared to be stuck at the event horizon it would somehow have to emit an infinite number of photons just before it fell through.

[ajb]

A black hole is an object whose mass is surrounded by an event horizon at which time is dilated infinitely. Such a state cannot be reached in a finite time. An event horizon is infinitely remote from an observer. This alone is sufficient to realize that black holes are merely mathematical constructs that do not exist in reality even if the universe is infinite in age.

All this shows is that a distant observer cannot observe the formation of the event horizon. Thus, you cannot equate the 'time taken as observer by a distant observer' as having any meaning as 'the duration of gravitational collapse'. As far as a local observer comoving with the collapsing material is concerned the event horizon is formed in finite time. You need to be careful when defining meaningful durations in situations like this.

Edited January 25, 2015 by ajb

[Rolando]

All this shows is that a distant observer cannot observe the formation of the event horizon.

 

To me, this means that there are no black holes in reality. Event horizons are the boundary of reality. They are infinitely remote from a distant observer.

 

 

Thus, you cannot equate the 'time taken as observer by a distant observer' as having any meaning as 'the duration of gravitational collapse'. As far as a local observer comoving with the collapsing material is concerned the event horizon is formed in finite time. You need to be careful when defining meaningful durations in situations like this.

 

There are coordinate transformations by which one can get rid of the boundary (if one restricts oneself to just one black hole), but if one does that one transcends reality.

 

The question that remains is whether physical "reality" and "existence" are relative notions. Can something in a physical sense be existing and real for an observer who is falling into a gravitational well while it is non-existing and merely imaginary for a stationary observer?

 

This problem does not arise with gravastars.

Edited January 25, 2015 by Rolando

[Strange]

There are coordinate transformations by which one can get rid of the boundary, but if one does that one transcends reality.

 

So only the coordinate system that supports your claim is valid?

[Rolando]

No problem, As this is on topic, here is related posts. I would love to get this guy on this forum.

 

https://briankoberlein.com/2015/01/21/lies-teacher-told/

 

Thanks for this link.

 

So only the coordinate system that supports your claim is valid?

 

I am reasoning in terms of the coordinate system that is not specific to a particular black hole.

[Strange]

I am reasoning in terms of the coordinate system that is not specific to a particular black hole.

 

The Schwarzschild metric? That is only valid for a non-rotating, isolated, eternal and unchanging black hole. Not a completely realistic model.

 

And, of course, it can be proved that alternative coordinate systems are mathematically equivalent. So you have no basis for picking one and rejecting the others.

[Rolando]

 

The Schwarzschild metric? That is only valid for a non-rotating, isolated, eternal and unchanging black hole. Not a completely realistic model.

 

And, of course, it can be proved that alternative coordinate systems are mathematically equivalent. So you have no basis for picking one and rejecting the others.

 

I do have a basis for my choice. I am reasoning in terms of the coordinate system that is not specific to one single black hole alone. All the familiar alternatives apply only to single black holes - not to the whole set of black holes that might be there.

And my reasoning is applicable whenever there is an absolute event horizon. It is not necessary to go into the details of the Kerr metric, which I judge to be appropriate. The Schwarzschild metric is just easier to discuss.

By the way, can it really be proved that a metric that covers more than the space exterior to the event horizon is mathematically equivalent to the Schwarzschild metric (exterior sulution)?

Edited January 25, 2015 by Rolando

[Mordred]

To answer that would take far too long. You'd be surprised just how much we can model.

 

Here is a very lengthy and technical paper that covers measurement and modelling the accretion disk including as close to the EH a possible. You might want to study the ZAMO and ZAVO metrics inside the article.

 

 

http://arxiv.org/abs/1104.5499:''Black hole Accretion Disk'' -Handy article on accretion disk measurements provides a technical compilation of measurements involving the disk itself.

[Strange]

 

I do have a basis for my choice. I am reasoning in terms of the coordinate system that is not specific to one single black hole alone. All the familiar alternatives apply only to single black holes - not to the whole set of black holes that might be there.

 

I don't understand what you are saying. The Schwarzschild metric applies to one single black hole. As do all the other solutions (as well as those mentioned earlier, there are the Lemaitre coordinates, the Gullstrand–Painlevé coordinates and others). They all describe the same thing in different ways, so I'm not sure how you are saying some are valid and others aren't.

 

 

The Schwarzschild metric is just easier to discuss.

 

The Gullstrand–Painlevé metric is arguably more intuitive.

[MigL]

I think he's referring to frames by 'co-ordinate system', Strange.

He's implying that a distant frame can be applied to any black hole.

And that a local one cannot.

 

I, myself, don't see why not.

[Rolando]

Thanks, MigL.

 

Consider this figure by Brian Koberlein.

( https://lh3.googleusercontent.com/-NgZK-Ee7RAk/UJPrJIsviFI/AAAAAAAABWs/BmVe1s0Y7Hw/w480-h480/freefal.png )

 

The black curve is valid for an outside observer - the red one for an observer who is falling into the black hole.
Now, according to my reasoning, black holes (in the strict sense of the word) do not exist. The black curve ends at the event horizon (or a little above if the universe has only existed for a finite time).
Since I believe in only one reality, in that the existence/non-existence of objects is independent of the observer, the red curve has also to end at the event horizon if it is to describe reality only. The fact that the course of the line can be calculated also at shorter distances from the singularity is not a valid argument for its reality. This part of the line does not proove that motion across the event horizon is possible in reality. It presupposes it (erroneously).

Edited January 25, 2015 by Rolando

[Strange]

Since I believe in only one reality, in that the existence/non-existence of objects is independent of the observer, the red curve has also to end at the event horizon if it is to describe reality only.

 

This is true, and as the black hole exists from the perspective of someone falling into it, it must exist from our point of view as well.

 

(Your link is broken, so I don't know what the diagram shows.)

[Rolando]

The Gullstrand–Painlevé metric is arguably more intuitive.

 

May be, but hardly easier to discuss, and it is also "local", in the sense of MigL.

 

 

(Your link is broken, so I don't know what the diagram shows.)

 

I inserted it in plain text. It may have been too long.

Edited January 25, 2015 by Rolando

[Strange]

May be, but hardly easier to discuss, and it is also "local", in the sense of MigL.

 

I still don't understand what that means. Both metrics describe space-time outside a spherically symmetrical object. What does "local" mean?

[Rolando]

 

I still don't understand what that means. Both metrics describe space-time outside a spherically symmetrical object. What does "local" mean?

 

I am not sufficiently familiar with the Gullstrand-Painlevé metric, but I think that it is not useful for understanding what goes on within one or several additional gravitational wells at a distance.

 

Trivia:

I once read Painlevé's original paper on this topic - a short essay that was easy to read.

Then I had a look at Gullstrand's paper - sveral pages full of equations and essentially no words. Terrible.

 

Edited January 25, 2015 by Rolando

[Mordred]

Thanks, MigL.

 

Consider this figure by Brian Koberlein.

( https://lh3.googleusercontent.com/-NgZK-Ee7RAk/UJPrJIsviFI/AAAAAAAABWs/BmVe1s0Y7Hw/w480-h480/freefal.png )

 

The black curve is valid for an outside observer - the red one for an observer who is falling into the black hole.

Now, according to my reasoning, black holes (in the strict sense of the word) do not exist. The black curve ends at the event horizon (or a little above if the universe has only existed for a finite time).

Since I believe in only one reality, in that the existence/non-existence of objects is independent of the observer, the red curve has also to end at the event horizon if it is to describe reality only. The fact that the course of the line can be calculated also at shorter distances from the singularity is not a valid argument for its reality. This part of the line does not proove that motion across the event horizon is possible in reality. It presupposes it (erroneously).

This sounds like the information paradox though not precisely.

 

you might want to read this paper as it's directly related to this discussion and what you've been trying to describe.

 

 

http://arxiv.org/abs/gr-qc/0609024

 

Observation of Incipient Black Holes and the Information Loss Problem.

The problem with using the Schwartzchild metric is that it is invalid past the EH. The coordinate system no longer works.

 

 

On a personal note, I keep in mind the expression " The universe doesn't care how we measure it"

 

In this context just because we cannot see the BH form from our observable perspective, doesn't preclude it from forming in a finite time from a different observer perspective. Ie the infalling test particle.

 

As the mathematics can easily model the latter example it is also equally valid.

 

 

Here

 

"As we mentioned above, astrophysical BHs are the product of

gravitational collapse, and then are not eternal black holes. A qual-

itative view of the spacetime of a realistic black hole is shown in the

Finkelstein diagram in Fig. 1.4, "

 

This paper will step you through the solution

 

http://www.roma1.infn.it/teongrav/leonardo/bh/bhcap12.pdf

[Strange]

I am not sufficiently familiar with the Gullstrand-Painlevé metric, but I think that it is not useful for understanding what goes on within one or several additional gravitational wells at a distance.

 

It describes exactly the same thing as the Schwarzschild metric.

 

 

Then I had a look at Gullstrand's paper - sveral pages full of equations and essentially no words. Terrible.

 

Yes, science can be hard.

[ajb]

To me, this means that there are no black holes in reality. Event horizons are the boundary of reality. They are infinitely remote from a distant observer.

You are interpreting this too harshly.

 

All coordinate systems are valid, even if they seem to be contradictory. There is of course no real contradiction here, just a poor use of one coordinate system to insist that all observers will absolutely agree on a duration.