Black Holes Disputed

JScarpace writes: "Researchers at the Los Alamos National Laboratory in New Mexico and at the University of South Carolina in Columbia have proposed the existence of "gravastars" which are bubbles of superdense matter. If they are correct, the idea of a black hole with a singularity at the center may be just a fantasy."

Comments

But if there are no black holes...

[Oroborus]

Where do all my left socks go from the dryer?

Here's their paper

[pepik_knize]

4 pages in your choice of formats here.

Re:Here's their paper

[KjetilK]

Thanks a lot! Saved me the trouble of searching! :-) However, it should be emphasized that this is a pre-print, it could have changed substantially when going through peer-review.

Also, folks, don't slashdot the site unless you know a bit about cosmology (if you don't know what I'm talking about when I say "line element" forget it) - this is a site that is very important for physicists in their daily work.

no singularity...

[Cryptnotic]

No one ever said that there had to be a singularity for there to be a black hole. All a "black hole" means is that the mass of the object is so great that the escape velocity exceeds the speed of light. Nothing can go faster than light, so you'd never make it out of such a place. Since light can't be reflected off of it, it's called a "black hole".

A "singularity" is a point at which the gravitational force is infinite. This logically doesn't even make sense, so it's no wonder that it's disputed.

Cryptnotic

Re:no singularity...

[ndevice]

there's also the people who think that the singularity never forms in the observable lifetime of the black hole because as the center mass contracts, things slow down (because time / speed of light is 'apparently' slower in gravitational fields)

Re:no singularity...

[MillionthMonkey]

Because your light cone has tilted completely in the direction of the singularity. Space-time sort of "flows inward" inside the event horizon, meaning if you're inside it at a distance R from the center, no message you transmit can ever reach any observer who is further from the singularity than R. All your possible world-lines are moving toward the singularity and have their final state there. You can't use a rocket to get out of the hole any more than you can prevent today from turning into yesterday.

Re:no singularity...

[harlows_monkeys]

Yeah, that's the physics geek explanation. When I posed this question to one of my physics professors at Caltech back in my student days, he came up with nothing better than that, either. Basically, as far as we could tell, (1) the "escape velocity is greater than speed of light, and nothing can go faster than light" explanation has nothing really to do with it, and (2) there is no simple intuitive explanation of why you can't get out of a black hole (where "simple intuitive" means comparable to the escape velocity explanation).

This reminds me of the explanations in books for the general public on how airplanes work. The usual explanation is that because of the curve on top of the wing, when the airflow splits at the leading edge, the part that goes over the top has to go farther to meet up with the corresponding part that goes under the wing, and since it has to go farther in the same time, it has to go faster. We know from Bernoulli that a faster airflow has lower pressure, so we get a pressure differential, and the wing rises.

Anyone see the problem with that? The first problem is that no reason is given for the airstream over the top to have to meet up with the airstreem under the bottom. Why can't it just flow straight back?

The real reason wings work is that they cause vortices to be generated that, because of the shape of the wing, go down, and since vortices have momentum, by the usual laws of elementary physics, there is force upward on the wing.

There is no simple explanation of why wings cause these vortices, but no one likes to say in their books for laymen that it takes a PhD in aerodynamics to understand how airplanes work, so we get the totally irrelevant Bernoulli explanation.

It would be interesting to compile a list of various areas in science and engineering where there are explanations for the general public like this, that are basically wrong, and are so widespread that even scientists and engineers use them without thinking about them when writing for the general public.

Re:no singularity...

[harlows_monkeys]

Thought experiment for you: take an object and start moving it upwards at 10 miles/hour (it can be a rocket with a large fuel supply, or you can supply energy from the ground, e.g., with a launching laser). Keep supplying enough force to keep the object moving away from earth at 10 miles/hour.

Question: what happens to this object?

Answer: it gets arbitrarily far away from earth. After a year, for example, it is about as far away as the moon.

What this demonstrates: you don't need to reach escape velocity to get out of a gravity well.

Re:no singularity...

[alfredw]

because time / speed of light is 'apparently' slower in gravitational fields

Indeed. This prediction is a result of trying to integrate a function through points where the value is infinite, and then dividing that result by another infinite number.

The end is result is, to use the beautiful terminology of a Mathematician, "indeterminate."

The conventional interpretation of this results is that the theory cannot reliably predict the behaviour of the universe at points like this. Given the total lack of experimental evidence in regards to these phenomenon, I'd say that's a safe bet.

The same sort of thing happens when you try to calculate the total energy in a cavity (due to heat) using a classical theory. It tells you "infinity." What that means is "you should have used Quantum Mechanics."

Re:no singularity...

[(void*)]

When I posed this question to one of my physics professors at Caltech back in my student days, he came up with nothing better than that, either.

That in fact is a very good explanation for what is going on.

The "escape velocity" explanation is basically expressing an energy requirement. It is not insightful, like the light cone explanation becuase the light cone (when visuallized) shows you exactly which trajectories are possible. They all head towards the blackhole. The whole idea is that you cannot overtake the speed of light.

So why is the answer not satisfying? Being a physics TA, I have to understand the misunderstandings of students. It would be very helpful to me to understand why the answer is not satisfying.

Need to shine a little experimental light.

[ArcSecond]

All this sounds a bit like philosophers arguing about how many angels can dance on the head of a pin. Unfortunately, the lack of an empirical solution (just go an look at one) is holding us back from really nailing down the true nature of "black holes".

What I do find interesting is that this gravastar model, like the black hole model, implies that the universe and black holes/gravastars are similar in nature: that they belong to the same class of objects. It is a wonderful puzzle to look into a black hole wondering "what's IN there", when the answer might be something that has qualities similar to the life-cycle of our own cosmos.

Until we get some solid predictions about ways to differentiate one from the other, this is going to be a purely theoretical debate. Hopefully someone can advance the debate into the experimental realm soon. Maybe the new gravitational observatories can "shed some light" on this shadowy subject. ;)

Re:gravastar?

[nomadic]

Fine, I shall rename them.

From now on they will be referred to as Doom Spheres.

Re:What is the difference?

[BlueUnderwear]

A black hole is just a place where the escape velocity exceed the speed of light. There is no dispute over whatever such a thing can actually exist, as all you need is enough mass for it to happen.

Escape velocity not only depends on the mass, but also on the distance from the center. In a black hole, escape velocity only exceeds the speed of light if you get closer than the event horizon.

Now, if for some reason the necessary mass would not fit into the event horizon, no black hole could occur. This new theory stipulates that if you have such a huge mass, it will actually form a hollow sphere where much of the mass is actually concentrated outside of the event horizon. Now, a hollow sphere has the following "interesting" properties (or would have, in classical mechanics):

1. outside the spere it is the same as for a point mass: mM/r^2
2. within the shell it would be approximately linear in r: mM*(r-r_inner)/r_outer^2/(r_outer-r_inner)
   (approximative formula, for "thin" shells)
3. inside, it would be zero

Probably this is not 100% exact, as we're in a relativistic context here, rather than a classical one, but we can still presume that gravity inside the sphere would be much weaker than outside.

This would basically mean that you would not have an escape velocity exceeding the speed of light anywhere:

• not outside of the sphere, because you'd be outside of the event horizon
• not inside either, because inside a hollow spere gravity caused by the sphere is basically zero.

It's called the Kutta condition

[Goldenhawk]

I'm an aerodynamic engineer.

>Anyone see the problem with that? The first
>problem is that no reason is given for the
>airstream over the top to have to meet up
>with the airstreem under the bottom. Why
>can't it just flow straight back?

See here for one of many explanations of the Kutta condition, one of the foundational principles of aerodynamics. This has nothing to do with an explanation for the layman. Basically, it states that the air MUST meet smoothly at the back of the wing.

Logically, if you spend some time thinking about the flow, you cannot possibly construct a situation where the air above the wing somehow slips past the air below. Remember that a jet moves so fast that its wing is only passing through a portion of the air for fractions of a second - it's simply not possible to make the air move fast enough to slip like this.

This principle has been demonstrated NUMEROUS times. You can demonstrate it very easily with a line of smoke through which a wing passes, among a zillion other simple experiments.

Your own reference seems to contradict you

[BlueUnderwear]

Go to the smoke experiment, and scroll down to the bottom of the page, to see what happens when the angle of attack becomes too big. Yes, the upper and the lower flow no longer meet. Hence the reasoning that the top flow must be faster simply because it has a longer way to do is not really correct. Conceivably it could come out behind the bottom stream, or, as observed, ahead.

Hey, it even says so, in bold: Stating that the fluid flowing above the airfoil is accelerated with respect to the fluid flowing below it ``because it must travel for a longer route in the same time'' is then definitely wrong. Betrayed but your own reference texts, eh?

As harlows_monkey says, in order to understand why the streams do meet if there is a correct angle of attack, you do need deeper insight into aerodynamics than is spelled out in the simple "lay-man's" explanation.

We've already proved the existence of black holes

[guttentag]

A famous former surgeon general discovered the first of these monsters a few years ago, and named it drkoop.com (the .com designation is often used to help identify black holes). Then there was altavista.com, webvan.com, and many others. The escape velocity exceeds the speed of VC money. Since nothing can go faster than VC money... Enron, by the way, is not a black hole. It's a pulsar -- a dead star that regularly flashes us with reminders that it's dead ("Enron doesn't have any money," "Enron doesn't have any money," Enron doesn't have any money," etc.).

How to form such a beast?

[Captn Pepe]

According to the article (in gr-qc/0109035, not the horrible thing linked from the /. article above), we essentially have a phase transition that results in an inflationary subspace inside this thin shell! I grant that, for some odd assumptions, this might be a stable solution, but I kind of doubt it. It has been proposed before that the collapse to a singularity triggers internal inflation, which is plausible but still gives a black hole, not a "gravstar".

Anyway, and I quote from their own article, "Here we forgo any discussion of the details of the quantum phase transition and present only the solution of Einstein's eqs." Mazur and Mottola have no clue how to make such a beast, either. If nothing else, the energy density wouldn't approach that needed for a phase transition until long after the entire assemblage was well within its own event horizon, again giving -- you guessed it! -- a Schwartzchild black hole. Recall, when a solid mass reaches the density required to fall within its own event horizon, the total density isn't much above nuclear densitites. During big bang baryosynthesis, densities are easily this large and inflation obviously didn't occur then (or else we'd have no protons in the universe).