What's new in black hole theory?

Just wondering how this column holds up to current theory.


Note added by Moderator: Cecil has responded to this post with a new column: http://www.straightdope.com/columns/read/2909/what-would-happen-if-you-were-swallowed-by-a-black-hole-revisited (27-Nov-2009) – CKDH

There’s not really much room for advances in black hole theory: They’re pretty much the simplest objects in the Universe. That said, there’s at least one flaw in the article (which was known even when it was written): From the reference frame of an external, non-falling observer, an infalling object takes an infinite amount of time to reach the horizon, but from the reference frame of an infalling observer, it takes a finite and very short amount of time to both cross the horizon, and to reach the singularity in the center.

It should also be noted that an accretion disk isn’t an inherently necessary feature of a black hole, so you could in principle fall into one without a disk (and therefore not be cooked by it). Likewise, the point of spaghettification is not necessarily the event horizon, and for a sufficiently-large black hole, it’d be possible to cross the horizon without harm (though crossing the horizon guarantees you a date with destiny shortly afterwards).

Honestly, that column didn’t hold up well to theory that was current when it was published. It is an example of Cecil’s desire to often titillate rather than inform. There is nothing wrong with having a little fun, but that column doesn’t really give the reader any good information about what would actually happen if he approached a black hole, to wit for anything smaller than a galactic-mass black hole the tidal forces would tear him molecule from molecule, and then the molecules apart.

As for what has changed, aside from some novel but untested (and, at this time, untestable) theories about quantum gravity (i.e. how gravity interacts on the quantum level), there haven’t really been any fundamental changes on the General Relativity side of the house, and Misner, Thorne, and Wheeler’s Gravitation, first published in 1973, is still in print and considered the authoritative work on gravity from a relativistic perspective. The classical thermodynamics of black holes are relatively simple, although there have been some developments in information theory across a event horizon. However, improvements in computational modeling based on GR have permitted higher fidelity and applying very strange non-linear topologies that result from gravitational singularities with various properties. In particular, the interaction between two or more gravitational singularities–traditionally a very difficult problem to solve analytically–can be modeled and examined.

One poster, Chronos, is doing doctoral research in this field and can no doubt offer a more authoritative answer on this topic.

Stranger

The interactions between black holes, as studied via computer, is an area of numerical relativity. It’s only in the past couple of years that folks have figured out how to model an entire collision between two black holes. I don’t really know many details about how that’s done, though: It’s not really my field.

None of that is relevant for a person falling into a black hole, though: The person’s mass will be negligible compared to that of the hole, so all of the annoying fiddly nonlinear bits that make the two-hole problem so tricky are small enough to be ignored, and the problem can be solved without use of a computer at all.

There is an interesting bit in the movie version of the “A Brief History of Time”, where Hawking describes hypothetically entering a black hole… He talks about “spaghettification” and what not… it is very interesting… A very good movie for people who are not so well versed in cosmology, but are still interested in the subject… although the movie is more biography than cosmology it is quite good…

Well that’s not even close. I get about 0.3 light years across, or 3 x 10[sup]12[/sup] km. Not sure what units you’d get 10 billion from.

What you need to realize is that the density of a black hole is not proportional to the mass. The Schwarzschild radius is directly related to mass, while the density is inversely proportional to volume, which increases as a cube of radius, so the larger a black hole gets, the less dense it is. The black hole that is in the center of the galaxy is approximately 4 million solar masses and has an average density of less than Earth’s atmosphere. I don’t know about the 10 billion light-years, which I suspect is a typo (should probably be kilometers or miles), but I don’t have time to look up the numbers to run the calc.

Sure; I was just expanding the scope to address the general question posed by the o.p. (“What’s new in black hole theory?”). The answer is that there is more refinement than revelation, which doesn’t mean that there isn’t important and scientifically valuable work being done, just that no great new insights have been found and tested to a point of being generally accepted.

Stranger

No I don’t need to realize that. There’s a reason I bolded the part about the black hole diameter, and not the part about the density.

Not exactly black holes, per se, but Scientific American had an article last month that a quantum effect called vacuum polarization may prevent a slowly collapsing mass from creating a black hole and form what the authors call a black star instead.

The article made my brain hurt.

Ooh Ooh, that’s right, Cecil saw my thread and wrote a whole new column to address the question, that’s right, I’m bad, you betcha!

A link to the new column.

I think Cecil still missed the point. He covered the spaghettification issue when entering a typical stellar-mass hole, at least. But then he went on to the more interesting case where it’s possible to survive crossing the EH if the BH is large enough to make the tides negligible. He didn’t say anything about what that experience would be like.

*** Ponder

I agree completely with this - Unca Cece still really didn’t address the issue. Although I’ve always thought that Stanley Kubrik pretty much nailed it. :slight_smile:

In that extreme case, nothing very interesting will happen, except that (I think) you’ll see a quick replay of the entire history of the interior of the black hole as you pass the front of the light that’s trying to get away but can’t. But I don’t think you’ll be able to see it in a coherent form, since your eyes aren’t flat.

A very old book I have on black holes describes the hypothesis that a spinning black hole should have a toroidal event horizon. And passing through said torus, without touching it, may allow some space-time jumping without spahettification.

Haven’t heard anything about this for a while, so has this hypothesis been rebuffed?
Googling has been inconclusive.

Well, I think I found the answer to my own question:

Basically, black holes may well have a toroidal “singularity” (my googling was coming up blank because I was looking for toroidal event horizon).

Not all scientists agree such a structure could be stable though. And if they do exist, it’s pretty much the consensus that you could not travel through the “hole”.

Why are scientists such spoilsports? Why can’t they crunch the numbers and find that A City of Wonders is in the middle of the torus?

I recently heard a podcast featuring Neil DeGrasse Tyson (I’m sure I can find it if anyone’s interested… it’s a Pub talk and he’s having a beer and answering audience questions).

He was discussing the fact that BHs evaporate and that there is an “accounting” for the matter that enters the black hole. Specifically, the matter that enters the BH will be radiated out as the BH evaporates. Obviously not in the same form as it went in since we’re talking about Hawking Radiation.

So his conclusion was that a BH is a dead end.

Although the new column is more accurate, I miss the brilliance of the original’s final paragraph:

This is the classic Straight Dope conundrum. Funny is easy, factual is easy. Lotsa luck getting both.

He calls it “Spaghettification”

http://www.youtube.com/watch?v=uNc-JLysk9Y

As does everybody else, including several people who posted in this very thread.

Welcome to the Dope, but please take the time to read the threads thoroughly before you post in one.