A black hole is a condensed star so a black hole must be made of condensed matter. In what form is that matter contained? Is it neutrons? In that case, what a distinguishes a black hole from a neutron star?

Well unfortunately if you peek inside one you’ll never be able to tell anyone outside what is in there.

Nevermind what the matter was before entering a black hole. On the way in the matter is stripped to its most basic constituents (basically quarks) and then squished into an infinitely dense point. Doesn’t matter if it was a star or a cheese sandwich. All the same inside a black hole.

What does that infinitely dense point looks like? Nobody knows. Physics falls apart at the singularity.

One thing I have never reconciled is that for anyone falling in to the Black Hole the singularity is *always* in their future. To me that sounds like in a manner of speaking they never quite reach the singularity. But if they cannot reach the singularity then how can there be a singularity (i.e. nothing ever gets there to squish together)?

::head explodes::

Thirty years ago the prevailing theory was called “black holes have no hair”, meaning that there are very few details about them. Not that few were known, but that fundamentally black holes had to be very simple. They have a mass, an angular momentum, and a net charge. None of these things can be created or destroyed, so they must stay constant from the stuff that formed the hole to the hole itself.

The current issue of Scientific American has an article on Naked Singularities that paints a much more complicated picture.

Yeah, that’s the thing, nobody knows. It must be a different kind of matter though because of the force of the black hole doesn’t behave like anything else that we have witnessed.

The fact is we don’t know, and probably will never know, what’s going on in there. However, knowing what we know about how things behave, it’s probably reasonably safe to assume it’s neither infinitely dense nor a point. It likely has some spatial extent, albeit very small and therefore some finite density, albeit very large. Generally when your math spits out infinites, you know something’s not exactly quite right. Nevertheless, black holes is plenty weird.

You might want to pick up Death from the Skies!, the latest book by Phil Plait (Bad Astronomer).

Among the other ways we could die, he discusses black holes, and what happens when you are pulled into one.

It’s an amazing look at our galaxy, as well.

I don’t know what this means. Gravitationally, they look like any other object of similar mass. If the Sun were to turn iinto a black hole right now, it would certainly go dark, but the Earth’s orbit (and those of all the other planets and cosmic junk) would remain unchanged.

Doesn’t string theory say something about matter being incompressible beyond the Planck length?

Thanks,

Rob

So, conceivably, they could have hair on the inside?

How do Black holes evaporate? what happens to their mass., plus-enerby that falls into a BH-what happens to it?

There was some peculation years ago, that the analogue to Blakholes (White Holes) existed-has anyone ever found one?

When people say that the singularity is always in the future if they’re inside a black hole, what they means is that anyone who’s inside a black hole *will* run into the singularity, guaranteed, no matter what they do. However, this doesn’t mean that it takes an infinite amount of time (as measured by an observer) to get to the singularity; in fact, you can show that the amount of time elapsed before you hit the singularity is always finite.

Here are a couple of old threads that might be of interest to the o.p.:

[thread=422479]How is the “size” of a black hole defined?[/thread]

[thread=469304]The Universe, Black Holes and Entropy[/thread]

Neutron stars (and other exotic hypothetical celestial bodies like quark stars) are still theoretical. Because of the tendency of unbound neutrons to decay it is unclear how such a body would behave or exactly what type of matter it would be composed of. Mass inside of a singularity, compressed by the curvature of space into an unimaginably small point, much denser than even quarks pressed “edge to edge”. We can’t really say what form mass takes in that form; indeed, a black hole can in many ways be considered a giant composite quantum particle.

From the subjective viewpoint of an infalling black-hole-diver, external time will slow as he or she is accelerated asymptotically to **c** when approaching the singularity “surface”. Note that the singularity is where the curvature of spacetime becomes infinite, or at least larger than can be measured down to quantum scales (Planck length). The singularity for a rotating black hole (and we can readily assume that all naturally created black holes will have some amount of rotational momentum conserved from the original matter it condensed from) is actually a ring rather than a point, though the size of the ring for a stellar mass black hole is immeasurably tiny, and the diver falling into such a hole would be torn to component atoms by the tidal shearing forces long before.

It is tempting to say that the outside observer would see the hole-diver as reaching the singularity surface in a finite (and actually very short) time, but as the observer will never see anything that occurs within the Schwarzschild radius, as escape speed now exceeds **c**. (This also means that the hole-diver will not see what goes on in his past, as the light cone now folds in on itself.) Although we can mathematically determine, given the initial momentum properties, when a mass falling into a black hole will contact the singularity, for all practical purposes it is “gone” from our universe once it passes the Schwarzschild radius, and is now a part of the “no hair” region that is the intimate core of the black hole.

It is possible to enter the ergosphere (the area of space that is distorted by frame dragging) of a very massive, quickly rotating black hole and exit out again; one can even plot paths that go backwards in time, though in order to escape without falling in you’ll have to turn around and come forward to at least your entry time. You can also fall into an orbit somewhere outside the Schwarzschild radius (3 Schwarzchild radii is the minimum radius for a stable circular orbit) which will keep you in a permanent and inescapable orbit, such that your personal future is bounded by some energy “surface” that you have insufficient ability to achieve.

Stranger

In reading Wikipedia about black holes, I read this article on the largest (mass-wise) one we know of: OJ287

http://en.wikipedia.org/wiki/OJ_287

A different wiki article says:

So. Will the “merge” of this pair be spetacular?

*spectacular :doh:

Respectfully request spellchecker

Very small rotating black holes evaporate (Bekenstein-Hawking radiation) due to virtual particle pair creation near the Schwarzschild radius; one member of a pair is absorbed but another flies off, taking with it a large amount of kinetic energy (in comparison to total rotational kinetic energy of the black hole). This energy loss reduces the mass of the black hole, and eventually the mass disappears (or is at least too small to form a measurable curvature of space). The size of black hole we’re talking about here is very small and incapable of being formed by the normal gravitational collapse, and instead must be due to some kind of quantum effects (hence why they are often called “quantum black holes”). Once the mass to surface area ratio of a black hole becomes too large the quantum effect no longer dominate, and while such a black hole will continue to radiate the amount of mass-energy loss is insignificant. Note that no one has ever observed Bekenstein-Hawking radiation, as it would be vastly less than the amount of normal radiation (due to interactions from infalling material) coming from any observable black hole, but the concept is widely accepted in the astrophysics community and is consistent with black hole thermodynamics as we know it.

The concept of “white holes” spewing matter and energy out is problematic; for one, a black hole is an area of space that contains maximal entropy within the event horizon, which cannot radiate away. Once matter goes into such a region, it cannot emerge back into our universe, or at least, not in any area of it we could observe without seriously interfering with causality and thermodynamics as currently accepted. It may be that the material enters some new region, isolated from our universe, and indeed, the expansion of our universe appears in many ways to be a black hole running in reverse, or at least, cockeyed.

Stranger

“18 billion solar masses!” I can’t get my mind around this. Will anyone describe what this would look like?

Like this: .

Sure. Like this → .

Except a lot bigger

ETA: Argh…beaten to it! Curse you! ::shakes fist::

Black holes evaporate via a process called Hawking radiation; for a somewhat intuitive picture, think of it like this: in quantum mechanics, there is no such thing as ‘empty space’ – the Heisenberg uncertainty principle allows for spontaneous generation of particle/antiparticle pairs, *provided* they annihilate again after some short time (basically, they live on borrowed energy, and borrowed energy, in this case, means also borrowed time). Now, if such a process happens just on the edge of a black hole horizon, it might just happen that one of the pair falls right into the black hole, while the other manages a narrow escape; however, then, the two can obviously never recombine and return their borrowed energy, in blatant (or so it would appear) disregard of fundamental physical principles *and* common decency.

But, if one looks at the whole process a little closer, one finds that for this to be allowed to happen, the particle that fell into the black hole must actually have had *negative* total energy, and thus its falling into the event horizon has effectively reduced the black hole’s total energy – by just the amount that’s now being carried away by the other particle!

So, at the end of the day, the cosmic account keeper has nicely balanced books, at the cost of things looking for all the world like the black hole just spit out something in stark contrast to its usual, sucky nature.

As for white holes, no, there have never been any observed, and they’re mostly thought to be mathematically sound, yet unphysical solutions to the equations of general relativity.

ETA: Yes, well, or what **Stranger** said better.

I want this book. So badly I can taste it (It tastes like chicken). Today’s my birthday, and I’m hoping it is waiting at home for me…

Yes, also socks that get lost in the dryer, pens that my cats have gotten hold of, and the pair of glasses that mysteriously vanished from my purse in 1997. But I’m not going in there to check.