Suicidal singularities?

September 8, 2010 7:27 AM

Can a black hole unblackholeify itself when it 'sucks in' less dense matter?

I was wondering whether a black hole, in the process of attracting matter to itself, can experience ontological issues when this matter comes in 'contact' with the black hole, the idea being that it might lower the overall density of the whole thing to the point where the escape velocity becomes less than the speed of light.

Presumably this could only occur in small black holes, since I have some idea that matter drawn in gets eventually 'spit out,' but I'm not quite sure about the dynamics of this process.

It came up in a discussion over whether black holes ever 'go away,' as an alternative to Hawking radiation.

posted by vckeating to science & nature (19 answers total)

No.
posted by kjs3 at 7:31 AM on September 8, 2010 [3 favorites]

"less dense" matter just gets made more dense.
posted by rmd1023 at 7:36 AM on September 8, 2010 [5 favorites]

What matters with a black hole is the density of the black hole itself, rather than the density of any surrounding matter with which it may be in contact. A black hole surrounded by absolute vacuum is still a black hole; adding a thin cloud of matter does not make the black hole any less dense - even though one could calculate an average density for the total assemblage of matter which might be quite low. So let's say that you approach an interstellar dust cloud which contains a black hole inside it. The cloud is not a black hole and does not have an event horizon. But the black hole inside is still a black hole and still has an event horizon. The only known means by which black holes go away is Hawking radiation. But in theory, given a sufficiently long period of time (which would be ridiculously long) every black hole will evaporate.
posted by grizzled at 7:36 AM on September 8, 2010

All matter is less dense than that of a blackhole. Theoretically it has infinite density.
posted by JJ86 at 7:36 AM on September 8, 2010

Let me start by saying that all of my understanding of physics (and especially astrophysics) begins and ends with the physics class I took in high school and not much more than that.

That said, I don't think the density of the matter entering the black hole matters. Since a black hole is a singularity, its volume in infinitely small making its density infinitely large (mass per unit of volume). Assuming I'm right about that, then any matter that enters the black hole increases its density or at least its mass.

I don't think its a question of how dense the matter entering the black hole is but how fast mass leaves the black hole versus how fast mass enters it. I think. That sounds right to me but I'm not a physics expert by any means.
posted by VTX at 7:49 AM on September 8, 2010

The short answer: No.

Longer answer: First off, the fact that the escape velocity for a simple black hole is the same as the speed of light is a mathematical coincidence, not the basis for the event horizon being a one way membrane. Michell's formulation was fortuitous. If it were, you could build a ladder and simply climb out. You don't need to be at escape velocity to leave the Earth, you only need a very long ladder.

You've heard that mass distorts spacetime, right? Well, what does that even mean? Imagine you are approaching a non-spinning black hole with no electric charge. Its enormous mass distorts spacetime, and that doesn't just mean falling. That means that your potential paths begin to point towards the black hole, more and more. When you are just above the event horizon, you can fire off a photon, tangent to the event horizon, and the photon will orbit the black hole, forever. The photon's "straight" is actually bent around the black hole.

Once you enter the event horizon, all of your paths point some fashion towards down. You think you're stepping to the left, but you're really stepping to the left and down. You go North, it's North and down. You're leaving? No, you're really going down. That is what the heavy distortion of spacetime means once you are in the event horizon. Prior to that, you still have a few "ups" left to you, but once you're in, the only way is down.

For destroying black holes, short of waiting for many 10^{double digit} years to pass, well, you can steal away some angular momentum from them, but that will only result in a non-spinning black hole. Black holes end up getting rid of their charge rather rapidly, too. What would be required would be exotic matter, matter with a negative mass. No, that is not antimatter. Antimatter still has a positive mass value.

Nobody has any exotic matter, but if you had some, you can do all kinds of goofy things with it, like hold open wormholes and the like. Once you have one impossible thing, many others follow.
posted by adipocere at 7:59 AM on September 8, 2010 [9 favorites]

You don't need to be at escape velocity to leave the Earth, you only need a very long ladder.

Isn't this just a slower way to achieve escape velocity? If you climb a ladder to geosynchronous orbit, once you're there you're going *googles* 3 klicks a second around the Earth just like everything else up there.
posted by ROU_Xenophobe at 8:18 AM on September 8, 2010 [1 favorite]

You never really escape, do you? Think about it. I could be a billion trillion miles from Earth and still feel the pull of its gravity ... to a very small amount. The same holds true with black holes or anything with mass. Gravity and the EM force go on forever. "Escape velocity" is a very fancy way of saying, "If I were flung away at this speed, with no additional thrust, I would fly away forever from that object." Implicit in that is that, if you ever stopped (relative to that thing) its attraction to you would again begin to make you "fall back" towards it ... unless something else were pulling on you harder.

That's why escape velocity is not really the concept that is of use here.
posted by adipocere at 8:25 AM on September 8, 2010

Density is a function of the electromagnetic forces that bond atoms and molecules. In ordinary circumstances this force is much stronger than the force of gravity, but in a black hole gravity has run amok. So things like atomic bonds and density cease to have any meaning because they are all defined by the EM forces which are overpowered by gravity. I'm not even sure if it makes sense to call it matter as we know it because it certainly wouldn't be in the form of atoms -- it would be more like just a random concentration of electrons, protons, and neutrons mushed up together in no discernible shape. Heck, I'm not even sure if it would be that, or whether it would just be pure energy.

In any case, no, there's absolutely no way that the density of an object before it fell into the black hole would have anything to do with anything.
posted by Rhomboid at 8:26 AM on September 8, 2010

"All matter is less dense than that of a blackhole"
It's more sensible to consider the density as the average over the volume inside the event horizon. That's useful for thinking about how much stuff you actually need to get in one place to create a black hole. In that sense, larger black holes are actually less dense than smaller ones.

A hole's density isn't determined by the make up of what falls in though - that all gets squished to the point like other posters have said. The density once it exists is unavoidably and simply fixed by the mass (give or take perhaps rotating holes etc).

More generally, if you throw something at a hole it goes in and you don't see it - there's always room inside to fit more in, and the more you throw in the bigger the hole gets.
posted by edd at 8:31 AM on September 8, 2010

The only working theory of gravity we have is General Relativity. General Relativity says that gravity isn't a "force" at all. Rather, mass distorts the fabric of space. Objects move in "straight lines" (geodesics) but because space is non-Euclidean, those geodesics follow paths which, in a Euclidean setting, would correspond to Newton's law of gravitation.

A black hole is a place where this distortion reaches its logical limit. The amount of mass is large enough so that the distortion of spacetime makes it so that no geodesic can emerge from within it. This has nothing to do with "density", it only has to do with the quantity of mass in that location.

There's no theoretical limit to the size of a black hole, either in terms of the amount of mass involved or in terms of the perceived diameter of the event horizon.
posted by Chocolate Pickle at 8:54 AM on September 8, 2010

Yeah, what most people above said. No matter how dense the matter coming into the black hole is, it's going to get crushed down to extreme density as soon as it enters the hole.

Now, if you could imagine something with negative density, maybe that could do what you're thinking about and un-black-hole-ify a black hole. But nothing like that exists, because it doesn't really make sense.
posted by auto-correct at 9:42 AM on September 8, 2010

perhaps if we knew what dark energy was, you could do it with this.
posted by jrishel at 10:20 AM on September 8, 2010

There's also the tricky problem that the tidal and magnetic forces close to high-density objects are powerful enough to rip normal matter to bits. You don't even a black hole for this. Both white dwarfs and neutron stars rip infalling gas (at relatively low densities) to plasma and compress it to degenerate matter (at extremely high densities.)
posted by KirkJobSluder at 11:39 AM on September 8, 2010

That's why escape velocity is not really the concept that is of use here.

adipocere, you're right that escape velocity is not the relevant concept, but not for the reason you cite.

Inside the event horizon of a black hole, escape velocity is infinite. It is not possible to escape at any achievable (finite) velocity, at all.

Outside the event horizon, escape velocity is finite, and as relevant as with any other massy object (like Earth, or the Sun).
posted by IAmBroom at 12:24 PM on September 8, 2010

adipocere mentions two things I'd like to expand on--

first, waiting a long time: this is useful because of Hawking radiation. Although at the semi-classical level this radiation looks just thermal (that is, totally evenly distributed and containing no information), we know at the quantum level, you get info back. You have to wait very very long for this, though, and the info will be all scrambled up.

Little black holes evaporate due to Hawking radiation faster than big ones, btw, so really if you create a tiny black hole and then watch a short time you might be able to "see it unblackify" in some sense. (Mostly if you could measure this, you'd see some radiation and then the b.h. would be gone).

second: changing the spin of a black hole. If you have a spinning black hole, you actually can pull some energy out; read the Wikipedia article on the Penrose process for more details. Not quite what you're asking for, OP, but still interesting and a way to reduce the energy in a bh.
posted by nat at 10:02 PM on September 8, 2010

Thanks for all the comments. I guess the problem I had was over the use (or in this case, uselessness) of the escape velocity concept.

One of the questions I have coming out of it (and perhaps this is for another post - I'm unsure about the rules here) is where does all the energy for the smashing up/crushing/spaghettisation/densification come from? Is it just one of those things we don't know because of the weirdness of the whole singularity thing?
posted by vckeating at 2:36 AM on September 9, 2010

"This has nothing to do with "density", it only has to do with the quantity of mass in that location."
Density is how much mass is in a given location (volume, but in this case the same thing really). The Sun is not a black hole because its mass is spread over a sufficiently large volume. If it were compressed down to higher densities, it would be one. In that sense it has everything to do with density.

"perhaps if we knew what dark energy was, you could do it with this."
That seems a bit unlikely. It has positive energy density too, just negative pressure. I'm not quite sure exactly how you're thinking it might help, and I wouldn't say there's no way it could, but I can't see a plausible way you could somehow do something like this.

"There's also the tricky problem that the tidal and magnetic forces close to high-density objects are powerful enough to rip normal matter to bits. You don't even a black hole for this."
As I said above, really big black holes aren't all that dense. They don't exert much in the way of tidal forces as you fall through their event horizon either, so not only do you not need a black hole, a black hole doesn't even guarantee it.

"where does all the energy for the smashing up/crushing/spaghettisation/densification come from?"
It's just gravitational potential energy. It comes from the same place as the energy that makes a coyote recently deprived of ground accelerate to the bottom of a canyon.
posted by edd at 5:00 AM on September 9, 2010 [1 favorite]

It's just gravitational potential energy.

To elaborate on that, any system where you have objects spread out from each other instead of clumped together is at a higher potential state, because it takes energy to move things out and apart against the force of gravity. The big bang stretched the springs so to speak, and the crushing is just them being allowed to snap back together.
posted by Rhomboid at 6:45 AM on September 9, 2010