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# Largest Black Holes as dense as air?

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Someone please tell me this guy is talking out of his bum. I thought the opposite was true: the more massive a black hole the denser it is. Well, this guy is saying the opposite:

http://blogs.discovermagazine.com/outthere/2013/08/20/the-baffling-simplicity-of-black-holes/#.UhQkkmT9nsc

For an ordinary sphere–a bowling ball, for example–the mass increases as the cube of the radius. If one bowling ball is twice the diameter of another it will weigh eight times (2 cubed) as much. The rule breaks down a bit for large objects like planets, but in a very straightforward way. Their incredible bulk compresses their insides, so as planets get more massive their interiors tend to get more dense, assuming you are making an apples-to-apples comparison of the same type of planet. Some planets around other stars have masses several times that of Jupiter, but they are similar in size because of this gravitational squishing.

Black holes do something completely different, however. Their radius increases in direct proportion to the mass. Double the mass of a black hole, and its diameter doubles as well. (I’m using the event horizon–the point-of-no-return that defines the shape of the black hole–as its “surface” in this discussion.) The math of calculating the diameter of a black hole could not be easier. A black hole with the mass of the sun has a diameter of about 12 kilometers, or 7 miles. Want to know the diameter of the black hole at the center of the Milky Way? Based on the motions of stars circling around it, the black hole has a mass of 3.6 million suns. Just multiply 7 x 3.6 million and you’ve got your answer: It is 25 million miles wide.

The direct relationship between size and mass has a funny effect. The more massive a black hole is, the less dense it is–and the dropoff happens rapidly, as the square of the radius. (Again, I’m using the event horizon to define the surface of the black hole.) A solar-mass black hole crams the sun’s entire 865,000-mile-wide bulk into that 7-mile wide sphere, corresponding to a density of nearly ten quadrillion times the density of water. It’s a staggering number. The black hole at the center of the Milky Way has a mass of 3.6 million suns, which means its density is (3.6 million x 3.6 million) times lower. That translates to about 1,000 times the density of water–still very high, and about 100 times the density of lead, but no longer so incomprehensible.

Other black holes are much more massive than the central one in our galaxy, though, which means they are also much puffier. The galaxy M87 contains a monster black hole that astronomers have measured as having the mass of 6.6 billion suns. Its density is about 1/3,000th the density of water. That is similar to the density of the air you are breathing right now!

Those internal oddities are so incomprehensible that Albert Einstein did not believe that black holes were a real physical possibility.

It's all hypothetical and the experts will most likely be saying something very different in ten years time. I'm more concerned about small black holes that swallow items in one room only to deposit them at random elsewhere in the house.

GreyGhost said:

It's all hypothetical and the experts will most likely be saying something very different in ten years time. I'm more concerned about small black holes that swallow items in one room only to deposit them at random elsewhere in the house.

I'm pretty sure these are closely related to the ones that cause all our socks to disappear in the laundry.

Quantum physics mans. ITS ALL UNPREDICTABLE!!!

I’m using the event horizon–the point-of-no-return that defines the shape of the black hole–as its “surface” in this discussion.

He doesn't have any idea what he is talking about.

Perhaps black holes don't have an interior, they are all surface.

This makes sense to me, isn't the whole thing with black holes that they essentially have infinite gravity at the singularity? The event horizon is the place where light can no longer escape, but all the mass, outside of what is currently being drawn into it, is in a point at the center. Of course it's not going to be a dense object if you're defining the black hole as everything from the event horizon in.

andrewj said:

Perhaps black holes don't have an interior, they are all surface.

Current thought is exactly this; though, it would be more accurate to discuss it in terms of a sort of duality.

It can both be thought of as a singularity with all its mass concentrated at a single point, and yet, the nature of that singular mass is completely and accurately described by the state of the event horizon.

It has lead to increasing interest in the holographic theory. That is to say, our universe seems to some extent to work in the exact same way. It could very well be that any separated region of spacetime is described by physics on its higher-dimensional boundary.

If something is bigger, the bigger the mass is, not less.

I hope nobody says "It's in space so it's weightless".

SavageCorona said:

I hope nobody says "It's in space so it's weightless".

I think everyone here is aware of the fact that mass and weight are not the same thing.

And if they don't: If I were in space my weight would be zero, but my mass would still be..whatever horribly low number it is. Gravity gives weight to an object's mass. The strength of the gravity determines the weight.

I might be just bordering on obese if I go to Jupiter.

I'm a complete ignoramus in these matters, but are you telling me a hole has mass?

I've always been under the impression it's not a hole, but because of the way it acts like one, they were named as such.

I don't know if this is relevant to the thread or not. It's Stephen Hawking talking about black holes, good enough for me.

OK, it's nothing to do with density. Never mind.

Does a black hole absorb the mass of an object it pulls into it?

So, he's basically saying that a black hole might be super-dense, perhaps with all of its mass concentrated into a single "point", but that if you count as its volume all the regions of space where it exercises its inescapable gravitational pull ("event horizon"), then it's as if it was really-really-really dilute?

That's pretty much like putting a lead weight inside of a paper box, and claiming that now, the lead weight is only as dense as the paper and air inside the box, because now the entire box is the lead weight? Bah.

Maes said:

So, he's basically saying that a black hole might be super-dense, perhaps with all of its mass concentrated into a single "point", but that if you count as its volume all the regions of space where it exercises its inescapable gravitational pull ("event horizon"), then it's as if it was really-really-really dilute?

Well, no, because we simply cannot know the configuration of matter beyond the horizon. But black holes do become 'less dense' the more massive they get based on the radius of the event horizon.

No, you've misunderstood the entire point of black holes. Black holes have sufficient mass that they have collapsed down to a single point due to gravity. So your notion of "density" is meaningless because all the mass is concentrated at the singularity (from Wikipedia: "The singular region can thus be thought of as having infinite density.").

What black holes do have is an event horizon. This is just defined as the point beyond which nothing can escape the black hole's gravity, even light. It's not the "radius" of the black hole, nor is it a "surface".

fraggle said:

No, you've misunderstood the entire point of black holes. Black holes have sufficient mass that they have collapsed down to a single point due to gravity. So your notion of "density" is meaningless because all the mass is concentrated at the singularity (from Wikipedia: "The singular region can thus be thought of as having infinite density.").

What black holes do have is an event horizon. This is just defined as the point beyond which nothing can escape the black hole's gravity, even light. It's not the "radius" of the black hole, nor is it a "surface".

But what's the harm in using those terms to put it into terms we can identify with?

I see no problem with this.

Everyone, welcome our newest member Event-Horizon

Hellbent said:

Everyone, welcome our newest member Event-Horizon

I did think the timing was very convenient when I first saw his name. Perhaps it can shed some light on what its density is.

EDIT: Offtopic, but this being a sciency thread, seems like a good place to ask what Grazza's new custom title is. Is there some secret message in it like his last one? At a first glance, it does look like something to do with gravity.

fraggle said:

What black holes do have is an event horizon. This is just defined as the point beyond which nothing can escape the black hole's gravity, even light. It's not the "radius" of the black hole, nor is it a "surface".

It does however represent a finite (?) volume in space, though not necessarily perfectly spherical. Start at any point outside this volume, and you can still escape, no matter how much energy must be expended in order to do so. Start at any point inside this volume, and you're toast.

In that sense, you could consider a black hole to have a large, if finite, extension (which might even be simplifiable as a sphere's radius), and thus coupled with its finite (if large) mass, would give a non-infinite density, which might even be as thin as air.

Of course, this approach creates the following problem: if some entity was to gather a dense-as-air mass of the same "extension" as a black's hole even horizon, would that aggregate ALSO work as a black hole? In other words, can you apply Gauss's law for gravity to a black hole's event horizon "boundary" (defined above)? Or is there something else at play here?

My educated guess is that a black hole cannot have such an extended event horizon as to render its "collective" density as low as that of air (that would mean that you're well away from any such inescapable event horizon boundary.

Of course, YMMV: any attempts to solve ill-posed ambiguous problems will only result in more ill-posed ambiguous solutions, as the solvers will try to fill-in the gaps, each for themselves.

Hellbent said:

Everyone, welcome our newest member Event-Horizon

<note to self - don't cross him>

Rejoyce, rejoyce, my trusty disciples, for salvation lies within: enter the concept of the Schwarzschild radius, which neatly sums up what I wrote so far. YES, there IS such a concept as an event horizon volume/boundary. YES, it might be spherical in some simplified cases. And yes, there might be an ambiguous grey area between large concentrations of non-degenerate mass (e.g. a bunch of super-heavy stars) inside the critical Schwarzchild radius, and "true" black holes.

Cool beans Maes.

I've been wondering if a million is really such a big number (trying to put into context the vast distances and numbers in the universe). Here is a map that shows the globe with countries sizes distorted to reflect their populations (sorry Canada):

One square = 1,000,000 people. You may be aware that India has 1,000,000,000 people, therefore 1000 squares are inside of India. Suddenly one million doesn't seem so big. I wonder how many squares are in the map.

Also, how many grains of sand do you think fill an 8oz cup?

EDIT: Now don't get me wrong, there are still a lot of stars in the known Universe. According to one person's calculations, there are about as many grains of sands in the world's beaches are there are stars in the Universe:

http://science.kqed.org/quest/2008/11/21/stars-and-sand-grains/

Hellbent said:

I wonder how many squares are in the map.

I'm going to guess about 7,106

Technician said:

I'm a complete ignoramus in these matters, but are you telling me a hole has mass?

I've known people who were massive a-holes, so yes.

Bloodshedder said:

Including sea squares and taking the text quadrilateral near the bottom into account, I make it 15,404 ± 400, allowing for miscounting.

Gez said:

I've known people who were massive a-holes, so yes.

I'm a bonafied masshole. Never realized that it's an oxymoron until now.

Maybe you're a new type of black hole in the field of quantum bogodynamics, absorbing everything you can read and then emitting a large amount of bogons ;-)

Eris Falling said:

Offtopic, but this being a sciency thread, seems like a good place to ask what Grazza's new custom title is. Is there some secret message in it like his last one? At a first glance, it does look like something to do with gravity.