# Schwarzlöcher (Black Holes)

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## Main Question or Discussion Point

What are black holes?

A theory I have is simple, and thus must be wrong. It seems to me that black holes are just extremely dense balls of matter, not holes at all, really. Just superdense balls.

So, what is the real explanation, likely involving 105 vectors and 31 dimensions?

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When matter is compressed to a (zero dimensional) point and the resulting gravitational force is so large that it creates a region where the velocity needed to escape the gravitational pull is larger than the speed of light.

Stellar black holes are formed in some supernovae.

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So instead of 31 dimensions, I get zero...

Wouldn't any amount of mass at zero length, width, and depth, implying zero volume, have an infinite density?

And so... in order for a black hole to exist, it must have a sufficient mass within that point, correct? How is this mass determined? Is the equation $g=\frac{G m_1 m_2}{r^2}$ used to determine this?

I'm not looking for the simple answer, don't worry.

And so... in order for a black hole to exist, it must have a sufficient mass within that point, correct? How is this mass determined? Is the equation $g=\frac{G m_1 m_2}{r^2}$ used to determine this?
The http://en.wikipedia.org/wiki/Schwarzschild_radius" [Broken] of an object tells us the mass-volume ratio required for a black hole to exist. I think of it in basic terms, a black hole is region of space from which light cannot escape, so what does this imply? Well the greater the ratio between mass and volume, the stronger the gravity, and thus the higher the escape velocity. At some point the escape velocity exceeds c, and that is when a black hole can exist.

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well when enough mass is compressed into a small enough volume it collapses in on itself to form a singularity. the singularity is actually where current laws of physics break down: if you use the
$$g=\frac{G m_1 m_2}{r^2}$$
equation, then you are plugging in a 0 for r (because its a point), which means that you are dividing by 0, which means
$$g=\infty$$. Infinite is obviously a problem...

Nabeshin
Interestingly enough, a black hole need not be very dense at all. For example, supermassive black holes, like those proposed to exist at the centers of galaxies, can have an average density less than that of water or air.

As far as production is concerned, one merely needs to cram matter into a small enough (nonzero!) volume, at which point a horizon forms around the object beyond which no light escapes (and conversely, any probing of the interior of this horizon is impossible! That is to say, we cannot experimentally know what goes on inside the horizon!). However, General Relativity tells us that the matter within this horizon MUST continue to fall to the center of the object until it all accumulates at a point. This is the so-called singularity.

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Well, we could know what happens in the horizon... but we wouldn't be able to communicate what we know to anyone.

So... does a black hole have a definite, measurable mass?

Nabeshin
Well, we could know what happens in the horizon... but we wouldn't be able to communicate what we know to anyone.
True true. But we tend not to consider this possibility!

So... does a black hole have a definite, measurable mass?
Yes. One method of measuring would be gravitational interactions, if we see a neutron star or something similar orbiting a BH.

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Would it make you laugh if, when you said that a black hole could be less than water, I pictured a black hole floating in a giant tub of water?

And of course, is there a black hole at every galaxy's center?

Nabeshin
And of course, is there a black hole at every galaxy's center?
I think the current belief is that the majority of all galaxies have a supermassive black hole at their centers. Of course, it's difficult make a decision about ALL galaxies, and I'm not exactly in the loop of galaxy formation, but definitely most.

I have to ask what is a Schwarzlöcher black hole? Did you mean to type Schwarzschild? I was taught that it is incorrect to imagine black holes as having a surface on which one could stand, even notionally. Is this correct? As Nabeshin mentioned it would be hard to say with 100% confidence that all galaxies contain within their center a SMBH. I believe over a million galaxies have been analyzed and that the strong majority of them possess a super massive black hole at their core, so that is strong evidence for that assumption.

The thing about density is a fun, for example a scoop of Saturn material would float about in your tub provided you could contain it in some sort of rigid shape. I am going to buy my children floating Saturns and a floating black thing to put in the bath, who needs ducks.

Joe

marcus
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I have to ask what is a Schwarzlöcher black hole? Did you mean to type Schwarzschild? I was taught that it is incorrect to imagine black holes as having a surface on which one could stand, even notionally. Is this correct?
das Loch means the hole in German
die Löcher means "the holes" (plural)

Schwarzloch = black hole
Schwarzlöcher = black holes

You were taught right, Joe. The event horizon is a mathematically defined surface ("of no return") but it is not a surface you could stand on. Even notionally.

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Correct Marcus. When I can't think of an interesting name for a title, I default to German. It's a little quirk.

I would love to get a miniature Saturn representation for a bath...

Wait! Has anyone sold planet representations at the density they would be? That'd be a great way to teach that part of planetology. And it can't be astrology, because that's the study of the stars.