Density

[Barry Scott]

There are very dense things here on earth, but a teaspoon of matter from a white dwarf would weigh a ton on earth and a teaspoon of neutron star would weigh a billion tons on earth! Black holes i even denser but im not sure how much. Are black holes the densist ( if thats a proper word :confused: ) object in the universe, and if you could get a teaspoon of it, roughly how much would it weigh.

Thankyou :)

[Qfwfq]

Density is the ratio of mass to volume and the only volume that it makes sense to ascribe to a black hole is that given by its Swarzschild radius. This depends on its mass, in direct proportionality, so volume is proportional to mass cubed and density will be inversely proportional to mass squared. The radius is:

 

[math]R=2\frac{GM}{c^2}[/math],

 

so you can work it out by calculating the volume, with:

 

[math]\frac{4}{3}\pi R^3[/math].

 

Putting it all together, you get:

 

[math]V=\frac{3c^6}{32\pi G^3 M^2}[/math]

[UncleAl]

Density is mass/volume. Fermi exclusion of electrons limits free matter to about 22 g/cm^3. High explosive shock compression might get you ~10X that. Gravitation is the biggest sustained squeeze.

 

Neutron stars have densities of 8x10^13 to 2x10^15 g/cm^3 compared to nuclear density of 2x10^14 g/cm^3. Surface gravity of about 10^12 gees. Note that about 1/3 the expected mass of a neutron star is lost as gravitational binding energy.

 

You have a big problem with the "density" of a black hole. While the external circumference of its event horizon is sloppy but computable, it has an infinite diameter measured across. One might then argue that the overall density of a black hole is asymptotic to zero. Like Dr. Who's tardis, a black hole is vastly bigger in the inside than it is on the outside.

[Barry Scott]

Oh, so its near enough impossible to calculate a black holes density then? So what object which we know about does have the highest density... a neutron star?

[InfiniteNow]

Oh, so its near enough impossible to calculate a black holes density then? So what object which we know about does have the highest density... a neutron star?

Bear in mind that there isn't total conformity in any of these objects, and they will all have differing densities. Your question is a bit like asking if a planet is denser than a moon, and the answer will always depend on "which planet, and which moon?"

 

What's denser, a marble or a bowling ball? Well, tell me what they're made of and offer some other parameters and we'll calculate it. :)

 

However, if one were to generalize, a black hole was so dense that it tore a hole in spacetime (that's what made it a blackhole), so I'd figure it to be denser than a neutron star since the neutron star has not yet torn such a hole...

[Jay-qu]

If you consider the possibility of all matter been squeezed into a singularity then there is no space.. infinite density.

[Barry Scott]

Are you saying that black holes could have infinite density Jay-Qu, i understand how you came to that conclusion but didnt UncleAl say earlier that you could argue black holes have a density of zero :circle:

Also, i dont know much about Quasars, but i do know that there are quite 'powerful'... should'nt they have a higher density than most things?

[Jay-qu]

Do a search of the site about quasars there is some good threads about it, one theory is that quasars may actually be black holes!

 

Saying that black holes have infinite density isnt really correct, what you would get is a divide by zero error, ie it is undefined.

[Mercedes Benzene]

I once heard that a black hole the size of a grain of sand could weigh 300 million (or maybe billion??:shrug:) times greater than the sun....

 

I'm not an astronomer or physicist, and I can't give a source for that statement... its just what I heard.

Please don't jump on me.

[InfiniteNow]

Are you saying that black holes could have infinite density

More appropriate would be a statement like, "Within black holes, there is a hypothesized point of infinite density which we call a singularity."

 

 

I once heard that a black hole the size of a grain of sand could weigh 300 million (or maybe billion??) times greater than the sun....

It's certainly possible. The idea is basically that the size of the object is not relevant to it's weight. Think of a marshmellow versus a lead ball bearing. They are the same size, but since lead is denser than (whatever marshmellows are made of...) sugar, it has a higher mass while occupying the same amount of space.

 

So, essentially, while our sun is enormous, the matter inside a black hole is denser and would weigh much more. Anyway, another quote I've heard is something like "a teaspoon of matter from a black hole would weigh 300M times more than a teaspoon of matter from the sun..." or other similar example which assists the reader in obtaining a better visual understanding.

[UncleAl]

Black hole density is

 

1) Finite and calculable (Qfwfq)

2) Asymptotic to zero (Uncle Al)

3) Infinite (Jay-qu)

 

That pretty much covers it. One should differentiate between the nascent naked singularity itself (1) and its equilibrium physical manifestion external to its event horizon (2).

[Jay-qu]

3) Infinite (Jay-qu)

 

I beleive I may have jumped the gun saying that, a singularity of 0 volume and incredibly large mass M has density = M/0, which is undefined.

[Qfwfq]

How do you get the infinite diametre Unk? :doh: The field? If that's perchance your line, then it goes for any massive object. :)

[Barry Scott]

If the density of a black hole is infinite, would that mean that it would have an infinite mass basically. E.g some matter the size of an atom from a black hole would weigh more than the whole of our galaxy. I know this isnt very scientific but potentially would this be the case?

[InfiniteNow]

Barry,

 

It's important to remember that, while related, density and mass are not the same.

[Jay-qu]

it would only be infinite due to 0 volume, though I think this would be an invalid thought experiment.. general relativity breaks down at zero volume

[TheBigDog]

It is my belief that nothing in the universe can exist in less than 4 dimensions. All matter has measureable dimension and exists for a span of time. None of these dimensions can be zero or that matter would simply not exist as matter. Following that logic I would state that a black hole at its core has dimension. And it follows that it has volume. And the volume would be relative to the total mass of the black hole in some fashion.

 

Infinity is a convenience of mathmatics for expressing very large numbers, but it doesn't exist in any part of nature with determinable boundaries. The universe itself may have undeterminable boundaries, and as such be expressed as infinite. But any bounded object if finite, and only in theory can it be expressed in any terms as infinite.

 

I would love to find a way of following up on this mathmatically. But it is not going to happen this weekend. I will give it a few years for anyone who wants to stick around and see my results.

 

Now, while on the topic of black holes, what is the matter within a BH called? I know that matter loses all of its characterizing properties when it gets swallowed by the hole. So there should exist in the hole a single "element" or state of matter. What it it? And what are its properties?

 

Bill