Gravity with mass or density?

[banzemanga]

Hello everyone! I am new here. I am a little confused about gravity and black holes.

 

I know that gravity is calculated by the mass of the two bodies divided by their distance. So that the more mass there is the stronger is the gravitational force. Gravity can bend light and the stronger the gravity is, the greater is the curvature for light's space-time direction (or whatever is called).

 

It is said that you can take anything and compress it into infinite density, it will eventually become a Black Hole. However, even if you change volume and density of an object by compressing it - it doesn't change the amount of mass. How could possibly the force of gravity of this object increased by increasing the density when the mass is just the same?

 

Thanks.

[GAHD]

Because it (generally) absorbes more mass too :)

See: Accretion disk

 

Also, one can assume that black holes have varying sizes to their event horizons. If compressed enough an object's local gravity may become enough to both keep the matter compressed and at the same time greate a deep enough well that any radiation emitted is retained. But then again, even our sun "could" become a black hole: it would just need to be so compressed that it's local gravity became rediculous; Earth's orbit would likely remain unchanged since the high gravity would only be very close to the event horizon of the Sol-Hole.

 

In short, it's not more gravity, it's denser gravity. ;)

[ronthepon]

Let me show that in the language of maths.

 

The gravitational force felt by an object is:

 

[math]F = {\frac{G{m_1}{m_2}}{r^2}}[/math]

 

Right? Now suppose that you have both masses m1 and m2 being one Kg each. And the distance between them really small, say [math]10^{-100}[/math]meters. That's ridiculously small, but it's just for an imaginary condition.

 

The force will be really, really high.

 

The same way with the earth. Normally, you don't have that small distances being effective. The radius of the earth is 6400,000 meters, and you'll never get closer than that without going underground.

As you go underground, the mass that was once below you begins to get above you, and it can be shown that all the mass above you won't contribute to gravity force.

 

In the formula, not only does r reduce, but so does m(of the earth, effectively). And it can be proved that m reduces faster then r^2 increases.

 

Hence you don't see the black hole effect.

 

Suppose the radius of the earth was a centimeter. Then, it would be a black hole. If you got to a close enough distance (the surface, actually) the force would be as high as it gets.

[Qfwfq]

How could possibly the force of gravity of this object increased by increasing the density when the mass is just the same?

It doesn't.

 

In the Schwarzschild solution, the field at a given radius r depends only on the total mass, providing it's in a spherically symmetric distribution around the centre and all at distance less than r. Compressing it further in doesn't change the field at that radius r. The interesting matter is when you decrease r, down toward the Schwarzschild radius.

 

It's different if the distribution is spherically symmetric but not packed into the Schwarzschild radius. When you go into it, only the mass at distance less than r counts. If you burrow down into Earth, the field decreases and will be zero when you get to the centre.

[arkain101]

I have a question and thought related to this topic.

 

Theoretically according to how gravity works, at the center of a body of mass, the gravitational forces will stop acting in a inward and compressed way. Gravity is a measurable force between two objects with mass.

 

So if we look at the earth as an example, the center of it is very dense, and very 'massive'. There is a strong magnetic (correction edit: gravitational force not magnetic :confused: ) force. However the greater towards the center an object is the greater the force of gravity will be surrounding it. Thus would it not be theoretically and technically plausible to predict that at the center of the earth there is a massive area where the gravity forces cause an area at the very center to become less dense than the surrounding center. I do not assume there would be a void because the pressure is very immense inwardly, but there should be a less dense area at the center of the earth where its dimensions would be calcuable in use of the mass of the earth, the size of the earth, and the density.

 

This being correct, even atoms themselves should techinically act in such a way where there is a void in its center, or less dense area.

[ronthepon]

Probably not, because the outer layers do not actually pull the inner ones.

 

There is a theorem, which states that inside a spherical shell, any point feels no gravitational force due to the mass of the shell. It involves quite a simple proof, and I think that Newton was the first to arrive at it using calculus.

 

So we can consider the earth a sphere consisting of many concentric shells, placed one outside the other, right from the center to the outer crust.

[arkain101]

Probably not, because the outer layers do not actually pull the inner ones.

 

Hi Ron,

 

I do not expect the outter most layers of the earth would pull the inner ones. I expect the inner core of super dense material to be acting on itself in such a way that the attraction forces should vary from exact center to outter center.

 

One example is. If we have two magnets stuck together and we then add two powerful magnets that we can switch on and off and place one on one side of the magnets stuck together, and the other on the opposite side of these magnets.

 

As we switch on the outter magnets the inner magnets that are stuck together will be attracted to the outter ones. Lets assume the experiment is set-up in such a way that the outter magnets are capable to tear the inner magnets apart and attract one inner to each outter magnet.

 

However, in the whole of the forces, the outter magnets are also pulling eachother together (if poles are arranged correctly that is) but there is also a tearing apart action taking place in the center.

[ronthepon]

Well, it's late and this is flying over my head. But I'd strongly suggest that you start a separate thread on this... it seems worth it... before it becomes hijacking another thread.

[arkain101]

Yah it was related in a sense so I will try to connect it in with the thread.

 

I apologize for my hi-jacking habbits.

[CHADS]

i think banza its because the energy in the mass has to flutuate in a very small area than its used too and the sum of its mass/energy exceeds the dimentional parameters that can support a centre so it has to release its energy into a singularity and pulls everything close by , in with it for being cheeky.

[infamous]

However the greater towards the center an object is the greater the force of gravity will be surrounding it. Thus would it not be theoretically and technically plausible to predict that at the center of the earth there is a massive area where the gravity forces cause an area at the very center to become less dense than the surrounding center.

This thought brings another question to mind. At the center of a black hole, is the influence of it's gravity reduced to zero? Strange thought that never occured to me before because we most often think of black holes as being infinitely small singularities. However, black holes can theoretically be, of any size. Take for instance a collection of matter within the schwartzchild radius of a few light years having enough mass to make excape velocity equal to the speed of light. It would still be a black hole but unlike the singularity, planets and stars might still exist in their own right. A space traveller passing into this region of space wouldn't even know that he had entered a black hole until he tried to escape it. And unlike the typical black hole so often theorized about, he wouldn't be torn apart by tidal forces and quite likely live through the experience even though he would never be able to return to his former world. Just a wild thought.............Infy

[hallenrm]

As a casual reader of this this thread, I was enticed with the word schwatzchild, I had never come across this word earlier perhaps because I never care to read a lot about Black holes.

 

So I googled, and hit on a website, which I believe will be useful for readers of this threa like myself. Here's the link:

 

http://www.astronomy.eku.edu/Yoder/l16_BH.htm

[banzemanga]

First, i want to thank everyone for their opinions.

 

Because it (generally) absorbes more mass too ;)

There is a recent contradiction of this theory. That Hawking radiation decays the Black Hole so fast that the Black Hole disintegrates before it can absorb everything in the universe - otherwise, the rate which Black Holes increases could be so fast that we could be possibly be much nearer or inside of one Black Hole right now already(maybe exaggerating a little bit :p).

 

In short, it's not more gravity, it's denser gravity. ;)

I agree with you in that one - i thought about it too.

 

[math]F = {\frac{G{m_1}{m_2}}{r^2}}[/math]

Right? Now suppose that you have both masses m1 and m2 being one Kg each. And the distance between them really small, say [math]10^{-100}[/math]meters. That's ridiculously small, but it's just for an imaginary condition.

The force will be really, really high.

Yeah... I got mixed up with the density, mass and volume formula. But then again, it means that without taking in count the amount of mass, anything including quarks, as long as we get close enough into the distance close to zero then anything would be a black hole? :evil:

 

About whether being closer to the center of Earth gravity contribute factors and affect density or not. With pressure of all the matter above that gravity is pulling down, it might compress matter close to the center but then again it might not because it is already really dense already and for it to compress it even more; it requires really a great amount of mass. Another things are the layers, the might reduce the pressure of matter too.

 

Also about layers, i thought that there could be a possibility that while Black Holes absorb mass, they would be less and less denser for every layer and generally become a new astral body like another star. I also read somewhere that scientists found newborn stars orbiting the Black Hole in the center of our Milkyway galaxy.

 

Last but not least, i totally forgot about Schwarzschild radius. For something to be a black hole, it have to have the mass of a radius of about ten suns. I got side tracked by something i read, that if you take anything and compress it into infinite density it becomes a mini black hole. But nah, it still needs the mass of Schwarzschild radius.

 

Thanks everyone again.

[CraigD]

This thought brings another question to mind. At the center of a black hole, is the influence of it's gravity reduced to zero?

For star and galactic core –mass black holes, I think the answer is – nobody knows with certainty. Some theories suggest singularities or near singularities in which physics break down so much that gravity and the other fundamental forces cease to make ordinary sense, others suggest a big sphereoid of neutronium or quark matter, in the center of which the gravitational force would have zero magnitude, like the center of a planet.

Strange thought that never occured to me before because we most often think of black holes as being infinitely small singularities. However, black holes can theoretically be, of any size. Take for instance a collection of matter within the schwartzchild radius of a few light years …

 

A space traveller passing into this region of space wouldn't even know that he had entered a black hole until he tried to escape it. And unlike the typical black hole so often theorized about, he wouldn't be torn apart by tidal forces

This is fertile ground for thinking, especially if you think really big, say, taking the whole visible universe.

 

Taking the equation for Schwarzschild radius, [math]r = \frac{2Gm}{c^2}[/math] and some common approximations for the visible universe (Eg: wikipedia article “Observable Universe”), we get:

Radius of visible universe = 7*10^25 m

Mass of visible universe = 3*10^52 kg

Event horizon of visible universe = 4*10^25 m

 

Which doesn’t quite work, since the event horizon is smaller than the radius – but not by much. If the universe is only slightly denser than estimated (and ignoring complicating factors like “dark energy” and expansion), the entire visible universe could be a black hole. Note that it’s not necessary to have any sort of super-massive body or singularity at it’s center – though it is necessary for it to have a precisely defined center.

 

Imagine what this would look like to inhabitants such as ourselves. All of local space – assuming you’re not on the “edge of the universe”, would be as usual. However, the trajectory of light from a very distant/old body could follow a curve around the volume between the most distant bodies and the event horizon, so that the body is visible in many different directions, each direction having a different path length, and hence different spectral characteristics, etc.

 

Wild thinking, but not untestable – though such experiments take extraordinary skill and imagination to design. I’m aware of at least one astronomical study along these lines, a Scintific American cover article, though I can’t recall the issue, and don’t have time to look it up now.

[infamous]

 

Wild thinking, but not untestable – though such experiments take extraordinary skill and imagination to design. I’m aware of at least one astronomical study along these lines, a Scintific American cover article, though I can’t recall the issue, and don’t have time to look it up now.

Thanks for your knowledgeable input CraigD. There is another wild thought that just occured to me:

 

If the universe can be vaguely discribed as a black hole, then some black holes might be composed of other black holes without each losing it's singular identity.

 

In the case of galactic black holes, observation has been made of one black hole absorbing another. Common reasoning suggests that what is now created is only a single black hole with the gravitational/mass of the combined two. This may in fact be the case, however, it may not be?

 

In the case of the very large black hole that we formerly discussed, it might be possible for the larger black hole to absorb a smaller one without cancelling out the smaller ones identity.

 

Another wild thought: Taking these possibilities into consideration, might we have within our own universe, local areas of space which appear to be void but are infact, very large black holes. Large enough to be classified as small universes in and of themselves. These structures could be observed if we could ever build a gravitational detector sensitive enough to measure them. Curious thought, universes within universes? And this may also fit the bubble or foam universe theory but altering it to include extra universes within our own.

 

If I've allowed my imagination to wander too far afield CraigD, correct any of the misconceptions I may have stumbled over. I certainly admire your expertise relating to these subjects.....................Infy

[Qfwfq]

And unlike the typical black hole so often theorized about, he wouldn't be torn apart by tidal forces and quite likely live through the experience even though he would never be able to return to his former world. Just a wild thought.............Infy

Why not?

 

The gradient of the field always goes toward infinity, approaching the Schwarzschild radius.

[infamous]

Why not?

 

The gradient of the field always goes toward infinity, approaching the Schwarzschild radius.

Einsteins's theory of GR predicts that though the gravitational field around a massive black hole is stronger on the large scale, it will exert weaker tidal forces than it's smaller counterpart. Taking this factor to the extreme, say in the case of a black hole with a radius on the order of a significent fraction of our universe, crossing the event horizon would subject our space traveler to very small tidal forces. Once inside this supermassive black hole, things might appear much the same as they do in the heavily congested area of the inner portion of our own galaxy. A black hole of this size would not need to consist of super dense material. All thats required is enough gravitational force to limit light energy from reaching excape velocity. This concept can be applied to our own universe, it consists of mostly space yet, can be vaguely defined as a gigantic black hole................................Infy