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Re: Finite Matter, Infinite Density...

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Posted by nåte on March 2, 1999 00:40:46 UTC

: : : According to theory, Black Holes have infinte mass and infinte gravity. : : : How then, can there be an event horizon? If the force of gravity from the Black : : : Hole is infinite, then it would not dissipate with distance and therefore would be : : : the same everywhere, for an infinite distance. If my understanding of infinity is : : : correct, then Black Holes must have a finite mass and gravity if there is to be an : : : an event horizon.....right?

: : actually I think you have the theory off a bit.

: : BH have (at their singularities) infinite density, : : not infinite mass; for the reason you state in the : : latter portion. : )

: : this is why there are different sizes of BH as well.

: : For instance the gravitational field surrounding a : : BH may as well be the mass of 50 of our suns, and : : the event horizon be 1500 feet across.

: Thanks, the finite mass aspect clears a few things up.

: It still doesn't deal with the infinite gravity though. : How is it possible to measure the dimensions of a : gravitational field under conditions of infinite gravity?

A BH does not have infinite gravity whatsoever. I'm not sure where you heard this from.

Let me give you an example...

Take for instance a massive star in its dying phase. As the nuclear fusion begins to slow, the equalibrium balance of the force of fusion expelling outwards falls below the force of gravity collapsing inwards.

As the crushing force is creating a denser mass, keep in mind the mass does not change, thus the gravitational field has not changed.

Now, imagine the star has collapsed to the critical size called the, "Schwarzchild radius". This is the diameter of the star where space has become "potentially" infinitely curved. Thus light cannot even escape this boundary. IOW, the escape velocity required to escape this event horizon becomes infinitely close to the speed of light.

Now, lets take a step back and look at this BH. Has its gravity increased? No... In fact, imagine this particular BH side by side with its earlier form as the star. The BH and star have exactly the same mass (accounting the BH hasn't drawn anything in) as the star does. (in its early, pre-collapse stage) The only thing that has increased is the density. If you measured the gravity attraction of the BH at a given distance, you would measure the same gravity measurement of the same star at the same distance...


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