# Is Gravity a function of mass or density

1. Aug 17, 2010

### curiouschris

I know I have a very simplistic understanding of gravity. but nonetheless I was contemplating black holes when an anomaly struck me.

Gravity (strength of) in my simplistic terms...
Gravity is a function of mass the more mass (matter) the stronger the force called gravity.

Creation of a black hole as I understand it...
A star at least 3 times greater than ours explodes into a supernova it then collapses back onto itself under its own gravitational influence. no longer supported by its nuclear reactions it continues to collapse eventually crushing the matter inside it to a super dense object. the gravity from this object is so strong even light cannot escape it.

Oooh I just thought of something else. but I digress.

Back to the original train wreck of thought....

Where does the "extra" gravity come from?

Surely the strength of the gravity is the same before and after the collapse. There is approximately the same amount of matter. Only its density has changed.
(I imagine a little got sucked back in from the surrounds if there was anything nearby. At the same time I imagine an amount was ejected never to return).

Does this mean that gravity is actually a function of density?
In other words as the density increases so does the strength of the gravity.

My digression is about the nature of gravity. If gravity is a wave and it travels in waves at the speed of light then how can gravity itself escape a black hole?

Personally I don't think gravity is a wave, though a perturbation in the field can travel as a wave at the speed of light.

I haven't asked this in cosmology as I felt except for the reference to black holes this is more of a general physics question.

CC

2. Aug 17, 2010

### Janus

Staff Emeritus
The total gravity of the black hole does not increase. If you were a given distance from a black hole before and after it forms you would not feel any difference in gravity.

However, gravity does increase as you get closer to the object's center as long as you don't go below its surface. When a black hole shrinks, its surface becomes closer to its center. This allows you to get a lot closer to its center without ever hitting its surface.

With a black hole, the original mass has shrunk so small that you can get so close to its center that you reach a point where the gravity is strong enough to stop even light from escaping.

3. Aug 17, 2010

### curiouschris

Thanks Janus

If that's the case which I did surmise. but seemed to be at odds to what I know of black holes.

Then black hole in a celestial sense must be tiny. The event horizon must be a lot smaller in diameter than the original star that created it. I thought or perhaps assumed black holes were huge objects consuming whole galaxies and the odd battle cruiser.

Instead are they mere kilometers or even meters across?

CC

4. Aug 17, 2010

### curiouschris

Oh cool I have just found a link that bears out this discussion

Of course I realise their is supermassive ones as well. I guess they are the ones that filled my imagination.

CC

5. Aug 17, 2010

### loseyourname

Staff Emeritus
I think you're confusing the whirpool of matter that gets sucked into a black hole (which can be very large) with the black hole itself, which in classical theory has no size at all but is a singularity of infinite density. You might also be thinking about the radius of the event horizon, which can range from a few millimeters to many thousands of kilometers.

Edit: I guess you realized that. The Schwarzschild Radius is the radius of the event horizon.

6. Aug 17, 2010

### curiouschris

My thoughts were that the "whirlpool" of matter sucked into the black hole would be the same whirlpool of matter sucked into the star before it became the blackhole.

As Janus explained for a given distance from the star the pull of gravity does not change as the sun descends into blackholeness.

That was what I had suspected was true, if gravity was mass reliant rather than density reliant.

What I had believed prior was that for some reason the gravity increased as a blackhole formed. sucking more and more of the neighbourhood in and therefore continuing to grow.

Anyone willing to respond to the question about how does gravity escape from a blackhole? I know it seems a silly question. but if gravity follows similar laws to light then surely it could not escape either.

7. Aug 18, 2010

### loseyourname

Staff Emeritus
That exact question was actually addressed on NASA's "ask an astrophysicist" FAQ page.