Thread: gravity question

Originally Posted by grapes

In this case, anything less than 400,000 kilometers is close!

Yes, like that asteroid

But wouldnt it all be about the center of mass ... if two systems have the same center of mass relative to the object being attracted and same mass ... then from a distance it would be the same ... like the sun vs a 1 sm bh.

Originally Posted by grapes

Of course. The gravitational field for the earth plus the mass of the moon would be a lot different than the earth and moon system now. That's Newtonian, even.

Originally Posted by tom

But wouldnt it all be about the center of mass ... if two systems have the same center of mass relative to the object being attracted and same mass ... then from a distance it would be the same ... like the sun vs a 1 sm bh.

If they're spherically symmetric. Otherwise, no, although approximately yes at a distance. As in my example, it can make a lot of difference the closer in. JFalz1024 asked about different dimensions, so I was thinking of oblong shapes.

Even with spherical shapes, there is a difference at distances smaller than the radius of the larger body, right?

This is great. Reading this stuff makes me feel like a kid.

One of my questions I had way back was about the center of mass of a non spherical black body at close range.

Originally Posted by grapes

If they're spherically symmetric. Otherwise, no, although approximately yes at a distance. As in my example, it can make a lot of difference the closer in. JFalz1024 asked about different dimensions, so I was thinking of oblong shapes.

Even with spherical shapes, there is a difference at distances smaller than the radius of the larger body, right?

Originally Posted by tom

One of my questions I had way back was about the center of mass of a non spherical black body at close range.

Why black body, necessarily?

And what was the question? Was it answered?

The question I had was that could we determine how energy / mass is distributed from inside the EH of a non spherical BH? The question initially arose from the idea that I had that a BH would never fully collapse relative to an external observer ... so most of the mass of a black hole must be eitehr at or near its surface OR evenly distributed inside. If you were close by in theory you should be able to measure density differences ... Like say in the case of a black hole consuming a massive neutron star. At the point of impact would the BH be lopsided? ... this stemmed into my new question which I just posted in the main forum of if BH keep their shape during these types of collsions OR do they eventually become spherical.

Originally Posted by grapes

Why black body, necessarily?

And what was the question? Was it answered?

As I've said earlier, side-to-side differences can make a difference in the gravity field. However, radial differences do not. From the outside, in the newtonian limit, you cannot tell the difference between a uniform ball, and one whose mass is entirely at its surface. I'm not sure whether GR makes a difference.

Originally Posted by grapes

As I've said earlier, side-to-side differences can make a difference in the gravity field. However, radial differences do not. From the outside, in the newtonian limit, you cannot tell the difference between a uniform ball, and one whose mass is entirely at its surface. I'm not sure whether GR makes a difference.

It's an interesting thought. I wish I knew the answer As you say, some mass distributions are indistinguishable from the outside; however, there are others that can be detected by watching their effect on objects in close orbit. If the asymmetric object began to collapse into a black hole, maybe we could keep detecting the asymmetry for a while. Maybe for a long time.

I don't really know. And I don't know if it's ATM or not

Do BH's have a 3d shape or structure inside the EH, or does the warpage of space continue to collapse into a singularity? Does matter past the EH spiral in or "fall" in?