Picture a satellite orbiting at a latitude of the Great Lakes. Now the Earth condenses to the size of a marble. Its position somewhere near what used to be the equator. Now go back to the satellite orbiting at what used to be the latitude of the Great Lakes? It wouldn’t keep circling empty space above the equator because there is no mass there to orbit. It’s been repositioned south to what used to be the equator. The satellite would have to adjust to this new center of gravity, no?
It’s not going to draw in anything that wasn’t already in its orbit. However, all masses act on others no matter how minute. Earth does currently have some impact on Venus’ orbit. And when you change the center of mass, that’s going to have some effect
Btw I’m typing like I know what I’m talking about. I don’t. I’m just a layman positing something I find fascinating
I feel like you mean latitude and satellites can’t orbit a latitude (other than the equator), and the equator is not at the center of mass of the Earth.
If you did change the center of mass it would, but I believe we are generally speaking the Earth condensed at its center of mass, so that remains unchanged.
And it’s not exactly zero, but I don’t think Venus is much affected by Earth at all and it would continue more or less the same if Earth completely disappeared.
There’s no universal law of volume though. It’s just gravity. In your example the center of gravity hasn’t changed. (This is hard to believe until you work it out for yourself. Also makes a good excuse for torturing undergraduates.)
The shell theorem thing is basically just saying you can take a ball of stuff and replace it with a smaller ball of the same mass and nothing changes as far as gravity is concerned. You can make the ball as small as you want. Even a single point.
I mean ok I’m starting to see some minor complications because the earth isn’t uniform in density and stuff like that but I don’t think we’re near worrying about that.
Your explanation is starting to make some sense to me. But back to my original question… If that’s true of the earth, then why not for a massive star that collapses on itself to form a black hole? According to you guys, mass doesn’t change, center of gravity doesn’t change. Nothing changes. Same as Earth?
Yes, I think you’re there! Sun or earth, whatever you want.
The shell theorem thing is basically just saying you can take a ball of stuff and replace it with a smaller ball of the same mass and nothing changes as far as gravity is concerned. You can make the ball as small as you want. Even a single point.
A collapsing star goes through a bunch of transitions before it collapses into a black hole and those transitions shed lots of mass/energy. It is very different from what happens if you were to create a tiny black hole on earth that then proceeds to swallow the entire earth.
It might make more sense if it’s framed this way: a thin shell of uniform mass can be replace by a point mass. So imagine the Earth is just an infinite number of shells, they can all be replaced by a point mass. Which makes calculating the motion of all the planets way easier.
Probably because Uranus is so big (gas giant) and because gravity gets weaker the farther away you get from something. So even though there’s more mass, you’re so much farther away from most of it that the net pull is less.
@moderators Can we extract black hole content to a black hole thread? While interesting, two months out from the election it probably would be better to keep this thread more strictly related to important relevant discussions.
