Comment by imglorp
1 day ago
> I guess it looks like it's down since it's a slice but the effect is an inward sphere?
Yes, gravity is a vector field: every point in space near a heavy body has a vector pointed at the center of the body with a magnitude of the field strength. https://en.wikipedia.org/wiki/Gravitational_field
Whenever someone uses fabric sheet analogy, they need to shout that the X-Y of the sheet is a 2 dimensional analog of X-Y-Z space, and the Z direction of the sheet is the field magnitude, with the slope indicating direction.
All models are wrong, but some are useful (for understanding).
Other thing is I believe they say gravity is strongest at the center of the sphere/core but I would think the mass is split evenly away from the core eg. maybe 2/3 radius from the center where it's equal mass on each side. But probably doesn't make sense wouldn't be a ball.
Since the strength is represented by the slope of the sheet (not the depth), it should still line up. Underneath the ball at the very center the sheet will be level, to match as you say, that the field strength is 0 there. The exact shape will probably be wrong though since it's mostly determined by the shape of the bottom of the object.
Inside a solid uniform sphere, gravity is exactly equal to the gravity of the sphere of material under your feet, as though any mass at any "altitude" higher than you did not exist. It's a pretty standard calculus problem. (the opposite is true, inside a solid hollow shell gravity is exactly zero everywhere)
The next thing they do after showing you the sheet is to roll a ball around the stretched part to demonstrate an orbit. Explaining how that analogy works starts to take more math than the actual field you’re trying to explain!