alt Hacker NewsThis sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body.
Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just part of earth's single gravitational force. All the airplanes in the sky are subject to the pull of the earth, but they're also a part of the gravitational pull that pulls other things to earth.
When shaking cereal, the big flakes rise to the top, but tiny bits of dust from each flake also separates and settle at the bottom. But earth, as a whole, has big bits and little bits everywhere all flowing freely. And gravity seemingly treats all those bits as a single object. But with sufficient distance between objects (e.g. different planets), it treats them separately. And with greater distance (e.g. galactic scale), it treats them as one again.
Replies
> gravity seemingly treats all those bits as a single object
Understanding the tides depends very much on disabusing oneself of this scaffolding.
There’s a bit of math you can do where you calculate the gravitational effect of standing on a sphere. You can redistribute the mass within the sphere and it won’t affect the gravitational pull, assuming it’s radially symmetrical.
One of those unintuitive results.
It probably only works in classical gravity but it’s still neat.
> And gravity seemingly treats all those bits as a single object.
It does not; but when you learned about gravity in school or wherever, you were only presented with scenarios in which it was valid to do so. Specifically, for Newtonian gravity, an object whose density is spherically symmetrical may be treated as a point mass at its center (w/r/t other point masses farther from the center than any point in the first object); this can be seen by integrating Newton's equation.
There's no reason to attribute this to special behavior by gravity with respect to "objects" which you identify. You could decompose a (spherically symmetrical) mass into several different spherically symmetrical "objects" and sum up their influence on a test mass, and the result is the same as if you had treated the original mass as a point mass. The hypothesis that gravity is "treating" the object one way or another now fails to distinguish all these possibilities; it's no longer physically meaningful.
At sufficiently large distances, or for sufficiently large ratio of large to small mass, the difference between, say, a planet and a spherically symmetrical mass is so small with respect to the main effect that it can simply be ignored.