zlacker

> If space is expanding why aren’t the radii of fundamental particles and their orbits and molecules also expanding? And if that we’re the case we couldn’t notice space expanding.

> Does space only expand somewhere else? Only between me and the Andromeda galaxy, and not _within_ me and the Andromeda galaxy? How would it know to do that?

If you start with expanding space in general relativity, and then carefully take the limit where you get back to Newtonian gravity, then it just corresponds to a classical force, specifically a very tiny force that weakly pulls everything apart, growing with distance.

This doesn't expand small objects, because they're rigid. It's the same reason that I can't make my laptop get bigger by gently pulling on the ends. On the other hand, it would pull apart two independent, noninteracting objects (such as the Milky Way and Andromeda).

>>knzhou+(OP)
On top of that, FLRW spacetime is a large-scale approximation: More realistic models should probably follow the 'swiss-cheese' approach, where local conditions can look rather different.

>>knzhou+(OP)
So molecules are like bits of glitter stuck to the surface of the balloon?