system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the Universe’s “local” geometry (e.g. on the surface of the Earth) so similar to Euclidean geometry?

I’m not sure if the wording of my question is correct or clear enough; I’ll try to elaborate more below. Also, I apologize if I’m inaccurate or unclear in what I say below too; I just started reading about this.

From what I’ve gathered/understood from articles and more-or-less nontechnical books on general relativity, it seems as if the “global” geometry of the Universe is not Euclidean, implying things like the shortest distance between two points in space is not a straight line. However, here on Earth, the path of shortest distance between two points (even if they are separated by the Earth itself) is almost indistinguishable from a straight line. I’m wondering why this is the case.

Edit: I didn’t think this was going to blow up and generate so much discussion. Thank you for taking time to respond! :)
gpt
It's the same as how the Earth looks flat even though we know it's a sphere. If the curvature is gentle enough, you can only see it on a really large scale. It's only e.g. near black holes and neutron stars that it gets strong enough to be visible.

But in a sense, this is sort of a backwards argument. General Relativity was derived with the *assumption* that space-time is a "locally Euclidian manifold". The theory is built from the observation that space-time appears to be Euclidian on small scales. But *why* this is the case is a matter of very speculative theoretical work.