ELi5: Physicists say space is ‚flat‘, but how can 3D space be flat?

Question

My assumption is that the short answer is that light travels in a 45 degree angle in a space-time diagram?

In: Physics

Answer 1

Its flat as in Euclidean geometry applies. On curved surfaces geometry works differently (parallels for example only exist in a flat space). Now of course it’s uncommon to think of 3D space as a surface but whatever you can say about a 2D flat or curved sheet you can extend things into one dimension higher.