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

Flat here doesn’t mean flat like a piece of paper, it means the universe zero curvature. Flat geometry just means Euclidean. It’s the geometry we’re all most familiar with in our everyday lives, where parallel lines never meet and the sum of angles in a triangle is 180 degrees. If the universe has a curvature, those things would not be true.