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

We say a 2d shape is flat if it doesn’t curve in a third dimension. We say that 3d space is flat if it doesn’t curve in some 4th dimension. As others say, that would show by triangle’s angles not adding up to 180 degrees.

Now, we know that space is curved locally, as heavy object’s gravity warps spacetime, but on larger scales the universe appears to be flat, at least to the extent we can measure it.