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

I don’t know what physicists you’re referring to, but I didn’t think anyone serious claims that “space is flat”. We acknowledge curvature in spacetime as a fundamental consequence of General Relativity. That’s how we can explain the Doppler Shifts of starlight, time dilation experienced by GPS and other high-velocity probes, etc.

What you might be confusing is the claim of space being *locally flat*. That’s more of an abstraction to explain relativistic behaviors of subatomic particles. Personally, I like to think of it as the difference between modeling terrestrial dynamic observations you can see with your eyes (microscopic) versus from a satellite (macroscopic). This “local flatness” is mathematically equivalent to creating a N-dimensional linear approximation.