- #71

PeterDonis

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yoron said:How do you differ between a flat space and a flat space time?

Because "spacetime" is the entire history of the universe, including all times, and "space" is just the universe at one instant of time. At one instant of time, the universe (according to our best current model) is flat; but when you include the time dimension, the universe is expanding, i.e., changing in time, and that means spacetime is curved.