how can scientists say the universe is flat, when it seems to go in all directions?
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“Flat” basically means “the shortest path is a straight line”.
If you look at the shortest path on the Earth’s surface from London to Tokyo, it’s not a straight line, because to be a straight line it would need to go through the Earth itself, not just remain on the surface.
If you look at the universe and ignore weird stuff like black holes and the theory of relativity, then the shortest path between two points is indeed a straight line. That what’s scientists mean by saying “the universe is flat”.
(If you try to account for gravity as understood since Einstein, then the universe is no longer fully flat and the shortest path is not always exactly “the straight line”. There are some well-known examples with a black hole where there are multiple differents shortest paths from one point to another.)
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