A question about the flatness of the universe.

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Discussion Overview

The discussion centers around the concept of the "flatness" of the universe, specifically questioning whether a flat universe implies the existence of an edge. Participants explore this idea through analogies and geometric reasoning, considering implications in both two and three dimensions.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions if the flatness of the universe, where angles in triangles sum to 180 degrees, necessitates an edge to the universe.
  • Another participant counters this by suggesting that a flat universe could extend infinitely, using the analogy of a flat blackboard.
  • A further analogy is introduced involving a sheet of paper that can be rolled into a cylinder, illustrating that flat surfaces can exist without edges even when angles sum to 180 degrees.
  • Participants discuss the concept of a toroidal shape, noting that it can be flat everywhere and finite, suggesting that the universe could potentially have a similar topology.
  • There is a clarification that the dimensionality of the space does not affect the possibility of a flat toroidal shape, emphasizing that such a shape can exist without net curvature.

Areas of Agreement / Disagreement

Participants present multiple competing views regarding the implications of a flat universe and whether it must have an edge. The discussion remains unresolved, with differing interpretations of geometric analogies.

Contextual Notes

Some assumptions about dimensionality and topology are not fully explored, and the implications of curvature in higher dimensions are left open for further discussion.

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A question about the "flatness" of the universe.

I may be out of my depth here or overthinking it but...

Does the fact that the universe is flat (angles in triangle add upto 180deg) mean that the complete universe has to have a edge?

If it had positive curvature, then it doesn't necessarily have to have a edge (like the surface of the earth)

Its just been bugging me that's all.

sid
 
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$id said:
Does the fact that the universe is flat (angles in triangle add upto 180deg) mean that the complete universe has to have a edge?

No; for a 2-dimensional analogy, imagine a flat blackboard that goes on forever.
 


Consider a two dimensional analogy. Take a sheet of paper, draw a triangle on it. Now roll it into a cylinder. The paper didn't have to stretch, so the sum of the angles of the triangle is still 180 degrees. The two dimensional beings living on the paper can't really visualize the operation, even though they may have taken topology at their two dimensional university. Now try to join the ends of the cylinder. We can't do that in our three dimensional space without the paper stretching and the angles no longer summing to 180 degrees.

But if we can go into a fourth spatial dimension, we can join the ends into a torus without stretching. Unlike a three dimensional donut, this torus is flat everywhere. And yet it is finite.

Now repeat the above, starting with a three dimensional solid cube...
 


B.C. said:
But if we can go into a fourth spatial dimension, we can join the ends into a torus without stretching. Unlike a three dimensional donut, this torus is flat everywhere. And yet it is finite.
A little bit of nitpicking here: actually, it doesn't matter how many dimensions you have. The important point is that torus itself has no net curvature, and so it is possible to think of an idealized torus without any. There's no reason why our universe cannot have this kind of shape.
 

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