There are stars and galaxies and whatever space objects everywhere we look weither it’s up, down, left or right. Yet, scientists say that the universe is flat? How?
In: Physics
“Flat” is subjective. The Earth is flat if you only look in 2 dimensions for less than 8 miles or so. But that “flat” surface curves away in a 3rd dimension, forming a sphere. The idea, or so i believe, is that the 3 dimensional universe may curve away in a 4th dimension, so that it appears to be 3D where we are, but like the earth’s surface it curves so that you can reach your starting point by going in one direction. The only way this would be possible without us knowing it right away is if the universe is immensely huge so that we can’t feasibly percieve the curvature. So it would technically be a curving “flat” surface instead of one huge round 3D shape.
What scientists mean by that is that the universe has a flat geometry. If you draw a triangle on a piece of paper, it’ll have angles that add to 180° and the same is true of the universe when we measure as large a triangle as we can. If you draw a triangle on the surface of a globe or a saddle or a donut, by contrast, that isn’t the case. The angles add up to a different number.
Obviously, a piece of paper is 2D and clearly flat; the surface of a sphere is 2D and clearly curved. That’s where the terms comes from. Scientists and mathematicians working on 3D (or higher) geometry still use “flat” and “curved” even though it’s harder to imagine.
“Flat” is not the right word but we have no words in the English language that extends the concept to a three dimensional object. If you have a line you would call it straight. If you have a surface you would call it flat. But what would you call a similar three dimensional object? So people are just reusing the term “flat” for three dimensional objects as well, which pretty much only cover space itself. What this means is that even though there are some local bumps in the structure of space itself mainly caused by gravity on a larger scales there is no curves or bends.
It’s thought that the universe could be flat, but that doesn’t mean skinny. If you stand on an infinite table and keep walking in a straight line you never get closer to your starting point. However, if you do that on a sphere you will come back to your starting point.
A flat spacetime has three spatial dimensions at right angles to each other. Spacetime around here is flat. If it were curved, you could travel far enough in one direction to return to your starting point. The Earth surface is 2D, and not flat. It seems flat, if you’re in a parking lot, but if you go East far enough you will get back to where you started.
Can you link this article please?
Flat means asymptotic flat. The precise definition require Penrose diagram: the structure of infinity of spacetime looks like the structure of infinity of Minkowski spacetime, the archetypical example of a perfectly flat spacetime.
In simple term, this means on average over large distance, all the curvature from the stars and such become negligible. It’s like if someone say that Kansas is flat, it doesn’t mean there are no bumps while driving through Kansas.
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