# How do we know we live in 3 dimensions? [duplicate]

How do we know that we are really living in 3 dimensional world?

We can draw 3D in a paper that is one dimension. So, maybe the world around us just looks 3D! how can we prove that we're living in 3 dimensional world not one dimensional?

• Can you walk forwards and backwards, left and right? Can you walk up the stairs a down the escalator? If so you might want to consider yourself living in a 3 dimensional world. – Horus Sep 22 '15 at 5:09
• So we can draw a line up and down, left and right and forward and backward on a paper. I know it looks three dimensions but I want a physical proof that we live in 3 dimensions – David 2000 Sep 22 '15 at 5:10
• You can draw forward and backward on a piece of paper? That seems... unlikely. – march Sep 22 '15 at 5:22
• Proving the obvious is sometimes a little tricky, but one method of proof is relation of gravity and distance. Gravity weakens by the square of the distance and that, believe it or not, is proof of 3 dimensions. in 4 dimensions, planetary orbits would behave differently than they do in our solar system. Black holes would form faster, yada yada yada. The volume to radius ratio would be greater than the 3rd power. (I like John Rennie's show lace answer too) that also works. And, yes you can draw lines a number of ways, but they need to be at 90 degree angles to represent dimensions. – userLTK Sep 22 '15 at 7:43
• Paper isn't one-dimension, even if you pretend it's got zero thickness. you need to learn a LOT about geometry and topology. – Carl Witthoft Sep 22 '15 at 11:53

The simple proof is that you can tie your shoelaces.

This is a proof because you can only tie a knot in an (approximately) 1D object in three dimensions. In lower than three dimensions it's impossible because that would require the shoelace to self intersect. In higher than three dimensions it's impossible because there's always a way for the knot to untie itself.

• So, when we establish that shoelaces do indeed untie themselves, what conclusions can we draw from that? ;) – Ernie Mar 14 '17 at 6:13
• But what if shoelaces are not 1D objects (approximately), but rather 2D objects in a 4D world? Maybe physics does not allow us to move in the 4th dimensions, but how can we know it does not exist? – Gill Bates Dec 17 '17 at 0:41
• I agree with Eddie : Isn't it possible for shoelaces to untie themselves in 3D also? Aren't they only held together by friction, rather than a topological impossibility? – sammy gerbil Oct 6 '18 at 13:39

Think what one dimension means: everything going forward or backwards in a line from -infinity to infinity.

1) A car would flatten you in one dimension, and there are billions of cars people etc in motion. As we exist there is an extra dimension. So we need two dimensions at least.

2) Walking in two dimensions you would hit your head at the first projection, the house would be on top of you. A third dimension is necessary.

This should be physical proof enough available to anybody that we live in at least three space dimensions.

There may be many more, as in string theories, but we live in a three dimensional space projection as far as our physics goes.