## 6-dimensional Sphere

Of course a 3-dimensional thing is bigger than a 2-dimensional thing. Just like a beach ball is bigger than a circle cut from paper.  And it’s equally obvious that a 4-dimensional thing would be bigger than a 3-dimensional thing.

Or … is anything obvious? Following is a lesson in multi-dimensional reasoning.

A 2-dimensional “ball” is a circle. Two-dimensional points with two coordinates comprise the 2-ball. Any pair of rectangular coordinates on the circle satisfy:*

$\dpi{300} \bg_white \sqrt{x^2+y^2}=1$.

For example the pair (√½, √½) is on the circle. And the pair (√¼, √¾) is also on it. But (½, ½) isn’t.

A 3-ball is made up of the 3-points whose three rect-coords satisfy

$\dpi{300} \bg_white \sqrt{x^2+y^2+z^2}=1$.

That’s just a normal sphere from real life.

And a 4-D ball would consist of any array of four values whose Pythagorean sum** is 1:

$\dpi{200} \bg_white \sqrt{x^2+y^2+z^2+w^2}=1$

A 5-ball is likewise the set of all 5-points with Pythagorean norm 1. It’s bigger than a 4-ball, of course.

A centered unit 6-sphere is the set of all 6-points with norm 1. It’s smaller than a unit 5-sphere.

• You: What?
• Me: Don’t ask what, you heard me.

### A 6-dimensional ball is smaller than a 5-dimensional ball.

The 7-ball is smaller still, and higher dimensions keep getting smaller in volume.

True story.  I am not making this up.

If you don’t believe me, just do the sextuple integral. The volume of a 6-sphere is 5.6771 and the volume of a 7-sphere is 4.7247. A 13-ball is less than 1 unit volume.

* I’m talking about a “unit circle” with radius 1, but that could be radius 1 mile or radius 1 nanometer. Or, like, whatever.

** Pythagorean sum? I’m being sly. Hinting at future posts about

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