Math Fun Facts!
hosted by the Harvey Mudd College Math Department created, authored and ©1999-2010 by Francis Su
Subscribe to our RSS feed   or follow us on Twitter.
Get a random Fun Fact!
or
No subject limitations
Search only in selected subjects
    Algebra
    Calculus or Analysis
    Combinatorics
    Geometry
    Number Theory
    Probability
    Topology
    Other subjects
  Select Difficulty  
Enter keywords 

  The Math Fun Facts App!
 
  List All : List Recent : List Popular
  About Math Fun Facts / How to Use
  Contributors / Fun Facts Home
© 1999-2010 by Francis Edward Su
All rights reserved.

From the Fun Fact files, here is a Fun Fact at the Medium level:

High-Dimensional Spheres in Cubes

How good is your intuition in high dimensions? Take a square and divide it into its four quadrants. Inscribe a circle in each. Now, draw a circle whose center is at the center of the big square and whose radius is just big enough to touch the four circles you just drew. We can perform an equivalent operation in a cube, inscribing spheres in each of its eight octants and then placing a sphere in the middle, just large enough to touch the other spheres.

It's easy to see that the central circle is much smaller than the square and that the central sphere is much smaller than the cube. What happens if we keep doing this in higher dimensions? Will the central sphere grow or shrink in diameter, relative to the side-length of the cube, as you change the dimension?

Our intuition suggests that the central sphere doesn't grow. After all, its boundary is determined by spheres which lie inside of the cube, hugging each of the corners.

It turns out though, that in the ninth dimension the central sphere is tangent to the cube, and in much higher dimensions the volume of the sphere is actually larger than the cube!

Presentation Suggestions:
Draw the case for the square (and if you're artistic enough, the cube as well) on the board. Ask the students what they think happens in higher dimensions. Maybe the diameter of the central sphere is always less than some constant 'a' (a<1) times the side of the cube, or maybe the central sphere keeps growing and gets arbitrarily close to touching the cube, or maybe it gets much bigger. Take a vote.

The Math Behind the Fact:
We'll make the numbers easy by using a cube (in dimension N) with side length 2. Then when we cut the cube into orthants, each sub-cube has side length 1. Let's just look at one of those sub-cubes. The sphere S inscribed in that sub-cube has diameter 1. When we draw the central sphere, its center is on a corner of that subcube. Draw the diagonal from that corner to the opposite corner of the sub-cube. That diagonal has length Sqrt[N]. The part of the diagonal going through the sphere S has length 1 because that is the diameter of S. Of the part that is left over, half of it is in the central sphere, and in fact forms the radius of that central sphere. So the central sphere has radius (Sqrt[N]-1)/2.

If N=9, the radius of the central sphere is 1, so it is just tangent to the cube. If N>9, then part of the central sphere bulges outside the cube! And, eventually the volume of the central sphere is actually larger than the cube.

For a related surprise, see Volume of a Ball in N Dimensions.

How to Cite this Page:
Su, Francis E., et al. "High-Dimensional Spheres in Cubes." Math Fun Facts. <http://www.math.hmc.edu/funfacts>.

Subjects:    geometry
Level:    Medium
Fun Fact suggested by:   Joel Miller
Suggestions? Use this form.
4.39
 
current
rating
Click to rate this Fun Fact...
    *   Awesome! I totally dig it!
    *   Fun enough to tell a friend!
    *   Mildly interesting
    *   Not really noteworthy
and see the most popular Facts!
New: get the MathFeed iPhone App!

Brings you news and views on math:
showcasing its power, beauty, and humanity

Want another Math Fun Fact?

For more fun, tour the Mathematics Department at Harvey Mudd College!