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We begin our lesson with that timeless question, what is the fourth

dimension?Now, that sounds kind of spooky.

We're going to jack it up a level and answer the question, what is the nth

dimension? But we're going to start at the

beginning. Let's say n equals 1.

What is that? Well, that's simply the real number line

that you all know and love. We could coordinatize it by some

variable, x. When we move up to the second dimension,

well now, we have two variables to state where we are in the coordinate plane.

Call it x and y. Three dimensions is a little bit harder

to draw, but no less difficult to understand.

We have x, y, and z. But now when we get to the fourth

dimension, it's a little difficult to draw pictures.

We can, however, simply add another coordinate.

You might be tempted to use t for time, or some other variable name.

But why don't we just use subscripts x1, x2, x3, x4.

The reason for this being that when we move to the nth dimension, well, for

large values of n, we don't have enough letters, but we do have enough

subscripts. Now, drawing pictures in the nth

dimension is hard, therefore, we're going to proceed by exploring through volumes

and shapes. Three simple shapes, in particular.

Cubes, Simplexes which are analogs of triangles or pyramids, and balls.

Now, what do we mean when we say volume in high deminsions?

Now lets take a moment to think about that.

And as dimension of a object goes from zero to n, we know what three dimensional

volume means. That's simply a volume.

But we also know what two dimensional volume means.

That is area. What is one dimensional volume?

Well that is really length. Okay, you've got those three down.

What is zero dimensional volume? Well a zero dimensional object is simply

a collection of points. How many points?

That is the volume. Zero dimensional volume is counting.

And now with these in place We can move to n-dimensional volume, which we will

call hyper-volume if we're feeling epic, or we might just simply call it n-volume,

that will work. Let's build our inutition for

n-dimensional volume by looking at an n-dimensional cube.

This is going to be a cube, where all the sides have unit length.

So in dimension, 3, it's the familiar object.

What is a 2 dimensional cube? It's simply a square with side lengths 1.

What is a 1 dimensional cube? Well, it's got to be some interval.

An interval of length one. What's a zero dimensional cube?

It's a zero dimensional set whose value or count is equal to one.

It's just a simple point, and from that we can extrapolate to higher dimensional

cubes. But at this point, pictures fail and we

need to translate two equations. Therefore, we'll define the unit n

dimensional cube as those points in n dimensional space whose coordinates x sub

i satisfy the inequalities x sub i. Bigger than or equal to zero, and less

than or equal to one for all i. Now how do you visualize that?

Well it's hard to do with your eyes, but you can do it with your hands.

If you think of each coordinate as an independent parameter, then it is

remarkably similar to what happens when you slide an equalizer or slider bars up

and down perhaps you've played with something like this on a sound system,

each of those slider bars is like a coordinate.

In the n dimensional cube, where n is the number of slider bars.

Each can go up or down independent of the other, until you hit the boundary where

it has to stop. Now with that in mind, let's take a look

at volumes. Is we consider what happens in each

dimension. How do we get from 1 cube to the next?

At each stage what we're doing is taking the lower dimensional cube and then

crossing it with an interval, making a one parameter family.

Of such objects. Now, in every case, the volume is 1.

We're used to that in area - length times width - or in 3D volume - length times

width times height. In each case, it's 1 times 1 times 1.

In fact, this is really the basis for how you should think but n dimensional

volume. The surface area, well that's a little

bit harder to wrap your brain around, but we want to look at the boundary of the

cube, and say how much n minus one dimensional volume is there.

So if we take a one-dimensional interval, and say what's the zero-dimensional

volume of the boundary? That is, how many points on the boundary?

Well, simply two. Surface area for a two-dimensional object

is what we used to call perimeter. And in the case of a square, the

perimeter is four. There are four unit edges.

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Of course the surface area for a three-dimensional cube is what?

Well you look at each of the six faces, and compute its unit area.

Adding together gives six. Now in general, one could argue correctly

that the surface area of the n dimensional cube is, in fact, Two n, the

number of boundary faces that you have. And that pattern works all the way down,

even to dimension zero. The diagonal of a cube is the distance

between opposite corners. We know from Pythagoras

what that is for a two-dimensional cube. That's square root of two.

What about for a three-dimensional cube? Well, we would have to apply the

pathagreon theorum twice to obtain the square root of three as the length of

this long diagonal between opposite corners.

If we continue inductively, we can show that the diagonal of the indimensional

cube has length square root of n. That's a little crazy because for large

values of n, you can have a very, very small unit cube such that the opposite

corners are very very far apart. That's a little strange.

But this pattern continues down even to dimensions one and zero.

Lastly, if we count the number of corners in a dimensional cube we see 4 squared.

There's four. For cube, there is eight.

And in general, it's not hard to show that there are 2^n corners.

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Let's move on to a different shape, one that requires some calclus to understand.

This is the simplex, this is an in-dimensional generalization of a

triangle or a pyramid. The unit simplex is defined algebraically

as a subset of the unit cube that satisfies an additional constraint, this

being that the sum of the coordinates is less than or equal to 1.

But what does that mean in terms of our slider bar analogy?

This means that you can take any of the individual bars and slide it all the way

up to 1. However you can't do this independently.

If you want to move the other slider bars up you have to do so in a way but the sum

of the values does not excede the treshold of 1.

That means that this is a highly constrained set.

It's not a large subset of the n-dimensional cube.

It feels much small. We expect to see that reflected in the

volume. Let's see how that works.

First, lets explore a few properties and then we'll compute the [UNKNOWN] volume.

The number of corners of an n-dimensional simplex is much less than that of a cube.

The n-dimensional simplex has n plus 1 corners.

What is the volume? Well, we know, for a single simplex, it's

just a point. The number of points is one.

We know for a one dimensional simplex since its the same as a one dimensional

cube. We just get a length of 1.

Now, a triangle as we all know, gives us area one-half.

When we look at a three dimensional simplex, it's a cone over that triangle.

We know the volume of a cone is going to be 1 3rd, the height 1 times the the area

of the base, 1 half. Now, we start to see a little bit of a

pattern here. What if I told you that the 4 dimensional

simplex had 4 dimensional volume equal to 1 24th.

That's true. And knowing that you would be convinced

of the pattern, namely that the volume of the n dimensional simplex v sub n must be

one over n factorial. Now that's a good guess, let's see if we

can show it. Our strategy for computing volumes of the

n dimensional simplexes is the same as that Of a cone.

We're going to slice in a direction parallel to the base.

And what we're going to see is that when we slice and n+1 dimensional simplex,

what we'll get in an n-dimensional simplex whose size is rescaled.

By a factor of x in each coordinate, where x is the distance to the top on the

simplex. So for a one dimensional simplex the

appropriate volume element of the slice that's nothing more than dx.

In a two dimensional simplex, the appropriate area element is what?

It's simply x d x. In the three dimensional case, well,

we've done this before. This is going to be 1/2 x times x d x and

in general, the difficult step. Is to argue that the volume element, for

the n plus one simplex, is the volume of the base and simplex v sub n, times x to

the n. Since we're re scaling each coordinate.

By a factor of x. But once we have that, and then

multiplying by the thickness dx, we can compute this n plus one dimensional

volume as the integral of the volume form.

That is the integral of v sub n times x to the n d x.

Integrating as x goes from 0 to 1. This is a trivial integral since v sub n

is a constant. Routine x to the n plus 1 over n plus 1,

evaluated from 0 to 1. That gives us v sub n over n plus 1 and

so we can write down all of these volumes by induction and argue that v sub n is in

fact 1 over n factorial. That's a nice application of simple

integration. Let's move on to an n-dimensional ball of

radius 1. These are a little difficult to draw.

In 2D this is simply a disk of radius 1. In 1D it's a disk of radius 1, well it's

really an interval of length 2, and in 1D it is again a simple point.

Higher dimensional balls are not so easy to draw.

Now how do we define it rigorously? The unit ball is defined as those set of

points with coordinates x sub I between negative one and positive one satisfying

the additional constraint that the sum of the squares of the coordinates is also

less than or equal to 1. This is what we're used to in 2D.

When we say x squared plus y squared, less than or equal to 1, this is simply

the generalization of that. Now, in terms of a slider bar analogy.

Now, all of the individual bars can go from negative 1 to 1.

Each can go to the very top, or the very bottom.

But, in between well, you have some freedom to move the individual sliders up

and down. But you can't move them all past a

certain point where the sum of the squares is less than or equal to one.

Nevertheless, it feels like there's a lot of room inside of there to move around,

How do we compute the volume? Well again, for a radius 1 ball in

dimension n, what is volume going to be? In dimension 0, there is the single

point, volume 1. In dimension 1, this interval has length

2. In dimension 2, well, we know the

formula, pi r squared. In this case, r equals 1.

In dimension 3, volume is Four thirds pie.

Moving up to dimension n. Well what are we going to do here, lets

call that volume of the unit ball v sub n.

And to determine what that is lets consider what happens when the radius is

not one, but r in this case. The length of the one-dimensional is 2

times r. The area of the two-dimensional ball is

pi r squared. Volume, 4 3rds pi r cubed.

In general, having a ball of radius r and dimension n is going to give you the

volume. Of the uniball times r to the nth power.

That's going to be helpful for us, as we'll see.

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The surface area is what? Well in the one-dimensional case it's

two. In the two-dimensional case we're looking

at the circumference. That's 2 pi r.

In the three-dimensional case, the surface of the ball is 4 pi r squared.

Do you see a pattern? Yes.

It's related to the derivative. In fact, it's going to be in the

n-dimensional case, n times V sub n times r to the n minus 1.

You'll be able to prove that result in multi-variable calculus.

What's the diameter? Well in all cases it's equal to 2 times r

or in the unit case is 2. Now, let's see if you can figure out what

this n dimensional volume of the uni-ball these are then is.

Well we're not going to able to prove it in here, we will prove it in the bonus

lesson. It's wise to say there's is some work one

can show that the volume, the n-dimensional ball of radius one is, when

n is an even number, let's say two times k, then the volume is pi to the k over k

factorial. When n is odd, that is 2k plus 1, then

the volume is pie to the k k factorial 2 to the n over n factorial.

that's going to complicated we will show you how to get this in bonus material.

For now, the question I want you consider is, what happens to the volume of the

unit-ball as the dimension increases? Well, let's see.

N, and thus k, are getting bigger and bigger and bigger.

But there's a factorial in the denominator.

What happens then? This means that the volume does not get

bigger as the dimension increases. In fact, the volume goes to 0 as

dimension increases, and it goes to 0 rapidly since factorials beat powers.

This is caused for some alarm or some puzzlement.

What does this mean? Well, let's think in terms of the

difference between a ball and cube in dimension n.

Let's say we have them fight. Who wins?

Well, in low dimensions, the ball of radius 1 definiately has more volume, or

area, than the cube of side length 1. This is true in 2D.

It's even true in 3D, but it is not true in all dimensions because of those

corners in the cube. Those corners eventually stick out from

the ball even when the two are concentric.

And all of the volume inside the n-dimensional cube lives in those

corners. That's why cubes beat balls.

This lesson was neither short nor simple. It may take a little time for things to

sink in. Don't worry.

You're not going to be asked any questions about hyper volumes of balls.

On the final exam for this course. And in our next lesson, we're going to

return to the more familiar low-dimensional world.

But step back for a moment. Think about what you've done.

We have, with rational thought and calculus, measured objects that you

cannot see. Smell, taste, touch, or experience with

your senses. That's not a bad day's work.