Name: 神奇的四邊形 - 數字愛好者 (Fantastic Quaternions - Numberphile)
Uploaded: 2021-01-14T10:15:47.000Z
Duration: 12 min 25 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

How do you rotate an object in three dimensions? So, you take an object like that,

and rotate it. That would be something that's useful, you can imagine, in computer

graphics. But how are we gonna do this? Now, one of the neatest ways of doing this

mathematically uses a new type of number, beyond the regular numbers you know.

So this is beyond the real numbers, as we call them.

They're even beyond the complex numbers—if you've heard of complex numbers before—it is even

beyond that. They are called quaternions. They are fantastic! So imagine that I want to move

along in one dimension. So you are living somewhere here and if I said move +5.

+5 means that you move 5 units to the right along this line, so I'd end up over here.

If I said instead move −3, you might understand that means move 3 units, but that time

move 3 units to the left. So that's how you move in one dimension. That's fairly easy, isn't it?

Let's talk about moving around in two dimensions and say I want to end up at a point over here.

What that might be... 3 units this way—let's call that a 3—and I move 4 units up like that.

One way you could write that as a set of instructions is to write it as 3+4i. Some of you may know what's

coming here, some of you may have seen this 'i' before. But for now, let's just say 'i' means

turn 90° and go up and down. For now, that's what it means. So that's one way I can travel to that point.

Alternatively I could look at it this way—I could travel 3 across and 4 up,

or I could rotate myself to this angle here and move along this diagonal line here—

and that would be 3+4i as well. I could use the same

instructions to make that point. That's how you travel in two dimensions. I mean, if I

want to then go further, and travel maybe a bit further on... So let's say I wanted to go maybe

1 more across and then 2 up like that. What I can do then by adding a further

1+2i, I end up at this point, and I can say that this point is the combined result, which means I have actually

traveled—what?—3 across and then a further 1, that's 4 across and... 6 up. Or, if you like, it's

the combined result of going from here up to here. So we can add these numbers together, and

we should think of them as numbers, right? You can add them together and you can

move around in two dimensions. So this time let's rotate this line. Let's take

my combined result here. What have I got? That's 4+6i. Let's rotate this 90°. If I

rotate it 90°, I think it's gonna end up something like that, and it will end up here.

It will end up on a point here, so this is going to be a 90° rotation. This

point is actually −6+4i, and that's where I end up if I rotate. These look similar to each other.

Why do they look similar? How can I get from this number to this number?

What I've done actually is multiplied. Instead of adding the numbers together, I've actually

multiplied this time by the number—we call it a number—the number 'i'. So there is a

special rule with this. The special rule is  'i' times itself, i×i, is equal to −1. Just to

show you what I did, I took where I was, 4+6i, I multiplied it by 'i', and what did I get? I got

4i+6... well, let's call it i². Oh, what is that? It's 4i−6, or in other words, where I ended up here,

−6+4i. So multiplying by that number 'i' means you rotate 90°. If you

do it twice, you get −1, which does make sense. So if you rotate 90°, and

then rotate 90° again, you are 180° now—you're

facing the opposite direction—so you're going in the negative direction, so it does make

sense. And it turns out that if you multiply these numbers together,

you get rotations. When you multiply the two numbers together, you add their angles

together. So if I want to rotate 45°, I would get my triangle for 45°.

1/√2 there, 1/√2 here. So that's a nice nice 1 now.

That is a 45° angle. Written as a number, that is  (1/√2)+(1/√2)i. So you are going that

distance across and then that distance up. If I multiplied by that number, I would rotate

by 45°. So adding them together makes you travel around in two dimensions,

multiplying them together makes you rotate in two dimensions.

Now if you're ahead of me and have seen this sort of thing before,

you may recognize these as complex numbers—which, by the way, is a

terrible name, "complex" numbers, which makes them sound like "difficult" numbers, which I don't think is

the intention. I think the idea is that they are "compound" numbers. They are actually

two-dimensional numbers. That's a way of thinking of complex numbers. Real numbers

are one-dimensional numbers, complex numbers are two-dimensional numbers, and you

can use them to travel in 2-D and rotate in 2-D. So the question is, how do we

rotate in 3-D? We must need three-dimensional numbers, right? So we must

have to introduce an 'i' and a 'j' as well. So we have three dimensional numbers, and

that's what people thought. It turns out, we don't need three-dimensional numbers,

we need four-dimensional numbers. To rotate in three dimensions, bizarrely, you

need a four-dimensional number. Let me show you how this works.

(STAMMERS) So there was an Irish mathematician called William Hamilton,

and he was thinking about this problem, and he couldn't solve it.

He said, "it doesn't work in three dimensions! I can't get it to work!"

And the story goes that he was having a walk with his wife along the canal in Dublin,

and then suddenly inspiration struck. He realized, "I don't need three dimensions,

I need four-dimensional numbers! I need 'i', 'j', and 'k'!" So flushed with this,

he immediately walked over to a nearby bridge over the canal, and he carved in the

rules that he needed to make this work. These are the rules that he carved into that bridge:

It's very similar to the complex number idea. Complex numbers, i² = −1—that's what

we needed for that. It's very similar, but no, we need 'i', 'j' and 'k'.

He carved this into the canal bridge, which unfortunately—graffiti, what a vandal!—unfortunately

it's not there anymore. But now there is a plaque celebrating

the rules, his discovery of quaternions. The rules are there, and every year,

mathematicians around Dublin have a pilgrimage to the spot where he had this idea for

quaternions for the first time. So by multiplying these four-dimensional numbers, you can now

rotate—like the 2-D version—you can rotate things in three dimensions,

and this is how things are done using computer graphics. Not only that, it's

how things are worked out for the rotation or the orientation of a space shuttle

going up, or it's how they work out the orientation or the rotation of

your smartphone. So let's say I want to rotate an object in three dimensions. Well, I pick a line—like

this pen here—I stab the object with this pen, and then I can rotate around

this pen here. So I might stab it this way, and then I can rotate the object like that. Or I might

stab this way, so it rotates the object this way, or this way, so we can rotate like that. So, how do

we write this? I take a line like that, and then I wanna rotate around that line. Let's do an angle, 'θ', like that.

Let's call it 'v'—this line, we're calling 'v'. So it's a three-dimensional thing.

It has three coordinates. Let's call them v₁, v₂, and v₃.

Let's take a point that's getting rotated around. Let's call that 'P'. That's just x, y, and z,

for that point, in three dimensions, this is how things work.

But now we're gonna rotate it using a quaternion. What does a quaternion look like? It looks like this:

A quaternion, let's call it 'h', would be something like a+bi+cj+dk,

and actually, for this I do need another little thing, something called h*, and h*=a−bi−cj−dk.

Right, if I want to rotate by an angle of θ, then a, b, and c are defined as:

a=cos(θ/2), b is going to be written as... v₁—that's come from my vector here, that's my

stabbing pen—times sin(θ/2), c=v₂⋅sin(θ/2), and d=v₃⋅sin(θ/2).

So it turns out we need four components for our number that we're gonna use. We need four

components to have the freedom to do the stabbing and then the rotating around that vector.

And how do you work out the rotation then, finally? If P, which is the point I'm rotating,

you write P as a quaternion, its coordinates are x, y, and z, so it's actually xi+yj+zk,