we have a notation for language, we have a notation for dance,

we came up with a notation for juggling.

And the really cool thing was that there was some unexpected

mathematics underneath that then let us predict the existence of juggling tricks that, as far as we knew, had never been done before.

Well, the one I'm going to talk about is called siteswap. If you go on the Wikipedia page, it does

have a couple of alternative names.

But really the place to start is with a little bit of juggling, so that you can see what it is

we're trying to capture. I'll just do three to start with, and a lot of people think that they'll go in a circle.

But, in fact, the easiest thing to do is to throw them

so that they all come down in the same order. And if you do that,

they've got yellow, green, orange,

and then it will be the yellow one's turn again.

But if my hand's taken in turns, that means the yellow has to change hands. So let me show you.

Yellow with this hand, green with that hand, orange with this hand.

So yellow green orange yellow green orange yellow

green orange yellow green orange. And you'll see that they have to change hands

because the balls are taking it in turns, and so are my hands.

And so it just works out that that's the way it goes.

So the first thing to be able to do is at least to be able to describe that juggling pattern

in any notation that you use, and then, think to yourself,

well, can I then also describe other ones?

So what I'm gonna do is I'm gonna have my left hand here, and my right hand here.

And I'm gonna have time running in this direction. So imagine that I'm walking

forwards as I juggle, and this, more or less, is where my left hand is, and this, more or less, is where my right hand is.

I'm gonna leave out some of the detail of my hands moving from side to side.

But I'm gonna say, as I'm walking forwards,

I'm gonna throw with the right hand from here, and that ball's gonna go over to the other hand.

I don't know when that balls gonna come down, but it's gonna go over to the other hand. And a moment later,

I'm gonna throw with the left hand, and then a moment later,

I'm gonna throw with the right hand, and then I'm gonna throw with the left hand, then I'm gonna throw with the

right hand, and then with the left hand. And I've got this this,

this rhythm about the whole thing, this, this constant metronomic beat going on here.

[tick tick tick tick tick tick tick]

So I'm gonna put the, the colors onto here now. So this one was with the yellow ball,

and this will also show you that they, they have to change hands, because if I throw the yellow one here,

and then I'll throw the green one next, and then I'll throw the orange one next, and then I've got to throw the yellow one here.

And so it has to be in the other hand.

And in truth, it comes down a little bit before that, so if we look at when it comes down, it might come down there,

and then it's in my hand for that moment there.

So there's the yellow ball going across to that hand there.

And then the green ball does the same thing. It goes over here, in that direction,

and then it's in my hand for a while, there. So that will be the green ball there.

And then the orange one gets thrown. It goes across to the other hand,

and so that's over here in this hand, and then that's the orange ball being thrown, and then the yellow one again,

and then the green one again, and then the orange one again.

[tick tick tick tick tick tick]

And, in fact, what we end up with here is a plait.

And if you juggle and walk forwards, or, more particularly, juggle and walk backwards, look at the paths that the balls leave in the air.

They literally form a braid.

It's ephemeral, because the balls don't leave, actually, leave the trails behind,

but if you could imagine that. And a guy called Henry Segerman has made 3D printed models of

juggling tricks that are then pushed through time, pushed through space, so you can see the intermingling, interleaving of the balls.

But here, what's happening now is, you can see that with three ball juggling, what's happening is

I throw the yellow ball, green ball, orange ball, then I throw the yellow one again, so the length of time

from that throw of the yellow ball to that throw of the yellow ball is

three beats of the clock, three ticks of the underlying metronome.

[TICK tick tick TICK tick tick TICK]

So, if you've got this tick, and it goes yellow green orange, yellow green orange,

then the yellow ball will be thrown every third time. So it's every third beat that the yellow ball gets thrown.

Another little comment about this is that the actual flight time, the time that the ball spends in the air, has to be less,

to allow for some time in the hand, what we call the hold of the juggling ball,

and that's the dwell, the, the time that it's in the hand, and then we have the flight time here,

and you'll see that flight time is a little bit less

than this time that it takes the balls to cycle around, and this is what we call the cycle time.

These names are not universally agreed. Some people call it the beat time, some people call it the underlying native time.

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And that's for three balls.

Now let's move on to four balls, and see what happens. Yellow pink green

and white.

So yellow pink green white, yellow pink green white, so here we go. We go,

yellow pink green white yellow pink green white yellow pink green white yellow pink green white yellow pink green white yellow pink green white yellow

pink green white. And now, you'll see that the balls are actually staying in the same hands.

So let's actually draw the diagram for four balls.

I've got left hand and right hand. Throw it here, throw it here, throw it here, throw it here, throw it here.

And I've got yellow pink green white.

And look, the yellow,

it's the yellow's turn, and it's the right-hand's turn again.

So the yellow has to come back to the same hand, and that's what we saw when I was actually doing the juggling. Yellow pink

green white.

So the yellow ball is thrown here, and next it has to be thrown there,

so it stays in the same hand. I'm gonna have it bob around, so it comes around. Again, again,

it has to come down that little bit early,

I catch it before I throw it. Obviously, it has to spend some time in the hand.

For simplicity, we often assume that that catch is exactly halfway between. In practice,

it's not. In practice, your hand is full for more than half the time.

Because the only time you can control the juggling prop is while you're holding it.

So you tend to hold it for as long as you can to give you the maximum control.

But you can see here, the yellow ball comes, and it's in air. And then the green ball does the same thing. It bobs

over the time that I'm holding the yellow ball.

and then it's in the hand, then the green, then the yellow ball gets launched again.

And this is actually how it feels. It feels like you get these two balls

bouncing over each other in the hand. And if I actually do yellow and green in the same hand, here,

you can see that they, they really do feel like they're bouncing, one over the top of the other.

It's almost like they're playing leapfrog. And, in fact, if you turn this side on, you can see that

they're playing leapfrog over each other. Although, it's leapfrog through time, not leapfrog through space.

But now, the interesting thing is, if we have a look at the time that the yellow ball gets thrown here,

and then the time the yellow ball next gets thrown is there, and look, how long is that?

Well, I've got a beat here, and then I go, one two three

four beats. And you can see that that really is

four beats of time from a throw of the yellow ball to the next throw of the yellow ball. And they're all doing the same thing.

So they will all have a four beat cycle.

In a moment, I'm gonna talk about throwing the balls to different heights. At the moment, they're all doing the same thing.

So at the moment, I can say that the pattern has four beats between throws. But shortly, I'll talk about an

individual throw being four beats.

Okay, so, having seen three balls and three beats, four balls and four beats,

we can fairly obviously go on and do five and six and seven, and there's no real interest in that.

We can go down and ask what does it actually mean, now, based on this kind of diagram,

can I draw a diagram for two balls? And what does then imply, that then imply,

for the physical juggling? And one ball, and perhaps even zero balls.

I'm not gonna do that yet, because I really want to get to the payoff.

I want to get to the actual notation, and why it's interesting and what happens.

So I'm gonna go back, and I'm gonna look at four ball juggling again. Yellow

pink green and white. So that's what's going on there. And the yellow ball will be thrown next over here,

and the pink ball will be thrown next over here, and the green ball thrown next over here, the white ball thrown next over here.

We know that that's what's going on. But now, let's cheat. Now, let's do something slightly different.

I'm looking at the yellow ball landing here, and I'm looking at the pink ball landing here, and I'm going, well, they're good friends.

Why don't they change places?

So, in fact, what we can have is the yellow ball

not come to here,

but actually go to there, where the pink ball would go. So the yellow ball actually goes

to where the pink ball would go. And the pink ball actually goes to where the yellow ball would go.

So they exchange

landing sites. They swap their sites. It is a site

swap, which is where we get the name of the notation from.

They are swapping the places that they go to.

And that's great on the diagram.

What does it mean physically? Well, if we look at this, the yellow would normally have this cycle time of four.

But now, the time it's next thrown is 1 2 3 4 5 beats of time in the future.

And the pink ball would be, normally 1 2 3 4, but in fact, 1 2 3, it's three beats of time in the future.

So what's happening is I'm doing lots of cycle time four

throws, so I'm doing 4 4 4 4 4. And then, suddenly and without provocation,

I'm doing a five and a three according to my diagram.

And what's really nice is that actually turns out to be exactly the kind of throw that you do when juggling five balls,

followed by exactly the kind of throw you do in juggling three balls.

So what's predicted from the diagram turns out to be the case in reality. And if I demonstrate that,

I need the yellow pink

green and

white, in that order. So, yellow pink green

and I'm gonna do the yellow and the pink as the

high and low, as the five and three, so if I just juggled four for a while,

and then I'm gonna do the yellow ball high, and the pink ball low, so we, counting down, so, three,

two,

one.

Five three.

And you'll see that they have changed hands, as is predicted by the diagram. The yellow did go high.

The pink did go low. And they landed perfectly in rhythm, and I could just carry on.

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