# Shannon’s Juggling Theorem

Claude E. Shannon is best known for his 1948 paper “A Mathematical Theory of Communication” in which he created the field of Information Theory, but he had many other important contributions in diverse areas.

What is lesser known is that he had a keen interest for gadgets and devices of all sorts and he invented some quite humorous ones, like “The Ultimate Machine” (also known as “The Most Useless Machine Ever”):
Video

Shannon was also an accomplished juggler. He came up with the following elegant theorem, known as

Shannon’s Juggling Theorem

(F+D)H=(V+D)N

F is the time a ball spends in the air (Flight)

D is the time a ball spends in a hand (Dwell), or equivalently, the time a hand spends with a ball in it

V is the time a hand spends empty (Vacant)

N is the number of balls

H is the number of hands

The theorem can be derived by looking at a complete juggling cycle first from the perspective of the ball, then from the perspective of the hand, then equating the two times. This is an application of one of the most useful general tricks in combinatorics: double counting. You count/measure something in two different ways (in this case the juggling time), and use the fact that the two results have to be equal.

We can read out from the theorem some obvious facts, such that if you throw the balls higher (increase F) then V will also increase (your hands will be empty for longer). If you increase D at the expense of F and V, until they become zero (you keep holding the balls in your hands), N and H have to be equal (one ball in each hand). No surprises here, except to note that the theorem assumes that there is at most one ball in one hand at a time, so it does not apply to multiplex patterns in which several balls are simultaneously held in the same hand (we would need separate Ds for hand and ball to fix this, but the simplicity of the theorem would be lost).

What if you want to juggle more balls (increase N) but you cannot change F, V or D (you cannot juggle any faster or throw the balls any higher)? No problem, just increase the number of hands (H). One way to achieve that is by becoming more social.

The Science of Juggling

A personal tribute to Claude Shannon