really knew the earth was round. For example, as a ship sailed away,

you could see its mast receding from view around the curve of the

earth, and Magellan's circumnavigation of the globe would seem to have

settled the question. But is a round earth the only possible

explanation for these phenomena? Actually, if you think about it,

there are many possible forms the Earth could have taken. For

example, suppose our planet were a large torus, or donut-shape.

People would have observed local curvature as they watched departing

ships, and again it would have been possible to travel west for a

while and return from the east. There are a few laws of physics that

tend to produce round planets, but in this podcast we don't care about

such trivial details, we're just thinking about the mathematical

possibilities. Thinking about such alternate possibilities is what

led mathematicians to pose the famous problem known as the Poincare

Conjecture, which was just resolved a few years ago.

Now suppose our planet had been a torus. Would there have been

ways to distinguish that situation from living on a sphere? Actually,

there are various fundamental differences between a torus and a

sphere. For example, if you circumnavigate the sphere, your path

partitions it into two halves, and anyone crossing from north of your

path to the south of it must cross your path at some point. On the

torus, if you were 'circumnavigating' the short way into the

donut-hole and back, your path would not bisect it, and another

traveller could make it to the other side without crossing your path. A

similar test you could do is to stretch a long rubber band along your

path, and tie it together when you return to your starting point:

when you are done, can you slide it along the surface and eventually

contract it to an arbitrarily small size? On a sphere, you will

always be able to, but on a torus, if you traveled into the hole and

back, you will never be able to fully contract the rubber band. The

branch of mathematics that studies basic properties like these of

surfaces is known as topology.

In topology, mathematicians study the essential features of surfaces

that do not vary when they are stretched, or "continuously deformed".

So you can think of a surface as a giant sheet of rubber: tearing or

gluing is out of bounds, but you can distort it all you want. In more

precise terms, two surfaces are homeomorphic, or topologically

equivalent, if there is a continuous, invertible 1-1 mapping between

them. The classic example is that a coffee cup is topologically

equivalent to a donut: both are continuous surfaces with a single

hole. You can imagine creating non-equivalent surfaces by adding

extra 'handles' to a sphere, or punching additional holes in a donut.

Add one handle to a sphere and you have a travel sphere convenient to

take to the airport, but with a little stretching it's also equivalent

to a donut. With two handles you have something equivalent to a kind

of figure-8 donut with two holes, and so on.

A surprising result of 19th-century mathematics was that if you

look at any closed, compact surface that can exist in 3-D space-- that

is, without any infinite protrusions or sharp edges-- it is guaranteed

to be homeomorphic to a sphere with a number of handles, or

equivalently, to a donut with some number of holes. So no matter how

crazy a surface you think you can construct in a 3-D world, in some

sense it is equivalent to a stretched n-holed donut. This result

first appeared in a paper in 1888, though it wasn't rigorously proven

until the 20th century.

Now where does the Poincare conjecture fit into all this? Well,

first we need to extend our vision by a dimension, and think about

discussing three-dimensional surfaces in four-dimensional space. Not

very easy to visualize, due to our daily lives occurring in our lame

3-D universe, but the basic concepts are the same. The question

Poincare asked is essentially whether, just like for 2-D surfaces any

closed, compact surface without a hole is homeomorphic to a sphere, is

any such 3-D surface in 4-space equivalent to a hypersphere? I'm

glossing over a few details here, but that's the basic concept. It

seems like a relatively simple question, but took our best minds over

a century to solve.

You may recall how back in episode 12 I talked about the fact that

this conjecture has now been proven, and about the odd decision of

Grigori Perelman, the eccentric Russian genius who solved the problem,

to refuse the Fields Medal. At the time, I wimped out of trying to

describe the theorem itself-- but since then I read an excellent book

on the topic, by Donal O'Shea, which gave me enough basics to attempt

this podcast. If you're still confused, an entirely likely

possibility, I highly recommend taking a look at that book, which is

linked in the show notes.

And this has been your math mutation for today.

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