Metrics Provide An Inner Product

Another post for the PoincarĂ© Project.

We've already seen that a one-form is a linear function from a vector to a (for our purposes) real number. On a manifold, one-forms correspond to stack-type vectors being applied to arrow-type vectors by counting how many "stacks" the arrow passes through.

In the previous post Metrics As Mappings Between Arrows and Stacks, we saw that a metric is an extra bit of structure that describes how to map between arrow-type vectors and stack-type vectors.

So, in summary:

• a metric tells you how to go from an arrow-type vector to a stack-type vector
• a stack-type vector can be applied to another arrow-type vector to get a real number

These two facts can be combined to let you take two arrow-type vectors and get a real number out of them.

This has parallels with currying in functional programming.

Recall that if a function "add" takes two integers and returns an integer, it can be viewed as a function that takes one integer and returns a function that takes one integer and returns an integer.

```add :: Int -> Int -> Int
```

Now, a one-form is a function that takes a vector and returns a real. In other words:

```Vector -> Real
```

So it is easy to see that if you curry a real-valued function that takes two vectors you get:

```Vector -> Vector -> Real
```

In other words, a function taking two vectors to a real is equivalent to a function from a vector to a one-form.

So if you have a metric that can convert between vectors and one-forms (or, in the context of a manifold, between arrows and stacks) then you also have a function from two vectors to a real.

Such a function is called an inner product or dot product. Often the notion of an inner product is defined first, before one-forms are introduced (if at all). In fact, some texts will define a metric to be an inner product. It is best for our purposes, though, to think of the metric's fundamental purpose as being converting between arrows and stacks (and back again) and the inner product as being an extra concept we get for free.

The original post was in the category: poincare_project but I'm still in the process of migrating categories over.