**General Relativity For
Tellytubbys**

**Calculus Of Variations **

**Sir Kevin Aylward B.Sc.,
Warden of the Kings Ale**

**Back
to the Contents section**

__Overview__

This section covers some bits and pieces that may not have been previously covered.

A straight line on a curved
surface is *defined* as the shortest
distance between two points. This bit is pretty obvious really. Get a map and
draw a line from London to New York. Try drawing that line on a globe. Yep, the
Concord flies over Greenland, well not anymore it don't, what with the crash
and all. So that’s the shortest path ain't it, but the flat map shows something
way different don’t it.

So, given a surface, one needs to find out the equation for these "straight" lines, which are called geodesics. To do this we first need to find out how to find the minimum of an integral. In the particular case of G.R., the integral will be the distance function.

We are going to do a bit of
hand waving here, but this reminds me to point out some useful information. For
those yanks reading this, you guys have got it completely wrong about Robin
Hood. You have this quaint and so naive idea about Europe, that it’s a wonder
you can tie your own shoelaces. Look, it’s a complete misconception that Robin
Hood stole from the rich and gave to the poor. In fact, what actually happened
was that he stole from the rich and *waved*
to the poor.

__Euler-Lagrange
Differential Equation __

Consider the integral

The job is to find the function that minimizes this integral, subject to certain conditions. This is technically described as finding the stationery value of the integral (because that sounds more impressive), which may actually be a local maximum, local minimum or point of inflection, in the following notation.

First the answer is:

where y_{x }means
the derivative of y wrt x

And no surprise to see that this extends to

When there are a number of variables, and where the x's are summed over the number of variables.

Now on to the derivation of the above.

__Euler-Lagrange
Derivation__

Consider the function, defined by:

i.e. vary y about a bit in order to get the best y.

the integral is now

This will be a turning point for our initial problem if

So

with

So

Integrating the second term by parts gives

But the 3^{rd} term
is zero by construction, so

But g(x) is arbitrary, so we must have

After replaying u with y, as required.

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