# Why lower index on components?

1. Aug 30, 2009

### enomanus

Hi everyone! I am trying to self- study GR and I am (very slowly) working through Schutz at the moment ( not the clearest at times for a " first course" text). Anyway I have noticed that several times when working with components of physical variables e.g. momentum/ energy, the 'up' index in say p alpha is deliberately 'lowered' in the workings to get p alpha 'down'.
for example in - deriving the Consevation law on geodesics so that we can spot if the metric is independent of any x component or - in Shutz page 170 where we work out the scalar product - p.p to show that energy is conserved in a stationary gravity field.
i can follow the lines and the equations BUT I don't understand why we keep lowering the index???
Is this something to do with physical variables or with 1-forms??
Thanks!!!

2. Aug 30, 2009

### atyy

We hypothesize that there is a metric - it eats two vectors and spits out a number - this number is the scalar product of the two vectors. When you compute the scalar product, you may choose to have the metric eat one vector before eating the second vector. This process is called lowering the index of the first vector.

3. Aug 31, 2009

### DrGreg

It is related to the fact that

$$\sum_{\alpha=0}^3dx_{\alpha}dx^{\alpha}=\sum_{\alpha=0}^3\sum_{\beta=0}^3g_{\alpha\beta}dx^{\alpha}dx^{\beta}$$​

is invariant (the same in every coordinate system), whereas

$$\sum_{\alpha=0}^3dx^{\alpha}dx^{\alpha}$$​

isn't.