At the end of my 8410 class today (see http://www.math.uga.edu/~pete/MATH8410.html if you care), one of my students asked me the following very interesting question:

Let $(K,|\ |)$ be a [normed field][1], with completion $(\hat{K},| \ |)$.  Suppose $\hat{K}$ is algebraic over $K$.  Must we then have $\hat{K} = K$?

As I have mentioned here before, I feel very lucky to be getting such penetrating questions.  This one I was not able to answer on the spot, although I remarked that it is true in all of the most familiar examples and that the (possible) lack of algebraicity of the completion is a key motivation for considering the <b>Henselization</b> instead.

<b>Edit</b>: the answer is <b>no</b>, as I have just heard from one of my students.  I have encouraged him to come to this site and register the answer.

To make the question more interesting, suppose we ask whether $\hat{K}/K$ can be finite and nontrivial?

[1]: http://eom.springer.de/n/n067360.htm