Date: May 13, 2013 9:32 PM
Author: mike3
Subject: What is the intuitive meaning of "non-Archimedean" for a valued field?
Hi.

I'm curious about this. The "Archimedean property" for an _ordered_

field F means that given any positive elements a and b in F, with a <

b, then there exists a natural number n such that na < b. Intuitively,

this means F has no "infinitely big" or "infinitely small" elements.

We could also say that "given any positive element a, then there is a

natural number n such that na > 1". If the property fails, then F

contains "infinitely small" elements.

Now, there is an analogous property for non-ordered, "valued" fields

(fields with an "absolute value" function added). The "Archimedean

property" here means that given any nonzero element a e F, that there

exists a natural number n such that |na| > 1. But what, intuitively,

does it mean when this property fails? In that case, there aren't any

elements with "infinitely small but non-zero" absolute value since the

absolute value functions are usually taken as real-valued, and the

reals are Archimedean (as an ordered field). Instead, what happens for

such a real-valued absolute value is that the triangle inequality

strengthens to |a + b| <= max(|a|, |b|) and not just |a + b| <= |a| + |

b|. This causes the space to behave really weirdly(*). But what is the

_intuition_ here, and how does this notion relate, if at all, to the

first one? Especially considering I see in papers like this:

http://www.math.ucla.edu/~virtanen/website_files/thesis.pdf

"Our usual notions of space and time are built from axioms of

geometry, which

started with Euclid but eventually were formalized by Hilbert. Of

particular

interest to us is the Archimedean axiom, which states that given a

line segment

of any length and a shorter line segment, successive additions of the

short line

segment along the long line segment will eventually surpass the long

line segment. ..."

which we recognize as the "Archimedean property". In Hilbert's axioms,

this Archimedean property seems more related to _ordered_ fields, yet

the paper goes on about non-Archimedean _valued_ fields, namely the p-

adic numbers. How can one intuitively grasp the geometry of a non-

Archimedean valued field and how does it relate to the notion in

Hilbert's axioms?