mike3
Posts:
2,396
Registered:
12/8/04


What is the intuitive meaning of "nonArchimedean" for a valued field?
Posted:
May 13, 2013 9:32 PM


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 nonordered, "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 nonzero" absolute value since the absolute value functions are usually taken as realvalued, and the reals are Archimedean (as an ordered field). Instead, what happens for such a realvalued 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 nonArchimedean _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?

