Unproven Fundamentals of Geometry

Points, Lines, Etc.

Hi, Han, I like thought-provoking questions like this.

I agree with you about the necessity of faith as it relates to our
knowledge of and interaction with the real world. In math, though, I
see things a little differently.

Math in itself is not intrinsically connected to the real world. It is
possible, and perfectly okay, to develop a mathematical system that
doesn't relate to anything in the real world. It is, as you say,
necessary to have "undefined terms" describing entities in the system,
and "postulates" (unproven facts relating those entities). But these
are not so much matters of faith as "rules of the game." They are
rules that we must adhere to if we are going to prove theorems within
the particular mathematical system.

We aren't allowed to introduce additional assumptions (undefined terms
or postulates) or alter them without explicitly stating the new
assumptions. When we do so, we are no longer working in the same
mathematical system. It may be a perfectly valid system, but it isn't
the same one once its rules have been changed, even the slightest bit.

Many mathematical systems - probably all until the last two centuries
or so - were motivated by attempts to describe and explain things in
the real world. At this point, math overlaps with science, and faith
becomes relevant. Do the undefined terms and postulates of our system
correspond to elements of the real world and their interactions? We
can't know. In all likelihood, they don't correspond exactly, but they
may make a good approximation.

For instance, a "point" in geometry can be thought of as something
with no length, width, or breadth. Everything in the real world has
some length, width, and breadth; we can only approximate a point by
making a dot with the sharpest pencil we can get. (Physicists now
think that electrons may actually be points, but electrons obey the
laws of quantum physics, which is rather more complicated than
ordinary geometry.)

Still, somehow, geometry is very useful in describing the real world,
even though strictly speaking, it describes things that don't exist in
the real world.

I said that you can change the rules and come up with a new system.
Euclid had 5 postulates in his system of geometry. You can see them
here, along with his undefined terms (he called them "definitions",
but not all of them are) and "common notions" (actually postulates
that are more fundamental than geometry):

Euclid's Elements, Book I (David Joyce)
http://aleph0.clarku.edu/~djoyce/java/elements/bookI/bookI.html

The fifth postulate was a lot more complicated than the others. It
wasn't very pretty, but it seemed to be needed in order to prove some
basic facts about real-world geometry - for instance, that the angles
in a triangle add up to 180 degrees.

Over the years, people tried to prove the fifth postulate, thinking
that something so complex must somehow follow from the simpler
postulates. They failed. In the nineteenth century, mathematicians
tried a different tack: try changing the postulate, and see what
happens. They found that they ended up with several varieties of
"non-Euclidean" geometry that were completely self-consistent, but
different from Euclid's geometry. Changing the "rules" made a new but
perfectly good game.

So what do you think happened next? Einstein came along and discovered
that these non-Euclidean geometries were just the thing to describe
the real-world interactions of objects with mass - that is, to
describe gravity. This is a case where the mathematical system was
invented with no consideration of the real world (and therefore no
faith element), but it turned out that this system does appear to
describe the real world.

The experiments to show that Einstein's theory of general relativity
do describe the real world better than any other mathematical system
are very tricky; it is still possible that another system would do
better. We can be absolutely sure that the results of general
relativity theory follow from its assumptions; the only question is
whether or not those assumptions match the way the real world is.

Let me put it another way. There are two kinds of truth; I'll call
them mathematical truth and real-world truth. Mathematical truth means
that a statement is consistent with the assumptions of a particular
mathematical system. In a sense, people created that system, and they
can tell absolutely whether the statement is true within that system.

(However, Kurt Goedel threw a monkeywrench in the works earlier in
this century. He proved that any sufficiently rich mathematical system
must include statements that are true but that cannot be proved within
that system. We can't tell what these statements might be. This is
mind-boggling!)

Real-world truth is of a different order: it means that a statement is
consistent with the particular system that is the real world. There is
only one real world, and no human created it; no one knows exactly
what the rules are. Scientists try to make rules that seem to describe
the real world, but they can't possibly know whether these rules
really describe everything in the universe.

So yes, faith is necessary, because we did not create the real world,
so we can't know absolutely what the rules of this system are ...
unless someone from outside this system - the creator of the system -
lets us know the rules.

I know I went far beyond answering your question. I hope my references
to geometry as an example give you an idea of how undefined terms and
postulates work. Thanks for writing.