Thursday, August 22, 2013

Theory Thursday, with bonus Metaphysics: Gödel's Incompleteness Theorem

Gödel's Incompleteness Theorem is one of those things that makes pop science writers go gaga.  And, I'm not going to be contrarian: it's profound, it's meta, it's paradoxical, and it has a lot to say about what we can accomplish as human beings.  But, at its core, it's really a programming problem, albeit one with very interesting implications*.

In the 1920s, in the shadow of The Great War, David Hilbert set out to do something monumental, something that would bring together mathematicians from all over the world, in a grand challenge: to formulate all of mathematics rigorously, and provably, starting from a single set of axioms.  It came to be known as Hilbert's Program, and the name has an unintentional level of acuity: it really boils down to a system of encodable rules which one can use to generate mathematically true statements.  Then, by utilizing these rules in a fully deterministic fashion, you can prove mathematical theorems.  By iterating with those rules, you could theoretically enumerate all possible mathematically true statements: it's a programming language for generating All Mathematical Laws!

Except...for that "theoretically" bit.  Turns out, it's not true.  Kurt Gödel spent a lot of time trying to prove the completeness and self-consistency of such systems, but eventually wound up proving just the opposite: he demonstrated that you could construct the following types of statements in this mathematical language:

  1. Statements about the provability of other statements (e.g., Statement X is unprovable)
  2. Statements that reference themselves (e.g., This sentence has N characters.)
  3. By combining the two, you can construct statements that reference their own provability, e.g.:
This theorem is unprovable.

This is similar in construction to the classic form of Russel's Paradox ("Does the barber who shaves everyone who doesn't shave themselves shave himself?"), but with a twist: In this case, if the statement is provable, it's a paradox.  However, if it is actually unprovable, there's no paradox.  Therefore, it is both unprovable but also true.  Hence, Gödel showed that the system is incapable of proving a demonstrably true statement, and cannot be complete.  As you might guess, there's a lot of overlap between this finding and computability theory, and it can even be expressed in terms of the latter, which brings it strongly into the realm of computer science.

But, one of the things that has always stuck out to me about the Incompleteness Theorem is what it has to say about what it is we can possibly know about the world, and what this means about our perception of reality.  I was raised in a moderately religious household, and eventually, in the course of my own explorations, became very observant, went to Jewish summer camp, studied the Torah, and tried, to the best of my ability, to puzzle out what it was God wanted out of us.  But, over the years, the relentlessly empirical life of the academic scientist took a toll on my faith, at first subtly, but like a river grinding out the Grand Canyon, its effect was profound over the 10 years I spent as a physicist.  And, one day, I woke up and realized: there was no room in The Mechanical Universe for an omniscient, ominpotent being**.  Plus, the psychological and sociological reasons for us to invent such a being were too clear to ignore.  It was just too facile a solution to the difficult problem of What Do I Do With My Life.

But even so, I've never been able to call myself an atheist, because, in spite of all my book larnin', I know what I don't know.  On the one hand, at the most granular scale, almost everything I've ever learned about the physical world is premised on an unprovable hypothesis: that the things that we observed yesterday are a good guide to what we will observe tomorrow.  It's unprovable, because the only evidence that we have for the inductive principle is the inductive principle itself: it has worked well in the past, so it should continue to work.  I have no problem using the inductive principle as a guide, as a way to decide which insurance to buy, or how to spot a mu meson.  But in matters of spiritual life or death, can we really count on something so flimsy?

And then, on the grand, cosmic scale, we have the Incompleteness Theorem: I know that there are things out there that are true, but can't be systematically proven.  In fact, Gödel proved that there's an infinite number of them.  If that's true, I can prove and prove and prove until my hearts' content, but I can never be sure that I haven't failed to observe something which could change my entire understanding around.  There's just too much truth out there, and not enough time.

So, what are we to make of the universe?  Nothing is provable, and there's an infinity of unprovable true things that we'll never know.  Given all that, can we really rule out, conclusively, the existence of something outside of this mortal coil?  And so ends my catechism.

*However, while it's really cool, it's doesn't have something to say about everything: Carl Woese once asked me to write a paper on what the implications of the incompleteness theorem were for biological evolution, and Douglas Hofstadter happened to be giving a colloquium that week.  I went down early before the colloquium and accosted him over coffee to ask him his thoughts on the matter.  He considered it briefly, and then said, "I don't think there are any."

**The Mechanical Universe used to be shown on PBS in Chicago when I was growing up, and I found every single episode, from Newton's Laws to Relativity, to be absolutely mesmerizing.  It had a strong impact on my decision to study physics.

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