Hacker News Reader

Comment by shmageggy | Hacker News Reader

I'm trying to get an intuition about why this whole game can even be played, and it's proving (ha) rather tricky. I'm ok with the idea that things sometimes cross into the realm of the impossible when transitioning from discrete/finite to continuous/infinite, or maybe from countably to uncountably infinite, but the notion that there's some constant threshold seems so counterintuitive. The link at the end of the article is pretty good [1] as is wikipedia [2], but I'm clearly missing some intuition here. Help!

[1] https://johncarlosbaez.wordpress.com/2011/10/28/the-complexi... [2] https://en.wikipedia.org/wiki/Kolmogorov_complexity#Chaitin....

View on HN

You can't solve the halting problem. Therefore, if you take a program that takes a Turing machine as input, searches for a proof in ZFC (or your favorite alternative) that the program halts or that it doesn't, and if it doesn't find a proof either way then it runs forever, your program is not a halting solver.

Therefore, there are some Turing machines which can't be proven to halt or run forever in any given consistent system that can be computed.

(Because if every TM had a proof, then the program above would solve the unsolvable halting problem).

Therefore, there is a smallest such program that's independent of ZFC.

Does that help?

> Therefore, there is a smallest such program that's independent of ZFC.

http://www.scottaaronson.com/blog/?p=2725

    we give a 7,918-state Turing machine, called Z
    (and actually explicitly listed in our paper!),
    such that:

      Z runs forever, assuming the consistency of a large-cardinal
      theory called SRP (Stationary Ramsey Property), but

      Z can’t be proved to run forever in ZFC [...],
      assuming that ZFC is consistent.

edit: Just saw that this was linked at the bottom of the article. As pohl mentioned, the article really buried the lede.

Yeah a bit. This part

> searches for a proof in ZFC (or your favorite alternative)

is tripping me up though. It's not obvious to me that you can do this in an automated fashion. I assume this is a standard tool for theorists though? How does such a program work?

Proofs can be expressed using a string and verified to check every step is valid.

So brute force over every string, checking if it's a valid proof for the claim you want.

It's not letting me reply any deeper, but that makes sense, thanks!

I think it's that you can't reply to posts that are too new.

You can, you just have to click on the timestamp to visit the comment's item page first, something like https://news.ycombinator.com/item?id=11750351

The idea is that proof systems can be formulated as a grammar for a recursively enumerable language (Type 0 grammars in the Chomsky hierarchy): https://en.wikipedia.org/wiki/Chomsky_hierarchy#Summary, such that if there is a proof of a particular statement a corresponding word can be derived by the corresponding grammar. But this can easily be done by a Turing machine (either check through all possible derivations in the grammar or, more abstract, let the Turing machine let non-deterministically choose a possible derivation and use the fact that the expressiveness of non-deterministic Turing machines is the same as for deterministic Turing machines).

  Therefore, there are some Turing machines which can't be
  proven to halt or run forever in any given consistent
  system that can be computed.

I feel like it's worth clarifying the quantifiers here. For all consistent systems, there exist Turing machines whose halting behavior can't be proven in that system. Unless I have some disastrous misunderstanding...

Well a consistent system that can't be computed can prove things about all Turing machines.

E.g. ZFC plus an infinite set of axioms of the sort "machine X halts" for all machines that halt. We can't compute all the axioms, but the system is perfectly consistent. We could compute the axioms if we had a halting Oracle.

It wouldn't be able to prove the halting behavior of some Turing machines that have halting oracles attached, though.

May I ask if you are able to visualize things using your mind's eye at at what levels (size, detail, color/brightness)? Just curious about correlation with the subject matter.

Reply on news.ycombinator.com