BusyBeaver(6) Is Quite Large

If we assume ZFC to be consistent, then Gödel's 2nd incompleteness theorem tells us that it cannot prove its own consistency. So in particular it cannot prove than TM_ZFC_INC will never halt.

If you believe that ZF is consistent, then you believe that the machine cannot halt (assuming you trust its construction). But you cannot write a proof in ZF that the machine cannot halt. Such a proof must include a new axiom "ZF is consistent", or some stronger axiom.

Yeah, I’m quite familiar with Friedman’s work. I mentioned him and his Grand Conjecture in another comment.

> This ignores the fact that it is not so easy to find natural interesting statements that are independent of ZFC.

I’m not ignoring this fact—just observing that the sheer difficulty of the task seems to have encouraged mathematicians to pursue other areas of work beside foundational topics, which is a bit unfortunate in my opinion.

Right, Hilbert’s goal was (loosely speaking) to “find a finitely describable formal system” sufficient to “capture all truths”. When Gödel showed that can’t be done, that shouldn’t imply we just stop with the best theory we have so far and call it a day—it means there are an infinite number of more powerful theories (with necessarily longer minimal descriptions) waiting to be discovered.

In fact, both Gödel and Turing worked on this problem quite a bit. Gödel thought we might be able to find some sort of “meta-principle” that could guide us toward discovering an ever increasing hierarchy of more powerful axioms, and Turing’s work on ordinal progressions followed exactly this line of thinking as well. Feferman’s completeness theorem even showed that all arithmetical truths could be discovered via an infinite process. (Now of course this process is not finitely axiomatizable, but one can certainly extract some useful finite axioms out of it — the strength of PA after all is equivalent to the recursive iteration up to ε_0 of ‘Q_{n+1} = Q_n + Q_n is consistent’ where Q_0 is Robinson arithmetic).

Gödel's theorem shows that you need an infinite number of axioms to describe reality (given that available reality isn't finite), so any existing axiomatic system isn't enough.

Unfortunately no, ZFC isn't good enough to capture arithmetical truth. The problem is that there are nonstandard models of ZFC where every single model of second-order PA within is itself nonstandard. There are even models of ZFC where a certain specific computer program, known as the "universal algorithm" [1], solves the halting problem for all standard Turing machines.

https://jdh.hamkins.org/the-universal-algorithm-a-new-simple...

Not an expert, but I've read about this a bit because it bothered me also and I think this is the answer:

Most of these 'uncomputable' problems are uncomputable in the sense of the halting problem: you can write down an algorithm that should compute them, but it might never halt. That's the sense in which BB(x) is uncomputable: you won't know if you're done ever, because you can't distinguish a machine that never halts from one that just hasn't halted yet (since it has an infinite number of states, you can't just wait for a loop).

So presumably the independence of a number from ZFC is like that also: you can't prove it's the value of BB(745) because you won't know if you've proved it; the only way to prove it is essentially to run those Turing machines until they stop and you'll never know if you're done.

I'm guessing that for the very small Turing machines there is not enough structure possible to encode whatever infinitely complex states end up being impossible to deduce halting from, so they end up being Collatz-like and then you can go prove things about them using math. As you add states the possible iteration steps go wild and eventually do stuff that is beyond ZFC to analyze.

So the finite value 745 isn't really where the infinity/uncomputability comes from-it comes from the infinite tape that can produce arbitrarily complex functions. (I wonder if over a certain number of states it becomes possible to encoding a larger Turing machine in the tape somehow, causing a sort of divergence to infinite complexity?)

We need to distinguish between a computer that's equivalent to BB(n), and a computer big enough to compute the value of the number that is BB(n). By (terrible) analogy: a 4004 can be made to write a finite loop that describes how many FLOPs the number 1 supercomputer can compute without, itself, being able to usefully perform the computations of that supercomputer. (The 4004 will run out of memory/addressable disk space.) Similarly, we can no longer build decidable programs in ZFC that can compute the number BB(748). Scott is saying that they now think this "disassociation" might occur at BB(7)!

Looking at [3], they seem to argue that the system isn’t complete for the usual Gödel reasons, which, sure, it isn’t, but then they call the claim that the system fails to decide, which is a statement about probability, a “scientific fact”. This seems to me like a mistake?

Like, a TOE is not expected to decide all statements expressible in the theory, only to predict particular future states from past states, with as much specificity as such past states actually determine the future states. It should not be expected to answer “given a physical setup where a Turing machine has been built, is there a time at which it halts?” but rather to answer “after N seconds, what state is the machine (as part of the physical system) in?” (for any particular choice of N).

Whether a particular statement expressed in the language of the theory is provable in the theory, is not a claim about the finite-time behavior of a physical system, unless your model of physics involves like, oracle machines or something like that.

Edit: it later says: “ Chaitin’s theorem states that there exists a constant K_{ℱ_{QG}} , determined by the axioms of ℱ_{QG} , such that no statement S with Kolmogorov complexity K(S) > K_{ℱ_{QG}} can be proven within ℱ_{QG} .”

But this, unless I’m badly misinterpreting it, seems very wrong? Most formal systems of interest have infinitely many distinct theorems. Given an infinite set of strings, there is no finite universal upper bound on the Kolmogorov complexity of the strings in that set.

Maybe this was just a typo or something?

They do then mention something about the Bekenstein bound, which I haven’t considered carefully yet but seems somewhat more promising than the parts of the article that preceded it.

> that's one of the freakiest things about things like the Busy Beaver function

Every sentence ever spoken and every view ever looked at is also a number. It's not a freaky thing about "things like" busy beaver, it's a freaky thing about the concept of information.

But even though everything is a number, saying "it's crazy that a number can be X" is usually someone making a mistake, using the everyday concept of numbers in their head. If you replace "a number" with "some text and code and data", people wouldn't say it's surprising that "some text and code and data" can be unprovable in ZFC.

Technically a photograph is a number, but primarily it's something else. BB(748) is the same, technically a number but primarily it's a series of detailed computer calculations.

This integer only exists if you assume classical logic. Otherwise, there is no such integer a priori, and actually there is none in general.

Only if you believe that a number you can't count is a number. You can believe that, but it's a leap.

Don't forget identity. Its range is small but important!

The wording implies that Aaronson does not know what he's talking about.

>If you want to learn about actual Busy Beaver results [...]

This is saying there is no discussion of the results in the article, which is not true.

>Unlike Aaronson, he actually is on the forefront of Busy Beaver research [...]

This implies Aaronson has no (or lesser) authority on the subject and suggests we should listen to somebody else who purportedly has more.

Nowhere in @NooneAtAll3's comment is there an argument made against/for the contents of the article, an example of that would be:

"Aaronson mentions X but this is not correct because Y" or something along those lines.

Instead, the comment, in it's full extent, is either discrediting (perhaps unintentionally) and/or appealing to the authority of people involved. That's ad hominem.

But the comment is not just saying something negative.

It is implying that claims from the article like "Then, three days ago, Tristan wrote again to say that mxdys has improved the bound again, to BB(6)>9_2_2_2" are not real results. The justification for these not being real results is solely based off whether author is actually on the forefront of research.

The "first" link (after the home button) on bbchallenge is the header bar link to https://bbchallenge.org/story which cites Aaronson in the first sentence (double first!). I would not describe it like OP for someone trying to find the actual link ;)

"One Collatz Coincidence", the 2nd story on the blog, also mentions Aaronson