It boggles my mind that a number (an uncomputable number, granted) like BB(748) can be "independent of ZFC". It feels like a category error or something.
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Running for fewer steps is extremely well defined and I don’t imagine that enters into this.
That means there’s issue is “never stop”? That also seems pretty well defined to me. For BB(748) to vary based on your model, if the machines that run for fewer steps don’t change, then that means one of the machines that never stops in one model will stop in another. Or the BB winner for our model will never stop in another model.
How can changing your model make it so a specific Turing machine goes from stopping after 10,000 steps to never stopping, or from never stopping to stopping after 11,000 steps?
So in one model a Turing Machine called R never stops. In another model R stops after Q steps. But here's the issue... Q isn't an actual natural number, what it is is some mathematical object that satisfies all of the properties of a natural number in ZFC, but is not an actual natural number. What it actually is is some infinitely large object that satisfies all of the Peano axioms of what a natural number is as well as satisfies the following set of rules:
Q > 0
Q > 1
Q > 2
Q > 3
...
So within this model, the Turing machine R halts after Q steps, and since from within the model Q is finite then from within this model BB(748) is at least equal to Q.
If BB(748) is actually 10,000, then we can add this as an axiom to ZFC to get a new formal theory ZFC + "BB(748) = 10000".
In this new theory the previous structure that contained Q as an element will not satisfy the definition of a natural number, so we don't have to worry about Q anymore... however, there will exist some number T > 748 where BB(T) is independent of our new theory. For BB(T), there will exist some other model that has its own Q* which satisfies all of our axioms including the axiom that BB(748) = 10000, but also that
Q* > 0
Q* > 1
Q* > 2
Q* > 3
...
If there’s another model where this machine doesn’t stop, then that means that at some point during this process, you reach a particular machine state and tape contents and transition to a different state than you did in the first model. That has to happen, because otherwise the execution follows the same process as before, and halts at Q steps. But the mechanics of the machine don’t depend on your theory. They’re just state transitions and tape operations.
Q isn't a natural number because natural numbers must be finite, but Q is infinitely large.
>If you had unlimited time and paper, you could sit down and run the machine by hand, counting each step, until it reaches the halting state. You will have counted Q steps.
What if the machine never stops? How many steps will you run before you decide that the machine never halts?
>There’s no such thing as a machine that stops after a number of steps defined by an infinitely large construct.
There's no such thing as an actual machine that stops after an infinite number of steps, but that's not the issue. The issue is that ZFC has different models with conflicting definitions of what infinite is. In one model there is an object called Q that satisfies all of the properties in ZFC of being a natural number, but is infinitely large. In this model the Turing Machine halts after Q steps. But there is another model, called the standard model, and in this model there is no Q, all elements of this model are actually finite, and in this model the Turing machine never halts.
ZFC doesn't know which of these two models is the "real" model of natural numbers. From within ZFC both of these models satisfy all properties of natural numbers. It's only from outside of ZFC that one of these models is wrong, namely the model that contains Q as an element.
You can add more axioms to ZFC to get rid of the model that has Q as an element, but if the resulting theory containing your new axiom is consistent, then it necessarily follows that there is some other model that will contain some element Q* which is also infinitely large but from within the theory satisfies all of the new/stronger properties of being a natural number.
That doesn’t make any sense. A Turing machine can’t halt after a infinite number of steps. It either halts after a finite number of steps, or it never halts.
I’m sure there are models of hypercomputation and corresponding “what’s the largest number of steps they can run?” functions that would admit infinities, but those would not be Turing machines and the function would not be the Busy Beaver.
What the commenter above you said doesn't make sense in our daily life, but it makes perfect sense when in comes to non-standard models.
You got confused because you're thinking natural numbers as something we can count in real physical world, which is a perfectly sane mental model, and that is why there was a comment above said:
> People find that weird because they don't think about non-standard models, as arguably they shouldn't.
Q is not a number you can actually count, so it doesn't fit into our intuition of natural number. The point is not that Q exists in some physical sense in real life, like "3" in "3 apples" (it doesn't). The point is that ZF itself isn't strong enough to prevent you from defining random shit like Q as a natural number.
Sure, you can come up with a set of axioms where the natural numbers include infinities. You may be able to use it to prove interesting things. But all that does here it make it so that the set of numbers describing how many steps a Turing machine runs before it stops is no longer the “natural numbers.”