This question presupposes that the task will ever be done
Sure. It’s called super-tasks.
From mathematics we know that not all sequences converge. So the sequence of distributions that you gave, or my example of the sequence 0,1,2,3,4,… both don’t converge. Calling them a supertask doesn’t change that fact.
What mathematicians often do in such cases is to define a new object to denote the hypothetical value at the end of sequence. This is how you end up with real numbers, distributions (generalized functions), etc. To be fully formal you would have to keep track of the sequence itself, which for real numbers gives you Cauchy sequences for instance. In most cases these objects behave a lot like the elements of the sequence, so real numbers are a lot like rational numbers. But not always, and sometimes there is some weirdness.
From the wikipedia link:
In philosophy, a supertask is a countably infinite sequence of operations that occur sequentially within a finite interval of time.
This refers to something called “time”. Most of mathematics, ZFC included, has no notion of time. Now, you could take a variable, and call it time. And you can say that a given countably infinite sequences “takes place” in finite “time”. But that is just you putting semantics on this sequence and this variable.
So the sequence of distributions that you gave, or my example of the sequence 0,1,2,3,4,… both don’t converge. Calling them a supertask doesn’t change that fact.
Your question of “after finishing the supertask, what is the probability that 0 stays in place” doesn’t yet parse as a question in ZFC, because you haven’t specified what is meant by “after finishing the supertask”. You need to formalize this notion before we can say anything about it.
If you’re saying that there is no formalization you know of that makes sense in ZFC, then that’s fine, but that’s not necessarily a strike against ZFC unless you have a competitive alternative you’re offering. The problem could just be that it’s an ill-defined concept to begin with, or you just haven’t found a good formalization. Just because your brain says “that sounds like it make sense”, doesn’t mean it actually makes sense.
To show that ZFC is inconsistent, you would need to display a formal contradiction deduced from the ZFC axioms. “I can’t write down a formalization of this natural sounding concept” isn’t a formal contradiction; the failure is at the modeling step, not inside the logical calculus.
This is a sequence of natural numbers. This sequence does not converge, which means that the limit as n goes to infinite of S(n) is not a natural number (nor a real number for that matter).
You could try to write it as a function of time, S’(t) such that S’(1-0.5^n) = S(n). That is, S’(0)=0, S’(0.5)=1, S’(0.75)=2, etc. A possible formula is S’(t) = -log_2(1-t). You could then ask what is S’(1). The answer is that this is the same as the limit S(infinity), or as log(0), which are both not defined. So in fact S’ is not a function from numbers between 0 and 1 inclusive to natural or real numbers, since the domain excludes 1.
You can similarly define a sequence of distributions over the natural numbers by
T(0) = {i -> 0.5 * 0.5^i}
T(n+1) = the same as T(n) except two values swapped
This is the example that you gave above. The sequence T(n) doesn’t converge (I haven’t checked, but the discussion above suggests that it doesn’t), meaning that the limit “lim_{n->inf} T(n)” is not defined.
Thomas, please read and understand query’s response above. In attempting to dismantle a concept you don’t like, you’ve lost precision. Formalize your questions and concerns rigorously and then see if a seeming contradiction is still there.
From mathematics we know that not all sequences converge. So the sequence of distributions that you gave, or my example of the sequence 0,1,2,3,4,… both don’t converge. Calling them a supertask doesn’t change that fact.
What mathematicians often do in such cases is to define a new object to denote the hypothetical value at the end of sequence. This is how you end up with real numbers, distributions (generalized functions), etc. To be fully formal you would have to keep track of the sequence itself, which for real numbers gives you Cauchy sequences for instance. In most cases these objects behave a lot like the elements of the sequence, so real numbers are a lot like rational numbers. But not always, and sometimes there is some weirdness.
From the wikipedia link:
This refers to something called “time”. Most of mathematics, ZFC included, has no notion of time. Now, you could take a variable, and call it time. And you can say that a given countably infinite sequences “takes place” in finite “time”. But that is just you putting semantics on this sequence and this variable.
I don’t understand you.
Your question of “after finishing the supertask, what is the probability that 0 stays in place” doesn’t yet parse as a question in ZFC, because you haven’t specified what is meant by “after finishing the supertask”. You need to formalize this notion before we can say anything about it.
If you’re saying that there is no formalization you know of that makes sense in ZFC, then that’s fine, but that’s not necessarily a strike against ZFC unless you have a competitive alternative you’re offering. The problem could just be that it’s an ill-defined concept to begin with, or you just haven’t found a good formalization. Just because your brain says “that sounds like it make sense”, doesn’t mean it actually makes sense.
To show that ZFC is inconsistent, you would need to display a formal contradiction deduced from the ZFC axioms. “I can’t write down a formalization of this natural sounding concept” isn’t a formal contradiction; the failure is at the modeling step, not inside the logical calculus.
Define the sequence S by
This is a sequence of natural numbers. This sequence does not converge, which means that the limit as n goes to infinite of S(n) is not a natural number (nor a real number for that matter).
You could try to write it as a function of time, S’(t) such that S’(1-0.5^n) = S(n). That is, S’(0)=0, S’(0.5)=1, S’(0.75)=2, etc. A possible formula is S’(t) = -log_2(1-t). You could then ask what is S’(1). The answer is that this is the same as the limit S(infinity), or as log(0), which are both not defined. So in fact S’ is not a function from numbers between 0 and 1 inclusive to natural or real numbers, since the domain excludes 1.
You can similarly define a sequence of distributions over the natural numbers by
This is the example that you gave above. The sequence T(n) doesn’t converge (I haven’t checked, but the discussion above suggests that it doesn’t), meaning that the limit “lim_{n->inf} T(n)” is not defined.
Thomas, please read and understand query’s response above. In attempting to dismantle a concept you don’t like, you’ve lost precision. Formalize your questions and concerns rigorously and then see if a seeming contradiction is still there.