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by vitus 718 days ago
I am not remotely convinced by this argument.

The first flaw I see is that the author is imprecise by commingling probabilities (0%, 100%) with absolutes (possible, impossible, none, never, etc).

> After all, probability-zero events do happen. Not a problem! Just pick two new real numbers! And if this fails, pick again!

Probability-zero events happen all the time. The probability of getting any specific value selected uniformly at random from the unit interval (say, 0.232829) is zero.

Probability-zero events should not be conflated with properties that exist nowhere.

> We can now state that for any such mapping, none of the three reals is in the countable set assigned to the others. And this entails that we can prove that |(ω)| > |ω2|! In other words, we can prove that there are at least TWO cardinalities in between the reals and the naturals!

That's... not how cardinalities work. Just because you have two sets with different elements does not mean they have different cardinalities. For instance, consider the set of integers {..., -1, 0, 1, 2, ...} vs the set of half-integers {..., -1/2, 1/2, 3/2, 5/2, ...}. These clearly have different elements, but you can easily construct a bijection between the two (just add 1/2 to each element in your set of half-integers), so you can demonstrate that they have the same cardinality.

> We define f(x) to be {y | y ≤ x}

Um, no. This demonstrates the existence of one such mapping. It does not demonstrate that the set of such mappings covers any substantial portion of the entire space of possible mappings.

2 comments

Also, this entire argument seems to be founded on https://en.wikipedia.org/wiki/Freiling%27s_axiom_of_symmetry. It is not clear that Freiling himself accepts this axiom -- "Freiling's argument is not widely accepted because of the following two problems with it (which Freiling was well aware of and discussed in his paper)."
It is clear that Freiling accepts it:

> Freiling claims that probabilistic intuition strongly supports this proposition while others disagree.

> Probability-zero events happen all the time. The probability of getting any specific value selected uniformly at random from the unit interval (say, 0.232829) is zero.

I would strongly challenge that claim. First, you did not choose that number uniformly at random, you chose it from at best a countably infinite subset, or more realistically, from a finite subset. And secondly, I do not think you can describe a situation where a number is actually chosen uniformly and randomly from the unity interval.

But, if the set is infinite, won’t the odds of any individual number to be selected be effectively zero?
It is, and if you choose the number uniformly at random, it's not just "effectively" zero, it is precisely zero.

GP's point, as I understand it, is that it is not actually possible to choose a number from [0, 1] uniformly at random in "real life".

I think you could argue that, e.g. in the dartboard thought experiment, the probability of choosing individual points doesn't really matter: only probabilities of measurable subsets with positive measure matter.

I guess, but, the set is not even countably infinite. "Selecting at random" is something that happens in the real, non-infinite world, not in the mathematically rigorous would where infinities can exist. So, no, probability-zero events do not happen in either.
Not necessarily - you might just come up with a number you know is in the set, say pi/4. I know it's in the set because it satisfies the conditions that define it. Still, the odds of that particular number being picked up are zero.
If you picked that, you did not pick uniformly from the unit interval. You picked from something else, which was much smaller.
so how does the random function selecting the number select the number before the last number in the infinite set?
It wouldn't "select" as if it were an infinite deck of cards, but rather generate a number we know is on the infinite set. It can very well take an infinite amount of time to come up with the digits though...
> rather generate a number

How?

A series of random bits?
What last number?
exactly