Why Mathematicians Can’t Find the Hay in a Haystack

I would guess it means "in some sense". In a measure-theoretic sense, for example. You need to give it meaning somehow, as you cannot really pick a "random" real number with any algorithm (most real numbers are actually not even computable!).

It could also be a remark that in infinite sets 100% (in a measure-theoretic sense) does not mean all of the elements.

The chance to be irrational is 99.9999...% (99 followed by an infinity of 9). And mathematically it is the same number as 100.

Suppose that were to select one number at random from the set of integers in the range from [-1..8]. There is a 1 in 10 chance of selecting -1.

Now suppose you select from the range [-1..98]. There is a 1 in 100 chance of getting the single negative value. In the range [-1..99,998] there is a 1 in 10,000 chance. And so forth.

In the range [-1..x], as we raise x higher the chance of getting the negative value is smaller, and the higher x gets, the closer the chance gets to zero. That is what we mean when we say "the chance of selecting -1 out of [-1..infinity] is zero".

And that is precisely the sense in which the mathematical structures this article refers to (like irrational numbers, or non algebraic solids) have 100% chance of being selected when choosing a (number/solid/whatever) at random.

In statistics there's a slight difference between something that happens 100% of the time and something that always happens.

Or rather just because something has a probability of 0 doesn't mean it can't happen, events with probability 0 happen all the time.

They may also have been referring to the fact that picking a random number over the whole number line is not that as simple as it sounds since the uniform distribution doesn't exist.

An idealized 'dot' has zero 'volume', as does a countable number of such dots. The bulk of the real number line is made from irrational numbers, with a countable number of rational dots mixed in.

If you define the 'probability' of hitting a rational dot in terms of ratios of volumina, it will be mathematically zero.

The footnote can be found in the original article at https://www.quantamagazine.org/why-mathematicians-cant-find-...

The reason it’s 100 percent and not 99.9999 percent has to do with the strange things that happen once you start thinking about the probabilities when infinity is involved. Around the turn of the 20th century, the mathematician Henri Lebesque helped to develop “measure theory,” which provided a mathematically rigorous way for calculating the probabilities when infinite outcomes are possible.

the same result holds if you restrict to some subset of the real line, say the closed unit interval `[0, 1]`.

although there are infinitely many rationals and infinitely many irrationals in the unit interval [0, 1], from the perspective of cardinality, there are a lot more irrationals. there are countably many rationals in [0, 1], meaning you can put those rationals into correspondence with the natural numbers 1, 2, 3, ... , but there are uncountably many irrationals in that same interval.

the quote about "100 percent chance" is probably alluding to measure theory. measure theory is used to define integration. in this 1 dimensional case of the real line, or a subset of the real line, you can think of the measure of a set being something like defining its length. the measure of the whole set [0, 1] is 1. the subset of all rationals in [0, 1] has measure zero, i.e. it has the same measure as a single point. there's no measurable length to it. not all subsets can be assigned a sensible notion of a length.

if you think of random sampling a point from the set [0, 1] according to a uniform distribution, then you have a probability of 0 < delta <= 1 of randomly sampling a point that belongs to any pre-defined subset [-0.5 * delta + x, x + 0.5 * delta] of [0, 1] . Informally, if we make delta smaller and smaller, the chance of randomly sampling a point that is arbitrarily close to x is probability delta that becomes arbitrarily close to zero. when you randomly sample from uniform distribution on [0, 1] the chance of sampling any particular pre-specified real number is probability zero. probability zero isn't the same as impossible! see https://en.wikipedia.org/wiki/Almost_surely

on another hand, despite this, the rationals are dense in [0, 1]. this means that for any real number x in [0, 1], for any (small) radius r > 0 we can find some rational y in [0, 1] that is within a ball of radius r from x, i.e. there is some rational y such that |x-y|<r. this means we can pick some sequence y_1, y_2, y_3, ... of rational numbers that become better and better approximations to the target x (which in the interesting case, x is irrational). think a sequence of decimal approximations to the target number x. despite this ability to approximate any irrational with a rational arbitrarily well, there are still a lot more irrationals, in both the cardinality and a crude measure-theoretic sense (if we delete them all it doesnt change the amount of length we started with).

> The asterisk at the end doesn't seem to lead to an actual footnote. What is meant by this?

perhaps it is meant to provoke all people who like to see everything clearly defined

Slogan: "rational numbers exist but do not take up space."

This is in a measure-theoretic sense, so literally 100% (not just 99.5%). On the number line, the measure of an interval is the (absolute) difference between its endpoints (the "length" of the interval). For any measurable set, you can approximate its size from above by covering its points with intervals and shrinking them as much as possible.

But because there are only countably many rational numbers, you can cover all of the rationals with a set of intervals with a finite total sum of lengths.

To be a bit more specific, if your rationals are r_1, r_2, r_3, ..., then for any small number e > 0, you can form the set of intervals

(r_1 - e/2^2, r_1 + e/2^2), (r_2 - e/2^3, r_2 + e/2^3), (r_3 - e/2^4, r_3 + e/2^4), ...

which have a total sum of lengths e/2 + e/4 + e/8 + ... = e and contain all of the rational numbers.

So for any positive number e > 0, the measure of the rational numbers is less than e, meaning that the measure of the rational numbers is 0.

To turn this into actual probabilities requires a little bit more work, since the measure of the number line, and hence that of the irrational numbers, is infinite, not "100%". But you could look at the probability of selecting a rational number in the interval [0, 1] and use the same reasoning as above to get a probability of 100% for an irrational number and 0% for a rational number.

You will pick an irrational number "almost surely" or "with probability 1". It's a somewhat counterintuitive* concept, but very well defined mathematically.

https://en.wikipedia.org/wiki/Almost_surely

[*] (For example, any individual number has probability 0 of being picked (exactly, not approximately), yet one is being picked. Or: there are infinitely many rational numbers between 0 and 1, but the probability that any one of them is picked is 0. Thus, the probability that one of the remaining ones is being picked (an irrational number), is 1.)

"the proof is left as an exercise to the reader"

Another way to think of it, other than the other fine posts: Imagine you are selecting your random number by using a ten-sided die and writing down the number of each (fair) roll. Suppose you assume you are going to write down a rational number. What does that imply? It means that your fair die is going to roll some prefix, and then roll some infinitely repeating segment over and over again. As you roll the dice an infinite number of times, what is the probability that your die will ever deviate from the necessary strict repetition of the repeating segment? It is literally 100%, because no matter how many times it happens to conform to the repeating sequence, there remain an infinite number of rolls remaining in which to deviate.

If it absolutely never deviated, over the course of infinite rolls, that would itself be proof that the die is itself not a fair die. e.g., if your putatively "fair ten-sided die" rolls up precisely 1/3, by virtue of rolling an infinite series of 3s, then that would be proof that it is not in fact fair and random.

I think this helps the 100% intuition. It is simply impossible for a truly random selection process to produce a rational number.

For any fixed compression scheme it's trivial to come up with files that don't compress.

If you've been following the devlopment of the Unum format[0], you'll know that this comes with a whole set of problems of their own.

[0] https://en.wikipedia.org/wiki/Unum_(number_format)

> as long as the precision of how you record and calculate numbers is greater than that, the solution will be correct.

Well .. not quite, you have to be very aware of the kinds of operations that may result in loss of precision or error accumulation. Whether you're using floating point or longhand decimal.

Conversely algebra operates as infinite precision, allowing the safe manipulation of numbers with no finite representation.

Sometimes even 0.1 is not equal to 0.1.

>This leads to things like junior programmers losing their minds when 0.1 + 0.2 doesn't equal 0.3

Junior programmers lose their mind when 0.1 + 0.2 != 0.3.

Senior programmers lose their mind trying to explain this to the business side.

That's why as soon as you come up with a list of properties your hay should have, you construct an example of it.

In relation to the article posted, there is a conjecture which states all algebraic, irrational numbers are normal.

So you can't imagine a world in which our understanding of pi or e might impact or be useful to the real world?

More broadly, you can surely imagine how the study of transcendent numbers might inform results in other fields? I don't usually try to justify math by pointing at its applications, but for example, the transcendence of pi leads to a proof that squaring the circle is impossible. Even if things aren't constructable, they can still be valuable to think about.

> numbers are man-made objects

That's plain wrong, animals can count too.

> Never repeating

Meaning "non-periodic." Repetitions can occur as many times as one wishes, e.g.

  0.1230123001230001230000123...

is irrational.

A more interesting puzzle is to prove or disprove whether every algebraic, irrational number is normal (in some base).