this post was submitted on 23 Oct 2024
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Natural numbers being infinite, how it be possible for the values between 1 and 2 to be "more infinite" ?
It's called countable and uncountable infinity. the idea here is that there are uncountably many numbers between 1 and 2, while there are only countably infinite natural numbers. it actually makes sense when you think about it. let's assume for a moment that the numbers between 1 and 2 are the same "size" of infinity as the natural numbers. If that were true, you'd be able to map every number between 1 and 2 to a natural number. but here's the thing, say you map some number "a" to 22 and another number "b" to 23. Now take the average of these two numbers, (a + b)/2 = c the number "c" is still between 1 and 2, but it hasn’t been mapped to any natural number. this means that there are more numbers between 1 and 2 than there are natural numbers proving that the infinity of real numbers is a different, larger kind of infinity than the infinity of the natural numbers
Great explanation by the way.
I get that, but it's kinda the same as saying "I dare you!" ; "I dare you to infinity!" ; "nuh uh, I dare you to double infinity!"
Sure it's more theoretically, but not really functionally more.
It's like when you say something is full. Double full doesn't mean anything, but there's still a difference between full of marbles and full of sand depending what you're trying to deduce. There's functional applications for this comparison. We could theoretically say there's twice as much sand than marbles in "full" if were interested in "counting".
The same way we have this idea of full, we have the idea of infinity which can affect certain mathematics. Full doesn't tell you the size of the container, it's a concept. A bucket twice as large is still full, so there are different kinds of full like we have different kinds of infinity.
When talking about infinity, basically everything is theoretical
Please show me a functional infinity
Right, an asymptote I guess, in use, but not a number.
It's been quite some time since I did pre-calc, but I remember there being equations where it was relevant that one infinity was bigger than another.
This reminds me of a one of Zeno's Paradoxes of Motion. The following is from the Stanford Encyclopaedia of Philosophy:
It's weird but the amount of natural numbers is "countable" if you had infinite time and patience, you could count "1,2,3..." to infinity. It is the countable infinity.
The amount of numbers between 1 and 2 is not countable. No matter what strategies you use, there will always be numbers that you miss. It's like counting the numbers of points in a line, you can always find more even at infinity. It is the uncountable infinity.
I greatly recommand you the hilbert's infinite hotel problem, you can find videos about it on youtube, it covers this question.
Because the second one is bounded ?
Basically, if two quantities are the same, you can pair them off. It's possible to prove you cannot pair off all real numbers with all integers. (It works for integers and all rational numbers, though)
How many infinities you accept as meaningful is a matter of preference, really. You don't even have to accept basic infinity or normal really big numbers as real, if you don't want to. Accepting "all of them" tends to lead to contradictions; not accepting, like, 3 is just weird and obtuse.
I thought the same but there is a good explanation for it which I can't remember
It's called countable and uncountable infinity. the idea here is that there are uncountably many numbers between 1 and 2, while there are only countably infinite natural numbers. it actually makes sense when you think about it. let's assume for a moment that the numbers between 1 and 2 are the same "size" of infinity as the natural numbers. If that were true, you'd be able to map every number between 1 and 2 to a natural number. but here's the thing, say you map some number "a" to 22 and another number "b" to 23. Now take the average of these two numbers, (a + b)/2 = c the number "c" is still between 1 and 2, but it hasn’t been mapped to any natural number. this means that there are more numbers between 1 and 2 than there are natural numbers proving that the infinity of real numbers is a different, larger kind of infinity than the infinity of the natural numbers
Your explanation is wrong. There is no reason to believe that "c" has no mapping.
Edit: for instance, it could map to 29, or -7.
Give me an example of a mapping system for the numbers between 1 and 2 where if you take the average of any 2 sequentially mapped numbers, the number in-between is also mapped.
Yeah, OP seems to be assuming a continuous mapping. It still works if you don't, but the standard way to prove it is the more abstract "diagonal argument".
But then a simple comeback would be, "well perhaps there is a non-continuous mapping." (There isn't one, of course.)
"It still works if you don't" -- how does red's argument work if you don't? Red is not using cantor's diagonal proof.
Yeah, that was actually an awkward wording, sorry. What I meant is that given a non-continuous map from the natural numbers to the reals (or any other two sets with infinite but non-matching cardinality), there's a way to prove it's not bijective - often the diagonal argument.
For anyone reading and curious, you take advantage of the fact you can choose an independent modification to the output value of the mapping for each input value. In this case, a common choice is the nth decimal digit of the real number corresponding to the input natural number n. By choosing the unused value for each digit - that is, making a new number that's different from all the used numbers in that one place, at least - you construct a value that must be unused in the set of possible outputs, which is a contradiction (bijective means it's a one-to-one pairing between the two ends).
Actually, you can go even stronger, and do this for surjective functions. All bijective maps are surjective functions, but surjective functions are allowed to map two or more inputs to the same output as long as every input and output is still used. At that point, you literally just define "A is a smaller set than B" as meaning that you can't surject A into B. It's a definition that works for all finite quantities, so why not?
because I assumed continuous mapping the number c is between a and b it means if it has to be mapped to a natural number the natural number has to be between 22 and 23 but there is no natural number between 22 and 23 , it means c is not mapped to anything
because I assumed continuous mapping the number c is between a and b it means if it has to be mapped to a natural number the natural number has to be between 22 and 23 but there is no natural number between 22 and 23 , it means c is not mapped to anything
Then you did not prove that there is no discontiguous mapping which maps [1, 2] to the natural numbers. You must show that no mapping exists, continugous or otherwise.
I'm confused as well. Isn't that like saying that there is more sand in a sandbox than on every veach on the planet?
We're talking about increasingly smaller fractions here. It's more like saying if you ground up all the rocks on earth into sand you would have more individual pieces of sand than individual rocks.