the problem is that no matter how you choose to describe real numbers, there are only countably many possible descriptions (e.g. only countably many programs that spit out strings of digits), but uncountably many real numbers! it's wack
Conversation
this makes some people very uncomfortable (and i think that discomfort is justified). what is "real" about the vast majority of the real number line being inaccessible to any form of description whatsoever???
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(and i mean *vast* majority - in a precise technical sense the probability of a randomly chosen real number being describable is literally zero)
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for the purposes of simpler questions in euclidean geometry you can get away with working with a much smaller set of numbers, the algebraic reals, which are all describable
but you actually need all of the real numbers to do calculus. and we need calculus for a million things
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so the real numbers, as usually constructed, are (this is very much in-my-opinion) this philosophically unsatisfying technical kludge we put up with because it lets us put geometry and calculus and a million other things on a rigorous foundation
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i have hopes that someday someone will find a more philosophically satisfying replacement for the real numbers but it's very unclear to me what that would look like
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in the meantime this is a good opportunity for me to highlight another thing i wish people talked about more: ime it's rarely useful (at first pass) to ask "what is X?" in mathematics and usually much more useful to ask "what does X do?" or "what is X for?"
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actually another meta-level pet peeve of mine: i think a lot of people trying to learn undergrad-to-grad-level math don't really internalize that all this stuff is tools that specific people invented in order to solve specific problems, not like eternal timeless stuff
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the whole modern axiomatic method, not to mention specific axiomatizations like metric spaces or topological spaces or whatever, all that stuff is contingent. in other timelines it could've gone another way. aliens might not have it. it can be evaluated and discarded if desired
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and imo that's up to and including the modern axiomatization of set theory and of the real numbers! we brought it into this world and we can take it out!
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QC Explains Why You Should Become A Math Crank
Replying to
Qiaochu, can you recommend any books or resources on history of mathematics, philosophy of mathematics or similar -- basically anything that covers the kind of topics you discuss here? I have this amazing Russian book but I'm looking for others
Replying to
QC can you give a similar explainer for Conway's surreal numbers?
They confuse me
TIA
