statements using it could be reconverted (using mathematical operations) into ones that do pay rent.
If statement A can be converted into statement B and statement B pays rent, then statement A pays rent.
Only if the mathematical operation is performed by pure logically entailment, which—if a meaningless definition of infinity is used and that definition is scrapped in the final statement—it would not be. We will just go on about what constitutes a mathematical operation and such, but all I am saying is that if there is a formal manipulation rule that says something like, “You can change the infinity symbol to ‘big enough’* here” (note: this is not logical entailment) then I have no objection to the use of the formal symbol “infinity.”
*ETA: or just use the definition we agree on instead. This is a minor technical point, hard to explain, and I’m not doing a good job of it. I’ll leave it in just in case you started a reply to it already, but I don’t think it will help many people understand what I’m talking about, rather than just reading the parts below this.
I reject infinity as anything more than “a number that is big enough for it’s smallness to be negligible for the purpose at hand.”
Is a terrible one for most purposes, because for them, no matter how big you make a finite number, it won’t serve the purpose.
For example? Although, if we agree on the definition below, there’s maybe no point.
The immediate sense that a word means something is not, itself, the meaning, but only a reliable intuition that the word means something.
That’s why I said “could potentially pay rent.”
The epsilon-delta definition in my highschool textbook didn’t use infinite sequences, except in the sense of “you could go on giving me epsilons and I could go on giving you deltas.”
But then it did use infinite sequences.
That definition of infinity (if we’ll call it that) directly means something to me: “this process of back and forth is not going to end.”
Unlike your original definition, this is a good definition (at least, once it’s been appropriately cleaned up and made precise).
Looks like we’re in agreement, then, and I am not a finitist if that is what is meant by infinite sequences.
But then, to take it back to the original, I still agree with Eliezer that an “infinite set” is a dubious concept. Infinite as an adverb I can take (describes a process that isn’t going to end (in the sense that expecting it to end never pays rent)); infinite as an adjective, and infinity the noun, seem like reification: Harmless in some contexts, but harmful in others.
For example? Although, if we agree on the definition below, there’s maybe no point.
A very early appearance of infinity is the proof that there are infinitely many primes. It is most certainly not a proof that there is a very large but finite number of primes.
I can agree with “there are infinitely many primes” if I interpret it as something like “if I ever expect to run out of primes, that belief won’t pay rent.”
In this case, and in most cases in mathematics, these statements may look and operate the same—except mine might be slower and harder to work with. So why do I insist on it? I’m happy to work with infinities for regular math stuff, but there are some cases where it does matter, and these might all be outside of pure math. But in applied math there can be problems if infinity is taken seriously as a static concept rather than as a process where the expectation that it will end will never pay rent.
Like if someone said, “Black holes have infinite density,” I would have to ask for clarification. Can it be put into a verbal form at least? How would it pay rent in terms of measurements? That kind of thing.
Like if someone said, “Black holes have infinite density,” I would have to ask for clarification.
Actually, the way I learned calculus, allowable values of functions are real (or complex), not infinite. The value of the function 1/x at x=0 is not “infinity”, but “undefined” (which is to say, there is no function at that point); similarly for derivatives of functions where the functions go vertical. Since that time, I discovered that apparently physicists have supplemented the calculus I know with infinite values. They actually did it because this was useful to them. Don’t ask me why, I don’t remember. But here is a case where the pure math does not have infinities, and then the practical folk over in the physics department add them in. Apparently the practical folk think that infinity can pay rent.
As for gravitational singularities, the problem here is not the concept of infinity. That is an innocent bystander. The problem is that the math breaks down. That happens even if you replace “infinite” with “undefined”.
This isn’t really correct. Allowable values of functions are whatever you want. If you define a function on R-{0} by “x goes to 1/x”, it’s not defined at 0; I explicitly excluded it from the domain. If you define a function on R by “x goes to 1/x”… you can’t, there’s no such thing as 1⁄0. If you define a function on R by “x goes to 1/x if x is nonzero, and 0 goes to infinity”, this is a perfectly sensible function, which it is convenient to just abbreviate as “1/x”. Though for obvious reasons I would only recommend doing this if the “infinity” you are using represents both arbitrarily large positive and negative quantities. (EDIT: But if you want to define a function on [0,infty) by “x goes to 1/x if x is nonzero, and 0 goes to infinity” with “infinity” now only being large in the positive direction, which is likely what’s actually under consideration here, then this is not so dumb.)
All this is irrelevant to any actual physical questions, where whether using infinities is appropriate or not just depends on, well, the physics of it.
Yes, and of course which theory will be appropriate is going to be determined by the actual physics. My point is just that your statement that “pure math does not have infinities” and physicists “added them in” is wrong (even ignoring historical inaccuracies).
But here is a case where the pure math does not have infinities
That is not a statement that the field of mathematics does not have infinities. I was referring specifically to “the way I learned calculus”. Unless you took my class, you don’t know what I did or did not learn and how I learned it. My statement was true, your “correction” was false.
Ah, sorry then. This is the sort of mistake I that’s common enough that it seemed more obvious to me to read it the that way rather than the literal and correct way.
As for gravitational singularities, the problem here is not the concept of infinity. That is an innocent bystander. The problem is that the math breaks down.
I never really got why the math is said to ‘break down’. Is it just because of a divide by zero thing or something more significant? I guess I just don’t see a particular problem with having a part of the universe really being @%%@ed up like that.
I guess I just don’t see a particular problem with having a part of the universe really being @%%@ed up like that.
What I think is more likely is that the universe does not actually divide by zero, and the singularity is a gap in our knowledge. Gaps in knowledge are the problem of science, whose function is to fill them.
Only if the mathematical operation is performed by pure logically entailment, which—if a meaningless definition of infinity is used and that definition is scrapped in the final statement—it would not be. We will just go on about what constitutes a mathematical operation and such, but all I am saying is that if there is a formal manipulation rule that says something like, “You can change the infinity symbol to ‘big enough’* here” (note: this is not logical entailment) then I have no objection to the use of the formal symbol “infinity.”
*ETA: or just use the definition we agree on instead. This is a minor technical point, hard to explain, and I’m not doing a good job of it. I’ll leave it in just in case you started a reply to it already, but I don’t think it will help many people understand what I’m talking about, rather than just reading the parts below this.
For example? Although, if we agree on the definition below, there’s maybe no point.
That’s why I said “could potentially pay rent.”
Looks like we’re in agreement, then, and I am not a finitist if that is what is meant by infinite sequences.
But then, to take it back to the original, I still agree with Eliezer that an “infinite set” is a dubious concept. Infinite as an adverb I can take (describes a process that isn’t going to end (in the sense that expecting it to end never pays rent)); infinite as an adjective, and infinity the noun, seem like reification: Harmless in some contexts, but harmful in others.
A very early appearance of infinity is the proof that there are infinitely many primes. It is most certainly not a proof that there is a very large but finite number of primes.
I can agree with “there are infinitely many primes” if I interpret it as something like “if I ever expect to run out of primes, that belief won’t pay rent.”
In this case, and in most cases in mathematics, these statements may look and operate the same—except mine might be slower and harder to work with. So why do I insist on it? I’m happy to work with infinities for regular math stuff, but there are some cases where it does matter, and these might all be outside of pure math. But in applied math there can be problems if infinity is taken seriously as a static concept rather than as a process where the expectation that it will end will never pay rent.
Like if someone said, “Black holes have infinite density,” I would have to ask for clarification. Can it be put into a verbal form at least? How would it pay rent in terms of measurements? That kind of thing.
Actually, the way I learned calculus, allowable values of functions are real (or complex), not infinite. The value of the function 1/x at x=0 is not “infinity”, but “undefined” (which is to say, there is no function at that point); similarly for derivatives of functions where the functions go vertical. Since that time, I discovered that apparently physicists have supplemented the calculus I know with infinite values. They actually did it because this was useful to them. Don’t ask me why, I don’t remember. But here is a case where the pure math does not have infinities, and then the practical folk over in the physics department add them in. Apparently the practical folk think that infinity can pay rent.
As for gravitational singularities, the problem here is not the concept of infinity. That is an innocent bystander. The problem is that the math breaks down. That happens even if you replace “infinite” with “undefined”.
This isn’t really correct. Allowable values of functions are whatever you want. If you define a function on R-{0} by “x goes to 1/x”, it’s not defined at 0; I explicitly excluded it from the domain. If you define a function on R by “x goes to 1/x”… you can’t, there’s no such thing as 1⁄0. If you define a function on R by “x goes to 1/x if x is nonzero, and 0 goes to infinity”, this is a perfectly sensible function, which it is convenient to just abbreviate as “1/x”. Though for obvious reasons I would only recommend doing this if the “infinity” you are using represents both arbitrarily large positive and negative quantities. (EDIT: But if you want to define a function on [0,infty) by “x goes to 1/x if x is nonzero, and 0 goes to infinity” with “infinity” now only being large in the positive direction, which is likely what’s actually under consideration here, then this is not so dumb.)
All this is irrelevant to any actual physical questions, where whether using infinities is appropriate or not just depends on, well, the physics of it.
They are limited by the scope of whatever theory you are working in.
Yes, and of course which theory will be appropriate is going to be determined by the actual physics. My point is just that your statement that “pure math does not have infinities” and physicists “added them in” is wrong (even ignoring historical inaccuracies).
Selective quotation. I said:
That is not a statement that the field of mathematics does not have infinities. I was referring specifically to “the way I learned calculus”. Unless you took my class, you don’t know what I did or did not learn and how I learned it. My statement was true, your “correction” was false.
Ah, sorry then. This is the sort of mistake I that’s common enough that it seemed more obvious to me to read it the that way rather than the literal and correct way.
I might call engineers “practical folk”; astrophysicists I’m not so sure. I’d like to see their reason for doing so.
I never really got why the math is said to ‘break down’. Is it just because of a divide by zero thing or something more significant? I guess I just don’t see a particular problem with having a part of the universe really being @%%@ed up like that.
What I think is more likely is that the universe does not actually divide by zero, and the singularity is a gap in our knowledge. Gaps in knowledge are the problem of science, whose function is to fill them.