matt grime
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I will concede this - I cannot state 'infinity is ...' and fill in something for the ... that is a definition in anyway that is very satisfactory. No mathematician would, or could without qualifying their statement. There was an interesting thread on sci.math about the role of infinity in mathematics, and the consensus was that mathematicians whilst using the term to illustrate concepts, would, when pressed to be rigorous, switch to another definition.
For instance, when we say there is an 'infinity of' possibilities, we actually mean, there is not a finite number of possiblities; given any finite number of options I can find another one'. When we say x(n) tends to x as n tends to infinity, what we actually mean is a statement that at no point includes the word infinity. Then there's the case of the sum to infinity, which is just the limit of a sequence as above, again with no infinity mentioned. Then the sum is 'infinite' if it is not finite, if there is no limit in the sequence of finite sums, that's all, agian we don't actually have an infinity there do we? Of course there is the point at infinity of the riemann sphere which neatly encapsulates the idea of being 'not finite', and which allows us to do many useful analytic operations. It is often called infinity, and can be related to the other examples, but is it 'infinity'? No, just like things such as multiplication it is contextual - the multiplication of real numbers isn't the multiplication of matrices is it? In short infinity is a useful concept, just as continuity is, but there is no object one can satisfactorily point to as infinity, just as there is no object one can point to and say that object is continuity.
Many cranks have this idea that infinity is actually something, something tangible, and that when we say the sum from one to infinity, we actually mean sum all the finite bits and then stop AT infinity just like we can stop at 7 or 20,445. If people learned the distinctions about these things we'd all be a lot better off. All this is compounded by the teaching that 1/0 IS infinity. It isn't, it is undefined in the ordinary arithmetic that they know, but it is true that 1/x can be made arbitrarily large, which is not the same thing at all.
For instance, when we say there is an 'infinity of' possibilities, we actually mean, there is not a finite number of possiblities; given any finite number of options I can find another one'. When we say x(n) tends to x as n tends to infinity, what we actually mean is a statement that at no point includes the word infinity. Then there's the case of the sum to infinity, which is just the limit of a sequence as above, again with no infinity mentioned. Then the sum is 'infinite' if it is not finite, if there is no limit in the sequence of finite sums, that's all, agian we don't actually have an infinity there do we? Of course there is the point at infinity of the riemann sphere which neatly encapsulates the idea of being 'not finite', and which allows us to do many useful analytic operations. It is often called infinity, and can be related to the other examples, but is it 'infinity'? No, just like things such as multiplication it is contextual - the multiplication of real numbers isn't the multiplication of matrices is it? In short infinity is a useful concept, just as continuity is, but there is no object one can satisfactorily point to as infinity, just as there is no object one can point to and say that object is continuity.
Many cranks have this idea that infinity is actually something, something tangible, and that when we say the sum from one to infinity, we actually mean sum all the finite bits and then stop AT infinity just like we can stop at 7 or 20,445. If people learned the distinctions about these things we'd all be a lot better off. All this is compounded by the teaching that 1/0 IS infinity. It isn't, it is undefined in the ordinary arithmetic that they know, but it is true that 1/x can be made arbitrarily large, which is not the same thing at all.
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