A philosophy problem about infinity

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Discussion Overview

The discussion revolves around the concept of infinity in mathematics and philosophy, particularly in relation to geometric shapes and the nature of infinite sets. Participants explore whether it is meaningful to define a circle or the universe in terms of infinity, and they delve into paradoxes associated with infinite quantities.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the validity of defining the shape of a circle as infinity using the equation x^2 + y^2 = ∞, pondering whether infinity can have a shape.
  • Another participant argues that since infinity is not part of the plane, the question lacks meaning unless the plane is extended to include infinity.
  • A participant suggests that if the universe is infinitely sized and shaped like a sphere, it raises questions about how one can determine the shape of something infinite.
  • Concerns are raised about the implications of defining the universe's expansion in terms of infinite size and shape, leading to perceived paradoxes.
  • Discussion shifts to the nature of infinite sets, with one participant noting the paradox of comparing the infinite set of even numbers to the infinite set of all numbers, questioning their cardinality.
  • Another participant references George Cantor's work on infinite sets, discussing the existence of different types of infinity and bijections between sets.
  • Clarifications are made regarding the concept of cardinality and the distinction between different infinities, including the idea that some infinities can be larger than others.

Areas of Agreement / Disagreement

Participants express differing views on the meaningfulness of defining shapes and sizes in terms of infinity. There is no consensus on the implications of these definitions or the nature of infinity itself, as multiple competing views remain throughout the discussion.

Contextual Notes

Participants highlight limitations in their definitions and assumptions regarding infinity, geometry, and set theory, but these remain unresolved within the discussion.

tmwong
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does it make sense if i define that the shape of a circle is infinity, which i write its equation as:

x^2+y^2 = ∞

i know that infinity means very very big until we cannot define how big is it. in this case, is the equation above still valid? can i still say that the shape of this infinity in the equation is a circle? or its size is not important or does not make sense already because infinity doesn't have shape since it's too big until we can't define what's the shape of that "thing"?
 
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Since infinity is not part of the plane the question, as a question of geometry, is meaningless. You need to extend the plane to include infinity, even then the most natural extension, that of the one point compactification of the Complexy plane, yields that the locus of points an "infinite distance from the origin" is the point at infinity.
 
e.g: if i say the size of the universe is infinity but its shape is a sphere. does it make sense?
 
It doesn't make mathematical sense unless you state it mathematically. Are you speaking topologically?
 
if yes? is that mean the universe is expanding from a singularity until infinity size in a sphere shape in my example?
 
however, if my statement just now was true, since its size is infinity, how could we know it's expanding in a sphere shape? and not in a cubid or other geometry shape? i found there is a paradox here.
 
Wow! You're right!

Also, I found another paradox. If there is an infinite amount of even numbers, and an infinite amount of numbers, are they the same? But one should be twice as big! Cool, huh?

Think I should send it to a math journal or something?
 
It's already been done, by George Cantor. Search for his name and you will find some pretty cool stuff about infinity/infinite sets.
 
DeadWolfe said:
Wow! You're right!

Also, I found another paradox. If there is an infinite amount of even numbers, and an infinite amount of numbers, are they the same? But one should be twice as big! Cool, huh?

Think I should send it to a math journal or something?

What you have noiced is that infinite sets can have proper subsets that are infinite (but this is nothin new as indeed the definition of an infinite set says somehting even stronger i.e. a set is infinite if there is a bijection between it and a proper subset of itself) . There is a bijection between the set of even numbers and the set of integers and there is also a bijection between the even numbers and the set of rationals, but there's no bijection between the set of even numbers and the ste of real numbers
 
  • #10
Right. Just to add to this: when a bijection between two sets is possible, then the cardinality of those two sets are equal. Thus, the cardinality of the set of even numbers equals the cardinality of the set of positive numbers/integers/rationals. Sometimes for short we call this "type" of infinity [aleph]_0. [aleph] is a hebrew letter.

What do I mean about different types of infinity? Well, there does not exist a bijection between the natural numbers and the real numbers. Google for "cantor's diagonal argument" for specifics. So in a sense, the cardinality of R (the reals) is larger than [aleph]_0. But both are infinity. Thus, the existence of different types of infinity.
 
  • #11
kreil said:
It's already been done, by George Cantor. Search for his name and you will find some pretty cool stuff about infinity/infinite sets.
I thought he was being sarcastic.
 

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