PrimaVera
Well-Known Member
_Z_ said:1. Infinity paradoxes. From the book infinity and the mind by Rudy Rucker.
From the book:
1.q. It is sometimes said that if infinitely many planets existed, and then every possible planet would have to exist, including one exactly like earth except with unicorns. Is this necessarily true?
I'm afraid I was "lost" at about this point, because I'm not seeing a paradox. This is, perhaps, a problem when we try to use Mathematical concepts that have very precise meanings within a broader philosophical context. The concepts associated with infinity are certainly seductive, but we must take care not to be equivocal.
In the case above, the "paradox" actually mixes two distinct meanings of "infinity" as used in Mathematics. The first concept is the number signified by a sideways "8". This is a "number" in the sense that it's used in equations (for example in the concept of a limit in Calculus).
The second concept, however, relates to sets, or rather, the cardinality of a set. When we talk about an infinite number of planets, we're really talking about a set, i.e. a collection of distinct objects that share some property that defines membership in the set.
The apparent paradox in this is the failure to realize that the cardinality of two different infinitely countable sets can be the same even though the two sets have different criteria for membership. Consider, for example, sets of numbers. The set of all whole numbers is an infinitely countable set. So is the set of all rational numbers (i.e. numbers that can be expressed as a ratio of two whole numbers). The set of all whole numbers is a subset of the set of all rational numbers, but the two sets have the same cardinality.
On the other hand, the set of all irrational numbers, that is numbers that cannot be expressed as a ratio of two hole numbers (e.g. PI), does not have the same cardinality as the set of all rational numbers. Consider a number line. Pick any two rational numbers. Between those two rational numbers, there is an infinite number of irrational numbers.
The cardinality of sets is denoted using Aleph (of the Hebrew alphabet) notation. Aleph-null is the cardinality of the set of all rational numbers. The set of all irrational numbers has a cardinality of Aleph-1.
Intuitively, it sometimes helps to think of cardinality as relating to the "density" of a set of objects. By that, I mean to say that one can, intuitively, think of the set of irrational numbers as being infinitely more dense than the set of rational numbers.
In terms of Mathematical proof, the difference between the cardianlity of two distinct sets rests on our ability to construct a logical one-to-one correspondence between elements in the two sets. If we can construct a logical one-to-one correspondence between the elements of two sets, then their cardinality is the same. If we can show that it's not possible to construct a one-to-one correspondence between the two sets, then the cardinality of the two sets is not the same.