[FOM] A few observations on probability, cardinality, and the continuum
joeshipman@aol.com
joeshipman at aol.com
Fri Dec 24 20:42:37 EST 2010
The problem with Freiling's argument is that it proves too much. It
depends on an intuition that any subset of the unit square or interval
has an associated probability of being hit by a "dart".
If this probability is to be countably additive, you are assuming a
real-valued measurable cardinal, which already disproves CH very
strongly without any need for Freiling's symmetry argument.
If it is only finitely additive, then such measures exist (and in 1 or
2 dimensions they are even compatible with Lebesgue measure and
invariant under rigid motions), but you can't really be sure countable
sets ought to have probability 0 of being hit.
If it is countably additive on points but not subsets, then you can ask
why shouldn't it be k-additive on points for any cardinal k smaller
than the continuum, in which case Freiling's symmetry axiom is
inconsistent.
I say it's better just to go with the intuition that every subset of
space has a "mass" which leads to a real-valued measurable cardinal. By
Banach-Tarski, such a measure can't be invariant under rotations, which
means that either our intuition about the isotropy of space has to be
thrown out or our intuition about the continuity and infinite
divisibility of space has to be thrown out.
There is an interesting alternative history here which has some useful
foundational lessons. Because Banach-Tarski was discovered after the
intuition of infinitely divisible space had been called into question
by the atomic theory, it was interpreted as evidence against infinite
divisibility rather than evidence against isotropy; however, if physics
had developed differently (for example if Riemann had lived long enough
to understand Maxwell's work and developed special and general
relativity before 1900) then it is possible that the intuition of
isotropy would have been discarded first, in which case Banach-Tarski
would not have been interpreted as evidence against infinite
divisibility and the RVM axiom would have seemed much more intuitively
plausible. (Of course the intuition of infinite divisibility would
eventually have been discarded too, but in the meantime RVM might have
become the basis for an alternative axiomatization of set theory, a
stronger one which proved among other things Con(ZFC), and CH might
have been considered unproblematically false.)
I don't believe that any alien mathematical civilization would ever
disagree with us about statements of arithmetic (they might think they
had proven X when we didn't, or think that we hadn't proven Y when we
claimed to, but there would never be a case where they thought they had
proven X and we thought we had proven not-X, if X was arithmetical and
the proofs had been sufficiently checked for mistakes). But they could
easily disagree with us about things like CH.
-- JS
-----Original Message-----
From: Tom Dunion <tom.dunion at gmail.com>
To: fom at cs.nyu.edu
Sent: Fri, Dec 24, 2010 12:44 am
Subject: [FOM] A few observations on probability, cardinality, and the
continuum
The principle of Indifference tells us that if neither outcome of
atwo-outcome experiment has any reason to be preferred, the
probabilityof each outcome is 50 per cent. I would denote as the
principle ofCoherence the idea that if an experiment (actual, or a
thoughtexperiment) can be “naturally” described within the framework of
twodifferent sample spaces, the probabilities of the various
outcomesshould come out the same.At the root of the difficulty of
genuine paradoxes of probability thatare not mere mistakes in reasoning
(such as the “Monty Hall Problem”)we often find lack of Coherence:
different but plausible sample spacesyield different results. (See,
for example, “Sleeping Beauty”; alsoJ. Bertrand’s chords of a circle
paradox, for simple illustrations ofthis problem.)With regard to the
thought experiment of tossing 2 random darts at theunit interval
(Freiling’s Axiom of Symmetry), I submit that what hasso disquieted a
lot of people is really a clash of these principles:neither dart should
be preferred to hit a point less than the other(Indifference), but
assuming the Continuum Hypothesis, a naturaldescription arises (by
means of any well-ordering of [0,1] of lengthomega_1) such that each
dart is thrown at a countable set, withprobability zero of hitting the
target, violating Coherence.There may be a way out of the conundrum
which has not been followedup. It may be that the cardinality of the
continuum is (say) aleph_2,yet the set of predecessors of each dart
turns out to typically be anonmeasurable set. The existence of such
sets of cardinality aleph_1together with not-CH is known to be
consistent with ZFC.This leaves us with an argument against the CH
which seems to leanheavily on the principle of Indifference. Can
anything more be said,even tentatively?Well, one claim about
nonmeasurable sets may be that they areproblematic, not because none of
them can ever be assignedintrinsically meaningful probabilities in any
experiment, but rather,they just don’t “play nice” with other sets.
Admittedly, to thinkthat *any* set of size aleph_1 has a probability of
one-half of beinghit by a dart thrown into an interval whose
cardinality is aleph_2 mayfeel like an assault on our intuition; but
suppose (for about 30seconds) the value of 2^{aleph_0} to be aleph_5.
Suppose further (andthis is still consistent with ZFC) that the
smallest cardinality of aset which is not of measure zero is aleph_4.
Does your intuition feelas violated now? If not, maybe it should not
have felt so “wrong”when thinking about those nonmeasurable sets of
cardinality aleph_1.Lastly, I don’t buy the claim that Freiling’s
argument is essentiallyself-refuting, i.e. throw 2 darts then
2^{aleph_0} cannot be aleph_1,but throw 3 darts, then (by a clever use
of mappings) it cannot bealeph_2, etc. That’s because one can “rig up”
an argument using justthe first two darts thrown, if one appeals to
further claims ofarguable plausibility. I’ll just leave it there,
since I don’t wantto go off on an excursus from the main point of all
this: Indifferenceplus Coherence seems to lead to a quandary that
heavy-duty users ofmathematics (think: statisticians and physicists)
can understand. Sohere is the “controversial” part of this posting,
but meant as aninvigorating challenge, not as a put-down -– why should
mainstreammathematicians be expected to cheer on the larger f.o.m.
community,when that community cannot see its way clear to a resolution
of such acomprehensible (to mainstreamers) argument as Freiling’s?Tom
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