Randomness of a bit string
Doug Merritt
doug at netcom.com
Mon Jan 24 22:38:30 PST 1994
Tim May said:
>I stand by my point that no number or sequence can be proved to be random.
To expand a bit on Perry's arguments, the bottom line of all this research
is that a claim regarding randomness can only be made *relative* to
a particular system for specifying algorithms.
In that sense, Tim's statement can be regarded to be correct, iff one
assumes that a context (an algorithmic specification system) is not
given. That is a huge qualifier, though, and not one to be taken for
granted.
A trivial example of this: pick some constant bitstring of length 32 or less.
Call it K. Now look at the class of algorithms specifiable by the
C code fragment printf("%x", K) --- i.e. print K as a hexadecimal number.
Relative to that particular set of (one) algorithms, that value of K is
trivially nonrandom, in the sense that the probability of of finding
that bitstring produced by that class of algorithm is precisely 1.
Next consider a program that computes an output by multiplying some
input by two. The probability that the output will be K, given any
possible (but unknown) input, is exactly zero if K happens to be odd.
If K is not odd, then the probability depends on the distribution
(randomness) of the inputs.
Proceeding in this fashion, it becomes increasingly clear that the
randomness of the output of an algorithm can only be measured relative
to the properties of the class of algorithms being considered. Randomness
in isolation is meaningless.
The best sources of intuition regarding randomness usually derive from
systems which shift the burden into an existing intuition on a slightly
different subject. For instance, flipping a coin can be regarded as a random
process in an intuitive sense, but only because it appeals to existing
intuitions about equiprobablistic outcomes.
Therefore one sees confused appeals to intuition about randomness, probability,
entropy, or related ideas, in cryptography, quantum mechanics, information
theory, statistical mechanics, philosophy (in regard to free will versus
determinism versus randomness), etc, etc, but given Chaitin/Kolmogorov/et al,
no intuition from any such subject should be taken at face value.
There's more, but I'll pause to allow flames. :-)
Doug
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