passive_fist comments on Estimating the kolmogorov complexity of the known laws of physics? - Less Wrong

10 Post author: Strilanc 08 July 2013 04:30AM

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Comment author: passive_fist 08 July 2013 08:41:13PM *  0 points [-]

It does make sense to talk of relative KC, actually, but it's not trivial to see why: http://en.wikipedia.org/wiki/Kolmogorov_complexity#Invariance_theorem

In particular, "If K1 and K2 are the complexity functions relative to description languages L1 and L2, then there is a constant c – which depends only on the languages L1 and L2 chosen – such that

\forall s\ |K_1(s) - K_2(s)| \leq c. "
Comment author: pragmatist 09 July 2013 05:46:16AM *  2 points [-]

Yes, I'm aware of this. It's still true, I believe, that for any two finite strings s1 and s2 one can find description languages L1 and L2 (with complexity functions K1 and K2) such that

K1(s1) > K1(s2)

and

K2(s1) < K2(s2).

So there is no language-independent sense in which s1 is more complex than s2 (or vice versa). To make the claim more concrete, consider the fact that for any finite string, one could construct a Universal Turing Machine that outputs that string when given the input 0 (the string is essentially hard-coded into the structure of the machine). This corresponds to a description language in which that string has minimal K-complexity.

This is all compatible with the invariance theorem. As a simple illustration, let the constant c associated with L1 and L2 be 5, let K1(s1) be 10, K1(s2) be 9, K2(s1) be 6 and K2(s2) be 8. In this example, both the inequalities I've given above are true, and the invariance theorem isn't violated.