# Turing Zeta FunctionsandUncertainty

Mike Stay

(Work done with Cristian Calude at University of Auckland, New Zealand)

# What is information?

• Liebniz, 1686: The year before Newton's Principia
• How to distinguish a world describable by science from one that isn't?
• "Lawful behavior"
• Data fitting
• Law is a smaller description of the data

# What's a description?

• Church, 1934: Lambda calculus
• Turing, 1936: Turing machines, compute the same functions as lambda calculus
• A description of x relative to a programming language is a program in that language with no inputs and whose output is x.

# Turing Machines

000000111101101000000000
......^.................
• Turing machines have
• a bi-infinite tape
• internal state
• A state is a row in a three column table:
• new bit
• new state
• right or left
or a special HALT state.

# Turing Machines

000000111101101000000000
......^.................
• This makes a Turing machine a partial computable function from binary strings to binary strings:
T:\{0,1\}^** \stackrel{\circ}{\to} \{0,1\}^**

# Universal Turing Machines

• A Turing machine U is universal if
• \forall Turing machines T,
• \exists c_T \in \{0,1\}^** such that
• \forall p \in \{0,1\}^**,
• \exists p' \in \{0,1\}^** such that
U(p') = T(p)
and
|p'| \le |p| + |c_T|.

# Universal Programming Language

• A programming language U is universal if
• for any other programming language T
• there's a program c_T, called a compiler such that
• for any program p written in T,
• there's another program p' written in U such that
and
p' is no longer than concatenating the compiler and the program for T.

# Algorithmic Information Theory

• Kolmogorov, Solomonoff, Chaitin in the 1960's.
• The complexity H_U(x) of a string x with respect to a TM U is the shortest program for U whose output is x:
H_U(x) = min(\{ |p| \ \ |\ \ p \in \{0,1\}^** and U(p) = x\})
• The smallest program p is called the elegant program for x.
• Work with prefix-free programs. Then can use Lebesgue measure to assign probability.

# Formal Axiomatic Systems

• Axioms and rules => Theorems
• Theorem examiner is a program that enumerates all possible proofs and asks of each proof:
1. Does this theorem prove that p is elegant for some x?
2. Is |p| > |\mbox{FAS}| + |\mbox{theorem examiner}|?
If yes to both, then return x.
• Contradiction, so no such long proofs exist!

# Chaitin's \Omega (1974)

\Omega = \sum_{p \mbox{ halts}} 2^{-|p|}
• Theorem: Bits of \Omega are random, i.e.
H_U(\Omega[m]) \ge m - c_U
where x[m] denotes the first m bits of the binary fraction x.

# Chaitin's \Omega (1974)

• Proof.
• Given \Omega_U up to 2^{-m}, we know the halting status of all programs \le m bits long:
• Run the programs in parallel:
      1
12
123
1234
1x345
1 3456
1 3x567
1 3 5678
.
.
.


# Chaitin's \Omega (1974)

• Add up contributions of halting programs
• When m bits match, no more p such that |p| < m can halt, because otherwise the contribution 2^{-|p|} > 2^{-m} and one of the bits would change!

# Chaitin's \Omega (1974)

• There is a program \Psi that
1. reads in \Omega_U[m]
2. computes the outputs of the halting programs
3. chooses a string x not in that set
Then
H_U(\Psi(\Omega_U[m])) = H_U(x) > m

# Chaitin's \Omega (1974)

• By universality of U, there's a program c_{\Psi} that implements it, so
|c_{\Psi}| + H_U(\Omega_U[m]) \ge H_U(x) > m
and setting c = |c_\Psi|,
H(\Omega_U[m]) \ge H(x) - |c_{\Psi}| \ge m - c
\square

# Chaitin's Incompleteness Theorem

• Given a program implementing any FAS and a theorem examiner that looks for proofs about the values of bits of \Omega_U[m], we can find its length
N = |\mbox{FAS}| + |\mbox{theorem examiner}|.
This program cannot prove the values of more than m = N + c bits. So a FAS like ZFC can't tell you more than a few bits of \Omega_U.
• So, there are infinitely many unprovable true statements, namely the values of the bits of \Omega_U.
• Calude et al. computed 64 bits of \Omega for a simple programming language.

# Chaitin's Incompleteness Theorem

• If you add k bits worth of axioms to your FAS, you'll be able to prove at most k more bits of \Omega_U.
• The bits of \Omega_U are irreducible information
• The answer to any finitely refutable problem is encoded in the bits of \Omega: Riemann hypothesis, Goldbach's conjecture, Fermat's last theorem.
• Given enough bits to cover a FAS and theorem examiner, you can tell (in principle) whether it will ever generate a proof of a given conjecture!

\Omega_U(s) = \sum_{p \mbox{ halts}} 2^{-s|p|}
• For computable real s>1, \Omega_U(s) is \frac{1}{s}-random, i.e.
H(\Omega_U(s)[m]) \ge \frac{m}{s} - c

# Stay: Zeta function of a Turing machine (2005)

• Let \mbox{bin}(n):\mathbb{N} \to \{0,1\}^** be the binary expansion of n without the leading 1:
   1       1       -
2      10       0
3      11       1
4     100      00
5     101      01
6     110      10
7     111      11
8    1000     000
.
.
.


# Stay: Zeta function of a Turing machine (2005)

\zeta_T(s) = \sum_{\mbox{bin}(n) \mbox{ halts}} n^{-s}
• For universal U,
zeta_U(s) is \frac{1}{s}-random.
• For any machine R that halts on all inputs (not just prefix-free ones),
\zeta_R(s) is Riemann zeta.

# Stay: Uncertainty principle

• Let \zeta = \zeta_U(1).
• Given \zeta[m], the uncertainty in \zeta is \Delta\zeta = 2^{-m}.
• What is the uncertainty in the elegant program for \zeta[m] given an unknown string s?
• If all the information we need is in s---that is,
U(s) = \zeta[m]
---then n=1.
• If s is independent of \zeta[m], then n needs to satisfy
U(\mbox{bin}(n)) = \zeta[m]
and since |\mbox{bin}(n)| > m-c, n > 2^{-m+c}.
• The difference between these extremes is the uncertainty, so \Delta n \ge 2^{-m+c_U}.