# Wick rotation and the Riemann zeta function

+ 6 like - 0 dislike
34 views

The goal of this question is to conceptualize in some way the fact that the Riemann zeta function $\zeta(s)$, and other zeta functions like it, have analytic continuations.

### Background

I have by now convinced myself that the following is a reasonable conceptualization of why the Riemann zeta function for real $s > 1$ is a natural object of study. First, the probability distributions $\mathbb{P}(X = n) = \frac{1}{\zeta(s)} \left( \frac{1}{n^s} \right)$ on $\mathbb{N}$ are the unique ones satisfying the following two conditions:

• Given that $n | X$, the probability distribution on $\frac{X}{n}$ is the same as the original distribution, and
• $\mathbb{P}(X = n)$ is monotonically decreasing.

Second, if one looks at the sequence of measures $\mu_s(A) = \sum_{a \in A} \mathbb{P}(X = a)$ of a subset $A \subset \mathbb{N}$ with respect to the above distribution, then the $s \to 1^{+}$ limit is the logarithmic density of $A$, which agrees with the natural density of $A$ if it exists.

One can also use statistical-mechanical language to describe the above distribution. There is a statistical-mechanical system, the Riemann gas, whose states are the positive integers $n$ and whose energies are the numbers $\log n$, and $\zeta(s)$ is its partition function (which then determines a distribution on $\mathbb{N}$ in the usual way). This explanation conceptualizes, among other things, the von Mangoldt function, whose Dirichlet series is just the average energy of the above system.

However, the language of probability distributions is insufficient for talking about $\zeta(s)$ for $s \le 1$ or for complex $s$.

### Question

Is there a way to conceptualize the values of the zeta function at complex values of $s$ as a "Wick rotation" of its values at real $s$? That is, is there some reasonable quantum-mechanical interpretation of numbers like the "formal" measure

$$\mu_{s+it}(A) = \frac{1}{\zeta(s+it)} \sum_{a \in A} e^{-(s + it)\log a}$$

(for $s, t$ real) as a probability amplitude, or something along those lines? Does this reasonable quantum-mechanical interpretation single out the critical line $s = \frac{1}{2}$?

This post imported from StackExchange MathOverflow at 2015-01-08 14:06 (UTC), posted by SE-user Qiaochu Yuan
en.wikipedia.org/wiki/Wigner_quasi-probability_distribution

This post imported from StackExchange MathOverflow at 2015-01-08 14:06 (UTC), posted by SE-user Steve Huntsman
I found a paper that mentions both Wick rotations and zeta functions. I have no idea whether it does what you want. J Casahorran, Quantum-mechanical tunneling: differential operators, zeta-functions and determinants, Fortschr. Phys. 50 (2002), no. 3-4, 405–424, MR 2003k:81061.

This post imported from StackExchange MathOverflow at 2015-01-08 14:06 (UTC), posted by SE-user Gerry Myerson
Thanks, Gerry. Unfortunately reading the abstract it sounds like the paper is using zeta-function regularization and not directly talking about zeta functions.

This post imported from StackExchange MathOverflow at 2015-01-08 14:06 (UTC), posted by SE-user Qiaochu Yuan

This post imported from StackExchange MathOverflow at 2015-01-08 14:06 (UTC), posted by SE-user mathphysicist

+ 6 like - 0 dislike

The Riemann zeta function $\zeta(s)$ at complex $s$ has the statistical physics interpretation of a partition function at complex temperature. This has no direct physical meaning in general, but for certain models it does. A notable example is the Ising model, where the real and imaginary temperature axes are related by a transformation from an hexagonal to a triangular lattice.

Quite generally, the zeroes of the partition function in the complex plane fall on lines rather than in areas. For ferromagnetic models this is the content of the Yang-Lee theorem. It is therefore natural to expect the Riemann hypothesis to hold, although the Yang-Lee theorem does not cover this case.

An overview of the older literature on complex temperature partition functions is: "Location of zeros in the complex temperature plane: Absence of Lee-Yang theorem", W. van Saarloos and D. A Kurtze, J. Phys. A: Math. Gen. 17 (1984) 1301-1311. A more recent paper is "Complex-temperature partition function zeros of the Potts model on the honeycomb and kagome ́ lattices", H. Feldmann, R. Shrock, and S.-H. Tsai, Phys. Rev. E 57, 1335 (1998). There are many more papers, it is a quite active field of study.

A very recent paper is http://arxiv.org/pdf/1110.0942

This post imported from StackExchange MathOverflow at 2015-01-08 14:06 (UTC), posted by SE-user Carlo Beenakker
answered Aug 22, 2011 by (180 points)

 Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead. To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL. Please consult the FAQ for as to how to format your post. This is the answer box; if you want to write a comment instead, please use the 'add comment' button. Live preview (may slow down editor)   Preview Your name to display (optional): Email me at this address if my answer is selected or commented on: Privacy: Your email address will only be used for sending these notifications. Anti-spam verification: If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:p$\hbar$ysicsOv$\varnothing$rflowThen drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds). To avoid this verification in future, please log in or register.