# Algorithmic Thermodynamics

```Dear Bruno and Friends,

The paper that I have been waiting a long time for.  ;-)```
```
http://arxiv.org/abs/1010.2067

Algorithmic Thermodynamics

```
John C. Baez <http://arxiv.org/find/math-ph,math/1/au:+Baez_J/0/1/0/all/0/1>,Mike Stay <http://arxiv.org/find/math-ph,math/1/au:+Stay_M/0/1/0/all/0/1>
```(Submitted on 11 Oct 2010)

Algorithmic entropy can be seen as a special case of entropy as
studied in statistical mechanics. This viewpoint allows us to apply
many techniques developed for use in thermodynamics to the subject
of algorithmic information theory. In particular, suppose we fix a
universal prefix-free Turing machine and let X be the set of
programs that halt for this machine. Then we can regard X as a set
of 'microstates', and treat any function on X as an 'observable'.
For any collection of observables, we can study the Gibbs ensemble
that maximizes entropy subject to constraints on expected values of
these observables. We illustrate this by taking the log runtime,
length, and output of a program as observables analogous to the
energy E, volume V and number of molecules N in a container of gas.
The conjugate variables of these observables allow us to define
quantities which we call the 'algorithmic temperature' T,
'algorithmic pressure' P and algorithmic potential' mu, since they
are analogous to the temperature, pressure and chemical potential.
We derive an analogue of the fundamental thermodynamic relation dE =
T dS - P d V + mu dN, and use it to study thermodynamic cycles
analogous to those for heat engines. We also investigate the values
of T, P and mu for which the partition function converges. At some
points on the boundary of this domain of convergence, the partition
function becomes uncomputable. Indeed, at these points the partition
function itself has nontrivial algorithmic entropy.

```
Now to discuss how this is useful to define a local notion of a measure for COMP.
```
--
Onward!

Stephen

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