Entropy? Honest!

Here we deconstruct, and then in a reasoned way reconstruct, the concept of "entropy of a system," paying particular attention to where the randomness may be coming from. We start with the core concept of entropy as a COUNT associated with a DESCRIPTION; this count (traditionally expressed in logarithmic form for a number of good reasons) is in essence the number of possibilities---specific instances or "scenarios," that MATCH that description. Very natural (and virtually inescapable) generalizations of the idea of description are the probability distribution and of its quantum mechanical counterpart, the density operator. We track the process of dynamically updating entropy as a system evolves. Three factors may cause entropy to change: (1) the system's INTERNAL DYNAMICS; (2) unsolicited EXTERNAL INFLUENCES on it; and (3) the approximations one has to make when one tries to predict the system's future state. The latter task is usually hampered by hard-to-quantify aspects of the original description, limited data storage and processing resource, and possibly algorithmic inadequacy. Factors 2 and 3 introduce randomness into one's predictions and accordingly degrade them. When forecasting, as long as the entropy bookkeping is conducted in an HONEST fashion, this degradation will ALWAYS lead to an entropy increase. To clarify the above point we introduce the notion of HONEST ENTROPY, which coalesces much of what is of course already done, often tacitly, in responsible entropy-bookkeping practice. This notion, we believe, will help to fill an expressivity gap in scientific discourse. With its help we shall prove that ANY dynamical system---not just our physical universe---strictly obeys Clausius's original formulation of the second law of thermodynamics IF AND ONLY IF it is invertible. Thus this law is a TAUTOLOGICAL PROPERTY of invertible systems!Comment: 27 pages, 11 figures. Published in the journal "Entropy" in June 2016. Abstracts from referee's reports quoted right after the abstrac

Leopardi “Everything Is Evil”

Giacomo Leopardi, a major Italian poet of the nineteenth century, was also an expert in evil to whom Schopenhauer referred as a “spiritual brother.” Leopardi wrote: “Everything is evil. That is to say, everything that is, is evil; that each thing exists is an evil; each thing exists only for an evil end; existence is an evil.” These and other thoughts are collected in the Zibaldone, a massive collage of heterogeneous writings published posthumously. Leopardi’s pessimism assumes a polished form in his literary writings, such as Dialogue between Nature and an Islander (1824)—an invective against nature and the suffering of creatures within it. In his last lyric, Broom, or the flower of the desert (1836), Leopardi points to the redeeming power of poetry and to human solidarity as placing at least temporary limits on the scope of evil

When--and how--can a cellular automaton be rewritten as a lattice gas?

Both cellular automata (CA) and lattice-gas automata (LG) provide finite algorithmic presentations for certain classes of infinite dynamical systems studied by symbolic dynamics; it is customary to use the term `cellular automaton' or `lattice gas' for the dynamic system itself as well as for its presentation. The two kinds of presentation share many traits but also display profound differences on issues ranging from decidability to modeling convenience and physical implementability. Following a conjecture by Toffoli and Margolus, it had been proved by Kari (and by Durand--Lose for more than two dimensions) that any invertible CA can be rewritten as an LG (with a possibly much more complex ``unit cell''). But until now it was not known whether this is possible in general for noninvertible CA--which comprise ``almost all'' CA and represent the bulk of examples in theory and applications. Even circumstantial evidence--whether in favor or against--was lacking. Here, for noninvertible CA, (a) we prove that an LG presentation is out of the question for the vanishingly small class of surjective ones. We then turn our attention to all the rest--noninvertible and nonsurjective--which comprise all the typical ones, including Conway's `Game of Life'. For these (b) we prove by explicit construction that all the one-dimensional ones are representable as LG, and (c) we present and motivate the conjecture that this result extends to any number of dimensions. The tradeoff between dissipation rate and structural complexity implied by the above results have compelling implications for the thermodynamics of computation at a microscopic scale.Comment: 16 page

Multivariable signatures, genus bounds and 0.50.5-solvable cobordisms

We refine prior bounds on how the multivariable signature and the nullity of a link change under link cobordisms. The formula generalizes a series of results about the 4-genus having their origins in the Murasugi-Tristram inequality, and at the same time extends previously known results about concordance invariance of the signature to a bigger set of allowed variables. Finally, we show that the multivariable signature and nullity are also invariant under $0.5$-solvable cobordism.Comment: 41 pages, 3 figure