Critical Phenomena in Complex Systems

Phase transitions and associated phenomena in physical systems have arguably been one of the defining subjects of physics since the second half of the last century. Indeed, a massive body of research has been conducted on fundamental physical models, both theoretical and computational, and in experimental studies. The success of the rigorous approach of physics has to a large extent been due to ingenuous applications of physics' reductionist methodologies. Models of large systems of simple interacting agents lead to emergent complexity observed in phase transitions - yet, these systems remain (at least to a degree) tractable by the methods of physics.

The wealth and complexity of the associated phenomena have encouraged the drawing of parallels between the physics of phase transitions and the complexity observed in real-life systems across many disciplines of science. In recent decades, criticality involving a host of associated scaling phenomena has been evoked (appropriately or not) in the context of a wide range of paradigmatic complex systems, including biological systems such as that of brain dynamics, regulatory systems such as heart rate control, human behaviour, financial and ecological systems and many more. In particular, in recent decades many natural and man-made complex systems have been identified as operating at or near criticality, prompting researchers to suggest analogies between such complex systems and elementary physics phenomena.

Contrary to model physical systems, the exact mechanisms of regulation or self-tuning in these phenomena are often difficult to pinpoint and remain unknown. Regardless of the nature of control and the exact mechanism of tuning towards phase transition, there exists a host of phenomena associated with alteration of a system's dynamics undergoing phase transition. These include an increase in long-range correlations, symmetry breaking, the appearance of soft modes and hard frequencies, and flickering, all constituents of fundamental thermodynamical slowing down. Paramount, yet under-explored, is the aspect of the predictive role of such phenomena. Indeed, critical slowing down is a profound marker of the approach of a vast change in the system's dynamics. The inhomogeneity, heterogeneity and non-stationarity of the dynamics of (critical) phase transitions leading to abrupt or gradual critical regime changes in real-life systems are, however, routinely encountered, often yielding these systems intractable.

Therefore, even though such phenomena are of profound importance in the characterisation of real-world complexity, still there remains a great divide between 'physics complexity' and real-world complexity, where neither the interactions nor the interacting agents are reducible to simple entities.

Of interest to this research topic are manuscripts dealing with criticality and phase transitions in real-world complex systems. This extends to contributions on criticality of:
complex topologies, e.g. networks, in multiscale systems, hybrid systems, heterogeneous systems, adaptive systems, learning and living systems, intelligent systems;
non-stationary, evolving and emergent, adaptive and motile agent systems, involving possibly intelligent agents;
hybrid multicomponent, multidimensional, multivariate structures, possibly involving active components, energy and entropy and information processors.
Both spatial and temporal critical phenomena and phase transitions are of interest for this topic.

All flavours of work are welcome, from purely theoretical contributions to experimental, simulations and data exploration based work. Review papers, opinion pieces and hypothesis papers are equally welcome.