Abstract. This article is preface to the SIGMA special issue "Tensor Models, Formalism and Applications", http://www.emis.de/journals/SIGMA/Tensor_Models.html. The issue is a collection of eight excellent, up to date reviews on random tensor models. The reviews combine pedagogical introductions meant for a general audience with presentations of the most recent developments in the field. This preface aims to give a condensed panoramic overview of random tensors as the natural generalization of random matrices to higher dimensions.
Why random tensors?General relativity, or classical gravity, is a theory of the ambient space-time geometry. The electroweak and strong interactions, that is the other three fundamental forces in nature, are described by perturbatively renormalizable quantum field theory [68,72,73,74,124,132,142,145] living on this geometric background. The main lesson of general relativity is that the ambient geometry is dynamical, and the main lesson of quantum field theory is that all the dynamical fields must be quantized. Thus, classical gravity predicts its own demise: a more fundamental theory, "quantum gravity", must come into play at some high energy scale.While classical general relativity is a field theory, it cannot be quantized the same way the standard model is: general relativity is not perturbatively renormalizable [70,143]. The lack of perturbative renormalizability of general relativity is a clear indicator that "quantum gravity" is quite different from the classical theory of gravity. This is why minimalistic approaches should be taken with a grain of salt: gravity is weakly coupled in the infrared, hence strongly coupled in the ultraviolet. This suggest that the fundamental degrees of freedom of quantum gravity are quite different from the geometric degrees of freedom perceived by low energy observers. It is far more likely that the geometric infrared degrees of freedom are just bound states of the genuine quantum gravity ultraviolet degrees of freedom.Over the years several candidate quantum gravity theories have been developed, most notably string theory. While much remains to be learned about the elusive fundamental theory of quantum gravity, one thing is certain: whatever this theory may be, it must make sense of an expression like: