Field patterns without blow up

Abstract

Field patterns, first proposed by the authors in [Milton, Mattei. Proc R Soc A. 2017], are orderly patterns of characteristic lines that arise in specific space-time microstructures whose geometry in one spatial dimension plus time is somehow commensurate to the slope of the characteristic lines. In particular, in [Milton, Mattei. Proc R Soc A. 2017] the authors propose two examples of space-time geometries in which field patterns occur: they are two-phase microstructures in which rectangular space-time inclusions of one material are embedded in another material. After a sufficient large period of time, the field patterns in both microstructures have local periodicity both in time and space, due to the PT-symmetry of the chosen space-time geometries. This allows one to focus only on solving the problem on a discrete network and to define a suitable transfer matrix that, given the solution at a certain time, provides the solution after one time period. Many of the eigenvalues of this transfer matrix have unit norm and hence the corresponding eigenvectors correspond to propagating modes. However, there are also modes that blow up exponentially with time coupled with modes that decrease exponentially with time. The question as to whether there are space-time microstructures such that the transfer matrix only has eigenvalues on the unit circle finds answer in this paper where we see that certain space-time checkerboards have the property that all the modes are propagating modes, within a certain range of the material parameters. Interestingly, when there is no blow-up, the waves generated by an instantaneous disturbance at a point look like shocks with a wake of oscillatory waves. This sets an arrow of time in the sense that it would be extremely difficult to produce the initial conditions necessary to create the corresponding time-reversed waves.