  Non-Hermitian effects have emerged as a new paradigm for the manipulation of
phases of matter that profoundly changes our understanding of non-equilibrium
systems, introducing novel concepts such as exceptional points and spectral
topology, as well as exotic phenomena such as non-Hermitian skin effects
(NHSEs). Most existing studies, however, focus on non-Hermitian eigenstates,
whereas dynamic properties of non-Hermitian systems have been discussed only
very recently, predicting unexpected phenomena such as wave self-healing,
chiral Zener tunneling, and the dynamic NHSEs that are not yet confirmed in
experiments. Here, we report the first experimental observation of rich
non-Hermitian skin dynamics using tunable one-dimensional nonreciprocal
double-chain mechanical systems with glide-time symmetry. Remarkably, dynamic
NHSEs are observed with various dynamic behaviors in different dynamic phases,
revealing the intriguing nature of these phases that can be understood via the
generalized Brillouin zone and the related concepts. Moreover, the observed
tunable non-Hermitian skin dynamics and amplifications, the bulk unidirectional
wave propagation, and the boundary wave trapping provide promising ways to
guide, trap, and amplify waves in a controllable and robust way. Our findings
unveil the fundamental aspects and open a new pathway toward non-Hermitian
dynamics, which will fertilize the study of non-equilibrium phases of matter
and give rise to novel applications in information processing.
