"V. Comments
1. Spontaneous formation of a time crystal represents the spontaneous emergence of a clock from a time-invariant dynamical system.
2. It is interesting to speculate that a (considerably) more elaborate quantum-mechanical system, whose states could be interpreted as collections of qubits, might be engineered to traverse, in its ground configuration, a programmed landscape of structured states in Hilbert space over time.
3. In general, fields or particles in the presence of a time crystal background will be subject to energy-changing processes, analogous to momentum-changing Umklapp processes of ordinary crystals. In either case the apparent non-conservation is in reality a transfer to the background. In our earlier model, O(1/N) corrections to the background motion arise.
4. The usual range of questions that arise in connection with any spontaneous ordering, including the nature of transitions into or out of the order at finite temperature, critical dimensionality, defects and solitons, and low-energy phenomenology, all pose themselves for time crystallization. In addition, there are interesting issues around the classification of space-time periodic orderings (roughly speaking, four dimensional crystals).
5. Semi-macroscopic oscillatory phenomena related in spirit to time crystallization are familiar in the a.c. Josephson effect. In that context, however, a voltage difference must be sustained externally."
to be continued.