3. Connected time crystals
From now on, the word “phase” will be used in two different senses: as a state of existence (such as a topological phase, vacuum phase, phase transition) and as a temporary phase, i.e., the fraction of a cycle in a periodic process (such as a phase difference, phase shift, phase entrainment). Usually, the meaning is clear from the context; otherwise, it will be provided.
Weakly coupled oscillators evolve asynchronously. They have different periods and disconnected phases. A phase shift imposed on one of them does not change the dynamics of the others. This gauge freedom is lost when the oscillators interact more strongly and become synchronized. Synchronization is a phase transition that can take various forms [
16,
17,
18]. Below, we will provide two examples: phase entrainment and amplitude quenching.
Phase entrainment (also known as phase locking) is a ubiquitous phenomenon that can occur between oscillators of the same type, as well as between oscillators that produce different waveforms (regular or chaotic), have different designs, and even use different principles of operation. Phase locking transforms the coupled oscillators into a coherent network, where the phases of the oscillators are tied to each other, and the oscillator phase differences emerge as an order parameter. The network -values form a field on the network, which we call the -field.
Phase-locked oscillators also lose frequency independence. After synchronization, their continuous spectrum becomes discrete, often consisting of only one line. In what follows, for simplicity, we will assume that the latter occurs by default.
Oscillator networks can be viewed as graphs, where oscillators occupy nodes, and bonds are represented by the links (edges) between them. The graph links are “weighted” by the phase difference
(in the process of link evolution) or
(after relaxation) and can themselves be considered self-organized systems. We will model their dynamics using the Adler equation [
19], which we have adapted [
15] to the formalism of iterated maps:
Here, is the oscillator cycle duration after synchronization, and are the phase differences between the oscillators at times and , separated by one cycle (time interval ); is the difference between the oscillator frequencies before synchronization (natural frequencies), and is the engagement coefficient, which characterizes the coupling strength between the oscillators and varies from (uncoupled) to (fully engaged).
Examples of phase-difference evolution trajectories, calculated using the iterated map (7), are shown in
Figure 1(a). They illustrate the quantum property of dissipative systems that we have mentioned in the previous section: regardless of the initial phase differences
, which occupy a continuous
interval, they asymptotically converge to one of the fixed values
, which are determined by the parameters ∆ω and κ. The trajectories asymptotically become exponential with time (
Figure 1(b). Accordingly, for link characterization, we use parameters similar to the oscillator parameters
,
,
and
, given by formulas (1)-(4), as well as the statistical distribution (5).
Examples of
plots simulated using iterated map (7) are shown in
Figure 2. All ground states (minima) correspond to oscillators operating in unison,
.
The ground states are the most stable. Links tend to gather near those, while environmental disturbances move them out. In addition, they repeatedly return to the ground states. This trend can be formalized by introducing elastic (restoring) forces
, which we define as
Elastic forces form the -field on the network. This field can be considered gauge if we pretend that the network has local gauge freedom and that the field arises as a network response to arbitrary changes in phase differences .
Under the action of the elastic force, the link changes the value of
not instantly but with some relaxation delay, which plays the role of inertia. As a result, elastic forces cause wave-like oscillations in the medium. The amplitudes of these waves are the phase differences
, which should not be confused with the wave’s own phase. This type of excitation is known as phase waves [
20]. We call them
-waves.
Near the ground states, the dependences
can be approximated by quadratic parabolas (an example is shown by the green dotted line at the bottom of the
curve in
Figure 2):
where
is the ground state value and
is a parameter, we call the stiffness factor. For weak engagement, when
,
. For stronger engagements,
increases faster than
, and for full engagement, when
,
.
Accordingly, with a small deviation from the unison, the elastic forces are proportional to the phase difference
:
and
-waves are harmonic. Equation (10) plays the role of Hooke’s law for oscillator links. If the linear approximation (10) is violated, other types of waves can arise, including solitary waves [
21].
The nonlinear behavior of the elastic forces is illustrated by the graphs in
Figure 3 in normalized coordinates. With the departure from the ground state, Hooke’s growth slows down, then it is replaced by saturation and (at sufficiently large
) a falling branch (
Figure 3). As
increases, the positions of the maxima of the curves approach the ground state
, and at
, the ascending segment and saturation disappear, and only the descending branch remains. If we recall the parallel between the parameter
and energy, then the softening of the force with increasing excitation, described by descending branches, resembles the behavior of color forces in chromodynamics, known as asymptotic freedom.
Dissipative systems are irreversible in time. Due to the lack of this symmetry, the time-crystal medium does not hold Lorentz invariance: irreversible time cannot be partially transformed into reversible space and vice versa. However, this does not prevent the medium from experiencing a universal time dilation, which is as profound as its counterpart is in relativistic gravity.
In time crystal networks, time dilation occurs in areas that move away from the ground state. It is directly related to slowing down relaxation. The time constant increases from the initial, ground state value and can progress to , which manifests a complete stop of the relaxation process. Time dilation concerns all processes in the affected part of the network. Looking ahead, we will model vacuum as a time-crystal network, where elementary particles and fundamental fields consist of connected network elements; thus, when excited, they experience time dilation. Any object that includes them as its parts will inherit this property. This applies to any type of clock/chronometer, even something as small and simple as an elementary particle itself.
It would be interesting to compare the time-crystal time dilation with the gravitational time dilation. To do this, let us compare the time dilation in a time-crystal link when it is excited from the ground state
to the maximum excitation
and the slowdown of a clock moving from infinity
(ground state) to the black hole horizon
.
is the distance between the clock and the center of the black hole, and
is the Schwarzschild radius. The normalized gravitational time dilation
as a function of normalized excitation
was extracted from the Schwarzschild metric and is given by the formula
It is shown in
Figure 4 by the blue curve. The normalized time-crystal time dilation
vs. the normalized link excitation
simulated with use map (7) is shown in the same figure by the red curve.
In time-crystal networks, phase entrainment equalizes the oscillation periods of all synchronized oscillators, and the resulting period unified plays the role of a global invariant. The local relaxation time can be measured in -units and used to uniquely determine the local time metric. This is also applicable to vacuum if it is a time-crystalline medium. However, since temporal periodicity does not entail spatial periodicity, there is no object that could play the role of a standard of spatial distance. Instead, we can use spatially extended objects, such as -waves, and, borrowing a postulate from the theory of relativity, endow all -waves with the same propagation speed . Then, the product can be used as a standard of spatial distance. Its size will follow the local time changes, as it does in Minkowski spacetime. However, it should be remembered that the link between spatial and temporal metrics is not an emergent property but is introduced a priori. Fortunately, our analysis of the time-crystal media does not include spatial distances.
Synchronization occurs as a trade-off between coupled oscillators, each trying to make the other oscillators operate in the same mode as itself. Their struggle is accompanied by positive feedback: the more oscillators operate in a given mode, the greater they attract other oscillators to this mode. Let us see how this changes the probability distribution
(see also [
15]) given
.
Suppose that the probability that an oscillator is in state
increases linearly with the number of oscillators already in that state, and
is the proportionality factor. Then, the synchronization-altered probability
can be written as
where
is the probability before synchronization. After separation of variables
Using expression (5) for the probability
in equation (13), we obtain the final expression for the probability distribution:
where
is a coefficient depending on
. The distribution (14) has a form similar to that of the Bose‒Einstein distribution.
Amplitude quenching [
16] is synchronization, in which the amplitude of the synchronized oscillators is reduced to zero. The affected oscillators group in the zero-amplitude state. In fact, they remove each other from the network.
To see how amplitude quenching will affect the statistical distribution, we can repeat the previous reasoning by making the following substitutions in equations (12)-(14): to distinguish between two synchronization scenarios, use index “
” instead of index “
” and instead of the proportionality factor
use
. Then, the probability, modified by the damping of oscillations, is equal to
which leads to the final distribution in a form similar to the Fermi-Dirac distribution:
where
is a coefficient depending on
.
It is noteworthy that, in contrast to quantum mechanics, quantum-like distributions (14) and (16) were obtained without the a priori assumption that the oscillators are indistinguishable from each other. Their alikeness is emergent.