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Dynamic Entropy Model: A Rigorous Mathematical Framework
Mitchell McPhetridge Dynamic Entropy Model (DEM) envisions entropy as an evolving, controllable quantity in open systems
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. Below we formalize this concept with a time-dependent entropy function, dynamic probability evolution, a feedback control mechanism for entropy, and entropy optimization principles. The formulation integrates information theory (Shannon entropy), thermodynamic laws, and control theory from complexity science.
1. Time-Dependent Entropy Evolution
Definition: Let a system have states i=1,2,…,Ni=1,2,…,N with time-dependent probabilities pi(t)pi(t). We define entropy as a function of time:
H(t)=−∑i=1Npi(t) lnpi(t),H(t)=−∑i=1Npi(t)lnpi(t),
analogous to Shannon entropy but allowing pipi to evolve with interactions
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. This treats entropy as a time-varying quantity (an entropy flowrather than a static number
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).
Differential Entropy Equation: Differentiating H(t)H(t) yields an entropy balance law:
\frac{dH}{dt} ;=; -\sum_{i=1}^N \frac{dp_i}{dt},\ln p_i(t), \tag{1}
using ∑idpi/dt=0∑idpi/dt=0 (probability conservation). Equation (1) links the entropy change rate to the probability flux between states. In the absence of external control (an isolated system), this typically reproduces the Second Law of Thermodynamics: entropy is non-decreasing
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. For a closed, adiabatic system one expects dH/dt≥0dH/dt≥0 (entropy production is non-negative). This formalizes the idea that without intervention, uncertainty in an isolated system cannot spontaneously decrease
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Interpretation: A positive contribution to dH/dtdH/dt arises when probability flows from more certain states to more uncertain ones (spreading out the distribution). Negative dH/dtdH/dt (entropy decrease) requires directed probability flow into fewer states (increasing order), which cannot happen naturally without external work or information input in an isolated system. In an open system with interventions, however, dH/dtdH/dt can be influenced externally (see §3). This time-based view of entropy addresses Shannon’s static assumption by acknowledging that observations and interactions continuously reshape the entropy landscape
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Example: If pi(t)pi(t) follow equilibrium dynamics (e.g. a relaxing Markov process), entropy will rise toward a maximum at equilibrium. Conversely, if an external agent begins sorting the system’s microstates (as in Maxwell’s Demon), dHdtdtdH can become negative, indicating entropy extraction from the system.
2. Probability Distribution Evolution
To model pi(t)pi(t) rigorously, we employ stochastic dynamics. Two common formalisms are:
- Master Equation (Markov Chain): For discrete states with transition rates Wij(t)Wij(t) from state ii to jj, the Kolmogorov forward equation governs the probability flow
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. For example, a continuous-time Markov chain satisfies:\frac{dp_i(t)}{dt} ;=; \sum_{j\neq i} \Big[ W_{ji}(t),p_j(t) - W_{ij}(t),p_i(t)\Big], \tag{2}ensuring ∑ipi(t)=1∑ipi(t)=1 for all tt. This master equationdescribes an open system’s probabilistic state evolution
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. If WijWij are constant, the system tends toward a stationary distribution (often maximizing entropy under constraints). If WijWij change (e.g. due to external influences or time-varying environment), pi(t)pi(t) adapts accordingly, reflecting emergent probabilities
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- Fokker–Planck Equation: For continuous state xx, p(x,t)p(x,t) can evolve via a Fokker–Planck PDE (the continuous analog of a master equation). For instance, with drift A(x,t)A(x,t) and diffusion D(x,t)D(x,t), one has:\frac{\partial p(x,t)}{\partial t} = -\nabla\cdot!\big(A(x,t),p(x,t)\big) + \frac{1}{2}\nabla^2!\big(D(x,t),p(x,t)\big), \tag{3}which is a form of the Kolmogorov forward equation for diffusion processes
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. This describes how probability density flows and spreads in state space over time.
Both (2) and (3) are stochastic evolution equations defining pi(t)pi(t) (or p(x,t)p(x,t)) trajectories. They embody open-system dynamics: probabilities can shift due to interactions, new information, or external perturbations, as DEM requires
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Entropy’s Time Derivative (via Master Equation): Substituting (2) into the entropy rate (1):
dHdt=−∑i,j[Wji pj−Wij pi]lnpi.dtdH=−i,j∑[Wjipj−Wijpi]lnpi.
This can be rearranged and interpreted. In detailed-balance conditions (e.g. closed equilibrium), one can show dH/dt≥0dH/dt≥0 (entropy increases until equilibrium). In non-equilibrium or externally driven conditions, the sign of dH/dtdH/dtdepends on the imbalance in transitions. The term corresponding to WijpiWijpi moving probability out of state ii reduces lnpilnpi (hence tends to increase entropy), whereas WjipjWjipj moving probability into state ii tends to decrease entropy if pipi was low. Thus the entropy change results from competition between dispersing probability (raising HH) and concentrating probability (lowering HH).
Reinforcement Learning Analogy: In a learning system, the probability distribution over actions or hypotheses pi(t)pi(t)is updated with experience. For example, in an entropy-regularized reinforcement learning policy, pi(t)pi(t) might follow a deterministic update that maximizes a reward plus an entropy term
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. Such dynamics can be written as gradient flows:
dpidt=η ∂∂pi[U(p)+αH(p)],dtdpi=η∂pi∂[U(p)+αH(p)],
where U(p)U(p) is a utility (negative loss) function and αα is a weight on entropy regularization. This drives pi(t)pi(t)toward an optimal distribution, demonstrating designed evolution of probabilities – a form of entropy-aware dynamics (high entropy is encouraged for exploration)
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Conclusion (Section 2): Equation (2) or (3) can be chosen based on the system (discrete vs continuous state). These provide a time-dependent probability model underpinning DEM: entropy now is simply a functional of p(t)p(t). Crucially, pi(t)pi(t) can itself depend on observations or feedback, enabling the next component – entropy feedback control.
3. Entropy Feedback Control
Concept: An open system can regulate its entropy via feedback loops
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. This means the system’s interactions or an external controller adjust transition probabilities in response to the current state of entropy or other signals, steering the entropy’s trajectory. We formalize this using control theory:
- Let u(t)u(t) be a control input (deterministic or stochastic) that can influence the probability dynamics. For example, in the master equation (2), the transition rates may depend on uu: Wij=Wij(u,t)Wij=Wij(u,t). As a simple case, one could add a controlled drift term to (2):\frac{dp_i}{dt} = \sum_{j}\Big[ W_{ji}(t),p_j - W_{ij}(t),p_i\Big] ;+; u_i(t), \tag{4}where ui(t)ui(t) is a feedback control term that directly injects or removes probability from state ii (subject to ∑iui(t)=0∑iui(t)=0 to conserve total probability).
- The control u(t)u(t) is derived from the system’s state or entropy. For instance, a feedback law might be ui(t)=Ki(p(t))ui(t)=Ki(p(t)) for some function/policy KiKi. A simple illustrative strategy: targeting low entropy: u(t)u(t) could push probability toward a preferred state (reducing uncertainty). Conversely, to increase entropy, uumight drive the system to explore under-represented states.
Lyapunov Function Design: We treat entropy (or a related measure) as a Lyapunov function to design stable feedback. Suppose the goal is to drive the system toward a desired distribution pi∗pi∗ (which might have a different entropy H∗H∗). We can choose a Lyapunov candidate as the Kullback–Leibler (KL) divergence V(t)=DKL(p(t) ∥ p∗)=∑ipi(t)lnpi(t)pi∗V(t)=DKL(p(t)∥p∗)=∑ipi(t)lnpi∗pi(t). This vanishes iff . Its time-derivative is:
dVdt = ∑idpidt lnpipi∗.dtdV=∑idtdpilnpi∗pi.
By designing u(t)u(t) such that for some , we ensure exponential convergence p(t)→p∗p(t)→p∗ (and thus ) by Lyapunov stability theory. For example, a proportional feedback could be:
ui(t)=−λ(lnpi(t)pi∗)pi(t),ui(t)=−λ(lnpi∗pi(t))pi(t),
with . Plugging into the dynamics yields , which is non-positive and zero only at equilibrium. This ensures (and thus the entropy difference) decays over time, achieving a controlled entropy evolution. Such control schemes leverage entropy as a feedback signal to maintain or reach a desired uncertainty level.
Maxwell’s Demon as Feedback Controller: Maxwell’s Demon is a metaphorical controller that uses information about particles (observing fast vs slow molecules) to reduce entropy by selectively allowing particles to pass
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. In our framework, the “demon” measures the microstate (feedback) and then applies to preferentially transfer probability (particles) between states, effectively biasing transitions in (2) to decrease entropy. The demon’s strategy can be seen as implementing a control law that keeps high-energy molecules on one side (maintaining an improbable low-entropy distribution). In control terms, the demon uses state feedback to achieve an ordering objective in defiance of natural equilibration.
Feedback in AI and Biology: Similarly, an AI system might monitor its internal entropy (e.g. uncertainty in predictions) and trigger adjustments when entropy is too high or low. For instance, an entropy-aware AI could slow down learning (reducing stochasticity) if entropy is rising uncontrollably, or inject noise/exploration if entropy falls too low (to avoid overfitting)
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. Biological organisms maintain homeostasis by feedback – consuming energy to reduce internal entropy (keeping order) in the face of environmental uncertainty
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. All these can be modeled by suitable u(t)u(t) in the probability dynamics.
Stability Analysis: Using control theory, one can prove conditions for entropy regulation. For example, using V=H(t)V=H(t) directly as a Lyapunov function: if we desire to hold entropy below a threshold, we want to be negative whenever exceeds that threshold. A feedback law is Lyapunov-stabilizing if it makes for some . This inequality ensures exponentially decays to . In practice, directly controlling might be indirect; it’s easier to control . But conceptually, a well-chosen control policy guarantees entropy will follow a stable trajectory (bounded or convergent), implementing entropy feedback control in line with DEM’s vision of “entropy-resisting” systems
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Finally, we note information-theoretic costs: Feedback control of entropy often requires expending energy or increasing entropy elsewhere (per Maxwell’s demon arguments). While our framework treats abstractly, a complete thermodynamic analysis would include the entropy cost of measurement and control actions to obey the Second Law globally. This links to Lyapunov functions in thermodynamics (free energy potentials) which ensure that while a subsystem’s entropy can be lowered by work/feedback, the total entropy including controller does not violate fundamental laws.
4. Entropy Engineering and Optimization
Concept: Entropy engineering refers to deliberately shaping and manipulating entropy flows in a system
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. This is achieved by optimizing system parameters or control strategies to achieve desired entropy outcomes (either minimizing entropy for order or maximizing it for exploration/diversity). We introduce optimization principles to guide this process:
- Optimization Objective: Formulate a cost functional that reflects the entropy goal. For example:
- Entropy Minimization: J=H(T)J=H(T) (entropy at final time ) or J=∫0TH(t) dtJ=∫0TH(t)dt. We seek controls u(t)u(t) minimizing subject to the probability dynamics (2) or (3). This yields an optimal control problem: minimize entropy accumulation over time.
- Entropy Maximization: Alternatively, maximize or include in the cost to promote uncertainty/spread. This is useful in, say, randomized algorithms or ensuring fair exploration in AI.
- Constraints: The optimization respects system equations and possibly resource limits. In a thermodynamic context, lowering entropy might require energy input; in AI, increasing entropy (randomness) might trade off with reward maximization.
Euler-Lagrange/Pontryagin Formulation: One can apply Pontryagin’s Maximum Principle for the control system with state . Define a Hamiltonian with co-state (Lagrange multiplier) for each state probability. For instance, if minimizing final entropy , the terminal condition is . The optimal control must satisfy stationarity conditions of the Hamiltonian. This yields feedback laws in terms of and . Solving these equations (generally nonlinear) gives the entropy-optimal strategy.
Example – AI Model: In machine learning, one can add an entropy regularization term to the loss function to tune entropy. For instance, in reinforcement learning, the soft actor-critic (SAC) algorithm maximizes expected reward plus an entropy bonus
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. This can be seen as solving an entropy-engineering problem: find the policy that maximizes . The solution uses stochastic gradient ascent on this objective, yielding a policy that deliberately maintains higher entropy (more randomness) for better exploration
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. This is entropy maximization in an AI system, improving adaptability.
Conversely, an AI system prone to chaotic behavior might include an entropy penalty to keep its decisions more deterministic, effectively minimizing entropy to reduce uncertainty in outcomes. Both cases are optimization-driven entropy manipulation. By adjusting (the weight on entropy) one can smoothly tune the system from greedy (low entropy) to exploratory (high entropy) regimes.
Example – Quantum System: In quantum control, one might want to cool a qubit system to a pure state (minimal von Neumann entropy). This can be framed as an optimal control problem: apply control fields to minimize the entropy of the density matrix at time . Researchers have proposed methods to steer a quantum system’s entropy to a target valueby time , using coherent (unitary) and incoherent (environmental) controls
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. The objective might be to hit a desired entropy , or simply to cool the system as much as possible. Constraints come from quantum dynamics (e.g. a Lindblad or Schrödinger equation). Solutions involve sophisticated algorithms (e.g. gradient-based pulse shaping or genetic algorithms
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General Optimization Principles: Whether in AI or physics, entropy engineering often boils down to:
- Define a performance index involving entropy (to minimize or maximize).
- Compute gradients of this index with respect to control variables or system parameters.
- Iteratively adjust controls/parameters to extremize the index (e.g. gradient descent or other optimizers).
- Ensure constraints are satisfied, often by augmented Lagrangian or projected methods (since probabilities must remain normalized and , controls might be bounded, etc.).
This approach aligns with how one might optimize an economic market’s policy to reduce volatility (where volatility can be seen as entropy of price distribution), or how one designs a feedback controller to reduce disorder in a power grid.
Entropy-Aware Interventions: McPhetridge’s vision suggests applying such principles across domains
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. Potential applications include:
- AI Bias Reduction: Interpret bias as emerging from low-entropy training data (over-concentrated in some features). By maximizing entropy of the data distribution (e.g. via data augmentation or re-sampling to a more uniform distribution), one can reduce bias. This is an entropy-increasing intervention to promote fairness
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- Robotics & Self-Organization: Robots can plan actions to maximize information gain (equivalently maximize entropy of their belief to explore) or to minimize uncertainty in their state estimation (minimize entropy). Both are solved by optimizing an entropy-based objective in the robot’s decision-making algorithm.
- Thermodynamic Computing: One could design computing elements that function by pumping entropy in and out. For instance, logically reversible computing minimizes entropy production; implementing such systems requires controlling entropy flow at a fundamental level via circuit design optimization.
- Complexity Management: In ecosystems or economies, interventions (like policies or feedback loops) can be seen as attempts to regulate the system’s entropy. A stable ecosystem maintains diversity (high entropy) up to a point, but not chaos; if an invasive species lowers diversity, managers may intervene to raise entropy (e.g. reintroduce predators) to restore balance. These actions can be optimized for effect and efficiency.
Theoretical Alignment: This optimization framework is consistent with information theory (e.g. maximum entropy principles
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), thermodynamics (engineering entropy flows with energy/work constraints), and complex systems theory (controlling emergent order/disorder). It treats entropy as a quantity that can be designed and controlled , much like energy or mass flows, heralding a shift from viewing entropy as merely an outcome to treating it as a control variable in complex systems
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Conclusion
The rigorous framework above extends Shannon’s entropy to dynamic, open-system contexts, providing: (1) a time-dependent entropy measure H(t)H(t) with a governing differential equation, (2) an evolution model for probabilities pi(t)pi(t) via stochastic dynamics (master or Fokker–Planck equations), (3) a feedback control paradigm to influence entropy in real-time (using control theory and Lyapunov stability to maintain desired entropy levels), and (4) optimization principles for entropy engineering to achieve entropy objectives in various applications. This aligns with McPhetridge’s post-Shannonian entropy vision
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and grounds it in mathematical theory.
Applications: The DEM framework can inform quantum computing(managing decoherence and information content), AI system design(entropy-regularized learning for adaptability
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), autonomous systems (actively gathering information or preserving order), and complex adaptive systems (ecosystem or economic interventions). By treating entropy as an evolving, controllable entity, we gain a powerful lens to analyze and design systems that harness uncertainty and order in tandem
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In summary, entropy in an open system is elevated from a static metric to a dynamical state variable with its own evolution equation, control inputs, and optimization criteria. This provides a foundation for future research in entropy-aware algorithms and thermodynamic control
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, bridging information theory and control theory in the study of complex, adaptive systems.
Sources:
- Shannon entropy (static) vs dynamic entropy concept
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- Time-dependent entropy formula and open-system interpretation
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mdpi.com
- Master equation for probability evolution (Markov processes)
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; Fokker–Planck (continuous case)
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- Entropy feedback mechanisms (Maxwell’s Demon, homeostasis, AI bias correction)
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- Research on entropy control in classical and quantum systems
pmc.ncbi.nlm.nih.gov
mdpi.com
- Entropy in AI learning (entropy regularization for exploration, adaptability)
spinningup.openai.com
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- McPhetridge’s DEM proposal for evolving probabilities, feedback-controlled entropy, and entropy engineering
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.