Microscopic reversibility: is it truly reversible?
Microscopic reversibility: is it truly reversible?
Today I had several doubts.
In contemporary physics, topology transitions are often treated as reversible at the microscopic level under Hamiltonian dynamics, while macroscopic irreversibility is discussed via Boltzmann-type arguments (e.g., the H-theorem). This is called the Loschmidt paradox. Biological systems also appear macroscopically irreversible.
(Here I use “topology transition” loosely to mean a qualitative reconfiguration of accessible state space/attractors under changing constraints.)
Q1. Why are biological systems irreversible?
Dissipation, history dependence, biological constraint asymmetry.
Q2. Hamiltonian reversibility assumes a closed system. Do “pure” closed systems exist?
Not as a literal physical reality. A “closed system” is an idealization: a system whose exchange of energy/matter/information across its boundary is negligible within a given modeling accuracy/time window. This is an idealization of the same level as asking whether there exists a substance/system in thermal equilibrium in reality.
Q3. Phonon traces depend on the presence of a medium (no phonons in vacuum).
What trace channels do not depend on that?
Photons (radiation), particle scattering (residual gas/impurities), spin-state changes, field fluctuations/vacuum noise (context-dependent), and interactions with surfaces/defects/impurities.
Q4. Then can we say the microscopic world lacks history dependence or leaves no traces?
Not in realistic settings: trace channels exist whenever the system is effectively open to environmental degrees of freedom.
Q5. Is Hamiltonian time-reversal symmetry “real” once non-equilibrium trace formation is considered?
Did it not retroactively apply an idealized closed system to a microscopic open system?
Time-reversal symmetry is a property of closed-system dynamics (full information). In practice, we model subsystems by tracing out environmental degrees of freedom or replacing them with effective terms (mean-field, friction, noise), yielding open-system behavior where reversibility becomes structurally non-implementable at the observable level.
Closing Philosophy Question.
Between statistical significance and simulation significance grounded in physically real micro-level minimal intrinsic properties (i.e., phenomena derived from the irreducible properties of the smallest units), which should we prioritize more?
