Time Asymmetries

Many of the most fundamental laws of physics are time-reversal symmetric (T-symmetric) or invariant. This means that if we reverse the direction of time, the laws would still hold. In other words, the physics described by the law would look equally plausible going forward or backward in time. Time reversal refers to the mathematical operation of flipping the direction of time in equations of motion. Many fundamental laws of physics, such as Newtonian mechanics, electromagnetism, and quantum mechanics, are symmetric under time reversal (T-symmetry).

If the fundamental laws are largely T-symmetric, why do we experience such a strong and obvious arrow of time? In our everyday experience, causes precede effects, creating a clear arrow of time. If time were truly reversed, our intuitive understanding of causality would be challenged - effects would precede causes. In this scenario, causality, the relationship between cause and effect, would be reversed, meaning that an event that normally follows another would precede it in the reversed timeline.

If the universe’s initial and final states are symmetric, as in a cyclic model, time symmetry might globally hold. The Standard Model is also time-reversal symmetric in the absence of specific interactions. CPT symmetry (combining charge conjugation, parity inversion, and time reversal) is always conserved, so T violations must be accompanied by CP violations (as observed in weak interactions). Local interactions, as in specific processes such as weak force decays, in which entropy increases in isolated systems, break T-symmetry. The overall structure and evolution of the universe, including the propagation of light across spacetime, the worldlines of particles may preserve time symmetry. For example, Wheeler-Feynman Absorber Theory proposes that electromagnetic radiation involves both advanced (future-directed) and retarded (past-directed) waves, canceling out asymmetries in isolated systems. Some proposals, such as transactional interpretation, use advanced-retarded waves to restore symmetry, but the collapse remains ad hoc in many frameworks.

The Second Law of Thermodynamics introduces an arrow of time due to increasing entropy. This asymmetry arises not from the laws themselves but from boundary conditions such as the universe’s low-entropy early state. Locally, entropy can fluctuate, but globally, it trends upward. The increase of entropy over time provides a macroscopic arrow of time, which is deeply tied to our perception of irreversibility. The weak nuclear force also violates CP-symmetry (charge-parity conjugation), which, via CPT invariance, implies T-symmetry violations. For example, certain kaon and B-meson decays exhibit time-asymmetric behavior. Circularly polarized light is another example of time asymmetry. When time is reversed, right-circularly polarized light becomes left-circularly polarized and vice versa, revealing a fundamental asymmetry.

Time symmetry is interrupted by wavefunction collapse, creating a privileged "now." Collapse interpretations introduce time asymmetry. The wavefunction collapse upon measurement breaks time symmetry. The wavefunction collapse is not fully time-symmetric. In collapse interpretations (Copenhagen interpretation), the measurement causes the wave function to collapse into one of its possible eigenstates. This process is typically treated as irreversible and asymmetric in time. After measurement, the wavefunction collapses, and information about the pre-measurement superposition is lost. Time symmetry is a foundational principle in physics, but its manifestation depends on scale and context. Symmetry is globally preserved in the universe’s overall structure and light propagation, as Wheeler-Feynman absorber theory proposed. Yet, asymmetries locally arise from boundary conditions such as entropy, specific interactions (weak force), and statistical behavior. The laws are symmetric, but the universe’s history and initial conditions give rise to the perceived arrow of time. This duality between symmetry in laws and asymmetry in phenomena is a key to perceiving time as it is.

Simple symmetries in EPR-correlated decays of kaon and B-meson pairs with CP violation
https://www.sciencedirect.com/science/article/abs/pii/0370269389910976

Circular polarization of light scattered from solid, rough surfaces is a double reflection mechanism, where the first reflection from a crystal grain leads to linear polarization of the incident light, which subsequently hits a neighboring grain.
https://www.sciencedirect.com/topics/engineering/circular-polarization