Time always flows forward, not backward {asymmetry, time} {time, asymmetry}.
agency
People can deliberately perform previous action to alter later event {agency, time}. By definition, agency is asymmetric in time. Agents cannot know everything about the past but only about accessible past. In particular, limited access to past can allow backward causation and backward dependence.
antiparticle
Antiparticles travel backward in time.
collisions
Particles collide and spread out. Entropy increases. Perhaps, time relates to spreading caused by collisions. One particle has no time, because time is relative.
behavior correlation
After two objects interact, their activities correlate at all future times. Before two objects interact, do object actions correlate or not? In quantum systems, correlations are not observable, but non-correlations are also not observable. In classical systems, correlations are not observable. Classical case does not necessarily derive from quantum case.
dependence
Later events depend on previous events. Perhaps, physical-law asymmetry can mediate dependent temporal asymmetry {third arrow strategy, time}. However, no physical-law asymmetry mediates temporal asymmetry.
dissipative structures
Larger-system subsystems {dissipative structure} can reduce entropy, if energy is available and systems use only their own processes. Perhaps, time is only about whole systems.
dark energy
In the past, all matter and energy distributed evenly, though gravity makes masses group together. Perhaps, space has repulsive property. How can universe contract to similar state, or how can time-reversed processes happen?
flow
Time is not flow but dimension, though dimension does have direction. Time is relative with space. Perhaps, time has more than one dimension, and time can take different paths.
phase transition
Universe phase transitions are about symmetry changes. All physical laws reflect symmetries. Time symmetry makes energy conservation.
rotating universe
Rotation drags light and space-time and so allows travel into the past. However, it is unlikely that universe rotates.
Time has direction, preferred series, orientation, and order along dimension, as does order in space. Events move forward from past to present to future {direction, time} {arrow of time}| {time's arrow}. The present contains records of the past. Time arrows are thermodynamic, electromagnetic, cosmological, or psychological.
Advanced causation cannot happen, because a later cause can happen when an earlier effect does not happen, and an earlier effect can happen when a later cause does not happen {bilking argument}|.
Radiation {coherent radiation}| travels outward from source, but coherent radiation does not travel inward to source, macroscopically. At classical levels, emitters add but absorbers cancel. Why is there radiation gradient, radiation flow, with respect to time? At quantum level, this condition does not necessarily hold. Perhaps, quantum-level emitters and absorbers can emit and receive coherent radiation, or coherence concept does not apply. Why are there coherent-radiation sources?
When isolated systems change state, they are more likely to change to state with higher-probability energy distribution. Statistically, motions become more random, and objects become more evenly dispersed {time, entropy} {entropy, time}. In isolated systems, entropy can decrease only temporarily and locally.
questions
Why was entropy low in the past? If time reversed, how can entropy decrease toward the past? Why is there entropy gradient, entropy flow, with time?
symmetry and heat
Higher heat means more symmetry. Heat release makes less symmetry.
curvature
Smooth curvature has less entropy than jagged curvature.
gravity
If no gravity, entropy is proportional to volume. If gravity, entropy is proportional to surface area. Perhaps, surface has no gravity, and gravity adds extra dimension.
addition
Entropy is an extensive quantity, because it is arrangement-number logarithm. If two systems merge, arrangement number is arrangement-number product, and total entropy is entropy sum.
low entropy in past
System is more likely to have higher entropy in the future. System is more likely to have had lower entropy in the past. If system had higher entropy in the past and low entropy now, intermediate steps were jumps down to lower entropy, which are unlikely. Universe entropy was lowest at first.
entropy change with time
Perhaps, at universe origin, everything was evenly distributed, with only one particle type, with lowest entropy. During inflation, there was negative gravity, making more even dispersal, with higher entropy, and less clumpiness, with low entropy. When inflation ended, many particles appeared, making more entropy. Particles spread through space, making lower entropy. Primordial gas had low entropy, and then gravity decreased entropy as it clumped matter, but overall entropy increased because potential energy changed into heat kinetic energy. Sun original gas cloud had medium-low entropy and temperature, and then Sun had higher entropy and temperature. Sunlight has medium entropy, and then heat has higher entropy. Food has medium-high entropy, and then waste has higher entropy.
black hole
Black holes have maximum entropy density: Boltzmann constant times surface area in Planck units divided by 4. Surface unit {Planck square} has one unit of entropy flux.
Two spatially separated correlated events can both be correlated with third event {fork asymmetry}.
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Date Modified: 2022.0225