Change shows time {time, physics}. Time orders changes.
direction
Time flows forward, not backward. Time changes are never symmetric. Locally, time changes can be almost symmetric if no change happens.
isosynchrony
Because universe is homogeneous, time flows almost the same everywhere.
physical laws
Physical laws are time-symmetric now, except for neutral kaon decay. Physical laws are always parity-charge-time symmetric. Reasoning works the same in both time directions.
imaginary time
Space-time has real time, which has direction. Space-time can have imaginary time {imaginary time}, which has no direction. If time has imaginary-number component, time is complex number and can have more than one dimension. Real-number time always increases, but imaginary-number time can be decreasing or increasing, just like spatial dimension. Real-number time is always positive, but time measured by imaginary numbers can be negative or positive. Having imaginary-number time dimension does not change physical laws. Having imaginary-number time dimension allows time to stand still. It also allows space not to have singularities.
origin
How did time arise? What causes number of time dimensions? Perhaps, time and space result from object interactions. Perhaps, motion necessity and nature create and require one long-range time dimension. More than one time dimension provides too many possibilities and cannot be stable. Less than one time dimension causes immobility. Perhaps, space and time result from induction.
origin: symmetry
Perhaps, the only allowable or most probable universes are asymmetric in time, though physical laws are symmetric in time. Perhaps, only asymmetric universes support life or intelligent life.
Spatial dimension can contain time {intrinsic time}.
Objects with no forces move at uniform speed, and observers can compare motions to other-object motions using object clocks {inertial clock}.
Events happen during times {duration}. Displacement can be over different paths, so distance between two points can differ. Similarly, time interval can be over different paths, so interval between two time points can differ. Perhaps, different paths solve paradoxes of time in quantum mechanics and relativity.
Moments {moment, time} cannot move in time and so cannot change, just like position cannot change. The present moment does not fade into the past or become the future. Just like all space exists everywhere, all time exists always, and all space exists always and all time exists everywhere. Time does not flow.
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}.
Outline of Knowledge Database Home Page
Description of Outline of Knowledge Database
Date Modified: 2022.0225