Arrow of time: Information-based systems have a preference for the order of events

[C C]

https://medium.com/the-infinite-universe...fc60101434

EXCERPTS (Tim Anderson): . . . The reason why things make sense in a particular order from cause to effect has to do with the 2nd law of thermodynamics which says that entropy, the level of disorder in a system, must always increase. Thus, a cause, it seems, must have lower entropy than its effect. Suppose I were to drop the mug of tea on the floor. The mug smashes into pieces. The tea sprays out over the floor. The entropy of the mug and the tea has increased by being allowed to smash and spill. One must be the cause of the other.

Quite a few solutions have been proposed to the dilemma:

• The universe simply began in a low entropy state. This is ad hoc and hardly explains why entropy increase is always observed.
  
• The expansion of the universe forces the thermodynamic arrow of time in one direction. Why this should be so is unknown, since the thermodynamic arrow applies to all systems no matter how small.
  
• Physical laws are incomplete and should reflect irreversible processes, things that are not time symmetric. Current physical laws depend on this symmetry, so it would be a major breakthrough to find that time symmetry is not respected (this would be called a Charge-Parity-Time or CPT violation and would be as impactful as showing that Lorentz symmetry is violated).
  
• Quantum decoherence — this interaction of pure quantum states (like isolated particles) with macroscopic objects — causes the arrow of time. To me this is just moving the question from entropy to decoherence. Why decohere in one direction and not the other when quantum mechanics is time reversible?

The most powerful resolution to the problem, however, focuses not on a defect of physical law but on us, or more properly, any information-based system. This is the information or memory-based arrow of time resolution which states that the information content of a system, i.e., its memory increases with increasing entropy. Thus, all information storage systems experience time as flowing towards increasing entropy despite time being reversible. Like the Silence from Doctor Who, any processes that evolve towards lower entropy are not remembered.

In 1950 Erwin Schroedinger, one of the founders of quantum mechanics, described this point of view using statistical mechanics. Suppose you have a system in non-equilibrium, such as a crystal that has gained energy and is in the process of melting into a liquid. Now, before it has reached an equilibrium, you divide it into four isolated pieces.

Schroedinger showed that each of these systems will pick a time direction and evolve to equilibrium in that direction. The evolution of the closed and isolated system will define a direction of time for itself. Therefore, the time t of Einstein, Newton, and even Boltzmann is different from the phenomenological, that is, observed, arrow of time which is t or -t. Unlike physical time, phenomenological time, the time we experience, is an entropy gradient, a path of increase that is defined through spacetime.

The information theory of time combines the entropy gradient arrow of Schroedinger with the information theory of [Claude] Shannon. [...] in the process of reversing entropy, a system must remove any correlations with its having happened. No correlations, no information, no memory. In Schroedinger’s example, therefore, those of the four closed systems that run in reverse time cannot be studied by physics because it is as if their evolution did not happen at all. So rather than seeing the systems as if they were running in reverse, we would not be aware of them. Our memory and the states of any other apparatus we used to study them would disappear.

Indeed, closed systems can run in reverse time all the “time” in our universe but we have no way of detecting them because they abscond with their information content. It would be like trying to see the future. An additional, and more strange conclusion, is that we would be able to “remember” those processes in the past and lose those memories in the future. That is, a process reversed from our own phenomenological direction removes information content (quantum correlations) about its existence, which suggests that in the past that information existed and decreased. Could this mean that we can remember the future? Well, yes and no, as macroscopic beings we are highly correlated with one another and one of the requirements for systems to be moving in opposite phenomenological directions is that they have to be almost perfectly isolated from one another... (MORE - details)

[Syne]

Quite a few solutions have been proposed to the dilemma:

• The expansion of the universe forces the thermodynamic arrow of time in one direction. Why this should be so is unknown, since the thermodynamic arrow applies to all systems no matter how small.
  
• Physical laws are incomplete and should reflect irreversible processes, things that are not time symmetric. Current physical laws depend on this symmetry, so it would be a major breakthrough to find that time symmetry is not respected (this would be called a Charge-Parity-Time or CPT violation and would be as impactful as showing that Lorentz symmetry is violated).
  

I really think the underlying physics of the metric expansion of space is fundamentally a time symmetry violation. But then, I also think that explains quantum weirdness.