How do Molecules Behave?
On molecular level, so-called deterministic chaos can be observed. Any minute perturbation of a molecule leads to divergent behavior of that molecule, which is a so-called Lyapunov instability. As a consequence, it is practically impossible to predict the behavior of a system with sufficient accuracy over some intermediate time interval, because the starting conditions of the system would need to be defined with an accuracy, which quickly increases as the prediction time is increased.
If the interactions between particles are included in the considerations, the situation gets considerably worse. It turns out that interactions even from the farthest particles will influence the behavior of an observed particle after at most 33ps (33 times a millionth of a millionth of a second). The effect is complete randomization after at most that short time. Also, the horizon that needs to be accounted for includes all particles within the observable universe and their behavior since the big bang. Thus, there exists a highly interlinked and quickly reacting network of interactions between all particles in the universe. On molecular level the behavior can thus only be regarded as unpredictable and random.
As a secondary consideration, the direction of time can be regarded. While most physical laws are invariant to the inversion of the direction of time, the initial conditions are not. In forward direction of time, the initial conditions can be chosen arbitrarily. In backward direction of time, as soon as more than 33ps shall be observed, they would need to fulfill the condition that all interactions develop backward in time properly and consistently until the big bang. The direction of time thus does not relate to the physical laws themselves but the definition of the initial conditions. The question on the direction of time of course only relates to our description of reality and not necessarily to reality itself.
These considerations assume a fully deterministic world view, which means that with the exact physical laws, a given starting situation will lead to an unavoidable and unique future development, which is solely defined by the physical laws and the initial conditions.
The discussion presented here has largely been previously published as:
Pfennig, A.: On the strong influence of molecular interactions over large distances.
Eur. Phys. J. D (2018) 72: 45. https://doi.org/10.1140/epjd/e2017-80293-4.