In the natural sciences, arrow of time is a term coined in 1927 by British astronomer Arthur Eddington. He used it to distinguish a direction of time on a four-dimensional relativistic map of the world, which, according to him, can be determined by a study of organizations of atoms, molecules, and bodies.
Physical processes at the microscopic level are believed to be either entirely or mostly time symmetric, meaning the theoretical statements that describe them remain true if the direction of time is reversed However, the opposite is true in the macroscopic world in that there is an obvious direction (or flow) of time. In others words process in our macroscopic environment are observed to be asymmetric with respect to the direction of time.
Entropy appears to be the only quantity in the macroscopic world that “picks” a particular direction. Therefore, it is used by science to define the arrow of time. As one goes “forward” in time, the second law of thermodynamics says, the entropy or disorder of an isolated system will increase when no extra energy is consumed. Hence, from one perspective, entropy measurement is thought of as a kind of marker that determines the direction of time.
However, one cannot apply the concept of entropy to the microscopic world of atoms to determine it because the entropy or disorder of system composed of a signal atom does not spontaneously increase as it moves through time. This is because the entropy of an atom moving to the left, while going forward in time is identical to one moving to the right, while going backwards in time. Therefore, the one cannot use it to define a direction for time in microscopic systems because it does not quantifiably change as one changes direction in time.
This points out one of the problems with using entropy to “pick a particular” direction for time because using it does not give a universally consistent direction for it. For example, on microscopic or atomic level the entropy or energy of an isolated atom as mentioned earlier does not spontaneously increase over time therefore, it cannot be used to define its direction. However, it can be in a macroscopic system because in those systems entropy does spontaneously increase over time.
Another problem in assuming the second law of thermodynamics is responsible for determining the arrow of time is that is statistical, so it does not hold with strict universality: any system can fluctuate to a state of lower entropy. This means the direction of the arrow of time in those system where it does will be reversed.
However, one could resolve this conundrum if one defines the arrow of time, as was done in the article “Defining what time is?†Sept. 20, 2007 only in terms of the sequential ordering of the causality of events instead of the entropy of a system. This would give a consistent direction in all macroscopic multi component systems as well as single component microscopic ones. This is because the ordering of the causality of all events would always move in the same direction forward.
For example, as mentioned earlier due to the statistical nature of the second law of thermodynamics it is possible for the entropy of gas molecules in a macroscopic environment to fluctuate or a move to a more orderly instead of a more disorganized arrangement. Therefore, in a system consisting of gas molecules, the direction of time can be reversed if one uses it to define the arrow of time. However, the sequential ordering of the causality of that event would move forward with respect to all other events because one cannot reverse the causality of an event without generating a new one. Therefore, each event would have a unique ordering of causality which could only move in one direction in time.
Therefore defining time only in terms of the sequential ordering of the causality of events would a provide a consistent direction for the arrow of time in both a macro and microscopic environment because the causality of a single an atom moving to the left in both single or multiple component system would always be proceeded by the causality of that the same atom moving to the right; even though the behavior of the atom is not qualitatively different in either case. This would be true in both our physical and mathematical perceptions of time.
Therefore, defining the time only as a measure of the sequential ordering of the causality of an event, as is suggested here would provide an unambiguous definition of arrow of time in both a macro and microscopic environment.
Later Jeff
Copyright Jeffrey O’Callaghan 2010