handofg0d Posted:

WASTE OF SPACE BOTH PHYSICALLY AND VIRTUALLY

The direction of time

The problem of the direction of time arises directly from two contradictory facts. First, the fundamental physical laws are time-reversal invariant. In other words, anything that can happen moving forward through time is just as possible moving backwards in time. Or, put in another way, through the eyes of physics, there will be no distinction, in terms of possibility, between what happens in a movie if the film is run forward, or if the film is run backwards. Second, our experience of time, at the macroscopic level, is not time-reversal invariant. Glbumes fall and break all the time, but shards of glbum do not put themselves back together and fly up on tables. We have memories of the past, and none of the future. We feel we can’t change the past but can influence the future.

The causation solution

One solution to this problem takes a metaphysical view, in which the direction of time follows from an asymmetry of causation. We know more about the past because the elements of the past are causes for the effect that is our perception. We feel we can’t affect the past and can affect the future because we can’t affect the past and can affect the future.

There are two major difficulties with this view. First is the problem of distinguishing the cause from the effect in a non-arbitrary way. The use of causation in constructing a temporal ordering could easily become circular. The second problem is with not the consistency of this view, but its explanatory power. While the causation account, if successful may account for some time-asymmetric phenomena like perception and action, it does not account for many others, like the breaking glbum described above.

The thermodynamics solution

The second major family of solutions to this problem, and by far the one that has generated the most literature, finds the existence of the direction of time as relating to the nature of thermodynamics.

The answer from clbumical thermodynamics states that while our basic physical theory is, in fact, time-reversal symmetric, thermodynamics is not. In particular, the second law of thermodynamics states that the net entropy of a closed system never decreases, and this explains why we often see glbum breaking, but not coming back together.

But in statistical mechanics things get more complicated. On one hand, statistical mechanics is far superior to clbumical thermodynamics, in that thermodynamic behavior, glbum breaking, can be explained by the fundamental laws of physics paired with a statistical postulate. But statistical mechanics, unlike clbumical thermodynamics, is time-reversal symmetric. The second law of thermodynamics, as it arises in statistical mechanics, merely states that it is overwhelmingly likely that net entropy will increase, but it is not an absolute law.

Current thermodynamic solutions to the problem of the direction of time aim to find some further fact, or feature of the laws of nature to account for this discrepancy.

The laws solution

A third type of solution to the problem of the direction of time, although much less represented, argues that the laws are not time-reversal symmetric. For example, certain processes in quantum mechanics, relating to the weak nuclear force, are not time-reversible, keeping in mind that when dealing with quantum mechanics time-reversibility comprises a more complex definition.

But this type of solution is insufficient because 1) the time-asymmetric phenomena in QM are too few to account for the uniformity of macroscopic time-asymmetry and 2) it relies on the bumumption that QM is the final or correct description of physical processes.

One recent proponent of the laws solution is Tim Maudlin who argues that, in addition to quantum mechanical phenomena, our basic spacetime physics (general relativity) is time-reversal asymmetric. He denies the definitions, often quite complicated, that underlie time-reversal symmetries, arguing that these definitions themselves cause the appearance of a problem of the direction of time.