Cycles of Time: An Extraordinary New View of the Universe

by Roger Penrose

In my opinion, there is a ‘can of worms’ here, if one demands that there should be a precise objective definition of physical entropy, applicable in all circumstances, with respect to which the Second Law is to be universally valid. I do not see why one should demand that there always be a well-defined, physically precise notion of ‘entropy’, that is entirely objective and consequently ‘out there’ in Nature, in some absolute sense,[1.11] where this ‘objective entropy’ almost never decreases as time progresses. Must there always be an actual entropy concept that applies to the slightly tinted viscous fluid between the cylinders, or to the configurations of nuclear spins that had appeared to become totally disorganized, though retaining a precise ‘memory’ of the order that they had had before? I do not see that this need be the case. Entropy is clearly an extremely useful physical concept, but I do not see why it need be assigned a truly fundamental and objective role in physics. Indeed, it seems reasonable to me that the usefulness of the physical notion of entropy has its origin largely in the fact that, for systems that we tend to encounter in the actual universe, it turns out that the normal measures of ‘macroscopic’ quantities give rise to coarse-graining volumes that do in fact differ from one another by stupendously large factors. There is a profound issue, however, as to why, in the universe that we know, they should differ by such enormous factors. These enormous fa...