Introduction

We observe entropy decrease towards the past. Does this imply that in the past the world was in a non-generic microstate? I point out an alternative. The subsystem to which we belong interacts with the universe via a relatively small number of quantities, which define a coarse graining. Entropy happens to depend on coarse graining. Therefore, the entropy we ascribe to the universe depends on the peculiar coupling between us and the rest of the universe. Low past entropy may be due to the fact that this coupling (rather than microstate of the universe) is non-generic. I argue that for any generic microstate of a sufficiently rich system there are always special subsystems defining a coarse graining for which the entropy of the rest is low in one time direction (the “past”). These are the subsystems allowing creatures that “live in time” – such as those in the biosphere – to exist. I reply to some objections raised to an earlier presentation of this idea, in particular by Bob Wald, David Albert and Jim Hartle.

The Problem

An imposing aspect of the Cosmos is the mighty daily rotation of Sun, Moon, planets, stars and all galaxies around us. Why does the Cosmos so rotate? Well, it is not really the Cosmos that rotates, it is us. The rotation of the sky is a perspectival phenomenon: we understand it better as due to the peculiarity of our own moving point of view, rather than a global feature of all celestial objects.

A vivid feature of the world is its being in color: each dot of each object has one of the colors out of a three-dimensional (3D) color-space. Why? Well, it is us that have three kinds of receptors in our eyes, giving the 3D color space. The 3D space of the world's colors is perspectival: we understand it better as a consequence of the peculiarity of our own physiology, rather than the Maxwell equations.

The list of conspicuous phenomena that have turned out to be perspectival is long; recognizing them has been a persistent aspect of the progress of science.

A vivid aspect of reality is the flow of time; more precisely: the fact that the past is different from the future. Most observed phenomena violate time reversal invariance strongly. Could this be a perspectival phenomenon as well? Here I suggest that this is a likely possibility.