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> A book review of time and cosmological entropy:
> 
> https://science.sciencemag.org/content/370/6522/1280?utm_campaign=toc_sci-mag_2020-12-10&et_rid=17445044&et_cid=3593354
> 
> "...are we headed toward the ultimate mundanity of equilibrium in a so-called heat death? In The Janus Point, Julian Barbour takes on this and other fundamental questions, offering the reader a new perspective—illustrated with lucid examples and poetically constructed prose—on how the Universe started (or more precisely, how it did not start) and where it may be headed.
> 
> "Barbour argues that there is no beginning of time. The Big Bang, he maintains, was just a very special configuration of the Universe's fundamental building blocks, a shape he calls the Janus point. As we move away from this point, the shape changes, marking the passage of time. The “future,” he argues, lies in both directions, hence the reference to Janus, the two-faced Roman god of beginnings and transitions.
> 
> ***
> 
> "One of the main thrusts of Barbour's proposal is an attempt to reconcile cosmological evolution with the second law of thermodynamics and an arrow of time. As a reminder, the second law is the one that tells us that systems tend toward increasing entropy, sometimes colloquially referred to as disorder. In a closed system, if we begin with order (low entropy), then disorder will grow until the entropy reaches a maximum. The change in entropy can be used to define past (low entropy) and future (high entropy).
> 
> "When applied to the Universe, the second law of thermodynamics evokes a number of questions. How did the Universe come to be in a state of low entropy, for example? Barbour's answer to this is the Janus point. In the three-body problem, the Janus point (i.e., a total collision) is an inevitability so long as the total angular momentum vanishes. Recall that angular momentum is among the sacred conserved quantities in classical mechanics, so vanishing at one time implies vanishing for all times. Barbour argues by analogy that the Universe, similarly restricted, must have a Janus point too. If true, the seemingly special configuration at the Janus point is not so special at all but in fact required by a conservation law.
> 
> "So, what happens when entropy reaches a maximum? Barbour dodges this question by arguing that entropy is not the right factor to consider and that we should instead look to complexity. With time, he argues, the building blocks of the Universe can arrange themselves into increasingly complex structures.
> 
> "Barbour does not buy the argument that the Universe is akin to a closed system whose entropy can reach a maximum and instead takes the optimistic outlook that complexity has no bound and our cosmic party can go on forever. But could this be so? The observed accelerated expansion of the Universe, the most likely explanation of which is a cosmological constant, suggests otherwise. A Universe with a cosmological constant is one that has event horizons—points of no return. And just as Stephen Hawking showed with black holes, the event horizon has a temperature and forms what can be thought of as a sort of box. Whether this means we should think of the Universe as a closed system remains unclear, but if that is the case, the entropy of the system will eventually be maximized. Change will cease, save for fleeting departures from equilibrium, and mundanity will prevail. Let us hope that Barbour is right."
> 
> Comment: Far out theories are fun to note, but this author runs counter to most current thought about time and the Big Bang and eventual heat death.