Time is Only a Side Effect

The Order of Time

Carlo Rovelli

A strange, slim, captivating volume. Its scope is wide-ranging, the writing dense in terms of content and reference, yet it would be slimmer even than it is (and far less captivating) if you dispensed with all the digressions, elaborations and poetic flourishes that bulk it out. The notes at the back are as impenetrable as the text is lucid, mischievously reversing the traditional order of things. 

     Rovelli, a theoretical physicist whose speciality is loop quantum gravity, considers here what physics implies about time. The discussion is in three parts. In the first, he demonstrates how relativity abolishes universal simultaneity, absolute time and even the relation between past, present and future, before moving on to show us how quantum mechanics eliminates even the flow of time. We are left with a set of events that we are able to distinguish from one another only because our experience of reality is blurred. There is only one physical support for the contention that time exists at all: the Second Law of Thermodynamics. 

     Having narrowed things down this far, Rovelli then does his best to demolish the ‘illusion’ of time generated by entropy. I don’t think he wholly succeeds, but I am not clever enough, nor sufficiently well read either in physics or philosophy, to mount a meaningful criticism. The best I can do is report that I was able to follow his logic quite easily but found it unconvincing in places. As to why, the reasons are better articulated here than I could express them myself.

    The second part of the book is very short and describes some of the physical implications of discarding time as a factor in our calculations.
     The third and most important part is an inquiry into how the sense of time that is so real and familiar to us comes into being. The discussion ranges through philosophy and psychology as well as physics. This is inevitable, since time is both metaphysical and – in so many of its aspects – subjective. Rovelli, who seems to be as interested in philosophy as in physics, argues that it is wholly so, emerging from the peculiar way in which humans (and some other animals) have evolved to operate in the physical world. 

     I think I grasped the way in which, according to him, time emerges from entropy, and I love the insight that it is the latter and not energy that really makes the world go round. This is one of a handful of intellectual thunderflashes Rovelli detonates before our eyes: time is an effect of gravity, time is made of emotion (he’s a fan of Proust, as well you may imagine), the world is made up of events not objects. However, it seems to me that he hasn’t quite worked out how all that happens; he modestly advances the proposal that it has to do with the particular way we have evolved to experience reality, which in turn defines the reality we experience. This argument is closely analogous to the weak anthropic rationale for the hospitality of the universe to intelligent life.

     The physics of how, in this entropic argument, the past is constructed out of traces of former states of a system that lie preserved in its current state is both speculative and abstruse. It also seems a bit risky, in survival terms, for organic evolution to have proceeded on such a basis. To Rovelli’s credit, he isn’t laying down the law here; this is how he thinks it all works, but he freely admits that his explanation is merely the best one he has found that fits the facts.

     Despite this softness at the centre, the book seems to hang together well. Rovelli’s model is certainly more comfortable, conceptually, than those ghastly block universes in which, absent time, every configuration of the world exists simultaneously. In Julian Barbour’s version of this concept even motion in absent; consciousness (or experience) is just a ball bouncing from one point in configuration space to another. Beat that for futility.

     One thing that Rovelli doesn’t really address, though, is how come we all share a common experience of time even though it appears to pass differently for each of us and our perception of it is based on our different individual histories, etc. This, of course, is an aspect of a bigger question: our experience of reality is constructed, but how? He does gesture at an answer by explaining that we partake of aspects of the world that are relevant to us, and since we’re much alike as entities these aspects are roughly the same. Sadly, this doesn’t take us very far before we stumble over the question of how to account for the differences.