When I studied physics at university in the 1970s, I became interested in the nature of time, particularly in light (pun intended) of its role in both special and general relativity.  I also read The Direction of Time by Hans Reichenbach (1891-1953) regarded as the leading empiricist philosopher of the 20^th century, as well as other philosophers and writers.  Later, in the 1990s I became a student of Zen Buddhism with Hogen Yamahata, who emphasised that the only reality is what is experienced here-now. This certainly describes my experience of time but is also fundamentally at odds with the usual understanding of the implications of modern physics. 

So I read Carlo Rovelli’s fourth book The Order of Time (2018) with great interest.  Unusually for a physicist he gives an accessible and very readable overview of the main findings of physics but also discusses the human experience of time and tries to integrate our common experience with the insights of modern physics. See here for my previous review of his first book Anaximander.

It’s a short book and is written for a lay audience, written in a very readable and accessible way. Its been criticized by some for not having enough hard science exposition and too much speculative stuff, particularly in the third section. However, I thoroughly enjoyed the mix and wholeheartedly recommend it if you are at all interested in the nature of time.

Rovelli opens with the claim that the nature of time is perhaps the greatest remaining mystery.  I’m inclined to think that consciousness and the fundamental constituents of reality are equally great mysteries, and indeed these may all be interrelated. The introductory chapter identifies a number of key questions about time:

• Does the universe unfold into the future, as time flows?  Does in fact time “flow”?
• Does the past, present and future all exist in the block universe of relativity, with our consciousness or perhaps the “present moment” moving through the blocks?
• Why do we remember the past and not the future? Put another way, why does time flow only in one direction, when the fundamental equations of physics have no preferred time direction?
• Is time a fundamental property of the universe in which events play out, or is time an emergent property, perhaps emerging only at a certain scale or degree of complexity?

The rest of the book is divided into three parts. In the first part, Rovelli summarizes the understanding of time in modern physics, and how this is radically at odds with our normal perceptions of time. Special and general relativity have conclusively shown that there is not a single universal time. Time passes at different rates at different places (faster where gravity is lower) and at for observers travelling at different speeds (slower when velocity is greater).  Additionally, there is no longer a single universal “now”. Between the past and the present there is an expanded “now” in which different observers will see events occurring with different time differences and possibly in a different order. The direction of time, the difference between past and future, does not exist in the elementary equations of the world and appears only in the second law of thermodynamics (entropy of a closed system can only increase or stay the same).

The second part, The World Without Time, delves into the fundamental nature of reality, drawing on Rovelli’s own field of research, loop quantum gravity in a shorter section. He argues that the fundamental constituents of reality are events (interactions) not things, and that space and time are emergent properties from these interactions. This is controversial, quantum loop gravity is but one of a number of contenders for the ’Theory of Everything’. There are physicists such as Lee Smolin (The Singular Universe and the Reality of Time) who have argued the opposite: that time is indeed a fundamental property of the universe. And more, like Sean Carrol, who conclude that neither he nor anyone else has a clue whether time is fundamental or emergent.

The third and final part attempts identify the sources of time and to understand how the non-universal time of relativity and the non-time of fundamental physics are consistent with our experience of time: “Somehow our time must emerge around us, at least for us and at our scale.” This to me was the most interesting and inspiring part of the book, though there are many reviewers on Amazon and Goodreads who really did not like it. Rovelli is up-front about the speculative nature of his “possible” answer. He is not sure it is the right answer, but it’s the one that he finds the most compelling and he doesn’t think there any better ones.

As have many before him, Rovelli locates the arrow of time in the second law of thermodynamics, that entropy can never decrease. Entropy is a macroscopic property of systems and essentially associated with the number of microscopic configurations that are consistent with the “blurred” macroscopic view. Rovelli repeatedly uses the idea of “blurring” to explain the emergence of time at the macroscopic level. You have to read him to (possibly) understand it. He also identifies another potential source of temporal ordering in the quantum uncertainty principle; that the values of variables such as speed and position depend on the ordering of their measurement. He then states that Alain Connes has shown that the emergent thermal time and quantum time are aspects of the same phenomenon. Time emerges from our ignorance of the microscopic details of the world.

Although I studied statistical mechanics, I never understood until reading Rovelli that entropy was a relative quantity, determined not only by the state of a system but also by the set of macroscopic variable with which it is observed. Rovelli addresses the issue of why the universe started in a low entropy state (allowing the emergence of time) by suggesting that the universe has pockets of high and low entropy by chance, and necessarily it is only in the regions with low entropy that time can emerge so that life is possible, together with evolution, thought and memories of past times. He is arguing an astonishing variant of the weak anthropic principle, namely, that the flow of time is not a characteristic of the universe as a whole but necessarily associated with local region(s) in which entropy is initially low. This suggested to me that perhaps there might be sentient life forms evolve in local regions of the universe which are not in initial low entropy state according to the macro-variables by which we and other earthly life interact with reality, but may be low entropy for life forms which interact via a different set of macro-variables.

Rovelli goes on to speculate that the emergence of time may have more to do with us than with the cosmos per se. Is this dangerously close to putting humans back at the centre of the universe? He then discusses how causality also is a result of the fact that entropy increases. This is what allows past events to leave traces in the present, and that we remember the past but not the future. Evolution has designed our brains to use the traces to predict the future. Causality, memory, history all emerge from the fact that our universe was in a particular state of low entropy in the past. That particularity is relative, dependent on the set of macroscopic variables with which we interact with the world.

And in the final chapters of the book, Rovelli turns to us, and the role we play with respect to time. This for me was the most fascinating and inspiring part of the book. Rovelli argues that humans (and the rest of the biosphere) have been hard-wired by evolution to accept and use the concept of time. The changing interactions are real, but the time in which all this seems to occur is a manifestation of the human (and probably animal) mind, which has evolved to make use of the memory traces of the past to predict the future. The movement is real, the changing is real, but the time in which all of this seems to occur is nothing more than a manifestation of human (possibly animal) mind and the illusion, in turn, is supported by the entropy generated in the functioning of our brains.

From his discussion of the three sources of time, he draws a number of conclusions about the implications for us:

• The self is not an enduring entity, but an emergent construct of the brain and memory, time is the source of our sense of identity.
• Reality is made up of processes or interactions not things. Things are impermanent.
• Being is suffering because we are in time, impermanent. “What causes us to suffer is not in the past or the future: it is here, now, in our memory, in our expectations. We long for timelessness, we endure the passing of time: we suffer time. Time is suffering.”

Rovelli explicitly notes how these are also some of the key insights of Buddhism: suffering, no-self, emptiness and impermanence. Rovelli here is NOT doing what people like Fritjof Capra and many new age gurus do: to use the strangeness of reality at quantum level as a justification for believing in macroscopic-level things like telepathy or clairvoyance.  Rather, he is arguing that his understanding of time is consistent with what I consider some of the most important insights of Buddhism. I have also realized that the lack of a universal present moment is not inconsistent with the Zen insight that only the present exists. My teacher Hogen-san has always used the phrase “here- now” to describe our only reality, not just “now”. So there is no conflict with the time of relativity.

While Rovelli addresses my key questions about the nature of time, I am not convinced of all his arguments and explanations, or even that I fully understand them. But it is an immensely thought-provoking read, and I will read it again and hopefully my brain will hold off hurting long enough for me to clarify my own thoughts and speculations on the nature of time.  In the meantime, I have been paying attention to the present moment as much as I can during my daily zazen, trying to simply experience here-now without thoughts about the past or speculations about the future.