The Order of Time (2017)
Carlo Rovelli (1956)
240 pages
evolution endowed us with intuition only for those everyday aspects of physics that had survival value for our distant ancestors, leading to the prediction that whenever we use technology to glimpse reality beyond the human scale, our evolved intuition should break down.
Max Tegmark, Our Mathematical Universe (363)
Rovelli opens his book with a sequence of five chapters grouped under the appropriately ominous section heading The Crumbling of Time. In each, he presents a particular expectation we have about time, one that seems unimpeachable based on our daily experience but which he goes on to quite thoroughly eviscerate. These include our impression that time: passes at a uniform rate everywhere; flows from the past, through the present, to the future; has a single now or present applicable across the universe; is independent of physical events and so represents a fundamental measure for everything in the universe; and, is continuous even at the smallest scales. Together these could be summarized as that our intuition tells us that time is an independent scale, around which everything in the universe aligns itself.He demonstrates, however, that these beliefs about time do not align at all with the understandings scientists have developed and, particularly over the past century, validated through experiment. Fortunately, despite the inherent complexity of the terrain he covers, Rovelli’s captivating prose helps guide us through the intricacies of the mind-blowing discoveries that have overturned our intuitive expectations about time.
We learn for example that time doesn’t pass at a uniform rate throughout the universe: the closer one is to a large mass the slower, in fact, time passes – for someone on top of a mountain, time passes more quickly than for someone down in a valley. Perhaps more strangely, it turns out that there is no single now throughout the universe – the now we experience at any given moment on Earth does not have an equivalent now-moment on a distant star, on another planet in our solar system, or even on the other side of a room, though the difference becomes more and more insignificant at such close range.
Such non-uniformities of time can seem strange and unexpected, but they come to feel like minor oddities compared to what follows, as Rovelli next dismantles our view of time as necessarily having a flow, from the past of the future. He notes that with the exception of the law of physics that says that heat cannot pass from cold to hot, all other physical laws are independent of time – they can run forwards and backwards, and so do not distinguish past from future.
Examining more closely the phenomenon of heat, Rovelli notes that the irreversible progress of heat in one direction is labeled entropy, a quantity that can only stay the same or increase, and that is “the only equation of fundamental physics that knows any difference between past and future … that speaks of the flowing of time.” (27) This connection, between entropy and time, becomes the basic building block for much of Rovelli’s subsequent discussion.
According to Rovelli, Ludwig Boltzmann made the critical association of entropy with increasing disorder – from configurations more special and particular to those less so – though he points out that this does not
clarify the difference between past and future … it just shifts the question [to] …: Why, in one of the two directions of time – the one we call past – were things more ordered … was entropy lower? (30)
The answer, Rovelli argues, lies in our inability to see all the details of a particular state of the universe, a concept he refers to as blurring. He explains by referencing a deck of cards: if we have a deck with the particular ordering of the first half red and the second half black, then shuffling them will, to our minds, disorder the deck – increase its entropy. But, Rovelli points out, if we pay attention to all the details of the cards in the deck, then any initial ordering is a particular, or special one, and shuffling will increase the disorder relative to it.
The end result is the understanding that any ordering of the deck can be considered particular if we are paying sufficient attention to all the details, and shuffling simply leads to a different particular configuration. Thus, crucially, if we comprehend all the details of a deck of cards, then as it is shuffled there is not actually any evolution from past to future, just transformation from one configuration to another. Extending this to the physics of the universe, Rovelli claims that only because we lack the ability to “apprehend the world in all of its minute detail” (33) do we perceive that a flow of events takes place from past to future; this becomes a key element later in the book in Rovelli’s reconstruction of how we perceive time.
Having demonstrated how our intuitive expectations about time have been misled by our interactions with the physical world, in the middle section of chapters Rovelli takes a brief detour to address a question at the heart of his own research work: if the universe is not governed by time, then … by what?
Summarizing the extended exploration of this topic in his book Reality is Not What it Seems (my review linked to at right), Rovelli points out thatthe distinction between past, present, and future is not an illusion. It is the temporal structure of the world. … [But] the temporal relations between events are more complex than we previously thought. (110)To address this question he begins, as in his earlier work, by looking back to the work of early philosophers in Greece, and from there builds up the history of science’s understanding, the fits and starts along the path to the models physicists are currently grappling with.
In recent years, Rovelli notes, a number of theories have been proposed to describe what happens at the level of the most fundamental building blocks of the universe, the bits, as it were, that “form, matter, photons, electrons, other components of atoms and the gravitation field.” (124) String theory is perhaps the most well know, but Rovelli leads a research team that is part of a cohort of scientists who propose what is referred to as loop quantum gravity as the appropriate model for the fundamental basis of the physical world.
He explains quantum gravity as tying together the fundamental elements of the universe, quanta or grains, in webs called spin networks. The key point – tying back to the question of what is time – is that these webs “transform into each other in discrete leaps,” (126) transformations that are described by, crucially, “equations without time.” (127) Employing the analogy of the deck of cards: at a detailed enough level, each shuffle is a transformation from one particular configuration to another, and the act of shuffling could take us in either direction between two successive configurations.
Having already demonstrated that our intuitive expectations of time do not align with physicists’ current understandings, and hinted at how the physical world could be described without a time variable, in the third and final section of chapters, Rovelli explores how we should think about time.
He describes time as not an intrinsic quantity, but rather something that emerges from the physical processes we experiences in our world. In particular, he recalls the concept of blurring described in the opening chapters – our inability to completely describe the state of a system at a microscopic level. He notes that the discoveries of quantum effects support the idea that we can only have a blurred vision of the world, since they point out the fundamental importance of the order of knowing the speed and then the position of a particle such as a molecule:
If what materializes first is the speed and then the position, the state of the molecule changes in a different way than if the order of the two events were reversed.” (138, italics in all quotes as in the original)This difference, Rovelli argues – this blurring at the microscopic level – creates a variable that we experience as time.
He refers to what emerges as thermal time, tied to the thermodynamics of the microscopic changes in the state of a system. He notes, however, that the ‘time’ variable described in this way does not yet have flow – it has no direction as we experience time, from past to future. This leads him to his central proposition: that the flow of time may be found in “the fact that the entropy of the world was low in the past,” (143) and our experience of time is then that of the shift from lower to higher levels of entropy.
To explain this, he recalls from the opening chapters the idea that “the entropy of a system depends explicitly on blurring … on what I do not register, because it depends on the number of indistinguishable configurations.” (145) Critically, this makes the level of entropy a “relative one” (145), dependent on how human beings as physical systems interact with the world. That is, for a state of the world based on another system’s different interaction with it (than ours), a different level of entropy may be experienced.
Here he returns again to his example of a deck of cards to provide a simple, but profoundly clarifying explanation. In particular, he recalls the situation in which the deck of cards is sorted so the first half are red, the second half then black – shuffling the cards then transforms them from the highly ordered, special initial state, to a more disordered one, one in which there are more indistinguishable configurations from our point of view, focusing on red versus black, and not other details of the cards.
But, he suggests, consider a deck shuffled into a random configuration, and memorize the first six cards. Now, as we shuffle further, we are again going from a particular, special configuration to a more disordered one relative to our particular starting configuration. We again observe ‘entropy’ to be increasing, but only relative to a configuration we determined to be particular, based on our memorization of the first six cards. Thus, Rovelli point out, the original point of ‘low’ entropy depends on our interaction with the system – what we consider to be the starting configuration based on our blurred understanding – and is not intrinsic.
Exploring in more detail how entropy leads to our experience of time, Rovelli examines the relationship between energy and entropy. He points out that though we commonly consider the energy from the sun as the source of what happens on Earth, the reality is that energy is conserved by a process, such as a plant growing or a machine operating. The actual driver of these processes, and all others, is the transformation of energy during the process from low entropy to higher entropy – ultimately to heat. The sun provides low entropy photons to the Earth, and
in turn, the sun is born out of an entropic configuration that was even lower: the primordial cloud from which the solar system was formed had lower entropy. And so on, back into the past, until we reach the extremely low initial entropy of the universe. (161)It is this transformation of energy – entropy – that causes the flow of events of the world and the universe, and that thus gives us the experience of time flowing form the past to the future.
Rovelli acknowledges that this leads directly to the question of why we, as a physical system, ended up in the fortunate position of having an interaction with the universe that we experience as having the special initial configuration of low entropy – a situation critically important for life as we know it. Here, though he doesn’t use the word, he resorts to a version of what has been referred to by some physicists as the multi-verse concept (see for example the book by Max Tegmark, from which I took the opening quote; my review at right), noting that
in the vastness of the universe, there are innumerable physical systems, and they may interact with each other in ways that are even more numerous. Among these, through the endless play of probabilities and large numbers, there will surely be some that interact with the rest of the universe precisely with those variables that find themselves having a particular value in the past.
It is hardly surprising that there are “special” subsets in the universe as vast as ours … and that someone [us in this case] wins the lottery. … For this subset the entropy of the universe is low in the past, the second law of thermodynamics obtains; memories exist … and there can be evolution, life, and thought.” (149)
(For more on the idea of multiverses see, for example, Max Tegmark's book, from which I took the opening quote; my review of that book at right.) Rovelli concludes with a captivating rumination on how this flow of events leads to our experience of time, noting that “inexorably … the study of time does nothing but return us to ourselves.” (170) He turns to those who study how human beings perceive time, how it emerges as part of our identity – neuroscientists, certainly, but also philosophers. He finds that our perception of time, finally, is built into how our minds are wired, the neurons and synapses through the formation of which are collected the
traces of the past … [containing] a blurred image, no doubt – think of how many millions of details our eyes see every moment that do not stay in our memory – but one which contains worlds.
Boundless worlds. (187)
Other reviews / information:
For those interested in meditation, and the exploration of how we perceive the world, Rovelli offers a his perspective on the feeling of 'self' that we feel we have:
The experience of thinking of oneself as a subject is not a primary experience: it is a complex cultural deduction, made on the basis of many other thoughts. My primary experience – if we grant that this means anything – is to see the world around me, not myself. I believe that we each have a concept of “my self” only because at a certain point we learn to project onto ourselves the idea of being human as an additional feature that evolution has led us to develop during the course of millennia in order to engage with other members of our group: we are the reflection of the idea of ourselves that we receive back from our kind. (177)
Along with Rovelli's book Reality Is Not What it Seems, mentioned above, I've also read his wonderful introduction to some of the key developments in physics over the past century, Seven Brief Lessons on Physics; my review linked to at right.
Have you read this book, others by this author, or even similar ones by other authors? I’d enjoy hearing your thoughts.
Other of my book reviews: FICTION Bookshelf and NON-FICTION Bookshelf
