Carlo Rovelli.The Order of Time.
Penguin. 2017
In his previous book, Reality Is Not What It Seems, Rovelli emphasises the need to remove the Newtonian model of a separately existing time from physical theories. In this new similarly slim and equally lucid volume, he delves deeper into what is this time thing, anyway.
He carefully picks apart the many different models of time in physics. Newtonian time has been replaced by many different models of time, all of which remove one or more ‘obvious’ properties. Special relativistic time depends on your speed, and asking what is now somewhere else “is like asking ‘What is here, in Peking?’”[p.37]: the present is defined just in a local bubble whose size depends on our precision. General relativistic time depends on the curvature of space, and so is different everywhere, and things fall because “the movement of things inclines to where time passes more slowly” [p.12]. Thermodynamics is the only basic physical theory that has an ‘arrow’ of time, of entropy increase, the existence of which depends on your scale; that “entropy exists because we describe the world in a blurred fashion”, and if we “observe the microscopic state of things the difference between past and future vanishes” [p.30]. Quantum mechanical (space)time is not continuous, but granular, as is everything else, and different times can coexist in superpositions.
Rovelli provides an interesting historical perspective on our current everyday intuitions of it being the same time in different places, and time always passing at the same speed: we didn’t always have these ideas. Clocks didn’t start started regulating our hours until around the 14th century. But these clocks were synchronised to local noon, not to each other. Then train timetables in the 19th century required synchronisation across distances. Time zones were invented in 1883, and cities gradually synchronised their clocks with each other. Then, in 1905, Einstein destroyed the idea of universal synchronicity. (I had known Einstein worked in a patent office; I was not aware he dealt specifically with patents related to synchonising clocks!) So, ironically:
[p56] only a few years passed between the moment at which we agreed to synchronize clocks and the moment at which Einstein realized that it was impossible to do so exactly.
Another interesting historical perspective: today we are accustomed to the idea the Newton’s view of an independently flowing absolute time, and think that Leibniz was some maverick suggesting relational time, of time being change. But actually, this view of time being dependent on change was the orthodox Aristotelian view, and it was Newton who was the maverick. We are just nowadays more used to the Newtonian view. Einstein synthesised the Aristotelian and Newtonian views: yes, spacetime is something real, yet it is relative, not absolute. And its reality is like other kinds of things that are real:
[p67] Spacetime is the gravitational field – and vice versa. It is something that exists by itself, as Newton intuited, even without matter. But it is not an entity that is different from the other things in the world – as Newton believed – is a field like the others. More than a drawing on a canvas, the world is like a superimposition of canvases, of strata, where the gravitational field is only one among others. Just like the others, it is neither absolute nor uniform, not is it fixed: it flexes, stretches and jostles with the others, pushing and pulling against them. Equations describe the reciprocal influences that all the fields have on each other, and spacetime is one of these fields.
Having spent the first part of the book bringing us up to date with current physics, Rovelli moves into more a speculative realm, a different view of time in terms of change. This is heady stuff. We should think of the world as a network of events, and “the simple fact that nothing is: that things happen instead” [p.85]. In this world there is no time as we currently understand it; instead it is
[p86] a world in which change is ubiquitous, without being ordered by Father Time; without innumerable events being necessarily distributed in good order, or along the single Newtonian timeline, or according to Einstein’s elegant geometry.
This gives a different perspective of what the world is made of:
[p86-7] We can think of the world as made up of things. Of substances. Of entities. Of something that is. Or we can think of it as made up of events. Of happenings. Of processes. Of something that occurs. Something that does not last, and that undergoes continual transformation, that is not permanent in time. The destruction of the notion of time in fundamental physics is the crumbling of the first of these two perspectives, not of the second. It is the realization of the ubiquity of impermanence, not of stasis in a motionless time.
In terms of events and processes, things are just (possible very) long-lived events: “‘Things’ in themselves are only events that for a while are monotonous.” [p.92]
Rovelli explains how some of the problems we have with this new physics is down to grammar: the human languages we use to talk about the world, with their simple past, present and future tenses, do not fit well with our current view of a more complex structure to physical time. But just because natural language, developed before we knew about this complexity, can’t cope, doesn’t mean our physical models are wrong: we just have to work harder.
Rovelli concludes his discussion with some thoughts on the origins of time: how it might emerge from the underlying granular, complex structure of spacetime events; from a particular blurring (ignorance) of macroscopic state; from non-commutative quantum operations imposing a natural (partial) order; from the fact that we have a point of view observing the universe while situated within it.
This is a beautifully written book, explaining complex concepts with great clarity and style. It is a translation. There is an amusing translation error on p193, which talks of “a degree of liberty”: after a moment of thought, I decided that this should be “a degree of freedom”. Despite the book’s slimness, there is a great deal to think about here; I have merely scraped the surface in my summary above. It is a wonderfully rich concoction of deep ideas and lucid explanation. Recommended.
For all my book reviews, see my main website.
Great review, thanks for sharing this. I really enjoyed "Reality Is Not What It Seems" and have been contemplating picking up this one as well - now it sounds like a must-read.
ReplyDelete