2015*Seven Brief Lessons on Physics*.2016*Reality Is Not What It Seems*.2017*The Order of Time*.

- What twentieth-century physics says about the world might be true. 2006. (In
*What is Your Dangerous Idea?*) - How Apparent Finality Can Emerge. 2013. (In
)*This Explains Everything*

These brief lessons are short essays of many important areas of fundamental physics, including quanta, cosmology, time, and black holes. They are indeed brief, and hence not very detailed, each being about ten small pages, but well written and readable.

Rovelli guides us through a brief history of the development of physical theories, to end up with a theory of quantum gravity, one of the big unsolved problems of today. The background, from ancient Greek natural philosophy, to modern quantum mechanics, is an excellent introduction to how the ideas gradually reformulated, bringing in concepts of discreteness, fields, and relationships. It is well worn material, but presented in a new, fascinating way. Moving into the final topics is necessarily more limited: the material is both highly technical, and not yet accepted as the standard paradigm.

Before we get to the quantum gravity, we move through Rovelli’s relational interpretation of quantum mechanics, which (modulo Leibniz) is a completely different way of conceptualising systems purely in terms of their relationships (interactions) with other systems: there is no state of an isolated system, only state revealed by its relationships and interactions. The concept of a Newtonian-like schema of a separately existing time that flows independently of everything else disappear in this relational model:

[p157]
If we want to understand the world widely,
if we want to understand how it functions in the less familiar situations where quantum gravity matters,
we need to abandon this schema.
The idea of a time *t* which flows by itself, and in relation to which all things evolve, is no longer a useful one.
The world is not described by equations of evolution in time *t*.
What we must do is simply to enumerate the variables *A*, *B*, *C*…. which we *actually* observe,
and write equations expressing relations between *these* variables, and nothing else:
that is, equations for the relations *A(B)*, *B(C)*, *C(A)*… which we observe,
and not for the functions *A(t)*, *B(t)*, *C(t)*… which we do *not* observe.

Finally, we get to the particular quantum gravity theory here: a complex superposition of quanta of space interacting and relating.

[p166-7]
Space is a spin network whose nodes represent its elementary grains,
and whose links describe their proximity relations.
Spacetime is generated by processes in which these spin networks transform into one another,
and these processes are described by sums over spinfoams.
A spinfoam represents a history of a spin network,
hence a granular spacetime where the nodes of the graph combine and separate.

That summary from the book makes sense in the context of all the material that has gone before. Go and read the whole thing yourself; it is beautifully written, includes fascinating perspectives on the development of physics, many great insights into contemporary quantum theory, and is as clear as anything about such a technical subject can be without actually invoking the technicalities.

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 synchronising 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 Einsteinâ€™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.