The Order of Time

by Carlo Rovelli

He looked for a plausible explanation and found one by imagining that the sun and the Earth do not attract each other directly but that each of the two gradually acts on that which is between them. And, since what lies between them is only space and time, he imagined that the sun and the Earth each modified the space and time that surrounded them, just as a body immersed in water displaces the water around it. This modification of the structure of time influences in turn the movement of bodies, causing them to ‘fall’ towards each other.1

Einstein. how planets influence each other. modification of the structure of time.

Here on the surface of our planet, on the other hand, the movement of things inclines naturally towards where time passes more slowly, as when we run down the beach into the sea and the resistance of the water on our legs makes us fall headfirst into the waves. Things fall downwards because, down there, time is slowed by the Earth.

Tendency

The question is meaningless. We might just as well ask what is most real – the value of sterling in dollars or the value of dollars in sterling. There is no ‘truer’ value; they are two currencies which have value relative to each other. There is no truer time. There are two times that change relative to each other. Neither is truer than the other.

Example. There is no truer time.

The world is not like a platoon advancing at the pace of a single commander. It’s a network of events affecting each other. This is how time is depicted in Einstein’s general theory of relativity. His equations do not have a single ‘time’; they have innumerable times. Between two events, just as between the two clocks that are separated and then brought together again, the duration is not a single one.

Einstein relativity

Physics does not describe how things evolve ‘in time’ but how things evolve in their own times, and how ‘times’ evolve relative to each other.

Time

The difference between past and future, between cause and effect, between memory and hope, between regret and intention … in the elementary laws that describe the mechanisms of the world, there is no such difference.

direction Of time

This is the only basic law of physics that distinguishes the past from the future.

Heat

Only where there is heat is there a distinction between past and future. Thoughts, for instance, unfold from the past to the future, not vice versa – and, in fact, thinking produces heat in our heads

Heat. Thoughts.

a name taken from ancient Greek, entropy: I prefer to take the names of important scientific quantities from ancient languages, so that they may be the same in all the living languages. I therefore propose to call entropy the quantity (S) of a body, from the Greek word for transformation: ἡ τροπή.6 Clausius’s entropy, indicated by the letter S, is a measurable and calculable7 quantity that increases or remains the same but never decreases, in an isolated process.

Entropy

ΔS ≥ 0 This reads: ‘Delta S is always greater than or equal to zero’, and we call this ‘the second principle of thermodynamics’ (the first being the conservation of energy).

Thermodynamic

Sadi Carnot thought that heat was a substance, a fluid. He was wrong. Heat is the microscopic agitation of molecules. Hot tea is tea in which the molecules are very agitated. Cold tea is tea in which the molecules are only a little agitated. In an ice cube, warming up and melting molecules become increasingly agitated and lose their strict connections.

What is heat.

This is what Boltzmann understood. The difference between past and future does not lie in the elementary laws of motion; it does not reside in the deep grammar of nature. It is the natural disordering that leads to gradually less particular, less special situations.

Disordering

We often say that causes precede effects and yet, in the elementary grammar of things, there is no distinction between ‘cause’ and ‘effect’.fn2 There are regularities, represented by what we call physical laws, that link events of different times, but they are symmetric between future and past. In a microscopic description, there can be no sense in which the past is different from the future.

Microscopic

For everything that moves, time passes more slowly.

Today, the slowing down of time can be observed in many physics experiments.

Google

when the hell is ‘now’ on Proxima b? The truth of the matter is that we need to give up asking the question.7 There is no special moment on Proxima b that corresponds to what constitutes the present here and now. Dear reader, pause for a moment to let this conclusion sink in. In my opinion, it is the most astounding conclusion arrived at in the whole of contemporary physics.

Time interesting conclusion

Our ‘present’ does not extend throughout the universe. It is like a bubble around us. How far does this bubble extend? It depends on the precision with which we determine time. If by nanoseconds, the present is defined only over a few metres; if by milliseconds, it is defined over thousands of kilometres. As humans, we distinguish tenths of a second only with great difficulty; we can easily consider our entire planet to be like a single bubble where we can speak of the present as if it were an instant shared by us all. This is as far as we can go.

Present

There is our past: all the events that happened before what we can witness now. There is our future: the events that will happen after the moment from which we can see the here and now. Between this past and this future there is an interval that is neither past nor future and still has a duration: fifteen minutes on Mars; eight years on Proxima b; millions of years in the Andromeda galaxy. It is the expanded present.8 It is perhaps the greatest and strangest of Einstein’s discoveries.

Past and future

The ‘present of the universe’ is meaningless.

Damn

This is the structure of spacetime that Einstein understood when he was twenty-five years old. Ten years later, he comes to understand that the speed at which time flows changes from place to place. It follows that spacetime does not really have the order outlined above but can be distorted.

Time distortion

If the present has no meaning, then what ‘exists’ in the universe? Is not what ‘exists’ precisely what is here ‘in the present’? The whole idea that the universe exists now in a certain configuration and changes together with the passage of time simply doesn’t stack up any more.

Google

The word ‘time’ derives from an Indo-European root – di or dai – meaning ‘to divide’. For centuries, we have divided the days into hours.3 For most of those centuries, however, hours were longer in the summer and shorter in the winter, because the twelve hours divided the time between dawn and sunset: the first hour was dawn, and the twelfth was sunset, regardless of the season, as we read in the parable of the winegrower in the Gospel according to Matthew.4 Since, as we say nowadays, during summer ‘more time’ passes between dawn and sunset than during the winter, in the summer the hours were longer, and the hours were shorter in wintertime

Gradually, time slips from the hands of the angels and into those of the mathematicians – as is graphically illustrated at Strasbourg Cathedral, where two sundials are surmounted, respectively, by an angel (one inspired by earlier sundials from 1200) and by a mathematician (on the sundial put there in 1400).

Strasbourg

It can hardly be pure coincidence that, before gaining a university position, the young Einstein worked in the Swiss Patent Office, dealing specifically with patents relating to the synchronization of clocks at railway stations. It was probably there that it dawned on him: the problem of synchronizing clocks was, ultimately, an insoluble one.

Holy shit. connecting the dots

This is the way in which we have traditionally conceived of time: counting the ways in which things change.

Change

So if nothing changes, if nothing moves, does time therefore cease to pass? Aristotle believed that it did. If nothing changes, time does not pass – because time is our way of situating ourselves in relation to the changing of things: the placing of ourselves in relation to the counting of days. Time is the measure of change:8 if nothing changes, there is no time.

Time cease to pass?

‘If it is dark and our bodily experience is nil,’ Aristotle writes in his Physics, ‘but some change is happening within the mind, we immediately suppose that some time has passed as well.’9 In other words, even the time that we perceive flowing within us is the measure of a movement: a movement that is internal … If nothing moves, there is no time, because time is nothing but the registering of movement.

Movement

Newton recognizes that a kind of ‘time’ exists that measures days and movements: the one treated by Aristotle (relative, apparent and common). But he also contends that, in addition to this, another time must exist: ‘true’ time that passes regardless, independently of things and of their changes. If all things remained motionless and even the movements of our souls were to be frozen, this time would continue to pass, according to Newton, unaffected and equal to itself: ‘true’ time. It’s the exact opposite of what Aristotle writes.

Time and true time

Legend has it that Leibniz, whose name is still occasionally spelled with a ‘t’ (Leibnitz), had deliberately dropped the letter from his name in accordance with his belief in the nonexistence of the absolute Newtonian time t.14

Lol

Don’t take your intuitions and ideas to be ‘natural’: they are often the products of the ideas of audacious thinkers who came before us.

To reply to the question ‘Where is something?’ means to indicate something else that is around that thing.

Where

the place of a thing is what surrounds that thing.

Where space

We can think of a great Newtonian canvas on which the story of the world is drawn. But this canvas is made of the same stuff that everything else in the world is made of, the same substance that constitutes stone, light and air: it is made of fields.

Field

Remember the clocks in Chapter 1 that slow down in the vicinity of a mass? They slow down because there is, in a precise sense, ‘less’ gravitational field there. There is less time there.

Reason

barely a year later it is Einstein himself who observes that this cannot be the last word on the nature of time and space, because of the existence of quantum mechanics. The gravitational field, like all physical things, must necessarily have quantum properties.

Gravitational fields must have quantum qualities

It is not possible to think of duration as continuous. We must think of it as discontinuous: not as something which flows uniformly but as something which in a certain sense jumps, kangaroo-like, from one value to another. In other words, a minimum interval of time exists. Below this, the notion of time does not exist – even in its most basic meaning.

Granularity

Granularity is ubiquitous in nature: light is made of photons, the particles of light. The energy of electrons in atoms can acquire only certain values and not others. The purest air is granular, and so, too, is the densest matter. Once it is understood that Newton’s space and time are physical entities like all others, it is natural to suppose that they are also granular. Theory confirms this idea: loop quantum gravity predicts that elementary temporal leaps are small, but finite.

Granularity

Spacetime is a physical object like an electron. It, too, fluctuates. It, too, can be in a ‘superposition’ of different configurations.

Superposition

Even the distinction between present, past and future thus becomes fluctuating, indeterminate. Just as a particle may be diffused in space, so, too, the differences between past and future may fluctuate: an event may be both before and after another one.

Interesting

But there is a strange aspect to this materialization of the electron: the electron is concrete only in relation to the other physical objects it is interacting with.

Relational

There is no single time: there is a different duration for every trajectory; and time passes at different rhythms according to place and according to speed. It is not directional: the difference between past and future does not exist in the elementary equations of the world; its orientation is merely a contingent aspect that appears when we look at things and neglect the details.

Main

The substratum that determines the duration of time is not an independent entity, different from the others that make up the world; it is an aspect of a dynamic field. It jumps, fluctuates, materializes only by interacting, and is not to be found beneath a minimum scale … So, after all this, what is left of time?

Time

Clocks that run at different speeds do not mark a single time, but the hands on each clock change in relation to the others.

Hmm. Interesting

The difference between things and events is that things persist in time; events have a limited duration. A stone is a prototypical ‘thing’: we can ask ourselves where it will be tomorrow. Conversely, a kiss is an ‘event’. It makes no sense to ask where the kiss will be tomorrow. The world is made up of networks of kisses, not of stones.

Things and events

The basic units in terms of which we comprehend the world are not located in some specific point in space. They are – if they are at all – in a where but also in a when. They are spatially but also temporally delimited: they are events.

Basic units

On closer inspection, in fact, even the things that are most ‘thing-like’ are nothing more than long events. The hardest stone, in the light of what we have learned from chemistry, from physics, from mineralogy, from geology, from psychology, is in reality a complex vibration of quantum fields, a momentary interaction of forces, a process that for a brief moment manages to keep its shape, to hold itself in equilibrium before disintegrating again into dust, a brief chapter in the history of interactions between the elements of the planet, a trace of Neolithic humanity, a weapon used by a gang of kids, an example in a book about time, a metaphor for an ontology, a part of a segmentation of the world that depends more on how our bodies are structured to perceive than on the object of perception – and, gradually, an intricate knot in that cosmic game of mirrors that constitutes reality. The world is not so much made of stones as of fleeting sounds, or of waves moving through the sea. If

Things are events

What works instead is thinking about the world as a network of events. Simple events, and more complex events that can be disassembled into combinations of simpler ones. A few examples: a war is not a thing, it’s a sequence of events.

we understand the world by studying change, not by studying things.

Main

The absence of time does not mean, therefore, that everything is frozen and unmoving. It means that the incessant happening that wearies the world is not ordered along a timeline, is not measured by a gigantic tick-tocking. It does not even form a four-dimensional geometry. It is a boundless and disorderly network of quantum events.

Main

If by ‘time’ we mean nothing more than happening, then everything is time. There is only that which exists in time.

There ia only that which exists in time.