The Order of Time

by Carlo Rovelli

Even the words that we are speaking now thieving time has stolen away, and nothing can return.

Let’s begin with a simple fact: time passes faster in the mountains than it does at sea level.

The ability to understand something before it’s observed is at the heart of scientific thinking. In antiquity, Anaximander understood that the sky continues beneath our feet long before ships had circumnavigated the Earth. At the beginning of the modern era, Copernicus understood that the Earth turns long before astronauts had seen it do so from the moon. In a similar way, Einstein understood that time does not pass uniformly everywhere before the development of clocks accurate enough to measure the different speeds at which it passes.

Children grow up and discover that the world is not as it seemed from within the four walls of their homes. Humankind as a whole does the same.

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.

Time has lost its first aspect or layer: its unity. It has a different rhythm in every different place and passes here differently from there.

Past and future are different from each other. Cause precedes effect. Pain comes after a wound, not before it. The glass shatters into a thousand pieces, and the pieces do not re-form into a glass. We cannot change the past; we can have regrets, remorse, memories. The future instead is uncertainty, desire, anxiety, open space, destiny, perhaps. We can live towards it, shape it, because it does not yet exist. Everything is still possible

Time is not a line with two equal directions: it is an arrow with different extremities. And it is this, rather than the speed of its passing, that matters most to us about time.

What exactly is this flowing? Where is it nestled in the grammar of the world? What distinguishes the past, its having been, from the future, its not having been yet, in the folds of the mechanism of the world? Why, to us, is the past so different from the future?

Shirazi is the author of those luminous verses that now stand at the entrance of the headquarters of the United Nations: All of the sons of Adam are part of one single body, They are of the same essence. When time afflicts us with pain In one part of that body All the other parts feel it too. If you fail to feel the pain of others You do not deserve the name of man. Perhaps poetry is another of science’s deepest roots: the capacity to see beyond the visible.

famous: if nothing else around it changes, heat cannot pass from a cold body to a hot one.

The crucial point here is the difference from what happens with falling bodies: a ball may fall, but it can also come back up, by rebounding, for instance. Heat cannot. This is the only basic law of physics that distinguishes the past from the future.

In the elementary equations of the world,5 the arrow of time appears only where there is heat.fn1 The link between time and heat is therefore fundamental: every time a difference is manifested between the past and the future, heat is involved. In every sequence of events that becomes absurd if projected backwards, there is something that is heating up.

why, in one of the two directions of time – the one we call past – were things more ordered? Why was the great pack of cards of the universe in order in the past? Why, in the past, was entropy lower?

The notion of ‘particularity’ is born only at the moment we begin to see the universe in a blurred and approximate way.

the difference between the past and the future refers only to our own blurred vision of the world.

blurred reflection of a mysterious improbability of the universe at a point in the past.

The flowing of time

The fact itself is quite simple. Instead of sending the two friends from the first chapter to the mountains and the plains, respectively, let’s ask one of them to stay still and the other one to walk around. Time passes more slowly for the one who keeps moving.

Our ‘present’ does not extend throughout the universe. It is like a bubble around us.

If I ask whether two events – one on Earth and the other on Proxima b – are happening ‘at the same moment’, the correct answer would be: ‘It’s a question that doesn’t make sense, because there is no such thing as “the same moment” definable in the universe.’ The ‘present of the universe’ is meaningless.

a common present does not exist: the temporal structure of spacetime is not a stratification of times such as this:

Near to a black hole, the lines converge towards it, like this:13

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. In other words, 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.

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.

The entire evolution of science would suggest that the best grammar for thinking about the world is that of change, not of permanence. Not of being, but of becoming.

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

For those standing below, things above are below, while things below are above … and this is the case around the entire earth.

Einstein’s phrase ‘the distinction between past, present and future is only a stubbornly persistent illusion’?

It is not a letter written to pontificate about the structure of the world: it’s a letter written to console a grieving sister.