The Order of Time

by Carlo Rovelli

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

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.

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.28 It is perhaps the greatest and strangest of Einstein’s discoveries. The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience.

It is like the point where the rainbow touches the forest. We think that we can see it—but if we go to look for it, it isn’t there.

The expanded present is the set of events that are neither past nor future: it exists, just as there are human beings who are neither our descendants nor our forebears.

a continuous trajectory toward the future returns to the originating event, to where it began.

the mass of the black hole slows time to such a degree that, at its border (called the “horizon”), time stands still.

The word “time” derives from an Indo-European root—di or dai—meaning “to divide.”

time slips from the hands of the angels and into those of the mathematicians—as

In 1883, a compromise is reached with the idea of dividing the world into time zones, thereby standardizing time only within each zone.

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:

Time is the measure of change:42 if nothing changes, there is no time.

If nothing moves, there is no time, because time is nothing but the registering of movement.

Clocks, for Newton, are devices that seek, albeit in a manner that is always imprecise, to follow this equal and uniform flowing of time. Newton writes that this “absolute, true, and mathematical” time is not perceptible.

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.

The space defined by Aristotle, the enumeration of what surrounds each thing, is called “relative, apparent, and common” by Newton. He calls “absolute, true, and mathematical” space in itself, which exists even where there is nothing.

We are used to saying “This glass is empty” in order to say that it is full of air.

Physicists call “fields” the substances that, to the best of our knowledge, constitute the weave of the physical reality of the world.

“gravitational” field: it is the origin of the force of gravity, but it is also the texture that forms Newton’s space and time, the fabric on which the rest of the world is drawn.

Spacetime is the gravitational field—and vice versa. It is something that exists by itself, as Newton intuited, even without matter.

Just like the others, it is neither absolute nor uniform, nor is it fixed: it flexes, stretches, and jostles with the others, pushing and pulling against them.

Time thus becomes part of a complicated geometry woven together with the geometry of space.

Nothing is valid always and everywhere.

Continuity is only a mathematical technique for approximating very finely grained things. The world is subtly discrete, not continuous.

The good Lord has not drawn the world with continuous lines: with a light hand, he has sketched it in dots, like the painter Georges Seurat.

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 ai...

“Time is composed of atoms, that is to say of many parts that cannot be further subdivided, on account of their short duration.”

Planck length: the minimum limit below which the notion of length becomes meaningless. Planck length is around 10-33 centimeters: a millionth of a billionth of a billionth of a billionth of a millimeter.

The second discovery made by quantum mechanics is indeterminacy: it is not possible to predict exactly, for instance, where an electron will appear tomorrow.

Between one appearance and another, the electron has no precise position,55 as if it were dispersed in a cloud of probability. In the jargon of physicists, we say that it is in a “superposition” of positions.

Spacetime is a physical object like an electron. It, too, fluctuates. It, too, can be in a “superposition...

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.

“Fluctuation” does not mean that what happens is never determined. It means that it is determined only at certain moments, and in an unpredictable way.

electron is concrete only in relation to the other physical objects it is interacting with.

the gravitational field—does not only have a dynamic influenced by masses; it is also a quantum entity that does not have determined values until it interacts with something else.

When it does, the durations are granular and determinate only for that something with which it interacts; they remain indeterminate for the rest of the universe.

Time has loosened into a network of relations that no longer holds togethe...

The picture of spacetimes (in the plural) fluctuating, superimposed one above the other, materializing at certain times with respect to particular obje...

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.

You got to deep-six your wristwatch, you got to try and understand, The time it seems to capture is just the movement of its hands

The world will go on turning, even without King Time.

the world is a network of events. On the one hand, there was time, with its many determinations; on the other, the simple fact that nothing is: things happen.

They are events, indeed: change, happening. This happening is diffuse, scattered, disorderly. But it is happening; it is not stasis.

Time, as Aristotle suggested, is the measure of change; different variables can be chosen to measure that change, and none of these has all the characteristics of time as we experience it. But this does not alter the fact that the world is in a ceaseless process of change.

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 un...

Thinking of the world as a collection of events, of processes, is the way that allows us to better grasp, comprehend, and describe it. It is the only way that is compatible with relativity. The world is not a collection of things, it is a collection of events.

The world is not so much made of stones as of fleeting sounds, or of waves moving through the sea.

Simple events, and more complex events that can be disassembled into combinations of simpler ones.

A cloud above a mountain is not a thing, it is the condensation of humidity in the air that the wind blows over the mountain. A wave is not a thing, it is a movement of water, and the water that forms it is always different.