The Order of Time

by Carlo Rovelli

The universe unfolds into the future, dragged by time, and exists according to the order of time.

What could be more universal and obvious than this flowing?

Something essential continues to draw us back to the nature of time.

What we call “time” is a complex collection of structures,2 of layers. Under increasing scrutiny, in ever greater depth, time has lost layers one after another, piece by piece.

A strange, alien world that is nevertheless still the one to which we belong.

the mystery of time is ultimately, perhaps, more about ourselves than about the cosmos.

We are seeing with “mad” eyes, like those of Paul McCartney’s Fool on the Hill: the crazed vision that sometimes sees further than our bleary, customary eyesight.

Things are transformed one into another according to necessity, and render justice to one another according to the order of time.

We might just as well ask what is most real—the value of sterling in dollars or the value of dollars in sterling.

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.*

If the world is upheld by the dancing Shiva, there must be ten thousand such dancing Shivas, like the dancing figures painted by Matisse.

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 toward it, shape it, because it does not yet exist.

Rebellion is perhaps among the deepest roots of science: the refusal to accept the present order of things.

Perhaps poetry is another of science’s deepest roots: the capacity to see beyond the visible.

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 every sequence of events that becomes absurd if projected backward, there is something that is heating up.

Within the reflections in a glass of water, there is an analogous tumultuous life, made up of the activities of myriads of molecules—many more than there are living beings on Earth.

Thermal agitation is like a continual shuffling of a pack of cards: if the cards are in order, the shuffling disorders them. In this way, heat passes from hot to cold, and not vice versa: by shuffling, by the natural disordering of everything. The growth of entropy is nothing other than the ubiquitous and familiar natural increase of disorder.

If we think about it carefully, every configuration is particular, every configuration is singular, if we look at all of its details, since every configuration always has something about it that characterizes it in a unique way.

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

all the phenomena that characterize the flowing of time are reduced to a “particular” state in the world’s past, the “particularity” of which may be attributed to the blurring of our perspective.

That it is only the blurred reflection of a mysterious improbability of the universe at a point in the past.

“Proper time” depends not only on where you are and your degree of proximity to masses; it depends also on the speed at which you move.

Our “present” does not extend throughout the universe.

The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience.29 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 “present of the universe” is meaningless.

There is an analogy between generations and the temporal structure of the world as revealed by relativity.

Special relativity is the discovery that the temporal structure of the universe is like the one established by filiation: it defines an order between the events of the universe that is partial, not complete.

In short, a common present does not exist:

It is the curious local structure of the present that produces black holes.

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 anymore.

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

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.

Newton’s time is not the evidence given to us by our senses: it is an elegant intellectual construction.

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.

Sometimes we speak of air as if it were something, sometimes as if it were nothing. Sometimes as if it were there, sometimes as if it were not there.

The existence of a complete void, without any physical entity except amorphous space, “absolute, true, and mathematical,” remains a brilliant theoretical idea introduced by Newton to found his physics on, for there is no scientific, experimental evidence to support its existence.

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, 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. Sooner or later, we always come across something that is new.

The world is subtly discrete, not continuous.

In the thirteenth century, the great philosopher Maimonides writes: “Time is composed of atoms, that is to say of many parts that cannot be further subdivided, on account of their short duration.”

Abstract thought can anticipate by centuries hypotheses that find a use—or confirmation—in scientific inquiry.

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.

Concreteness occurs only in relation to a physical system: this, I believe, is the most radical discovery made by quantum mechanics.*

In order to make advances in our understanding of the world, it is not always necessary to have new data.

Our different speeds define different surfaces of simultaneity.

The fact that there is nothing logically impossible about travels to the past is demonstrated clearly in an engaging article by one of the great philosophers of the last century: David Lewis,

Science is like affection: those who are dearest to us are those with whom we have the liveliest disagreements.

the place of a thing is the inner boundary of that which surrounds the thing,