The Order of Time

by Carlo Rovelli

We inhabit time as fish live in water.

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

In Hindu mythology, the river of the cosmos is portrayed with the sacred image of Shiva dancing: his dance supports the coursing of the universe; it is itself the flowing of time.

Do we exist in time, or does time exist in us? What does it really mean to say that time “passes”?

what modern physics has understood about time. It is like holding a snowflake in your hands: gradually, as you study it, it melts between your fingers and vanishes.

A world stripped to its essence, glittering with an arid and troubling beauty.

quantum gravity—is an attempt to understand and lend coherent meaning to this extreme and beautiful landscape. To the world without time.

One by one, we discover the constituent parts of the time that is familiar to us—not, now, as elementary structures of reality, but rather as useful approximations for the clumsy and bungling mortal creatures we are: aspects of our perspective, and aspects, too, perhaps, that are decisive in determining what we are.

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

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.

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.

This modification of the structure of time influences in turn the movement of bodies, causing them to “fall” toward each other.

a mass slows down time around itself. The Earth is a large mass and slows down time in its vicinity. It does so more in the plains and less in the mountains, because the plains are closer to it. This is why the friend who stays at sea level ages more slowly.

If things fall, it is due to this slowing down of time. Where time passes uniformly, in interplanetary space, things do not fall. They float, without falling.

on the surface of our planet, on the other hand, the movement of things inclines naturally toward w...

the slowing down of time nevertheless has crucial effects: things fall because of it, and it allows us to keep our feet firmly on the ground. If our feet adhere to the pavement, it is because our whole body inclines naturally to where time runs more slowly—and time passes more slowly for your feet than it does for your head.

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

There is no “truer” value; they are two currencies that have value relative to each other. There is no “truer” time; there are two times and they change relative to each other. Neither is truer than the other.

There is not one single time; there is a vast multitude of them.

We do not describe how the world evolves in time: we describe how things evolve in local time, and how local times evolve relative to each other.

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

the water flows more slowly near its banks, faster in the middle—but

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.

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.

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

In the elementary equations of the world,13 the arrow of time appears only where there is heat.

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 backward, there is something that is heating up.

Only where there is heat is there a distinction between past and future.

“entropy.” The equation provides the mathematical definition of the variation of entropy (S—S0) of a body: the sum (integral) of the quantity of heat dQ leaving the body at the temperature T.

Clausius’s entropy, indicated by the letter S, is a measurable and calculable quantity15 that increases or remains the same but never decreases, in an isolated process.

“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).

It is the only equation of fundamental physics that knows any difference between past and future. The only one that speaks of the flowing of time. Behind this unusual equation, an entire world lies hidden.

Heat is the microscopic agitation of molecules. Hot tea is tea in which the molecules are very agitated.

Scientists are not immune from talking nonsense.

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.

The growth of entropy is nothing other than the ubiquitous and familiar natural increase of disorder.

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.

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

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.*

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

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.

entropy, as Boltzmann fully understood, is nothing other than the number of microscopic states that our blurred vision of the world fails to distinguish.

there is nothing intrinsic about the flowing of time. That it is only the blurred reflection of a mysterious improbability of the universe at a point in the past.

For everything that moves, time passes more slowly.

A moving object therefore experiences a shorter duration than a stationary one: a watch marks fewer seconds, a plant grows more slowly, a young man dreams less. For a moving object, time contracts.

“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.

The notion of “the present” refers to things that are close to us, not to anything that is far away.