The Order of Time by Carlo Rovelli

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Summary

The difference between past and future exists only in our approximate, statistical perception of nature. If I observe the microscopic state of things, then the difference between past and future vanishes. If we could see exactly and take into consideration the actual dance of millions of molecules, then the future would be “just like” the past.

In the elementary equations of the world, the arrow of time appears only where there is heat. 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 backward, there is something that is heating up.

Heat, entropy, and the lower entropy of the past are notions that belong to an approximate, statistical description of nature. The difference between past and future is deeply linked to this blurring. So if I could take into account all the details of the exact, microscopic state of the world, would the characteristic aspects of the flowing of time disappear? Yes. If I observe the microscopic state of things, then the difference between past and future vanishes.

The present moment does not exist ubiquitously throughout the universe. It's like a bubble around us which extends to the precision with which we determine time (e.g. nanoseconds, milliseconds,..) For example, what you perceive to be happening in the present moment has to reach your eyes or ears through light or sound waves. That light or sound takes time to reach you, let’s say a few nanoseconds. We are always nanoseconds away from what is happening now.

In the end, instead of many possible times, we can speak only of a single time: the time of our experience—uniform, universal, and ordered. This is the approximation of an approximation of an approximation of a description of the world made from our particular perspective as human beings.

Book Notes

There is not one single time; there is a vast multitude of them. The difference between past and future exists only in our approximate, statistical perception of nature. If I observe the microscopic state of things, then the difference between past and future vanishes. If we could see exactly and take into consideration the actual dance of millions of molecules, then the future would be “just like” the past.

Speed slows down time: if you are moving, instead of being static, you age slower. Mass slows down time: if you leave in the plains, instead of the mountains, you age slower.

→ The present moment does not exist ubiquitously throughout the universe. It's like a bubble around us which extends to the precision with which we determine time (e.g. nanoseconds, milliseconds,..) For example, what you perceive to be happening in the present moment has to reach your eyes or ears through light or sound waves. That light or sound takes time to reach you, let’s say a few nanoseconds. We are always nanoseconds away from what is happening now. If we talk to someone on the other side of the world, their voice takes milliseconds to reach you, so you are even more "far" away from the present moment there.

Time is not continuous: it is a series of events. The events of the world do not form an orderly queue, like the English. They crowd around chaotically, like Italians.

→ It is only in the fourteenth century in Europe that people’s lives start to be regulated by mechanical clocks.

Before Newton, time for humanity was the way of counting how things changed. Before him, no one had thought it possible that a time independent of things could exist. Don’t take your intuitions and ideas (such as about what time is) to be “natural”: they are often the products of the ideas of audacious thinkers who came before us.

→ The entire history of the universe consists of this halting and leaping cosmic growth of entropy. It is neither rapid nor uniform, because things remain trapped in basins of low entropy (e.g. a stock of wood) until something opens a door onto a process that finally allows entropy to increase (e.g. a match). The growth of entropy itself happens to open new doors through which entropy can increase further.

→ Large or small portions of the universe remain isolated in relatively stable situations for periods that can be very prolonged.

The entire universe is like a mountain that collapses in slow motion. Like a structure that very gradually crumbles.

→ 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.

→  Our minds organize reality by grouping, segmenting, drawing lines, dividing into sections, establishing boundaries etc. We approximate the world by breaking it down into pieces, to continuously elaborate information and generate behavior.

→  To a large extent, the brain is a mechanism for collecting memories of the past in order to use them continually to predict the future. The possibility of predicting something in the future obviously improves our chances of survival and, consequently, evolution has selected the neural structures that allow it.

“Reality is formed only by memory.” - Proust

→  In the end, therefore, instead of many possible times, we can speak only of a single time: the time of our experience—uniform, universal, and ordered. This is the approximation of an approximation of an approximation of a description of the world made from our particular perspective as human beings who are dependent on the growth of entropy, anchored to the flowing of time.

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