Thread by @wilte

Reading: The Order of Time - Carlo Rovelli goodreads.com/book/show/4115…

Part I: What modern physics has understood about time

Part II: The world without time

Part III: "In a world without time, there must still be some thing that gives rise to the time that we are accustomed to"; what are the constituent parts of the time that is familiar to us?

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

Time passes more slowly in some places, more rapidly in others.

A mass slows down time around itself.

If things fall, it is due to this slowing down of time.

Things fall downward because, down there, time is slowed by the Earth.

Astronomy and physics have since developed by following this seminal lead given by Anaximander: by understanding how phenomena occur according to the order of time.

There is no "truer" time; there are two times and they change relative to each other. Neither is truer than the other

Times are legion: a different one for every point in space.

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

every time a difference is manifested between the past and the future, heat is involved.

Entropy (S) never decreases;
ΔS ≥ 0

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

This is the disconcerting conclusion that emerges from Boltzmann's work:

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

Not only is there no single time for different places-there is not even a single time for any particular place.

"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.

"Now" means nothing

If we want to represent all the events in the universe and their temporal relations, we can no longer do so with a single, universal distinction between past, pres ent, and future

What is time, Aristotle vs Newton

Legend has it that Leibniz, whose name is stil 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

Spacetime is the gravitational field—and vice versa.

Curved spacetime

Time thus becomes part of a complicated geometry woven together with the geometry of space. This is the synthesis that Einstein found between Aristotle's con ception of time and Newton's.

The gravitational field, like all phys ical things, must necessarily have quantum properties.

The three fundamental discoveries that quantum mechanics has led to are these: granularity, indetermi nancy, and the relational aspect of physical variables.

Each one of these demolishes further the little that was left of our idea of time.

Granularity is the most characteristic feature of quantum mechanics, which takes its name from this: "quanta" are elementary grains.

The "quantization" of time implies that almost all values of time do not exist.

a minimum interval of time exists.
Below this, the notion of time does not exist—even in is most basic meaning,

Indeterminacy: it is not possible to predict exactly, for instance, where an electron will appear tomorrow.

Blurred superposition of different spacetimes:

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

Only in interaction, cloud of probability "collapses" and materializes in a particular configuration

Time and space also behave in this way

Recap of Part I - The crumbling of time

Part II: the world without time

The World Is Made of (a network of) Events, Not Things

The difference between things and events is that things persist in time; events have a limited duration.

we understand the world by studying change, not by studying things.

The shapes of atoms will be eventually understood only with solutions to Schrödinger's equations describing how the electrons in atoms move. Events again, not things.

We understand the world in its becoming, not in its being.

Philosophers call "presentism" the idea that only the present is real, that the past and the future are not-and that reality evolves from one present to another, successsive one.

This way of thinking no longer works, however, if the "present" is not defined globally,

Changes are not arranged in a single orderly succession: the temporal structure of the world is more complex than a simple single linear succession of instants. This does not mean that it is non existent or illusory.

If we are not all in single file, it does not follow that there are no relations between us.

We are struggling to adapt our language and our intuition to a new discovery:

the fact that "past" and "future" do not have a universal meaning. Instead, they have a meaning that changes be tween here and there. That's all there is to it.

If we find a sufficient number of variables that remain synchronized enough in relation to each other, it is convenient to use them in order to speak of when.

We need a theory that tells us how the variables change with respect to each other.

The fundamental equations of quantum gravity are effectively formulated like this: they do not have a time variable, and they describe the world by indicating the possible relations between variable quantities.

The relations of spatial adjacency tie the grains of space into webs; "spin networks."
The name "spin" comes from the mathematics that describe the grains of space."

A ring in the spin network is called a "loop," and these are the loops that give "loop theory" its name.

Loop quantum gravity shows that it is possible to write a coherent theory without fundamental space and time and that it can be used to make qualitative predictions

Noncommutativity of quantum variables: position and speed do not commute, they cannot exchange order with impunity

Perhaps it is the very fact that the effect of these interactions depends on the order in which they take place that is at the root of the temporal order of the world.

Both the sources of blurring -quantum indeterminacynacy, and the fact that physical systems are composed of zillions of molecules- are at the heart of time.

Temporality is profoundly linked to blurring

Time is ignorance

The initial low entropy of the universe, and hence the arrow of time, may be more down to us than to the universe itself.

the flow of time is not a characteristic of the universe: like the rotation of the heavens, it is due to the particular perspective that we have from our corner of it.

Every glance that we cast toward the world is made from a particular perspective.

Many things that we see in the world can be understood only if we take into account the role played by point of view.

entropy increases. This growth is what we experience as the flowing of time.

What is special is not the state of the early universe: it is the small system S to which we belong (with low entropy at the start)

The truth is that there is plenty of energy and it is not consumed. It's not energy that the world needs in order to keep going. What it needs is low entropy.

What makes the world go round are not sources of energy but sources of low entropy.

Near to the Earth we have a rich source of low en trophy: the sun.

We have at our disposal an abundance of low entropy, and it is this that allows plants and animals to grow, enables us to build motors and cities-and to think and to write books such as this one.

It is the growth of this entropy that powers the great story of the cosmos.

The entire history of the universe consists of this halting and leaping cosmic growth of entropy. It is neither rapid nor uniform.

Photosynthesis deposits low entropy from the sun into plants.
Animals feed on low entropy by eating.

When we recognize in particular that two events have the same cause, we find this common cause in the past, not in the future.

No magical force of "causality" going from the past to the future.

It happens because the improbability of a correlation between two events requires something improbable, and it is only the low entropy of the past that provides such improbability.

The existence of common causes in the past is nothing but a manifestation of low entropy in the past.

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.

to make calculations about possible futures is what we call "deciding."

Here and now, there is no past and no future.

Where are they? Augustine concludes that they are within us.

The awareness of passing of time, is internal

Proust: "Reality is formed only by memory."

Final chapter (13) summarizing Part I & II:

No common present
No difference btwn past & future in elementary equations
Closer to mass or faster move ➡️ time slows down
Ours is world of events rather than of things

Summary Part III

The directionality of time is therefore real but perspectival: the entropy of the world in relation to us increases with our thermal time.

Spacetime as table: 2 dimensions the one that we call space, and the one along which entropy grows, called time.

All well ascertained:
Slowing down of time according to altitude & speed
Nonexistence of the present
Relation btwn time & gravitational field
Relations btwn different times are dynamic
Elementary equations do not recognize direction of time
Relation between entropy and blurring

in search for time, advancing increasingly away from ourselves, we have ended up by discovering something about ourselves

Not emotional dimension of time that prevents us from apprehending nature of time objectively

Perhaps the emotion of time is precisely what time is for us

"Every day countless people die, and yet those who remain live as if they were immortals."

Our fear of death seems to me to be an error of evolution.

Basically, I believe that the fear of death is the result of an accidental and clumsy interference between two distinct evolutionary pressures

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