The Order of Time

Summary

In this book Carlo Rovelli takes us on a journey through the nature of time, from the perspective of physics, philosophy, and human experience and shows how the three differ wildly. What we think time is like, is nothing like what it actually is. Time in a way does not exist, at least not in the way that we think it does. And Rovelli does a beautiful job of explaining what time really is, tying it back into a notion of beauty and humanness, somehow.

The main takeaway of the book is that Physics shows us that time, as we traditionally understand it, is an illusion. It's a construct of our minds, a way to make sense of the world around us. The universe doesn't care about time, it doesn't have the same notion of it that we have. It's wildly more complicated because of relativity, quantum effects, the ideas of entropy. Time, how it really exists, physically, is not useful to us anymore. Notions of causality, of "now", the "past" or the "future", don't make sense from a physicist's perspective. But that doesn't matter. Because they are important to us. Slicing up the world, into time, the way that we perceive it, is meaningful to us, and therefore it is very real, even if not in a physical sense.

In Rovelli's words:

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

And that is the important bit.

Detailed Notes

Part I - The Crumbling of Time

Chapter 1 - Loss of Unity

We inhabit time as fish live in water. 

Physics stripped the nature of time of its temporality. There is not really a thing that is like time as we experience it. 

How we perceive time is a mystery about us, not the universe. The universe knows not time, we do. 

Time passes faster in the mountains than it does at sea level. 

Time doesn't have an abstract absolute value. All time is relative to each other. 

Chapter 2 - Loss of Direction

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

Heat flowing from hot to cold, from packed energy in one place to dispersed energy in another, entropy in other words, is the only thing in physics that cares about the direction of time. Every other equation works just as well backwards as it does forwards. Entropy though, entropy always increases. 

We see the water in a glass like the astronauts saw the earth from the moon: calm, gleaming, blue. [...] 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.

Atoms frantically move around, there is a molecular frenzy that spreads to everything it touches. But in so spreading, it loses some of its momentum, becomes more spread out, becomes colder, more uniform. The universe at the beginning was like a deck of cards not shuffled at all, the process of time is continuous shuffling until everything looks the same and order is lost. But order of cards as a concept only applies if not looking closely enough. Because every deck of cards is a particular order, just this way or just that way. 

If looked at very closely though all regularity disappears. Single particles behave according to physical laws, that move them backward or forward in time. There is no cause and effect at that level. But we lose sight of that because we don't see single particle trajectories, we only look at an aggregate view of particles as a whole and therefore lose knowledge about the past and future. 

Chapter 3 - The End of the Present

Speed also changes time. Things that move faster experience less time passing for them. 

The idea of now is meaningless, since now depends on the observer. Things that occur "now" for a stationary person, are different than things that occur now for somebody moving. 

We only ever look into the past, because light takes time to reach us. What happened "now" we never see. If things are really far away then this difference between now becomes really large and so we lose it completely. 

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

The presence is a bubble that only extends a little bit outwards from our position, if we measure time more accurately that bubble shrinks, measure it 100% accurately, and every single point in space has its own notion of now, different from every other point. 

Analogy between family trees and time, asking whose descendant to whom makes sense, but asking if two people are "the same generation" doesn't make sense, because drawing a boundary of generation can be arbitrary. Cones of descendence can be drawn backward and forward in time for family trees. Like family members, moments of time are related by such cones. There is a partial order, each moment has moments that influenced it and came before, and moments that come after it, but there are also moments with no relationship to them, outside their current cone of the past. 

Chapter 4 - Loss of Independence

"How long is forever?" asks Alice. "Sometimes just one second", replies the white rabbit. There are dreams lasting an instant in which everything seems frozen for an eternity. Time is elastic in our personal experience of it. Hours fly by like minutes, and minutes are oppressively slow, as if they were centuries. 

Time didn't use to be measured the same way everywhere. The idea of time passing at the same speed during the whole time of the year was also not clear. Time was the time between sunrise and sunset. It changed with the seasons. And it was different from place to place because its progress was marked by sundials. Once faster modes of travel became possible things changed. One had to align, to synchronise clocks and the notion of time as being the same in places appeared. 

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

The place of a thing is what surrounds that thing. According to Newton space exists even if there is nothing. It's mathematical, absolute space. Space is a "container" for things to be in. This idea is recent it was not there before Newton. 

Time and space as Newton has conceptualized them are real. Einstein proved that, because he showed that they can bend and transform when in the presence of matter. 

The world is made out of fields. 

Space is the gravitational field—and vice versa.

The gravitational field can stretch and expand. Time and space can therefore stretch and expand to. They can even stretch and expand rhythmically, in a process we call gravitational waves. 

Time and space exist as absolute entities, yet they are not independent from the matter within them. The matter influences the behavior of space and time.

Chapter 5 - Quanta of Time

Quantum Theory leads to 3 properties of the universe: granularity, indeterminacy, and relations between physical variables.

Time is quantified, there exists a smallest time step. This is called the Planck Time (and the distance light can travel in that time leads to the Planck Length) They are the smallest distortions that the gravitational field can have. 

Continuity does not exist in physics. 

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

Time is also indeterminate. Like how electrons jump between places based on probability distributions, so does time, it fluctuates. And space I guess too?

It is only determined once it interacts with other pieces of time/space. When two different probability waves intertwine and interact, they both collapse and become determined for a brief moment (this idea makes no sense when viewed from the perspective of time...)

Part II

Chapter 6 - The World is Made of Events Not Things

The world is nothing but change.

The world is not static, but time as we know it still doesn't exist. It's a relation between the changes of variables.

There are no things for there is no time for them to exist in. They only "appear" as things when interacting, when changing, ceaselessly. Quantum Objects are "not defined", outside of the interactions.

The hardest stone, [...] is in reality a complex vibration of quantum fields, a momentary interaction of forces, a process that for a brief moment manages to keep it's shape, to hold itself in equilibrium before disintegrating again into dust brief chapter in the history of interactions between the elements of the planet, a have of Neolithic humanity, a weapon used by a gang of kids, an example in a book about time, a metaphor for an ontology, a part of a segmentation of the world that depends more on how our bodies are structured to perceive than on the object of perception—and, gradually, an intricate knot in that cosmic game of mirrors that constitutes reality. The world is not so much made of stones as of fleeting sounds, or of waves moving through the sea.

Thought: This reminds me a lot of the ideas of Alan Watts, which are inspired by Buddhism and Hinduism, namely that God or the Universe is playing hide and seek with itself, forgetting it's oneness to have fun.

Book Recommendation: Timaeus – Plato

We understand the world in its becoming not its being.

Chapter 7 – The Inadequacy of Grammar

Presentism: only the present moment is real, future and past aren't.

Eternalism: All of them are real all at once, forever.

Eternalism is the "block universe". Time is not only illusory, it's just like moving through a space in a different direction of a concrete 4 dimensional solid.

Thought: David Deutsch is a big proponent of this argument. Adding to it that there is a multiverse existing in this block state that describes every possibility that quantum events could have gone. All of them, there in this crazy mathematical structure, hanging forever.

Change doesn't follow a global order, it is more chaotic than that, with local and super local orders and different ranges of that order extending. But it's still there.

The word "exist" can be used in many different ways. Does a fictional character exist? What about rules in a game? Or a definite physical fact, that if looked at more closely isn't "real", like Newtonian Mechanics?

Words are relative to their position and use.

Chapter 8 - Dynamics as Relations

As human beings, we live by emotions and thoughts. We exchange them when we are in the same place at the same time talking to each other, looking into each other's eyes, brushing against each other's skin. We are nourished by this network of encounters and exchanges. But, in reality, we do not need to be in the same place and time to have such exchanges.

Thoughts and information can transcend time and space, as signals written down on a piece of paper, or electromagnetic waves traveling through the cosmos, or as waves of compressed air expanding in a room.

Space is quantized at the plank scale. These spatial units, form networks of close relations, spin networks, because of the mathematics describing their interactions. These spin networks can have loop like structures - hence the name "loop theory". The whole thing is written without a reference to the variable of time.

Part III - The Sources of Time

Chapter 9 - Time is Ignorance

What is Time? Where does our human perception of it come from if we so thoroughly deconstructed all of the notions of it to be unreal?

Properties of the universe, high and low, teams, cats and all other things, emerge from the fundamental objects that make up the universe. They are not fundamental, yet they somehow exist, as aggregates of the building blocks.

Cutting up the world into separate pieces is something that humans (and other living things) do in order to gain and convey meaningful information that is useful to survival.

Macroscopic states determine time. They determine energy and it's fluctuations which determines the usage and definition of "time", locally, in that macroscopic context.

The slicing of reality into the macroscopic system defines the order and direction of time, what happens to this "thing". Particles themselves don't care about time, but the particles in aggregate do. This conception of time is called thermal time. It's in regard to the arrow of time, defined by entropy and it increase in macroscopic systems.

Because quantum actions are non-commutative, they exhibit proto time, they are ordered.

Chapter 10 - Perspective

The entire difference between past and future may be attributed solely to the fact that the entropy of the world was low in the past.

Speed and entropy are both relative quantities. If one system has high entropy can only be defined by whether or not another system has low entropy. There is no "absolute" measuring stick.

Our interactions with the universe are particular. The entropy of the entire universe might have not changed at all, but the way that we are slicing up macroscopic systems from the whole can change that picture because we forget to count entropy in things that we don't care about.

Time might be a perspective effect, just like the "turning" of the sun. It's us that turns, leading to the misleading perspective that the sun is.

Every configuration of a pack of cards is special, it's only that we recognize a certain order of cards as more special than others because their pattern jumps out to us.

Thought: We are a system in the universe that is just as "special" as any other configuration of the universe, with one key difference. We can assign specialty to things that we care about, we are a configuration that is peculiar in so much, that it thinks of itself as peculiar and that it can slice up the universe into things peculiar to us. The universe doesn't care. We do. And the universe has no concept of entropy, we introduce it by saying look at this ordered state of things. There is no ordered state to begin with, all the states are equally likely, equally important.

Science tries to lose subjectivity, but subjectivity might mean something. It could be part of scientific theory and inquiry.

Even language has a sense of viewpoint built into many words. I, today, here, this, tonight. These are called indexical words.

Chapter 11 - What Emerges from a Particularity

Things don't need energy to work, they need low entropy in other words energy differences that can be used. The sun is providing this.

The reason why we believe that the past is causing things in the present and the future doesn't is because from our point of view the past was a special, lower entropy state, and when things move from low to high entropy it creates an illusion of cause and effect, explanations that go with it.

Chapter 12 - The Scent of Madeleine

We approximate the world by breaking it down into pieces.

We try to predict the world and in order to do our predictions efficiently we chunk up the world into chunks and concepts that are meaningful to us. We fixate on recurring structures, patterns in the world and pick up on different patterns - as a predictive machine.

The concept of ourself is a pattern we learn by experience, it is something which helps the predicting of ourselves, in relation to others. What would happen if "we" do this vs. that. Knowing about us and our capabilities is certainly useful for extracting patterns in the world that is experienced through our sensory organs.

We are histories of ourselves, narratives.

It is memory that solders together the processes, scatter across time, of which we are made. In this sense we exist in time.

Book Recommendation: Your Brain is a Time Machine – Dean Buonomeno

Book Recommendation: Confessions – Augustine

Existence is always in the present moment, yet we measure traces of the past in that moment and therefore can know about the past and integrate it into our perception right now.

Book Recommendation: Philosophicae Naturalist – William of Ockham

Book Recommendation: Critique of Pure Reason – Immanuel Kant

Boom Recommendation: Remembrance of Things Past

Time is the thing that gives rise to suffering and happiness, two sides of human life, intertwined by time.

Chapter 13 - The Source of Time

Time is intimately linked to our perception of it. Even if there is no real concept of it in physics that behaves anything like our intuition of it, the intuition still makes sense, locally.

Book Recommendation: The Direction of Time – Hans Reichenbach

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

We begin to see that we are time. We are this space, this clearing opened by the traces of memory inside the connections between our neurons. We are memory. We are nostalgia. We are longing for a future that will not come.

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

Hunger and thirst, curiosity, the need for companionship, the desire to love, being in love, the pursuit of happiness, the need to fight for a position in the world, the desire to be appreciated, recognized, and loved; loyalty, honor, the love of God, the thirst for justice and liberty, the desire for knowledge... Where does all this come from? From the way that we are made, from what we happen to be. We are the products of a long selection process of chemical, biological, and cultural structures that at different levels have interacted for a long time in order to shape the funny processes that we are.

In the end, human life is beautiful, filled with time, yet terminated by it, we will die but the things that make life beautiful are still there. The fear of Death is just our prefrontal lobes being to good at predicting the future which is inevitable, and means that we will die, sooner or later, to which another part of our brains responds with terror, like an animal confronted with a life and death situation. Yet living in spite of this fear, can be rewarding, cherishing the beautiful moments in between.