A Beginner's Guide To The Bestseller Time Does Not Exist
Carlo Rovelli
I want to try explaining the Italian physicist Carlo Rovelli’s bestselling book, “The Order of Time.”
Honestly, this is a fairly difficult thing to attempt. The provocative title is easy to grasp for someone with a background in physics, but for a beginner it runs completely against intuition. The correct advice would really be “just read the whole book.” Those who can understand it will, and those who can’t, won’t. But the content is so interesting that it would be a waste to leave it there, so I want to give it a try, if only to help spread the word. If I unpack the key points in order, I think even a beginner can reach a certain level of understanding.
My goal is for my wife (a former nurse who wants nothing to do with IT, startups, electronics, or physics) to understand it after reading this maybe 20 times.
Keep in mind that this book mixes together
- content that is already widely accepted
- claims from the author’s research group that are currently being debated in academic circles
- “hypotheses that were wrong but worth discussing,” put forward by past scholars
so that distinction needs to stay in view while reading.
Let’s begin.
The first thing to understand is that “absolute time does not exist.” This is relatively easy to grasp. For example: ① Time passes faster at higher altitude ② Clocks on airplanes run slow ③ If you could video-call someone on a star one light-year away and ask them “what are you doing right now?”, you could never hear back about the “now” you recognize
① and ② are mainly Einstein’s achievements.
Time can run fast or slow
“Gravity slows down time” is already an “absolute fact based on observation.” It has been measured in various places, and the fact that the passage of time differs by altitude has been confirmed again and again. There’s no way around accepting this. In other words, objects fall on Earth because time runs more slowly near the ground (gravity decelerates it more strongly there), and working out the mechanism governing this gravity is one of the biggest themes of modern physics.
The story about clocks running slow for fast-moving objects is often told through the example of twin brothers, one riding a spaceship back and forth to other stars while the other waits on Earth, and the one who waited ends up older (I was fascinated by this when I read a book about Einstein as an elementary school student), but what actually led Einstein to notice this was a question that arose while solving Maxwell’s equations (the tremendously exciting and elegant equations describing the relationships in electromagnetism) for something “in motion.” You inevitably cannot solve the equations without introducing a t’ distinct from the t that describes time. Einstein interpreted this as “the clock of the person in motion.”
In other words, even if two people are in the same place, there will be a subtle difference in how time passes for “a person standing still” versus, say, “a person repeatedly making tiny left-right oscillations.” This too has been experimentally verified. Despite being (nearly) in the same place, the passage of time differs. It becomes clear that there is no longer any “time” common to all things.
It’s a famous story that in his youth Einstein worked not at some prestigious research institute but at a patent office in Switzerland, and it was there, while handling patents related to the synchronization of clocks, that he realized perfect synchronization of time is impossible. That may well have been what drew him fully into this world.
What is “the present”?
Finally, ③ is, at its simplest, a matter of communication speed: you ask a question, and one year later the other person hears it, and their reply takes another year to reach you, so two years pass before the reply arrives (and it’s the other person’s words from one year after you asked) —but various alternatives can be considered here. Suppose, for example, you decided in advance on a date and time for reporting back. As noted above, though, once location and speed of movement differ, a fixed shared clock can no longer be maintained. In other words, your “now” and the other person’s “now” are already out of sync from the moment they started moving. Suppose there were a telescope that let you see someone one light-year away, and you thought: why not just call whatever you’re seeing right now your “now”? But if a method of traveling at the speed of light were ever developed, the moment you see that image, the other person might already be standing right next to you. In that case, which one is “the other person’s now” as you perceive it? After all, what you see through the telescope is not “now.” In other words, the “now” you recognize cannot possibly grasp the “now” of something far, far away.
And yet we go about our lives without feeling any of the inconsistencies raised so far. That’s because the gap in time humans can generally perceive is, at best, around 0.1 sec, and as long as we’re talking about events on Earth, the discrepancies that occur there are many orders of magnitude smaller than the limits of our perception. So as long as we limit ourselves to “humans on Earth,” it’s entirely possible to feel like we’re sharing a “now.”
<The history of “your own personal time”>
In fact, until the first division of standard time in 1883, “time being different from place to place” was simply taken for granted. Viewed from the angle of convenience, this is entirely natural.
In an era when communication was far less developed than today, personal clocks were not widespread, and people were loose about time, it’s easy to imagine that for most people, information like “how many hours until sunset (time to wrap up work and start dinner)” or “how many hours until the sun reaches its zenith (when should I light the kiln)” mattered far more than “what time is it by standard time (so as not to be late for an appointment).”
The “delay” we feel day to day in communication is not a discrepancy in time itself but the sum of things like the time taken for modulation and demodulation in communication, the time taken to process audio and video, and delays caused by processing speed being squeezed by other tasks on a computer— it is, in short, purely technical.
As far as the historical record goes, the first person to express a negative view of “your own personal time” was Aristotle (seriously, a monster). Time is a count of the number of changes in things; if things do not change, time does not pass. In darkness we experience nothing, meaning time does not pass. And yet we do think of things, meaning time does pass. That was roughly his line of thinking. At first glance this looks like an outlandish abstraction, but it actually aligns remarkably well with the worldview modern physics has since unraveled (truly a monster).
The age of Newton
Time moved on, and Newton affirmed the kind of apparent time Aristotle spoke of, but separately from that, he believed an absolute time existed (unobservable, appearing only in equations). Because Newton’s equations explained the physical phenomena of “almost everything” we see, the prevailing mood became “Newton is right.” This is where the idea of “your own personal time” became fixed in our minds. But many philosophers and thinkers of the time actually sided with Aristotle’s position and pushed back against this idea of Newton’s.
In other words, if we think “personal time exists,” Newton simply made us believe that. After that, in the world of physics, Einstein published the theory of relativity, quantum mechanics developed, and many phenomena that Newtonian mechanics could not explain were explained, and the theories were rewritten one after another.
No modern person denies the existence of elementary particles. Nor does it seem likely that any modern person would flatly deny the many findings from the field of quantum mechanics. So a modern person who affirms the existence of time is behind the times and mistaken.
Modern quantum gravity theory dates to a 1967 paper by Bryce DeWitt and John Wheeler. As they worked out quantum gravity theory, the time variable disappeared from the equations. They were confused, and left the problem for the future to solve.
<Digression: time is not smooth. It is discrete>
The question of what the smallest constituent of matter is has long driven physicists’ research. It eventually led to quantum theory, and today the common understanding is that the Planck length = 10^-33 [cm]. There is no such thing as a length smaller than that. Since no particle is smaller than that, there is no way to measure it. Likewise, time also has a minimum unit, the Planck time = 10^-44 [sec]. Since no finer unit of time than this exists, time is a discrete, granular value. Humans simply cannot perceive time at that fine a scale, so we merely feel as though it is smoothly and continuously connected.
It might feel a bit unsettling, but we live in a discrete world.
From here we cut deeply into the territory of modern physics. This is likely unfamiliar ground for anyone who has only studied physics in high school. This domain is so complex that even the great genius Einstein got parts of it wrong.
If you break things down, the smallest units go: atom > elementary particle > field. A field is an event. In other words, a thing itself is a “special state” of an event. For example, a stone is a special state of vibration in a quantum field. That acts on elementary particles, which act on atoms, and humans see, touch, and feel the “stone” state that happens to have become fixed.
Fields—elementary particles, photons, gravitational quanta—make up space. Time does not appear here. Time has no direction, nor is it a straight line, nor does it lie within a curved geometry as Einstein said.
Quantum states are sometimes described as “clouds of probability.” That phrase vividly captures the basic principles of quantum mechanics:
- everything is defined by a probability distribution
- the state is unknown until observed.
In the quantum world, nothing’s behavior is fixed except through interference with something else. Before that, all that exists is a number of probabilistically “possible states,” and “which state it actually is remains unknown”— this is difficult to grasp intuitively, yet it has been confirmed through numerous experiments.
In the microscopic world, this chain of interference and determination is constantly occurring. But because human perceptual ability cannot directly see or touch it, we cannot know the details. As a result, humans see only an approximation of an approximation of an approximation of reality, and mistake that for “the world.”
Time, too, in that world of approximation upon approximation upon approximation (the world we live in), can be perceived as a single unified thing, but in the quantum world, it is something that appears and vanishes only within interactions, only in relation to whatever it is interacting with.
Saying that time disappears might make things seem even more incomprehensible, but there’s nothing mysterious about it. Everything is born and vanishes. That is due to the increase of entropy (the second law of thermodynamics). It’s enough to understand entropy as a measure expressing that an ordered state is low and a scattered, disordered state is high.
For example, firewood has low entropy (its atoms are arranged neatly), but once it catches fire and burns, it transitions to a high-entropy state (atoms scattered about). In the universe, hydrogen has low entropy; reaching helium requires a stellar explosion, and that explosion requires a huge, contracted mass of hydrogen, which takes millions of years to form. A low-entropy state is fixed, and once something happens it moves toward increase, then stagnates again for a while, and when something happens again, a door opens once more. The workings of all life follow this flow.
Objects don’t fall because falling lowers their energy. Energy is conserved in total. The correct answer is that mechanical energy (low entropy) is transferred to the Earth as heat (high entropy).
The world is like a pile of sand slowly collapsing from the order of the universe’s birth into disorder.
And within such a world, time is not something real; the only true reality is the gravitational field.
The equations in the aforementioned paper by Bryce DeWitt and John Wheeler express the connections between individual events. In that world, there was no variable called time. And come to think of it, the famous Schrödinger equation, too, is an equation describing change.
Heidegger: “Time temporalizes only insofar as there is a being called Dasein there.” The self is born through the recognition of and interaction with the world.
Augustine said, “Time exists in the mind.” To begin with, why can we keep perceiving “the present” even though “the present” is something that flows by? The past is memory, and the future is prediction. The present is the self that perceives it. In the end, all of these exist within the self.
In quantum theory, unless there is an observer (something that interferes in some way), we cannot know what state an object is in, or even whether it exists at all. Time is the same kind of thing.
Time is a measure for gauging change. But there is no single variable t suited to measuring everything. This is exactly what Aristotle said, through and through.
The delay of time, the nonexistence of the present, the relationship between time and the gravitational field, entropy— these are all facts that have already been proven.
However, the quantization of the gravitational field remains at the level of theory. There are no experimental results. And the claim that the fundamental equations contain no time variable is not an established fact either; it is currently a hot topic under fierce debate.
Originally published in Japanese at https://clazytech.com/2021/08/673/. Translated with LLM assistance and reviewed before publication.