A Physicist's Bestseller Argues Time Does Not Exist
Carlo Rovelli
I want to try to explain the bestselling book by the Italian physicist Carlo Rovelli, “The Order of Time.”
Honestly, this is a fairly difficult thing to attempt. The provocative title is easy to grasp for someone with some background in physics, but for a beginner, it runs completely against intuition. So the correct recommendation would really be “just read the whole book.” Those who can understand it will, and those who can’t, won’t. But the content of the book is so interesting that it would be a shame to leave it at that, so I want to give this a try to help spread it further. If I unpack the key points one by one here, a beginner can get to a certain level of understanding.
My goal is for even my wife (a former nurse, someone who wants nothing to do with IT, startups, electricity, or physics) to understand it after reading this maybe 20 times.
Note that what’s written in this book is a mix of
- content that has already been broadly 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 scholars of the past
so some caution is needed when reading.
Now, let’s begin.
<There is no such thing as one’s own time>
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 couldn’t actually hear about the “now” that 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.” Measurements have actually been taken in various places, and it has been repeatedly confirmed that the passage of time differs depending on altitude. There’s no way around accepting this. In other words, objects fall on Earth, it’s thought, because time decelerates more near the ground (because time moves more slowly there), and figuring out the mechanism governing this gravity is one of the biggest themes in modern physics.
The story of clocks running slow for fast-moving objects is often told through the example of twin brothers, one of whom rides a spaceship back and forth to other stars while the other waits on Earth — the one who waited on Earth ends up aging faster. (I read a book about Einstein as an elementary school student and found this fascinating.) But what set Einstein on this path was a question that arose while he was solving Maxwell’s equations (the wildly exciting and elegant equations describing the relationships of electromagnetism) for something ‘in motion.’ He found that the equations could not be solved without introducing a t’ that differs from the t describing time. Einstein interpreted this as “the clock of the person in motion.”
In other words, even if two people are in the same place, “a person standing still” and, say, “a person repeatedly making tiny left-right vibrations” experience time passing at subtly different rates. This too has been experimentally confirmed. Despite being in (nearly) the same place, the passage of time differs. So there is no longer any “time” common to all things.
It’s a well-known story that Einstein, when young, worked not at some famous research institute but at a patent office in Switzerland, and it was there, while handling patents related to clock synchronization, that he realized “perfect synchronization of time is impossible.” That may well have been what drew him fully into this world.
What is “now”?
Finally, regarding ③: put simply, because of the speed of communication, your question reaches the other person a year later, and their reply takes another year to reach you, so altogether the reply arrives two years later (and moreover, it’s their words from one year after you asked) — but various alternatives can be considered for this problem. Suppose, for example, you decide in advance on a date and time for a report. As noted above, though, once location and speed of movement differ, you can no longer maintain a single shared clock. In other words, your “now” and the other person’s “now” are already out of sync from the moment they start moving. Suppose there were a telescope that let you see someone one light-year away, and you thought you could just treat what you’re seeing right now as “now.” But if a method of moving at the speed of light were developed, the very instant you saw that image, that person might be standing right next to you. In that case, which one would be “the other person’s now” as you recognize it? After all, what you see through the telescope is not “now.” In other words, your recognized “now” cannot grasp the “now” of something far away.
And yet, we go about our lives without feeling any of the inconsistencies described above. That’s because the discrepancies in time that humans can generally perceive are 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 “humanity on Earth,” it’s entirely possible to feel like we share a “now.”
<The history of ‘one’s own time’>
In fact, until the first standardization of time zones in 1883, “time differing by location” was simply taken for granted. From the standpoint of convenience, this is perfectly natural.
In an era when communication was far less developed than today and personal clocks were not widespread, people were loose about time, and for most people it’s easy to imagine that “how many hours until the sun sets (time to wrap up work and have 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 rather 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 purely a technical matter.
In fact, as far as recorded history goes, the first person to express skepticism about “one’s own time” was Aristotle (seriously, a monster of a thinker). Time is a count of the number of changes in things, and if things do not change, time does not pass. In darkness, we experience nothing, meaning time does not pass. But we do think of things. That means time does pass. This is roughly his line of reasoning. At first glance this seems like a far-fetched conceptual argument, but it actually aligns remarkably well with the worldview that modern physics has since unraveled (truly a monster).
Newton’s era
Time moved forward, and Newton affirmed the kind of apparent time that Aristotle spoke of, but separately from that, he believed there existed an absolute time (unobservable, appearing only in equations). And because Newton’s equations explained the physical phenomena of “almost all things” that we see, the view that “Newton is right” spread widely. This is where the idea of “one’s own time” became fixed in our minds. But in fact, many philosophers and thinkers of the time took Aristotle’s side and pushed back against this idea of Newton’s.
In other words, if we believe that “one’s own time exists,” Newton is simply the one who instilled that belief in us. Later in the world of physics, Einstein published the theory of relativity, quantum mechanics developed, and many phenomena that Newtonian mechanics could not explain were accounted for, and the theories were rewritten one after another.
There is surely no modern person who denies the existence of elementary particles. Nor does it seem likely that any modern person would flatly deny the many discoveries made in the field of quantum mechanics. So if there is a modern person who affirms the existence of time, that person is simply out of date and mistaken.
The birth of modern quantum gravity theory dates to a 1967 paper by Bryce DeWitt and John Wheeler. As they worked out their conception of quantum gravity theory, the time variable disappeared from the equations. They were confused, and threw the problem to the future.
The question of what the smallest constituent element of matter is has long driven physicists to research. Ultimately, this led to quantum theory, and today the common understanding is Planck length = 10 to the -33rd power [cm]. There is no such thing as a shorter length than that. Since no particle smaller than that exists, there’s no way to measure it either. In the same way, time also has a minimum unit, the Planck time = 10 to the -44th power [sec]. Since no finer division of time than this exists, we can say time takes discrete, discontinuous values. Humans cannot perceive time at such a fine scale, so we merely feel as though it’s connected smoothly and continuously.
It might feel a little eerie, but we are living in a discrete world.
From here we cut deeply into the territory of modern physics, territory that people who only studied physics in high school have probably never heard of. It’s such an extremely complex domain 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, an object itself is a “special state” of an event. For example, a stone is a special state of vibration of 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 are made of elementary particles, photons, and gravitons, and these form space. Time does not appear here. Time has no direction, is not a straight line, and is not, as Einstein said, embedded in curved geometry either.
Quantum states are sometimes described as a “cloud of probability.” That’s because the phrase vividly expresses the basic principles of quantum mechanics:
- everything is defined by a probability distribution
- the state is unknown until observed
In the quantum world, the behavior of anything only becomes determined through interference with something else. Before that, there are merely several probabilistic “possible states,” and “which state is actually the case is unknown” — this is a state of affairs confirmed by numerous experiments, even though it’s a state that is intuitively difficult to grasp.
In the microscopic world, this chain of interference and determination is constantly occurring. But because of the limits of human perceptual ability, we cannot directly see or touch it and so 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 way the world is.”
Even time, 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 only appears — and then vanishes — within interactions, only in relation to whatever it’s interacting with.
Saying that time disappears may make things seem all the more baffling, but there’s nothing mysterious about it. Everything is born and disappears. That is due to the increase of entropy (the second law of thermodynamics). It’s enough to understand entropy as an index expressing that an ordered state is low and a scattered 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; it “wants” to become helium, but that requires a stellar explosion, and that explosion requires a massive contracted mass of hydrogen, which takes millions of years to form. A low-entropy state is fixed, and when something happens it moves toward increase, then stagnates again for a while, and when something happens again, a new door opens. The workings of all life follow this flow.
Objects don’t fall because falling lowers their energy. Total energy is conserved. 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 shown in the paper by Bryce DeWitt and John Wheeler mentioned earlier expressed the connections between individual events. There, a world with no variable called time existed. Come to think of it, the famous Schrödinger equation is also 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 is in the mind.” To begin with, “the present” flows and yet we can go on perceiving “the present” — why is that? The past is memory, and the future is prediction. The present is the self that perceives it. In the end, all of these reside within the self.
In quantum theory, unless there is an observer (something that interferes in some way), it’s impossible to 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.
The delay of time, the nonexistence of the present, the relationship between time and the gravitational field, entropy — these are all proven facts.
But the quantization of the gravitational field is still only at the level of theory. There are no experimental results. And the claim that there is no time variable in the fundamental equations is also not a settled fact, but rather a hot topic currently under fierce debate.
Originally published in Japanese at https://clazytech.com/2021/08/673/. Translated with LLM assistance and reviewed before publication.