DEEP READING · READ 18a
The Order of Time · Carlo Rovelli · 2017
Every feature you attribute to "time" — that it is uniform, that it flows in one direction, that there is a single "now" everywhere, that it exists independently of things — modern physics peels away, layer by layer, until at the deepest level the very word "time" no longer appears in the equations. And yet you plainly feel time flowing, aging you, carrying you toward death. Rovelli's answer: that flow is not a property of the universe but something that we — creatures who remember, forget, and can only see the world in a blur — read into a world that has no time of its own. Time is not the stage on which the universe unfolds. Time is us.
Rovelli is an Italian theoretical physicist, a co-founder of loop quantum gravity — one of the main attempts to stitch gravity together with quantum mechanics. He also wrote the global bestseller Seven Brief Lessons on Physics. This 2017 book is his most ambitious: it uses physics at its frontier to dismantle every human intuition about "time," then turns around and asks — if there is no time in physics, where does our felt sense of its passing come from? The whole book is built like a symphony in three movements: first it breaks time apart, then it describes a world without time, and finally it lets time grow back out of that world and onto us. What makes it singular is that it refuses to merely dazzle with science; it pushes physics all the way into philosophy, into Proust, into the face of death.
The first cut removes "time is uniform." Einstein's general relativity showed long ago that the closer you are to the ground, the closer to a mass, the slower time runs; higher up, farther away, it runs faster. This is not mechanical error in a clock; it is time itself flowing at different rates in different places. Rovelli gives a crisp example: two friends, one who lives by the sea and one who moves up a mountain — when they meet again years later, the one on the mountain has aged a little more, because he sat farther from the center of the Earth and time ran faster for him. Even your head ages a touch faster than your feet.
This sounds like mysticism, yet it has been measured to the bone. Today's most precise atomic clocks can detect a difference in rate when two clocks are separated by mere tens of centimeters of height. (NIST demonstrated this with optical clocks around 2010.) GPS satellites must correct for this time difference every single day, or positions would drift off by kilometers. The conclusion is hard: there is no such thing as "a single time for the universe." No master clock ticks in the background; every object, every place, has its own rhythm. What we take for "uniform time" only seems so because we are all crowded onto the surface of one planet, all moving slowly, so the differences are too tiny to notice.
The second cut removes "the present." Our intuition harbors an enormous assumption: that at this instant the entire universe shares a single "now" that is happening — that Earth's now, Mars's now, the Andromeda galaxy's now are one and the same moment. Relativity says: this shared "now" does not exist.
Rovelli drives it home with Andromeda, which lies about 2.5 million light-years away. Ask "what is happening on Andromeda right now," and the question has no definite answer. Because simultaneity itself is relative: for observers in different states of motion, which two events count as "happening at the same time" differs. Between you and Andromeda there is a vast band that is neither your past nor your future — Rovelli calls it the extended present, a fog millions of years wide in which "before" and "after" have no meaning. So "now" and "here" are words of the same kind: "now" only means something for "here"; it is local, not cosmic. To speak of "the state of the universe right now" is as empty as standing on Earth and asking "where is the universe's 'here'?"
The third cut is the most startling: in the fundamental laws of physics, past and future are symmetric and indistinguishable. The basic equations of Newtonian mechanics, electromagnetism, relativity, and quantum mechanics all hold if you swap t for −t and run them backward. A film of planets orbiting, played in reverse, still obeys the laws of physics — you cannot catch it cheating. Fundamental physics does not know any difference between "past" and "future."
So where does our vivid sense that "time has a direction" come from — that a broken egg never reassembles, that people age but do not un-age? Rovelli points to a single source: the increase of entropy, that is, the second law of thermodynamics. The one natural phenomenon in the universe that distinguishes past from future is that heat always flows from hot to cold, never the reverse. Ice melts in warm water, perfume disperses through a room, energy spreads from concentrated to diffuse — these run in one direction only. Rovelli's crystallizing line: the entire difference between past and future comes down to just one thing — that entropy was lower in the past. The arrow of time is not an arrow of space or of gravity; it is a thermal arrow.
Why does entropy only ever rise? Thanks to Ludwig Boltzmann's stroke of genius, carved on his tombstone as S = k log W: entropy measures nothing more than how many microscopic arrangements look identical at the macroscopic level. A shuffled deck has an astronomical number of "disordered" arrangements and exactly one "sorted low-to-high" arrangement — so disorder is the norm and order is a miracle, and a system naturally slides toward the state with more arrangements (higher entropy), simply because that state is overwhelmingly more likely. Entropy increase is not a mysterious force; it is just probability: the universe evolving toward what is more probable. (Boltzmann was ridiculed in his lifetime for this "merely statistical" explanation and took his own life in 1906, never living to see himself canonized.)
Here Rovelli plays his deepest and most contested card. If the arrow of time comes from "entropy was lower in the past," then we must ask: why is the entropy "low"? Who decides what counts as "ordered" versus "disordered"?
His answer is seismic: entropy is not absolute — it depends on how you "blur" your view of the world. Back to the deck: "sorted" looks special only because we happen to care about suit and rank; to a being that cared about some other property, an entirely different set of arrangements would look "special." "Order" is always relative to the macroscopic variables some observer cares about. A system's entropy depends on how you "bundle" microscopic states into "the same kind" — and that bundling, that blurring (Rovelli's coarse-graining), is brought by the observer; it is not built into the world.
From this he draws a vertiginous conclusion: the entropy of the past may be low not as an objective fact of the universe, but as a consequence of the particular set of blurred variables that "we, this specific physical subsystem," happen to use when we interact with the world. In other words, the arrow of time may be an artifact of our angle on the world, not a property of the world itself. He puts it fiercely: "the passage of time springs from our ignorance of the world." The universe has no past or future of its own; it is creatures like us — able to grasp only a handful of macroscopic features and blind to the ocean of microscopic detail — who read out, from our particular blurred viewpoint, a world of rising entropy and flowing time. The arrow of time is not in the universe; it is between us and the universe.
The fourth cut removes "time is an independent container." Newton imagined time as a line that flows on its own, independent of everything, with the world playing out inside it. Rovelli sides with Aristotle: time is nothing but the measure of change itself — no change, no time. Einstein went further and welded time to space and to the gravitational field: spacetime is not a stage; it is itself the gravitational field, a "thing" in the universe that bends, trembles, and undulates with matter.
From this Rovelli gives the heart of his worldview: the world is not made of "things" but of "events." What you take for solid, permanent "objects" are all slow events. A stone is merely a process that lasts unusually long and changes unusually slowly — it is a "long event." A kiss is a short event, a stone a long one; the difference is only pace. A storm, a song, a family, and even you yourself are not "things" but happenings — processes, networks of relations. The reason "what is time?" bewilders us is that we keep trying to treat it as a thing; but it is not a thing at all — it is the very relation of ongoing change between things. This step reconnects physics to Heraclitus's ancient intuition that "everything flows," and to the Buddhist insight of impermanence and no-self: the world is not assembled from unchanging substances but woven from processes in interaction.
The fifth cut strikes continuity. Quantum mechanics tells us energy and matter come in discrete packets (quanta); Rovelli's loop quantum gravity goes further and claims time and space are granular too, with a smallest possible interval of time — the Planck time, about 10⁻⁴⁴ seconds. A "time" shorter than that is meaningless, just as you cannot own half a photon. Time is not a smooth line but a string of exceedingly fine grains; at the smallest scale it even "superposes" and grows fuzzy, quantum-style.
And the most astonishing finale: the equations describing the universe's most fundamental level contain no variable "time" at all. The Wheeler–DeWitt equation of 1967 — a basic equation of quantum gravity — has no t in it. This is not an oversight; it says that at the deepest level the universe can be fully described without any "master time" beating in the background. All you need is to describe how the variables change relative to one another — where the Moon has moved, how much sand has run through the hourglass, how many times the heart has beaten — each referring to the others. That is enough. No "true time" flows; there are only countless things changing with respect to each other. "Time" is not a fundamental building block of the universe; it is just one variable we have singled out from this web of mutual change and pressed into service as a ruler.
Having cut this far, a sharp question arises: if the universe has no unified time, no present, and the arrow of time is merely perspectival — then what is our so-vivid feeling that time is passing? Rovelli's answer is the book's most moving passage: the passing of time is not in the world but in us; and it is in us because "we" are ourselves made of time.
He borrows the famous madeleine from Proust's In Search of Lost Time — the cake dipped in tea that, in a single mouthful, brings a whole lost childhood flooding back — to make the point: the self is a knot tied out of the traces of memory. The brain ceaselessly weaves together the traces the past has left (memory) and its projections toward what is to come (anticipation), and it is precisely this weaving that manufactures the experience of "I persist, and time flows." Without memory there is no "I," and no felt passage of time. As Rovelli puts it (paraphrased): we are time — we are this small clearing opened by the traces that memory leaves in the connections between our neurons; we are stories, processes woven from time. So time is not a prison we are trapped inside; time is the very material we are made of. This is why the book comes to rest on death: the fear of death is, in a sense, an evolutionary illusion, and seeing clearly that "we are simply a stretch of time" may let us meet that ending with a little more peace — death, as the ancients said, being the sister of sleep.
The book is a journey of descent and re-ascent, in three movements:
One line to thread it all: physics takes time away from the universe and hands it back to us — because time was always something on "our" side, not the universe's.
1. There is no "single time for the universe": the closer to a mass, the lower down, the slower time runs — someone on a mountain ages faster than someone by the sea, and atomic clocks can now measure this across mere centimeters of height.
2. There is no "now" for the whole universe: "what is happening on Andromeda right now" has no definite answer; "now," like "here," is only a local word.
3. In the fundamental laws of physics, past and future are perfectly symmetric and indistinguishable — run time backward and the equations still hold.
4. The one phenomenon that naturally distinguishes past from future is heat flowing from hot to cold and entropy only rising; the arrow of time is a "thermal" arrow.
5. Entropy increase is not a mysterious force but mere probability: Boltzmann's S = k log W says the universe just evolves toward states that have more arrangements and are therefore more likely.
6. Rovelli's boldest stroke: entropy depends on how you "blur" your view of the world — the low entropy of the past may be not an objective fact but a product of our particular perspective. "The passage of time springs from our ignorance."
7. The world is made not of "things" but of "events": a stone is a "long event" that changes very slowly, and you and I are not objects but processes, webs of relation.
8. Time is granular (the smallest Planck time is about 10⁻⁴⁴ seconds), and in the equations describing the universe's deepest level the variable "time" simply disappears — leaving only things changing with reference to one another.
9. Our vivid sense of time passing comes from the brain weaving memory and anticipation together: without memory there is no "I" and no flow of time; we are not in time, we are time.
10. Physics takes time away from the universe and gives it back to us — time was always on "our" side; and seeing this may let us meet death (the sister of sleep) with a little more peace.