The Scale of Time

80 years. 2.5 billion seconds. The journey starts here.

Two and a half billion seconds. Said like that, it sounds enormous. And yet each passing day consumes 86,400 of them — and nobody feels them go.

A human life fits inside a ten-digit number of seconds. That's both immense and modest: immense because no other mammal of comparable size lives as long; modest because that number, measured against what comes next in this journey, will become invisible.

But for now, it's all we have. The most natural unit of measure there is — not the metre, not the kilogram, but a life. Everything that follows in this journey will be measured against it.

All lives end to end

An estimated 100 billion human beings have lived on Earth since our species appeared. Giving them an average life of around thirty years — accounting for infant mortality, wars, epidemics — that comes to roughly 3 trillion person-years of human life accumulated. (100 billion people × 30 years = 3 trillion years.)

The universe is 13.8 billion years old. Which means that if you laid every human life ever lived end to end, in a single unbroken chain, it would stretch to roughly 220 times the age of the universe. (3,000 ÷ 13.8 ≈ 220.) All human experience — every love, every loss, every sleepless night — amounts to 220 universes lived in parallel, accumulated in silence.

Humanity's oldest timekeeping device.

Before clocks, before calendars, the heart was our timekeeper. Musicians still count rhythm by it. Greek physicians measured the pulse of the sick.

A human heart beats around 72 times per minute — one pulse every 0.83 seconds, all life long, never willingly stopping. Over 80 years, that's roughly 3 billion beats. A number that's hard to feel, until you realize that each one of those beats was a real second — lived, and gone forever.

What's striking is that mammals seem to all receive the same quota. A mouse beats 600 times per minute and lives 2 years. A blue whale beats around 6 times per minute at rest and lives 80 to 90 years. The total number of beats converges, for almost all mammals, around one billion. Humans, with their 3 billion, are a remarkable exception — an animal that has somehow learned to outlive the pace its heart was built for.

The smallest unit of your presence in the world.

While you blink, 10 trillion chemical reactions happen in your body. You missed all of them.

A blink lasts between 150 and 400 milliseconds. Over a day, you blink between 10,000 and 20,000 times — about 30 minutes of total darkness, sliced into imperceptible fragments. The brain erases them: it "fills in" the missing image by extrapolating from what it saw just before. That's why you never notice the dark.

This mechanism — saccadic suppression — reveals something essential about our relationship with time: what we perceive isn't raw reality, but a continuous reconstruction. The experience of the present moment is already, in part, the past.

The jump that lights the lamps, the screens, the stars.

All non-thermal light is born from an electron jumping between energy levels. A candle flame, a neon sign, the light of a star — same mechanism, same duration.

Inside an atom, electrons occupy distinct energy levels — well-defined "floors." When an electron absorbs energy, it climbs a floor. Unstable, it immediately falls back down, emitting a photon — a particle of light whose colour depends precisely on the energy difference between the two levels. This is what gives sodium lamps their orange-yellow glow, or neon its red-orange: each atom has its own floors, so its own colours.

The duration of this jump varies by atom and transition: from a few femtoseconds (10⁻¹⁵ s) for highly energetic transitions, up to a few nanoseconds (10⁻⁹ s) for the slower ones that produce ordinary visible light. That last scale — the nanosecond — governs the light you see around you.

In 2023, the Nobel Prize in Physics recognised the mastery of attosecond lasers — pulses ten thousand times shorter than a femtosecond. For the first time, they make it possible to film electrons in motion as they jump, in real time.

Light oscillates. Colour is its rhythm.

Light is an electromagnetic wave: an electric field and a magnetic field oscillating as they propagate. Colour isn't a property of the surface you're looking at — it's the frequency of that oscillation.

Red light oscillates about 430 trillion times per second — a period of ~2.3 femtoseconds. Violet light oscillates about 750 trillion times — a period of ~1.3 femtoseconds. Between these two frequencies — from the red of a cherry to the violet of lavender — lies all the light the human eye can see. A narrow window.

Below red: infrared. You can't see it, but you feel it as heat — it's what a radiator or glowing embers emit. Above violet: ultraviolet. Also invisible, it's energetic enough to damage DNA — hence sunburn.

The same timescale, and vision

It's precisely at this scale — a few femtoseconds — that vision begins. When a photon strikes a pigment in your retina, a molecule called retinal changes shape in about 200 femtoseconds. That change triggers a nerve signal that travels to the brain, which produces an image. Light oscillates, strikes a molecule, and the molecule flips — in less time than it takes light to cross a few wavelengths.

Observing reactions this fast once seemed impossible. Ahmed Zewail solved it in the 1980s using two ultrashort laser pulses: the first to trigger the reaction, the second, slightly delayed, to photograph it at different stages. He received the 1999 Nobel Prize in Chemistry for founding femtochemistry.

During the blink of your eye — 200 milliseconds — green light will have oscillated 100 quadrillion times. A femtosecond is to a blink what a blink is to 1.3 million years.

The time it takes light to cross the heart of matter.

If crossing an atomic nucleus took the time of a blink, that blink would last tens of billions of years — longer than the universe has existed.

An atomic nucleus measures about one femtometre — one millionth of one billionth of a metre. Light, which travels 300,000 kilometres in a second, takes 10 zeptoseconds to cross it.

It's at this same scale — a few femtometres — that the strong force operates, the force that binds quarks into protons, and protons and neutrons into nuclei. This isn't cause and effect: the crossing time and the range of the strong force don't determine each other. But they inhabit the same territory. The strong force is an extremely short-range force — it simply switches off beyond a few femtometres, as if it didn't exist. 10 zeptoseconds is the time light takes to cover that domain. It's the size of the kingdom.

Without the strong force, atomic nuclei would instantly fall apart. Protons repel each other — they all carry a positive charge, and like charges push away. It's the strong force that overcomes this repulsion, but only at close enough range. The result: every atom in your body holds together because, at the femtometre scale, an invisibly short-range force maintains its cohesion — without ever stopping, since the beginning of the universe.

The particle that doesn't have time to feel the strong force.

The top quark lives less time than it takes light to cross a proton. And yet it weighs as much as an entire tungsten atom.

What a quark is

Protons and neutrons — the building blocks of the atomic nucleus — are not themselves fundamental. They are made of quarks, bound together by the strong force. There are six types of quarks, named by physicists: up, down, charm, strange, bottom, and top. The first two — up and down — make up all ordinary matter: a proton is made of two up quarks and one down quark, a neutron of two down and one up.

The other four quarks — including the top — don't exist in stable matter. They're so heavy and so unstable that they only form under extreme conditions: inside particle accelerators, or in the very first instants of the universe, when energy was sufficiently concentrated. The top quark is the heaviest of the six — it weighs as much as an entire tungsten atom, which is extraordinary for a fundamental particle.

Why it dies so quickly

Its lifetime — about 0.5 yoctoseconds — is so brief that it decays before the strong force even has time to bind it to other quarks and form a hadron. It isn't the strong force that destroys it: it's the weak force, which converts it into a bottom quark by emitting a W boson. But the consequence is striking — all other quarks, being lighter, live long enough to be captured by the strong force and form composite particles. The top quark stands alone: it appears, leaves a few traces in a detector, and vanishes.

This solitude makes it precious to physicists. A quark bound to others is difficult to study — the strong force muddies the measurements. The top quark decays before binding: it can be measured almost bare, like a pure fundamental particle. It's a rare window onto the deepest laws of matter.

If the top quark lived as long as a housefly — about a month — then on the same proportional scale, a human being would live 10⁴¹ years. A number that exceeds the current age of the universe by 31 orders of magnitude.

The smallest duration physics allows us to conceive.

Below this duration, the word "before" loses all meaning. Planck time isn't the smallest duration ever measured — it's the smallest that can even be conceived.

What it represents

Imagine time as a digital image. Each pixel is the smallest unit of detail the image can hold. Below a pixel, there is no blurry half-pixel — there is simply nothing. Planck time may be that pixel: not just a very short duration, but the ultimate granularity of time itself.

This number emerges naturally when you try to combine the two great theories of modern physics — general relativity and quantum mechanics. At this scale, the two fall into irreducible contradiction. Quantum gravity, the theory that would reconcile them, doesn't yet exist. Planck time is the limit where our understanding stops.

The distance to you

Between Planck time and a human life, there are 54 orders of magnitude. To get a sense of it: if Planck time lasted the time of a blink, that blink would last 2 × 10³⁴ years — about one million trillion trillion times the age of the universe. Everything this journey has just crossed — heartbeats, blinks, chemical reactions, electron jumps, light oscillations, a nucleus crossing, the top quark — fits inside that space.

We were only in the instant. We haven't moved yet.

300,000 years of humanity. Of which barely 5,000 years of writing.

All of humanity's written memory — Homer, Caesar, Shakespeare, Einstein — fits inside the last 1.7 percent of our existence. The rest left no recorded history. Everything was passed on orally, or was lost.

~1,550 years ago

In 476, Rome falls in the West. The city that had dictated the laws of the Mediterranean world for a thousand years yields to the Visigoths of Odoacer. Not an explosion — a slow disintegration, after two centuries of pressure. Sumer, the first great civilisation on record, had lasted about 3,000 years before disappearing in turn. Most civilisations don't reach that threshold: the average lifespan before collapse or major transformation runs around 300 to 400 years.

~2,600 years ago

The Greek alphabet, direct ancestor of our own, spreads through the Mediterranean basin. A rupture: for the first time, anyone can learn to read and write in a matter of weeks. Earlier scripts — cuneiform, hieroglyphs — demanded years of study and kept writing the domain of scribes.

~4,600 years ago

The Great Pyramid of Giza is built. Cleopatra will be born 2,491 years later, in 69 BC — and the iPhone will arrive 2,076 years after Cleopatra. The pyramid is therefore more distant from Cleopatra than it is from us. This reversal of perspective captures the depth of historical time: even what we call "Antiquity" is a continuum spanning several millennia, not a single block.

It's also around this time that a relict population of woolly mammoths goes extinct on Wrangel Island, in the Russian Arctic. Continental mammoths had disappeared much earlier, around 10,000 years ago. But this island group survived until around 2000 BC — while the Egyptians were building the pyramids. Two realities coexisted on the same Earth, thousands of kilometres apart, neither aware of the other.

~5,200 years ago

In Sumer, Mesopotamia, scribes carve the first cuneiform signs into clay tablets. Grain receipts, inventory lists — accounting before literature. Writing doesn't begin with poetry. It begins with trade.

~10,000 years ago

In the Fertile Crescent, some communities stop following herds and start planting. Agriculture doesn't invent itself in a single day or place: it emerges independently across several parts of the world, over a few millennia. But this is where the transition is best documented. Over a few generations, nomadic groups become sedentary. The first permanent villages appear. Human time changes in nature: people begin to think in seasons, harvests, years.

~12,000 years ago

The last ice age draws to a close. Glaciers retreat. The landscapes we know — coastlines, forests, rivers — take shape. Sea levels rise by tens of metres, swallowing inhabited land. It's in this context that the first permanent villages of the Levant appear, precursors of agriculture.

~17,000 years ago

In what is now the Dordogne, France, humans descend into underground galleries and paint horses, aurochs, deer by the light of fat lamps. The caves of Lascaux. Not a primitive gesture — an expressive power that speaks to a fully developed mind: capable of abstraction, representation, perhaps narrative. These people thought, dreamed, told stories. They were us.

~40,000 years ago

Neanderthals disappear from Europe. They had lived there for 400,000 years. Homo sapiens, arrived a few millennia earlier, takes their place — not by wiping them out, but partly by absorbing them, progressively pushing them into marginal zones. Modern genetic studies have confirmed it: a portion of their DNA still lives in each of us.

~300,000 years ago

Homo sapiens appears in North and East Africa. Not as an entirely new species sprung from nothing, but as a gradual transformation from close ancestors. These first modern humans looked physiologically like us — rounded skull, flat face, prominent chin. What distinguished them from earlier species emerged progressively: the capacity for complex articulated language, enabling the transmission of narratives, the coordination of groups, reasoning about the absent and the future; symbolic behaviours — art, rituals, personal ornament. These capacities didn't appear all at once. They seem to have consolidated over tens of millennia, carried by a brain whose deep structure hasn't changed since.

186 million years of dominion. Of which 0.16% is humanity.

If the reign of the dinosaurs lasted 24 hours, all of human history — from the earliest Homo sapiens to today — would occupy the last 83 seconds. And Tyrannosaurus rex would only appear in the final 17 minutes.

252 million years ago · The dawn

The worst extinction in the history of life has just ended. The Permian extinction — probably triggered by a cataclysmic volcanic episode in Siberia that saturated the atmosphere with CO₂ and acidified the oceans for hundreds of thousands of years — wiped out roughly 96% of marine species and 70% of land species. A world almost entirely erased. The dinosaurs didn't cause it: they inherited it. The first of them appear in a convalescing world, most ecological niches empty. They don't yet dominate — they coexist with other reptiles, feeling their way.

201 to 145 million years ago · The peak

A smaller extinction — probably linked to another intense volcanic episode at the Triassic-Jurassic boundary — eliminates the rival reptiles. The dinosaurs inherit the world. Their golden age: the continents fracture slowly, the climate is warm and humid, the forests dense. The giants appear — Brachiosaurus, Diplodocus, Allosaurus. Animals of 30, 40, 50 tonnes.

Why such size, never matched by any land animal since? Not simply because food was abundant — modern elephants, in equally rich environments, cap out at 6 tonnes. The difference lies in the deep biology of dinosaurs: hollow bones like birds (light for their volume), a highly efficient flow-through respiratory system, and rapid growth that let them reach those dimensions without spending decades vulnerable. The blue whale exceeds those masses today, but only because water carries its weight — on land, the mechanical constraints make such size impossible for a vertebrate.

145 to 66 million years ago · The mature reign

The Cretaceous is the longest of the three periods, and the most complex. Angiosperms — flowering plants — appear and transform the landscapes. Bees co-evolve alongside them. Birds, direct descendants of certain feathered dinosaurs, diversify. And in the very last 2 million years of this period, near the end, Tyrannosaurus rex arrives — the most famous species of the reign, and yet one of the most recent. It lived chronologically closer to us than to Stegosaurus, which preceded it by 80 million years.

66 million years ago, an asteroid 10 kilometres across strikes the Yucatán Peninsula. The impact releases energy equivalent to billions of nuclear bombs, triggers tsunamis, global wildfires, and hurls so much debris into the stratosphere that the Sun disappears for years. Within a few decades, three quarters of all species go extinct — including all non-avian dinosaurs. What survives, in underground and aquatic refuges: reptiles, birds, and small mammals. Their descendants, 66 million years later, are reading this.

From the first self-replicating molecule to the first mammal: 3.6 billion years of invention.

Life took 2 billion years to invent the cell with a nucleus. The slowest leap in all of evolution — and perhaps the most decisive.

~225 million years ago · The first mammal

In the late Triassic, while the dinosaurs are settling in, a small group of synapsid reptiles gives rise to the first mammals. They are tiny — mouse-sized. They probably live at night, when the dinosaurs are less active. Their brains are proportionally larger, their metabolism faster, their relationship to offspring different. They won't dominate the world for another 160 million years, after the dinosaurs are gone — but they are already there, discreet and persistent, throughout the entire reign of their immense contemporaries.

~350 million years ago · The first land vertebrates

Lobe-finned fish — capable of hauling themselves out of the water for brief forays — give rise to the first tetrapods. These four-limbed animals are the common ancestor of all living amphibians, reptiles, birds, and mammals. The vertebrate colonisation of dry land begins here. What drove them out of the water remains debated: predator escape, access to new resources, or simply the chance of an anatomy that lent itself to it.

~540 million years ago · The Cambrian explosion

Within a few tens of millions of years — a geological flash — nearly all the major groups of complex animals appear simultaneously in the fossil record. Eyes, mouths, articulated limbs, shells, nervous systems. Before the Cambrian, life was essentially microbial or gelatinous. After: a world populated by recognisable creatures. Why this sudden explosion? Atmospheric oxygen had finally reached a threshold sufficient to sustain an active metabolism. The evolution of eyes may also have played a role — once an animal can see its prey, all its neighbours must adapt or disappear.

~700 million years ago · The first multicellular organisms

Simple multicellular organisms — sponges, Ediacaran jellyfish-like forms — appear in the oceans. Technically animals in the phylogenetic sense, but nothing like the complex fauna of the Cambrian: no eyes, no limbs, no nervous system. It's the first time in the history of life that an organism is made of differentiated cells working together. The transition from single- to multi-celled life is fundamental: it opens the path to all the complexity that follows. It happened independently several times in evolution — suggesting it was not improbable, just slow.

~1.5 billion years ago · The nucleated cell

A bacterium engulfs another bacterium without digesting it. Both survive, and their descendants form a new type of cell — the eukaryotic cell, equipped with a nucleus and mitochondria. Every cell in your body is of this type. Not a gradual mutation: a fusion between two distinct organisms. The mitochondrion is still today, genetically, a bacterium living inside your cells. This symbiosis took 2 billion years to occur after life's appearance. It made all complex life possible — animals, plants, fungi.

~2.4 billion years ago · The Great Oxidation

Cyanobacteria — tiny photosynthetic organisms — have been releasing oxygen as a waste product for hundreds of millions of years. Oxygen accumulates first in the oceans, oxidising dissolved iron. Then it spills into the atmosphere. For nearly all living organisms at the time, it's a poison: oxygen is chemically aggressive, destroying fragile organic molecules. The Great Oxidation may be the largest extinction in the history of life — almost never discussed, because there were no complex animals yet to suffer it. And paradoxically, this catastrophe made everything that followed possible.

~3.5 billion years ago · The first fossils

In Western Australia, in rocks 3.5 billion years old, traces of stromatolites are found — layered structures formed by mats of photosynthetic bacteria. These are the oldest confirmed fossils. These bacteria already existed in organised communities, capturing sunlight, producing organic matter. Life at this time was already functional, already metabolically sophisticated. Not at its beginnings — already established.

~3.8 billion years ago · The origin

The oldest chemical traces of life date to around 3.8 billion years ago — carbon isotope ratios in metamorphic rocks from Greenland that suggest biological activity. The Earth was then 700 million years old. Nobody knows exactly how the first self-replicating molecule appeared — in a warm pond, in an underwater volcanic vent, on a grain of cosmic dust. Several hypotheses coexist. What is certain: something began there, and has never stopped.

700 million years of violence, fusion, bombardment — before the first breath.

Earth wasn't always a planet. First a cloud of dust and gas, then an aggregate of colliding rocks, then a ball of molten rock. It took 700 million years before the conditions for possible life were in place.

~4.54 billion years ago · Formation

Around the young Sun, a disk of gas and dust rotates. Dust grains slowly aggregate into rocks, rocks into asteroids, asteroids into planets. Earth forms by accretion over tens of millions of years — a violent process, marked by constant collisions. The heat released by those impacts melts the rock: the young Earth is largely liquid, a magma ocean beneath a sky of vapour.

~4.5 billion years ago · Birth of the Moon

A Mars-sized body — called Theia — strikes the proto-Earth in an oblique collision. The impact hurls billions of tonnes of material into space. That material enters orbit and accretes over a few decades to form the Moon. Without this impact, no Moon — and perhaps no life: the Moon stabilises Earth's axial tilt, regulating seasons and climate over millions of years. It also slows Earth's rotation, gradually lengthening the days.

~4.4 billion years ago · The first crust

The surface cools. The first grains of zircon — an extremely resistant mineral — form in the nascent crust. Some have been found in Australia: the oldest known terrestrial materials, dated to 4.4 billion years. They prove the solid crust already existed, and that liquid water may have been present at the surface. The conditions for prebiotic chemistry were beginning to come together — even if the violence of the Late Heavy Bombardment that followed wouldn't make things easy.

~4.1 to 3.8 billion years ago · The Late Heavy Bombardment

A period of intense meteoritic bombardment strikes Earth, the Moon, and the other inner planets. The craters visible on the Moon today date largely from this era. Giant impacts may have vaporised the oceans and sterilised the surface multiple times. And yet — perhaps because of those very impacts bringing water and organic molecules from asteroids and comets — life seems to have begun just afterward, or perhaps even during this period. The boundary between destruction and origin is blurred.

Our star is at mid-life. It has been burning for 4.6 billion years, with roughly as many left.

Our star is at mid-life — it has been burning for 4.6 billion years (Ga), with roughly as many remaining. Ga stands for gigaannum, one billion years — the standard unit in cosmology and geology.

Birth of the Sun

4.6 billion years ago, a molecular cloud of gas and dust — perhaps disturbed by a nearby supernova's shockwave — begins to collapse under its own gravity. Matter accumulates at the centre: pressure and heat build until the first nuclear fusion reactions ignite. The Sun switches on. It has been burning around 600 million tonnes of hydrogen per second ever since.

Formation of the planets

The rest of the cloud, flattened by rotation, forms a disk of gas and dust around the young Sun. In that disk, dust grains slowly aggregate: first millimetre-sized grains, then pebbles, then kilometre-scale bodies called planetesimals, then protoplanets. Collisions are violent and frequent. Jupiter forms quickly — massive enough to capture the surrounding gas before it disperses into space, explaining its gaseous composition. Then Saturn. The rocky planets — including Earth — form more slowly, by the gradual accumulation and merging of rocky planetesimals, a process spanning tens of millions of years.

The first stars had no planets, no possible life. They just lit up the universe.

For the first 180 million years of the universe, there were no stars. A gas of hydrogen and helium slowly cooled in total darkness. We call this era the Cosmic Dark Ages.

~13.6 billion years ago · The first stars

The first stars — called Population III stars — bear no resemblance to our Sun. They are made almost entirely of hydrogen and helium, the only elements that then existed: the Big Bang produced only these two light gases. They are probably very massive — tens to hundreds of times the mass of the Sun. A star that massive burns through its fuel at a prodigious rate: where our Sun will live 10 billion years, these giants lived a few million, then exploded as supernovae.

These explosions had a decisive consequence: they scattered through the universe the first heavy atoms — carbon, oxygen, nitrogen, iron, silicon — forged by nuclear fusion in their cores. Without those stars, dead 13 billion years ago, there would be no rocky planets, no water, no organic chemistry. Everything around you — and you yourself — is made of their ash.

~13 billion years ago · The first galaxies

Stars don't form alone: gravity groups them into ever-larger structures. The first galaxies appear as early as 300 to 500 million years after the Big Bang — earlier than models predicted before JWST observations — small, irregular clumps far from the elegant spirals we know. They merge over billions of years into larger and larger structures. The Milky Way itself is the product of many mergers and absorptions of smaller galaxies, a process still ongoing today.

The James Webb Space Telescope, operational since 2022, has detected galaxies as far back as 300 million years after the Big Bang — far earlier than models predicted. These observations surprised cosmologists: the universe structured itself faster than previously thought. The models are being revised.

~10 billion years ago · The peak

The rate of star formation across the universe peaks about 10 billion years ago. Galaxies are then at their most active: dense, bright, rich in gas. Since then, the rate has declined steadily — the gas available to form new stars is gradually exhausted, consumed or ejected by supernovae. Our Sun formed 4.6 billion years after this peak, inside a Milky Way already in decline. We live in the descending phase of a cosmic firework that reached its peak long before our birth.

The beginning of everything. Or at least: the beginning of what we can see.

The first 380,000 years of the universe are opaque to any observation. Light could not travel freely in a universe too dense and too hot. Everything we can directly observe begins there — the moment the universe became transparent.

What the Big Bang was not

The Big Bang was not an explosion in space — it was an expansion of space itself. No centre from which everything burst out, no edge beyond which there was emptiness. At every point in the universe, density and temperature were infinitely high, and they decreased everywhere simultaneously as space expanded. Counter-intuitive, but exactly what the equations of general relativity describe — and what observations confirm.

The first fractions of a second · Inflation

Before the first particles even form, the universe passes through an extraordinarily rapid expansion phase: cosmic inflation. Between roughly 10⁻³⁶ and 10⁻³² seconds after the Big Bang, space expands exponentially — by a factor of at least 10²⁶ in an imperceptible instant. It isn't matter moving: it's space itself growing faster than light (which is permitted, since no information is travelling). Inflation explains why the universe is so homogeneous at large scales — and why the slight density fluctuations observed within it form the blueprint from which galaxies will coalesce hundreds of millions of years later. It remains one of the best-supported hypotheses in modern cosmology, even though the precise mechanism that triggered it is still unknown.

The first minutes

In the first fractions of a second, energy condenses into fundamental particles: quarks, electrons, neutrinos. Within a few minutes, quarks assemble into protons and neutrons. This is when the ratio of hydrogen to helium in the universe is fixed — roughly 75% hydrogen to 25% helium, a proportion that matches precisely what we measure today in the oldest stars. One of the most precise and well-verified predictions in modern cosmology.

380,000 years later · The wall of light

For the first 380,000 years, the universe is a plasma so dense that photons cannot propagate — they are immediately reabsorbed. The universe is opaque. Then, as it cools, protons capture electrons to form the first neutral atoms. This shift makes the universe transparent within a few tens of thousands of years. Light propagates freely for the first time. This radiation — cooled since by 13.8 billion years of cosmic expansion — is still detectable today across the entire sky as microwaves: the cosmic microwave background. It is the oldest light we can observe — a birth photograph of the visible universe.

What the universe contains — and what we cannot see

This journey has described stars, galaxies, atoms, quarks. But all of that — all the ordinary matter that physics has described since Newton — accounts for just 5% of the universe's content. The rest is invisible, and still poorly understood. About 27% is dark matter: a form of matter that emits neither light nor radiation of any kind, but whose gravity is very real — without it, galaxies couldn't have formed so quickly, and the stars of spiral galaxies would fly off instead of staying in orbit. The remaining 68% is what we call dark energy: a form of energy associated with the vacuum itself, driving the expansion of the universe to accelerate. It's why distant galaxies recede from us faster and faster. Neither dark matter nor dark energy has yet been directly detected in a laboratory. They are there — their effects are measurable with precision — but their deeper nature remains one of the most fundamental open questions in physics.

If the Big Bang had happened at midnight — where would we be now?

Before moving to the future, a pause. Everything Act II has just crossed — from the first stars to the extinction of the dinosaurs — compressed into a single day. 1 hour = 574 million years. 1 minute = 9.6 million years. 1 second = 159,000 years. And all of recorded human history occupies roughly the last half-second.

The sky changes. The geography changes. The traces of humanity fade.

In 100,000 years, if a civilisation survives and looks up, it will recognise none of today's constellations. The stars will have shifted. Orion will be gone. The Great Bear will look like something else.

In 10,000 years

Earth's axis slowly rotates through a ~26,000-year cycle — the precession of the equinoxes. In 13,000 years, Vega will be the pole star. In 10,000 years, the celestial north pole will sit in a region with no bright star: future navigators will have to find other bearings. In 50,000 years, Niagara Falls will have eroded back to the Great Lakes and ceased to exist.

In 100,000 years

Almost all physical traces of humanity will have vanished. Concrete crumbles within a few thousand years. Steel rusts completely. The most durable cities will have been buried under sediment, vegetation, or glaciers. What will persist most reliably: a thin layer of radionuclides in the rock — the chemical marker of the nuclear age — and the waste repositories we are building today, hoping they hold. And perhaps radio signals continuing to race away through space at the speed of light.

In 1 million years

Erosion and tectonics wage a slow war: mountains wear down, but the plates push them back up almost as fast. What is certain: the map of the world will look nothing like anything we know. In 1 million years, tectonic plates shift 20 to 30 kilometres — enough to reshape entire coastlines.

The continents never stop. They never have.

In 250 million years, all the continents will join together again. It will be the sixth time in Earth's history. It's called Pangaea Proxima.

In 50 million years · The Mediterranean disappears

The African plate continues drifting toward Europe at 2 to 3 centimetres per year — the speed your nails grow. In about 50 million years, the Mediterranean will have disappeared, replaced by a mountain range taller than today's Alps. The collision between Africa and Europe is already underway: the Alps themselves are its product, formed less than 50 million years ago.

In 250 million years · Pangaea Proxima

According to current models, all continents will reunite to form a single supercontinent — a giant ocean on the other side. Like the Pangaea of 300 million years ago, but different: the continents never return to exactly the same place.

The Sun won't die in an explosion. It will grow, slowly, until it swallows us.

In 4.5 billion years, Andromeda will collide with the Milky Way. Not an explosion: stars are so far apart that the two galaxies will pass through each other like two clouds of smoke — in silence, over hundreds of millions of years.

In 600 million years · The last total eclipse

The Moon drifts from Earth at roughly 3.8 centimetres per year. In about 600 million years, it will be too far to cover the Sun's disk exactly. A remarkable coincidence — that the Moon today sits at precisely the right distance to cover the Sun perfectly — will be gone forever.

In 1 billion years · The oceans evaporate

The Sun's luminosity increases by roughly 1% every 100 million years. In 1 billion years, it will be about 10% brighter than today — enough to trigger a runaway greenhouse effect: the oceans will gradually evaporate. Complex life as we know it will become difficult, then impossible.

In 4.5 billion years · Collision with Andromeda

The Andromeda galaxy is approaching the Milky Way at about 110 kilometres per second. The two galaxies will collide in about 4.5 billion years and merge over several hundred million years. Our solar system will be flung into a different orbit — perhaps toward the outer edges of the resulting galaxy. But the stars themselves will almost never collide: the space between them is too vast.

In 5 billion years · The Sun becomes a red giant

The Sun will have exhausted its central hydrogen. Without the pressure of nuclear fusion to counterbalance gravity, the core contracts and heats — triggering fusion in the outer layers. The star's envelope swells considerably: the Sun will become a red giant, probably engulfing Mercury and Venus, and threatening Earth. What remains — the stellar core — will slowly cool over billions of billions of years as a white dwarf.

In 100 trillion years, the last star will go out. Night will become permanent.

Red dwarfs — the smallest, most fuel-efficient stars — will burn for trillions of years. When the last one goes out, the universe will be thousands of times older than it is today.

The universe stops forming new stars when there is no longer enough cold gas to collapse. This process is already underway — the stellar formation rate peaked 10 billion years ago and has been declining since. The existing stars will exhaust themselves one by one. In 10¹⁴ years, none will remain.

What remains then: white dwarfs slowly cooling over billions of billions of years, neutron stars, black holes, and cold planets drifting in total darkness. The universe will not be dead — but it will be silent. The Milky Way–Andromeda collision will have been completed long ago: the merged galaxy will itself have lost almost all its active stars.

If protons are mortal — and we don't yet know whether they are — then all ordinary matter eventually disappears.

A proton has been stable since the Big Bang. None has ever been observed decaying spontaneously. But some theories predict they will — in billions of billions of billions of years.

The Standard Model of particle physics does not predict proton decay. But grand unified theories — which attempt to unify the three non-gravitational fundamental forces — do predict it, with lifetimes of 10³⁴ to 10³⁶ years depending on the model. Underground experiments like Super-Kamiokande in Japan have been monitoring tanks of ultra-pure water for decades, waiting to see a proton decay. Nothing yet — which only sets a lower bound on their lifetime.

If protons decay, all ordinary matter — white dwarfs, planets, asteroids — will slowly reduce to light particles and energy. Only black holes will remain.

Hawking showed in 1974 that black holes are not eternal. They radiate. Imperceptibly.

A black hole's evaporation time is proportional to the cube of its mass. A solar-mass black hole takes ~2 × 10⁶⁷ years. A black hole a thousand times more massive will take a thousand times a thousand times a thousand times longer — 10⁹ times as long.

The mechanism

The quantum vacuum is never truly empty: it seethes with virtual particle pairs that appear and disappear, annihilating each other. Their total energy is zero. Near an event horizon, one of these particles can fall into the black hole while the other escapes. But for the energy balance to remain zero, the falling particle must carry negative energy in the general relativity sense — which amounts to removing mass-energy from the black hole. It pays the price: the escaping particle carries away energy taken from the black hole's mass. This is Hawking radiation. For a solar-mass black hole, this flux is currently undetectable and vastly overwhelmed by absorption of the cosmic microwave background. Only once the universe is much colder will this radiation become the dominant process.

The timescales

A solar-mass black hole evaporates in ~2 × 10⁶⁷ years. Supermassive black holes at the centres of galaxies weigh millions to billions of solar masses. A black hole of 10⁹ solar masses takes about 10⁹⁴ years to evaporate. These black holes are the last organised structures in the universe — and their complete evaporation marks, according to models, the end of all complexity.

The last thing that will ever happen.

Thermal death is not an explosion. It is a silence. A state where everything is so uniform, so cold, so empty, that nothing can happen anymore — because time only has meaning if there can be a "before" different from an "after".

What 10¹⁰⁰ means

10¹⁰⁰ is a googol — the number that gave Google its name. The number of atoms in the observable universe is estimated at around 10⁸⁰. A googol is therefore 10²⁰ times larger than that. To grasp this gap: 10²⁰ is itself a hundred billion billion. The googol exceeds every physical quantity it is possible to name within the observable universe.

Some reference points: the number of grains of sand on all Earth's beaches is around 10¹⁹. The number of seconds since the Big Bang is around 4 × 10¹⁷. Multiply those two together and you get ~10³⁶ — and 10³⁶ is still 64 orders of magnitude below the googol. You can keep multiplying astronomical quantities together for a very long time before approaching 10¹⁰⁰.

The impossible line

The calculation: 10¹⁰⁰ years ÷ 1.38 × 10¹⁰ years (age of the universe) ≈ 7.2 × 10⁸⁹. If Big Bang → Today = 200 pixels, the 10¹⁰⁰-year point lies at 200 × 7.2 × 10⁸⁹ ≈ 1.4 × 10⁹² pixels away. The diameter of the observable universe converted to pixels at 96 dpi is about 3.3 × 10²⁹ pixels. The ratio: 1.4 × 10⁹² ÷ 3.3 × 10²⁹ ≈ 4 × 10⁶² times the diameter of the observable universe. This point cannot be represented. It can only be named.

The final silence

When the last black hole has evaporated, nothing organised remains. A few very low-energy photons, neutrinos, electrons and positrons so far apart that the probability of them ever meeting tends to zero. A temperature infinitely close to absolute zero, uniform in every direction.

This is maximum entropy — the state predicted by the second law of thermodynamics: the universe tends inexorably toward total uniformity, until no exploitable energy gradient remains, no further process is possible. Without process, without change, time loses its operational meaning.

Not an explosion. Not a collapse. Something stranger and more final: a state in which nothing, ever again, can happen.