Stage 1 · The everyday picture
Time as we live it
Only now is real. The past has gone, the future has not arrived, and the present slides forward through everything at once.
This is now: one instant, spread across all of space. A star, its planet, a few lamps and you, all at once.
The instant slides forward. The planet moves on, a lamp flashes, and each moment gives way to the next. That sliding is what we call time passing.
Behind it, the past fades. It happened, but it isn’t anywhere anymore.
Above it there’s nothing yet. The future hasn’t happened, so it doesn’t exist.
This picture has a name, presentism: only the present is real. It’s how almost everyone lives.
It quietly assumes one thing: that this instant is the same instant everywhere, for everyone. Keep an eye on that.
Stage 2 · Space plus time
Stack the moments
Keep every “now” instead of throwing it away, stack them, and you get one object: spacetime. Anything that lasts becomes a thread through it.
Now break presentism’s rule. Instead of letting each moment go, keep it, like a frame of film.
Stack the frames in order, earliest at the bottom. The moments we let fade come back too.
Things that last become threads through the stack. A lamp that stays put is a straight line. The planet, circling its star, winds into a helix.
Fill the gaps between frames and the stack becomes one solid block: every place, at every time. Physicists call it spacetime.
Each thread is a worldline, the whole history of one thing. You have one too.
Nothing in this block moves. Motion is just the shape of a thread.
Stage 3 · Light cones
One speed for everyone
Light moves at the same speed for every observer, however they move. That fixes a cone around every event: what could have reached it, and what it could reach.
Pick one event: a flash, here and now.
On each moment that follows, its light is a ring, spreading at 300,000 kilometers a second, the same in every direction.
Stack those moments and the rings trace a cone. Its surface is everywhere the flash’s light will ever be.
Below is the past cone: everything that could have sent a signal to the flash. Outside both cones is “elsewhere”: events too far away to affect it, or be affected by it.
Two people at the same moment have overlapping past cones. The overlap, in white, is their shared history: everything both of them could know about.
Now look at the block from a frame moving at 45% of light speed. The grid skews and every worldline leans.
The cone doesn’t change. Light has the same speed for every observer. Michelson and Morley looked for a difference in 1887 and found none. Nor has anyone since.
If light is the same for everyone, “the same moment” can’t be. That is where the single now starts to break.
Stage 4 · Time dilation
Moving clocks run slow
There is no single cosmic tick. Two clocks that take different paths through spacetime disagree when they meet again, and the gap has been measured.
Twins on Earth. One stays home. The other flies to Proxima Centauri, 4.2 light-years away, at 80% of the speed of light.
Each bead is a birthday: one year on that twin’s own clock. The dashed rungs join equal ages, and they lean further every year.
At Proxima the traveler turns around. That turn is what makes the two paths different: home’s path is straight, the traveler’s bends.
Neither twin feels time slow down. Each clock ticks once a second for the person carrying it.
They meet again. Home has lived 10.6 years, the traveler 6.4. Both are right about their own lives.
Unwind the traveler’s bent path and stand it beside home’s. It is shorter. In spacetime, the straight path between two events is the one that takes the most time.
This is not just a thought experiment.
Fast muons at CERN live 29 times longer than muons at rest. GPS satellite clocks run 38 microseconds a day fast and are corrected for it. Atomic clocks flown around the world came back out of step with the ones that stayed behind.
So time is not one cosmic tick. Every path through the block keeps its own.
Stage 5 · What everyone agrees on
The bowl of equal ages
Send travelers out from one moment at every speed. Join the places where each has aged exactly a year: they form a bowl, and every observer agrees on its shape.
Start with one event. From it, travelers set off in every direction at speeds up to 85% of light. None of them can leave the light cone.
Mark where each one turns a year old by their own clock. The traveler who stays put gets there first. The fastest get there later, and farther out.
Join those birthdays and they make a bowl. Do the same at two years and three: bowls inside bowls.
Building this shape needs no agreement about whose clock is right. It uses only ages, and every observer agrees on how old each traveler is.
Follow two of them. A stays where the event happened. B moves at 60% of light speed. Both are two years old.
A’s now, on that birthday, is a plane that just touches the bowl.
So is B’s. It grazes the same bowl at B’s birthday, tilted by B’s speed.
Now redraw everything from B’s point of view. The bowls do not move at all. B sits at the bottom, and the two nows swap tilts.
That is what an observer’s now is: the plane that grazes the bowl of equal ages where they stand. Everyone has their own.
Stage 6 · Relativity of simultaneity
Whose now?
Two observers moving past each other slice spacetime into “nows” at different angles. Distant events that happen at the same time for one happen at different times for the other.
Here is the block. Seven lamps stand still, each flashing once (the numbered points). A stands at the centre. Up is the future.
A’s now is a flat slice through here-now. Lamp 6 flashes inside it: for A, that flash is happening at this very moment.
B passes A at half the speed of light. B’s path through the block leans.
B’s now tilts by exactly that lean. It passes through the same here-now, but it catches lamp 4, not lamp 6.
Change B’s speed and the order of distant flashes shuffles. For A, lamp 6 flashes before lamp 3. For B, it is the other way round.
Two pairs never swap: light from lamp 1 reaches lamp 3, and a messenger from 2 reaches 4. A cause comes before its effect for everyone.
Now draw the same block in B’s frame. B stands upright, A leans, and the two knives trade places.
One shape doesn’t change in either picture: the light cone. Every observer agrees where light can go.
Neither knife is wrong. Nothing in physics picks one of these nows as the real one.
Stage 7 · Same block, different photographs
A now is a rolling shutter
A moment is a slice through the block. Tilt the slice and you get a different picture of the same events, like a phone camera that reads its sensor line by line.
Here is a spinning rotor: three blades, one with a red tip, and two lights circling the other way.
Stack its moments and the blades sweep out a twisted column. Every position they ever take is in the block at once.
A’s now is a flat slice through that column. Its face is A’s photograph of the rotor: three straight blades.
B’s now is tilted. It cuts the left side of the rotor earlier than the right.
Lay the two photographs side by side. Same rotor, same block, and B’s blades bend.
Phone cameras do this with fast propellers. They read the sensor one line at a time, so each line catches the blade at a different moment.
For a real propeller and someone walking past it, the bend is far too small to see. Across the distance to another galaxy, it isn’t small at all.
Stage 8 · The Rietdijk–Putnam argument
Two walkers, one galaxy
Two people pass in the street. At the distance of Andromeda, their two “nows” are days apart. Which of those days is real?
On the left, Earth’s history. On the right, Andromeda’s, 2.5 million light-years away. Each bead is half a million years.
Here and now, on Earth. If “now” were the same for everyone, one flat slice would say what is happening on Andromeda at this moment.
Two people pass in the street, one walking toward Andromeda and one away. At walking pace their nows tilt by a hair. Here the tilt is exaggerated so you can see it.
Across 2.5 million light-years, the hair becomes a gap. Their nows meet Andromeda’s history at two different moments.
Look at Andromeda at true scale. The walker heading toward it is 4.3 days ahead; the other is 4.3 days behind. The gap is 8.5 days.
Roger Penrose’s version: the Andromedans launch a fleet on a day inside that gap. For one walker it has already left. For the other, the decision hasn’t been made.
Walking isn’t special. Let the passers-by move at any speed and their nows sweep up and down Andromeda’s history.
Close to light speed, the window opens to 2.5 million years either way, right up to the edge of the light cone.
The farther away a place is, the less “now” means there. The table runs from the Moon to the Virgo Cluster.
So which moment on Andromeda is happening now? Relativity gives no way to choose.
Stage 9 · The block universe
Everything, all at once
If every observer’s now is as real as any other, then between them they make every event real. No moment is special: past, present and future all exist.
Back to the block. A stands at here-now. The light cone marks what A can reach and what can reach A.
Picture everyone who could pass through this exact place and moment, at every speed. Each of them has a now, tilted by their speed.
Sweep through them all. Every lamp a slice touches is flashing “now” for someone standing right here.
Together their nows fill everything outside the light cone. Six of the seven lamps are someone’s now. Lamp 7 sits off to the side, where a sideways slice would catch it.
The argument takes one more step. Lamp 5 is real: it is someone’s now. And someone at lamp 5, moving at the right speed, has A’s future in their now.
So A’s future is real for someone who is real. If being real is something we all share, it is real for A as well, already.
Repeat that from every event, in every direction, and nothing in the block is left out.
That is the block universe. Past, present and future all exist, and “now” is like “here”: a place in the block, not the edge of reality.
Both steps rest on premises you can deny. Stage 12 is about the people who deny them.
Stage 10 · A life in the block
Your worldline
You are not a point traveling through time but a thread laid along it, from birth to death. What makes one end feel like the past is a slope in the block, not a motion.
Here is a life in the block: one thread from birth, at the bottom, to age eighty, at the top. Each bead marks a decade.
Nothing moves along it. Every moment of the life has its own place in the block, and all of them are there.
Look closer and it is two threads: your head and your feet, about 1.7 meters apart.
The higher a clock, the faster it runs, so your head ages faster than your feet. The inset joins the moments when each has lived the same number of years.
By eighty, your head is ahead by about half a millionth of a second. Optical clocks have measured this effect over a height of 33 centimeters.
So why does time seem to run one way? Here is a drop of ink spreading through water, drawn as a block.
Packed tight at the bottom, spread out at the top. That is entropy rising. The arrow of time is a slope across the block, not something moving through it.
Memories are records, and records only form toward the low-entropy end. Each slice of you holds traces of the slices below it, never of the ones above.
On the block view, that is what the passage of time amounts to: a thread whose every slice remembers only what lies beneath it.
Stage 11 · Gravity is the shape of time
The block bends
Clocks run slower closer to a mass. The block is not a rigid crystal but a curved one, and near a black hole every future points inward.
Gravity changes the block too. Here is one day around Earth. The planet’s path through time is a tube, and its spin twists the meridians into spirals.
Five clocks: at sea level, on Everest, on the space station, on a GPS satellite and in geostationary orbit. Each bead is three hours on that clock.
The flat sheet is where the sea-level clock reads 24 hours. Every other clock reaches its own 24-hour mark a little before or after.
Higher clocks run faster, so they get there first. The space station moves fast enough that its clock runs slow. On screen the rates are exaggerated 200 million times; the corner has the real ones.
GPS satellites gain 38 microseconds a day. The system corrects for it. Without the correction, positions would drift by about 11 kilometers a day.
Now a black hole. The glowing tube is its horizon. The line down the middle is the singularity.
These are flashes of light aimed straight out. Outside the horizon they escape, slowly. Exactly on it, one stands still. Inside, they fall in.
Follow someone falling in, with their light cone at each step. Far away a cone stands upright. Closer in, it tips toward the hole.
Inside the horizon, both edges of every cone point inward. The singularity is not a place ahead of them. It is in every one of their futures.
The block is not rigid. Mass bends it, and that bend is what we feel as gravity.
Stage 12 · Open question
Is it true?
Relativity rules out one universal now. Whether that means the future already exists is something physics has not settled. Here are the main answers.
Relativity rules out a single now shared by everyone. It doesn’t say, by itself, what exists. Here is the same block four times.
Presentism: only the present is real. The past was and the future will be, but neither is. It is the everyday view, and the hardest to fit with relativity. Whose present?
The growing block: the past and the present are real, and new moments keep coming into being. The future is open.
The moving spotlight: every moment exists, but one of them is lit up as now, and the light moves.
The block universe: every moment exists and none is special. “Now” works like “here”. It says where you are, not something about the world.
In 1968 Howard Stein pointed out that relativity still allows a kind of becoming. From any event, what has already happened can be its past light cone, and every observer agrees on that.
Quantum mechanics raises its own question. If some outcomes are truly random, perhaps the future is not settled until it happens. George Ellis calls this a crystallizing block.
Free will is not obviously at stake. In the block, your choices are still there, and they still cause what comes after them.
Settled: there is no single now that everyone shares. Not settled: whether the future already exists. That is still argued.
Stage 13 · From the Big Bang to here
The block of everything
Everything we will ever see lies on a single surface in the block: our past light cone, reaching back to the Big Bang.
Pull out as far as the block goes. Time runs up from the Big Bang to today, and distance runs across. The bright line is our galaxy.
Everything we see reaches us along our past light cone. Trace it back from here and now.
It widens as we look further back, but not forever. Billions of years ago, everything was closer together.
So the cone is a teardrop. At its widest it is under six billion light-years across: light we see from nearly ten billion years ago set off no farther away than that.
These are other galaxies, carried apart as space expands. All of them start together at the Big Bang.
We see each galaxy where its thread crosses our cone: not as it is now, but as it was when that light set out.
At the bottom, the cone meets the glow of the young universe. The microwave background we see set off just 41 million light-years away. That matter is now about 45 billion light-years away.
The rest of the block is out there too, beyond the cone: events whose light hasn’t reached us yet, and some it never will.
On the block view, this is all of it at once: the Big Bang, the first stars, the day you were born, the day you turn eighty. Nothing is waiting to happen. Each moment simply is, where it is.