Concepts explained

Interstellar — Where One Hour Costs Seven Years

Where gravity is strong, time runs slow. Not a science-fiction premise but a correction applied to GPS satellites every day. This film carried that theory over not as a number but as a face — a father younger than his daughter.

Questions this piece threads together 44 min readUpdated 2026-08-12

The scene is not an exaggeration. That time runs slow where gravity is strong is a century-old result, and GPS satellites are correcting for it as you read this.

This piece asks which theory the film took up, and how that theory was turned into a story.

The story, ending included

In the near future, agriculture on Earth is failing. Blight takes one crop after another and dust storms bury the towns. Cooper, a former pilot, farms corn with his daughter Murph and son Tom.

Murph experiences something in her room she cannot explain: books fall on their own, dust settles in stripes. She calls it a ghost. The two work out that the pattern is a set of coordinates, follow them, and find what remains of NASA, operating in secret.

Someone has opened a wormhole near Saturn, and beyond it lies a galaxy with three candidate worlds. Signals from an earlier expedition are still arriving. Cooper promises to come back, and leaves.

The first planet lies beside a vast black hole, Gargantua. One hour there is seven years on Earth. Trapped by the waves, they spend a few hours; back aboard, twenty-three years have passed. Cooper watches the accumulated video messages from his daughter in one sitting. The child is now his own age.

On the second world, Dr Mann — who arrived before them — is found to have falsified his data to summon a rescue. On Earth, Professor Brand confesses as he dies that there was never a way to take everyone: only the plan to found a new civilisation from frozen embryos.

Short of fuel, Cooper drops into the black hole with the robot TARS so that the remaining fuel can carry his colleague on. Inside the horizon he is not destroyed but arrives inside a structure: the back of his daughter's bookshelf, with time laid out like space. Murph's ghost was her father, from later.

Using the second hand of a watch he sends her, in Morse, the gravitational data from inside the black hole. The grown Murph solves the equation with it and humanity leaves Earth.

Cooper is recovered near Saturn and meets his daughter, old and dying. Then he leaves again, to find the colleague preparing the third world alone.

First theory — time is not the same for everyone

One result holds the whole film up: .

That time passes at the same rate everywhere seemed too obvious to question. It was broken twice, in 1905 and 1915.

Why gravity slows time

How Einstein got there is startlingly simple. He imagined a falling lift.

And time dilation follows from it. Send light from the ceiling of an accelerating lift to the floor and the floor rises to meet it. The light arriving below is slightly bunched up.

Bunched-up means time runs slower down there. And since acceleration and gravity are indistinguishable, time runs slower where gravity is stronger.

How the theory becomes a story

That is the theory. What the film did was carry it over as a face rather than a number.

What the film shows

Cooper watches twenty-three years of video messages in one sitting. The child is now his own age, speaking to him from the screen. He did nothing, and he missed her life.

what the theory says

What the equations give

A clock deep in a gravity well runs slower than one outside. How much slower is set by the depth, and the figure can be calculated.

A little further in

The seven-year ratio on Miller's planet was not a plausible-sounding invention. The physicist Kip Thorne worked out the conditions that would produce it.

The director asked for it first: make one hour equal seven years. Thorne asked what that would require. The answer: the black hole must be very massive — something like a hundred million suns — must spin at nearly its maximum rate, and the planet must sit just outside the horizon.

Second theory — a wave standing like a mountain

The waves on Miller's planet do not roll in and break. A mountain of water walks over the horizon.

What the scene borrows is .

Beside a black hole the effect is enormous. The planet's ocean bulges hugely on both sides and stays there, and as the planet rocks slightly, those bulges sweep across the surface. The wave in the film is less something arriving than the planet turning underneath it.

Third theory — what a black hole looks like

Gargantua was not drawn by an art department. Thorne's equations were fed in and the path of each ray of light traced.

It looks strange because of .

The software written for the calculation produced patterns nobody expected, and the team turned them into three published papers. The visual effects work became research.

In 2019, five years after the film, a black hole was photographed for the first time. The ring resembles the film's image with one difference: one side is far brighter.

Where theory stops

From the moment Cooper crosses the horizon, matters change.

What happens inside an is unknown. General relativity says there is a of infinite density at the centre, and an infinity in an answer usually means the theory has stopped applying.

Hawking showed that black holes radiate very slowly and shrink. Where the information that fell in goes is a question that followed, and the argument is not over. Gravity and theory are both needed here, and no theory yet joins them.

Where theory holds

Everything outside the horizon. Time dilation, tidal forces, bending light — all calculable and all confirmed by measurement.

the boundary

Where it does not

Inside the horizon. From here there is no equation to compute with. The film put his daughter's bookshelf there.

Only gravity crosses

In the film Cooper can send his daughter something only through gravity. That premise was not invented either.

Physics has an old puzzle: why is gravity so weak? A fridge magnet beats the pull of the entire Earth. Of the four forces, gravity alone is absurdly feeble.

The film's most argued-over line — that love is something reaching across space and time — belongs here too. The film does not claim love as a fifth force. It is closer to saying that gravity opens the channel, and no equation tells you whom to send the message to.

1905Einstein — moving fast slows time
1907The equivalence principle — acceleration and gravity are indistinguishable from inside
1915General relativity — gravity is not a force but curved space
1959Pound and Rebka — a time difference measured over the height of one tower
1971Atomic clocks flown around the world and compared on return
1974Hawking — black holes radiate and shrink
2014Interstellar — a black hole drawn by calculation
2019The Event Horizon Telescope — the first photograph of one

The 1959 experiment is the striking one. One tower at Harvard, 22.5 metres tall. The difference in the rate of time between its top and bottom was actually measured — about two parts in a thousand million million.

The question that remainsThe twenty-three years Cooper lost were exactly what the theory said they would be. It could have been calculated before he left, and it had been. How far apart are being able to calculate something and being able to bear it?