Concepts explained

Tenet — If Entropy Falls, Does Time Run Backwards?

Most of the difficulty people have with this film comes from one thing: hearing entropy decreasing and time running backwards as the same sentence. Entropy is decreasing inside your refrigerator right now. The refrigerator is not travelling into the past.

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

That single question is the whole of Tenet — and it is exactly where the film becomes difficult.

The story, ending included

The protagonist is never named. A CIA operative, he is captured during an operation at the Kiev opera house, tortured, and swallows a suicide pill — then wakes. He is told he passed a test, and is brought into an organisation called Tenet.

He is given two clues: a gesture with interlocked hands, and the word Tenet. And footage of a bullet coming out of a wall and returning to a gun. These objects were made in the future and have travelled back; they are called inverted.

The trail leads through an arms dealer to a Russian oligarch, Sator, who is in contact with the future and receiving inverted materials. He is assembling the pieces of a device called the Algorithm, which reverses the direction of time for the whole world. The future has been made unliveable by climate collapse and means to erase the past.

The protagonist approaches Sator through his wife, Kat. Through an incident at the Oslo freeport and a motorway chase, he learns that the inverted figure he fought was himself. Passing through a machine called a turnstile puts him into an inverted state, in which he can retrace events he has already lived.

The climax is a temporal pincer: one team advances forwards through the same ten minutes while another moves backwards, each reporting to the other what it has seen. The Algorithm is recovered and broken apart, and Neil, a colleague, goes back to save the protagonist and dies doing it.

One thing is revealed at the end. Neil was recruited by the protagonist himself, in the future. Their long friendship is already past for Neil and has not yet begun for the protagonist. The man who founded Tenet was the protagonist too.

First — the laws have no direction

Begin with the surprising part: most fundamental laws of physics do not distinguish the direction of time.

Reverse time in Newton's laws and they still hold. So do the laws of electricity and magnetism, and relativity. As far as the laws are concerned, going back is as normal as going forward.

Only one law carries a direction: the — entropy only increases.

Second — entropy is a count

What is needs no difficult definition.

So entropy increasing is less a law than an act of counting. It is not forbidden to decrease; it is absurdly unlikely to.

Third — and this is the point

Here is the key to the film.

Entropy is decreasing all the time, right now.

In a 1944 lecture Schrödinger described what living things do as feeding on , later shortened to negentropy.

Now the decisive question.

So what Tenet imagines is not a local decrease of entropy. Refrigerators already do that.

What it imagines is an object whose direction in time has itself been turned around. For that object our future is its past. Entropy falling is not the cause of the state but the symptom seen from outside.

A little further in

Where did that imagination come from, then? Not from nowhere.

Particles that go backwards really exist

In 1928 Paul Dirac wrote an equation for the electron and it returned two answers. One was the electron; the other was something identical to it but with the opposite electric charge.

Instead of discarding it he insisted it must exist. Four years later was found.

Twenty years after that, Feynman and Stückelberg offered another way to read these antiparticles.

That is where Tenet's inversion is borrowed from. A bullet leaving a wall and returning to a gun is Feynman's diagram enlarged to human scale.

What the theory has

A single particle can be described as moving backwards in time. The calculation works and is used daily. But such a particle is observed as an antiparticle; it does not walk backwards through our world.

what the film enlarged

What the film has

People, cars and bullets reverse whole. Countless particles would have to turn together and stay consistent, and the theory offers no way to do that.

The rules the film set itself

What the film takes trouble over is setting its rules and then keeping them.

Inverted characters wear oxygen masks. Why? The air is running forwards while their lungs run backwards, so inhaling would push air out. They must carry their own.

The grandfather paradox and its answer

If the past can be reached, a question follows at once. What if you go back and kill your own grandfather? Then you are never born, and never go back.

The film's answer comes in one line: what happened, happened.

Which is what makes Neil's end sad. He walks into the place knowing when he dies. And he tells the protagonist that for them this is the end of a beautiful friendship — but for him it is the beginning.

What is left unanswered

There are things the film cannot keep consistent. These are less faults than places where no theory gives an answer.

Which way does memory accumulate for someone inverted? Memory is a trace left as entropy rises inside a brain, so reversing the direction should erase it as they go — and then they cannot know what they came to do.
The theory offers no way for every atom of a person to turn together and stay consistent.
What happens when an inverted object meets an ordinary one is not settled either.

Back to the first question

Return to why only the shattering cup looked wrong.

Two billiard balls have only a handful of ways to scatter and to gather, so either direction is plausible. The fragments of a cup are hundreds of millions of grains, and there is effectively one way for them to return to their places and be a cup.

1824Carnot — heat flows only one way
1865Clausius — gives entropy its name
1877Boltzmann — entropy is a count of arrangements
1928Dirac — the equation's second answer: antimatter
1944Schrödinger — life feeds on negative entropy
1949Feynman — an antiparticle is a particle going backwards in time
1990Novikov — only self-consistent histories occur
2020Tenet

The question that remainsEntropy falls inside a refrigerator and time does not turn. So what are we sensing when we feel time passing? Are we watching the cup break, or remembering the order it happened in?