No Inversion Device Required: The Arrow of Time May Point Both Ways

by Liz Publika

Tenet is a 2020 sci-fi action film directed by Christopher Nolan. It follows an unnamed CIA operative, "The Protagonist," who joins a secret organization to stop a Russian oligarch from using a device built to invert the entropy of the entire world, obliterating everyone and everything that's ever lived. To do so he must master "time inversion," or moving backward through time.

While a time inversion device is still not on the radar, recent research suggests the arrow of time may point both ways. 

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Los Alamos physicist Luis Pedro García-Pintos and his colleagues didn't invent an inversion machine, and nothing in their lab moves backward through spacetime the way “The Protagonist" does in the film's climax. What they built is something narrower and, in its own way, stranger: a way to manipulate the one thing that actually makes time feel one-directional at the quantum scale — measurement itself.

To understand the significance of this development, we first have to start with the definition of dynamic systems. Simply put, a dynamic system is a mathematical model used to describe how a specific state evolves over time; a state is a complete snapshot of the system at one exact moment in time. It contains all the numerical information needed to predict the system's future. So, let’s break that down. 

Let's say you're attending an amateur tennis match and have your phone camera ready to go. A stray ball suddenly flies through the air. Your jerk reaction triggers the camera, capturing the ball hurtling mid-air at that precise moment — whether it's 5 meters up or 100. Whatever that single number is, it's not a complete state. If you try to predict where the ball will be one second later, you won't be able to.

That's because you don't know which way the ball was moving — up, down, or whether it came off the court at all. To predict its position a second later, you need two numbers: the ball's exact position (x) and its velocity (v). Together, x and v make up the state vector — the exact condition of the ball at that instant. Two numbers means this is a 2-dimensional state space: the ball's state is a single point moving around inside it.

Think of a state space as an imaginary mathematical "map" or room that contains every possible condition the ball could ever be in. As the ball moves, those two numbers constantly change. On your mathematical map, this looks like a single dot drawing a continuous line. This essentially means that you can completely describe where or how a ball is moving at any given moment using just two pieces of information.

Since knowing the state isn't enough on its own, you also need the rule that pushes that point around over time — in this case, gravity, which is pulling the ball back to earth. That rule is separate from the state itself: if the state is the snapshot, then gravity is what moves the snapshot forward. Put the two together — a state, and a rule for how it evolves — and you've got a dynamical system.

Give it a starting state and the rule, and you can calculate exactly where the ball will be at any future moment. Quantum systems have the same two ingredients, but with an interesting catch; you still have a "snapshot" and a "rule," but instead of mapping a single path, the rule — the Schrödinger equation — pushes the entire spread of possibilities forward simultaneously, dictating how the probabilities change.

In other words, instead of a clean position and velocity, the "state" is more like a spread of plausible possibilities represented by a wave function; the wave's height, squared, indicates the probability of finding the particle there. The particle doesn't have one definite location until something interacts with it. And the moment you check on it — take a measurement — you don't just get a peek at that state, you change it.

There's no way to look without disturbing what you're looking at. That disturbance is what García-Pintos and his team had to work around. It matters because it's what actually makes time feel like it only moves forward at the quantum scale. Measure a system, and you knock it into a new state that can't be undone — and that one-way disturbance is what creates a sense of "before" and "after."

García-Pintos and his team built a control Hamiltonian — a precise sequence of energy fields and pulses. By applying this tool as feedback, they can cancel out or overcorrect that measurement disturbance. As a result, the system's path can look stretched out, blurred, or run in reverse, instead of ticking forward the usual way.

Testing that setup, they found it doubles as something else entirely: a "measurement engine" that pulls usable energy directly out of the act of measuring — a quantum version of the Maxwell's Demon thought experiment in 1867, which challenged the Second Law of Thermodynamics by imagining a tiny creature sorting fast and slow molecules to separate heat from cold without using energy. 

Time travel in the physical sense — closed timelike curves, wormholes, anything from general relativity — requires the geometry of spacetime itself to loop back on itself. While García-Pintos's work never touches spacetime, it's a demonstration that the "arrow of time" most people treat as a fixed law of nature is, at this scale, more like a default setting than a hard rule.

So even though the controlled experiment did not introduce an inversion machine, as imagined in Tenet, it did reverse the statistical footprint of a single quantum system's measurement history. Nolan needed a machine to dramatize that. García-Pintos needed a control Hamiltonian. Next steps include running this on superconducting qubits. If it holds up, the same trick for reshaping time's arrow could feed into more capable quantum batteries and computers. 

Time's arrow isn't a law, it's a side effect of disturbance, and if you control the disturbance, you can bend the arrow.

Note* Image available via Fair Use and Public Domain.