“Scientists make quantum time flow backward” was one of 2026’s most shared physics headlines. The real work, from physicists at Los Alamos National Laboratory, is subtle and genuinely interesting: it is about the arrow of time, why it exists, and how measuring a quantum system can bend it. This post explains it carefully, because the headline and the physics are not quite the same thing.
Why does time have a direction at all?
Film a snooker ball bouncing off a cushion and play it backwards: it still looks possible. The basic laws of motion work equally well in both directions. But film an egg breaking and run it backwards, and everyone knows it is reversed. The difference is entropy, roughly a measure of how many ways the energy and particles in a system can be arranged. The second law of thermodynamics says that the total entropy of an isolated system does not decrease: eggs break, heat spreads from hot to cold, perfume spreads through a room. That one-way tendency is what physicists call the arrow of time.
Measurement gives quantum systems an arrow too
In quantum physics, measuring a system is not passive: it disturbs the system (this is called measurement back-action). Repeated measurements push a quantum system along a random path, and those paths also have a natural direction that can be described using entropy production. The Los Alamos team asked: if you measure continuously and respond immediately, can you control that direction?
[Diagram: a feedback loop showing the idea in outline — measure the system, feed the result to a controller, and apply a tailored field or pulse straight away. Depending on the strategy, feedback can cancel, amplify or overcorrect the disturbance caused by measurement.]
Their answer was yes, in theory. Using carefully designed control fields (a designed Hamiltonian, meaning a chosen set of forces acting on the system) together with feedback, they showed that a measured system can be steered along paths whose arrow of time is suppressed, stretched, blurred or even inverted, so that the paths look consistent with time running backwards.
“The tools we’ve constructed can manipulate the perceived arrow of time, leading to surprising, novel ways to control quantum systems.” — Luis Pedro García-Pintos, Los Alamos
Energy from measuring
The team also proposed a “measurement engine”: a way of extracting useful energy directly from the disturbance caused by quantum measurements. In their words, quantum measurements become “a thermodynamic resource that can be tapped to perform work.”
A disagreement can be resolved by better theory, not only by new particles. The work builds on Maxwell’s demon, a 19th-century thought experiment in which a tiny being sorts fast and slow molecules and seems to beat the second law. The resolution is that gathering, storing and erasing information has its own thermodynamic cost. The Los Alamos authors make the same point: “the second law is not violated when all sources of thermodynamic costs are accounted for.” Also note that this was a theoretical study in Physical Review X; experimental demonstrations with superconducting qubits are planned, not done.
Quick check: which of these is an accurate statement about the Los Alamos “time reversal” work — that a laboratory clock was made to run backwards, that it shows how measurement and feedback could make a quantum system’s path look time-reversed without breaking the second law, that it proves the second law is wrong, or that it produced unlimited free energy? Answer: it shows how measurement and feedback could make a quantum system’s path look time-reversed, without breaking the second law. Time for everything else runs normally, and once the costs of measuring and feedback are included, total entropy still does not decrease.
Why it could matter
- Quantum computing: qubits are constantly disturbed by their surroundings and by measurement. Better ways to steer, cancel or reuse those disturbances could help control them.
- Quantum thermodynamics: understanding when measurement can be a source of work helps define the true energy cost of computation.
- Fundamental physics: it sharpens the question of where the arrow of time comes from.
Thermodynamics and quantum measurement are among the most misunderstood topics in physics, and also among the most rewarding to actually understand rather than just memorise.
Sources
- ScienceDaily: Scientists make quantum time flow backward in stunning physics breakthrough (Jul 2026)
- Wikipedia: Maxwell’s demon
- Wikipedia: Arrow of time
Sources reviewed 17 October 2026.
Try it yourself
State the second law of thermodynamics in terms of entropy, and use it to explain why heat flows spontaneously from a hot object to a cold one but not the reverse.
Show the worked approach
- the total entropy of an isolated system never decreases (it increases in any spontaneous process)
- when heat flows from hot to cold, the entropy gained by the cold object is greater than the entropy lost by the hot object (ΔS = Q/T is larger at lower T), so total entropy increases
- heat flowing from cold to hot would decrease total entropy, so it does not happen spontaneously (it needs work to be done, as in a fridge)
What to focus on: State the law precisely, then use ΔS = Q/T to compare the two objects.
Common mistake: Saying entropy 'always increases' for any object. It is the total entropy of an isolated system that cannot fall; a fridge's inside can lose entropy because more is released outside.
Frequently asked questions
Did scientists really make time run backwards?
No. Physicists at Los Alamos National Laboratory designed control methods that make a measured quantum system follow a path that looks as if its 'arrow of time' points backwards, or is stretched or blurred. Time itself, for the lab and everyone in it, carries on forwards as normal.
Was this an experiment?
It was a theoretical study, published in Physical Review X in 2026 by Luis Pedro García-Pintos with Yi-Kai Liu and Alexey Gorshkov. The authors plan demonstrations using superconducting qubits, the building blocks of some quantum computers.
Does it break the second law of thermodynamics?
No. Like the famous Maxwell's demon thought experiment, it can look as if entropy decreases locally, but the researchers note that the second law is not violated when all the thermodynamic costs, including the measuring and the feedback, are counted.
What is the 'measurement engine'?
A proposed device that extracts useful energy from the disturbance caused by quantum measurements themselves, treating measurement as a thermodynamic resource.
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