Somewhere between the quiet decay of excited atoms and the abstract machinery of quantum theory, a question about time itself has come into view. Two physicists, Yakir Aharonov and Avshalom Elitzur, have worked through the mathematics of a device that exists only on paper and found that its power to run time backward is bounded, lasting perhaps a femtosecond in the simplest case. The boundary does not come from time resisting reversal, but from the surrounding world remembering what the light has already carried away. In that memory, and in the way it closes off reversal, may lie part of the r
Physicists Find Quantum Limit on Running Time Backward: Roughly One Femtosecond
The limit isn't imposed by time, but by the environment's memory of the light.
Mark
So what did they actually find?
Mimi
That in the simplest version of their theoretical machine, time can effectively run backward for about a femtosecond, a quadrillionth of a second, before the environment gets in the way.
Mark
Why the environment, and not something about time?
Mimi
As atoms emit light they interact with the electromagnetic field and leave traces. Once the environment holds too much information about the emission, the two histories can't interfere anymore.
Luke
Let me flag something. Nobody has built this machine. The femtosecond figure comes from a mathematical analysis of one optical example, not a measurement.
Mark
And it's a preprint, right?
Luke
Yes, on arXiv, not peer reviewed. So it's one pair of authors' calculation, however distinguished.
Mimi
The machine isn't new either. Aharonov proposed it in 1990. This is the follow-up asking how far the negative time can go.
Mark
Doesn't reversing time break the second law of thermodynamics?
Mimi
The authors say no. The reversal appears only in a small post-selected fraction of outcomes, and across all outcomes entropy still behaves.
Luke
Worth noting that's their argument, and it hasn't been tested. Also, longer reversals aren't ruled out outright. Bigger machines with many more branches could in theory do it, but with increasingly improbable success.
Mark
So why does it matter?
Mimi
It pinpoints where reversal fails, which may help explain how irreversible processes arise from the quantum world.
The Pulse
- A thought experiment of 100 excited atoms poses a test: if more than 80 come out excited, time has partly run backward.
- A paper-only quantum time-translation machine, first proposed in 1990, is pushed to its mathematical limits by its own authors.
- The environment's retained information about emitted light breaks the coherence that reversal depends on, capping it at roughly one femtosecond.
- Apparent violations of the second law vanish once post-selected outcomes are weighed against the full range of results.
- The work remains an unreviewed arXiv preprint, with no experiment yet able to test whether the machine could ever operate.
Somewhere between the quiet decay of excited atoms and the abstract machinery of quantum theory, a question about time itself has come into view. Two physicists, Yakir Aharonov and Avshalom Elitzur, have worked through the mathematics of a device that exists only on paper and found that its power to run time backward is bounded, lasting perhaps a femtosecond in the simplest case. The boundary does not come from time resisting reversal, but from the surrounding world remembering what the light has already carried away. In that memory, and in the way it closes off reversal, may lie part of the reason the past feels fixed and the future open.
Imagine 100 atoms, each holding energy it is slowly releasing. You wait until 80 remain excited, then pass all 100 through a machine that exists only on paper. If more than 80 emerge excited, the machine has undone some of the decay that already happened. That test anchors a new theoretical study by Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research, posted as a preprint that has not yet faced peer review. In the simplest model they examined, time could effectively run backward for only about a femtosecond, a quadrillionth of a second.
The machine dates to 1990, when Aharonov and colleagues proposed a quantum time-translation device that could, through superposition and post-selection, make a system behave as though it had experienced more time, no time, or negative time. Thirty-six years later, Aharonov and Elitzur asked how far that negative time could stretch. To judge whether reversal had truly occurred, they imagined the decaying atoms sealed in a cell with their emitted light and absorbing walls, so that the light could return to re-excite them.
A barrier appeared quickly. Atoms are never isolated; as they emit, they leave traces in the surrounding electromagnetic field. Reversal holds only while those traces leave the two possible histories still able to interfere. In their optical example, that window closes after about a femtosecond, not because time resists being turned around, but because the environment has remembered too much of the light. Longer reversals are possible in principle, but only by multiplying quantum branches, each with ever slimmer odds.
Even so, the second law of thermodynamics seems to forbid any of this. The researchers answer that the apparent reversal appears only in a tiny, carefully selected fraction of outcomes, and across all outcomes the system still obeys the law. Their larger hope is not a time machine but an understanding of why time seems to flow one way. "The arrow of time enters exactly where the machine stops," they write. No experiment has yet tested any of it.
Start with a thought experiment about 100 atoms. Each has been energized, and each is slowly letting go of that energy. You wait until some have released it, so that 80 remain excited. Then you run all 100 through a machine that exists, for now, only on paper. If more than 80 come out excited, the machine has undone part of the decay that already happened. That test sits at the center of a new theoretical study of how far quantum mechanics lets time effectively run backward.
The physicists behind it are Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research. Their work is in a preprint on arXiv, which means it has not been peer reviewed. For the simplest model they examined, they found that time could effectively run backward for only about a femtosecond, one quadrillionth of a second.
The machine is not new. In 1990, Aharonov and colleagues proposed a theoretical quantum time-translation machine, a device that would use quantum strangeness to make a system behave as though time had sped up, stopped, or reversed. It relies on superposition, where a system occupies several states at once. Picture a clock in superposition at two positions in a gravitational field, with gravity different enough between them to change how fast time passes. Deeper in the field, the clock runs slower. Farther out, it runs faster. When the two branches are recombined they interfere, and by selecting a particular interference outcome, a step called post-selection, the clock can behave as though it experienced more time than either branch, no time, or a negative amount of time.
Thirty-six years on, the pair asked how far that negative time could go. Since nothing has been built, they used mathematics to study four slightly different configurations of the machine. First they had to settle how anyone would prove time had run backward, and that is where the decaying atoms come in. An excited atom, with an electron lifted to a higher energy level, tends to settle back down, emitting a photon in a process called spontaneous emission. Ordinarily this goes only one way. Reversal would mean the emitted light returns to the atoms and re-excites them, but the photons might have already flown off or been absorbed elsewhere. So the researchers imagined the atoms sealed in a cell together with their emitted light and walls able to absorb it, and passed the whole cell through the machine.
Even then, a barrier appeared. The atoms are not isolated. As they emit, they interact with the surrounding electromagnetic field and leave traces of what happened. For the simplest machine, reversal works only while those interactions preserve the quantum coherence that lets the two possible histories interfere. In the researchers' optical example, that window lasts about a femtosecond. After that, the environment holds too much information about the emission for the two-branch machine to produce the reversal. The limit, in other words, comes from the environment's memory of the light, not from time itself. More elaborate machines could in theory reverse longer intervals, but only by adding many more quantum branches, with increasingly improbable chances of success.
The second law of thermodynamics seems to forbid all this, since the entropy of an isolated system cannot decrease. Arthur Eddington famously said that a theory against the second law deserves no hope and must collapse in deepest humiliation. The researchers' answer lies in post-selection. The apparent reversal shows up only in a tiny, carefully chosen fraction of outcomes. Considered across all outcomes, the system still obeys the second law.
The larger point is not building a time machine but understanding why time seems to move one way. Many fundamental equations work equally well forward and backward, yet everyday processes seem impossible to undo. By locating where quantum reversal fails, the work could help show how the environment produces irreversibility. In the researchers' words, "The arrow of time enters exactly where the machine stops." No experiment has yet tested any of this.
Notable Quotes
The arrow of time enters exactly where the machine stops.— Aharonov and Elitzur, preprint
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