Physicists have identified a limit on how far a quantum time-reversal machine can effectively run time backward, with the simplest model allowing a reversal of about one quadrillionth of a second before the environment’s memory of the past intervenes, according to a new preprint.
The finding, posted on arXiv by Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research, revisits a concept proposed by Aharonov and colleagues in 1990 called a quantum time-translation machine. That theoretical device would exploit quantum superposition to create a situation in which a system appears to experience time moving backward.
In the original proposal, a clock placed in a superposition of two positions in a gravitational field, one where time runs slower, one where it runs faster, could, through a process called post-selection, yield an outcome representing a negative amount of elapsed time. The new work investigates how far that negative time could go.
The researchers modelled the reversal using a sealed cell containing excited atoms and the light they emit as they decay. If the time machine could reverse the decay, more atoms would be left excited than remained after some had decayed. The key is that the emitted photons interact with the surrounding electromagnetic field, leaving traces of what has happened. The machine’s ability to reverse the decay depends on the interactions retaining the quantum coherence needed for the histories to interfere; the environment’s retention of information about the emission limits that coherence.
In the researchers’ optical example, the window for the simplest two-branch version of the machine to produce the reversal lasts around a femtosecond. “The arrow of time enters exactly where the machine stops,” the researchers write.
More elaborate configurations with many quantum branches could reverse longer intervals, but at increasingly low probabilities of success. The work does not violate the second law of thermodynamics, the authors note, because the apparent reversal occurs only in a carefully selected subset of outcomes; across all possibilities, entropy still increases.
The work could help physicists understand a fundamental puzzle: why time appears to move in only one direction even though many fundamental equations of physics work just as well backward as forward. By pinpointing where quantum time reversal breaks down, the study may help explain how irreversible processes emerge from the quantum environment.
Analysis
Why This Matters
- The limit suggests that even in theory, macroscopic time reversal is impossible without extreme quantum complexity.
- It provides a concrete boundary between reversible quantum dynamics and the irreversible arrow of time we experience.
- The work could guide future experiments testing the foundations of quantum mechanics and thermodynamics.
Background
Physicists have long known that many fundamental equations work equally well forward and backward in time, yet everyday experience is full of irreversible processes. The concept of a quantum time-translation machine, first proposed by Aharonov and colleagues in 1990, offers a theoretical way to create a time-reversal effect using quantum superposition and post-selection. The new analysis examines the practical limits of such a device.
What to Watch
- Experimental attempts to realize a quantum time-translation machine, though still theoretical, may emerge in small-scale quantum systems.
- Further theoretical work exploring more complex configurations that could push the reversal limit beyond a femtosecond.
- Implications for understanding the arrow of time and its connection to quantum decoherence.
Checked
12 claims in this story checked against 1 source: 12 supported.
Show each claim
- SupportedPhysicists have identified a quantum limit on how far time can effectively run backward.ScienceAlert: “A new mathematical exploration of this question has returned a surprising answer: The quantum environment itself imposes a limit on how far back time can effectively run.”
- SupportedThe simplest model allows time to run backward for about a femtosecond before environmental memory limits the reversal.ScienceAlert: “For the simplest model explored by physicists Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research, time could only effectively run backward for about a femtosecond – or one quadrillionth of a second.”
- SupportedThe new mathematical analysis was published as a preprint on arXiv.ScienceAlert: “Their work is detailed in a preprint uploaded to arXiv .”
- SupportedThe 1990 quantum time-translation machine was proposed by Aharonov and colleagues.ScienceAlert: “In 1990, he and his colleagues proposed a theoretical device called a quantum time-translation machine”
- SupportedThe proposed machine uses quantum superposition to make a system behave as if time had run backward.ScienceAlert: “which could exploit the strange properties of quantum mechanics to make a system behave as though time had sped up, stopped, or even run backward.”
- SupportedIn the clock example, post-selection could yield an outcome representing a negative amount of elapsed time.ScienceAlert: “That could mean more time than has passed for either branch. It could mean no time at all. Or it could mean a negative amount of time – as though the clock had been running backward.”
- SupportedThe researchers modelled possible reversal of atomic decay using a sealed cell containing atoms and emitted light.ScienceAlert: “Aharonov and Elitzur imagined enclosing the atoms inside a completely sealed cell, along with the light they emitted and walls that could absorb it.”
- SupportedIf decay were reversed, more atoms would be excited than remained excited after some had decayed.ScienceAlert: “If you end up with more than 80 excited atoms, the machine has effectively reversed some of the decay that had already taken place.”
- SupportedThe emitted light interacts with the surrounding electromagnetic field, leaving traces of what happened.ScienceAlert: “As they emit light, they interact with the surrounding electromagnetic field, leaving traces of what has happened.”
- SupportedThe simplest machine’s reversal depends on retaining the quantum coherence needed for the histories to interfere.ScienceAlert: “For the simplest version of the machine, reversing the decay depends on those interactions retaining the quantum coherence needed for the two possible histories to interfere.”
- SupportedMore elaborate versions could reverse longer intervals, but with increasingly low probabilities of success.ScienceAlert: “More elaborate versions of the machine could theoretically reverse longer intervals, but only by introducing vastly more quantum branches, with increasingly improbable chances of success.”
- SupportedThe researchers wrote, “The arrow of time enters exactly where the machine stops.”ScienceAlert: ““The arrow of time enters exactly where the machine stops,” the researchers write .”
