Quantum Time Reversal Limit
Physicists have calculated that the quantum environment limits effective time reversal to about one femtosecond in the simplest theoretical models, ScienceAlert reported.
Time flows in a single direction under relativity, even when slowed by strong gravitational fields or high-speed travel. Looking into quantum mechanics opens the possibility of time running backward, prompting a new mathematical exploration by researchers.
Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research detailed their findings in a preprint uploaded to arXiv.
Quantum Superposition and Decay
Superposition allows a clock to occupy multiple positions in a gravitational field at once, experiencing different time rates.
Post-selection lets physicists interfere those branches to produce outcomes outside normal ranges. To test how far this negative time could go, Aharonov and Elitzur examined atomic decay in a sealed cell.
Unexcited atoms emit light and drop energy levels through spontaneous emission. By putting decaying atoms through the theoretical machine, researchers checked if the device could reverse the decay process.
Environmental Memory Barriers
Atoms interact with the surrounding electromagnetic field while emitting light, leaving traces of what happened inside the sealed cell. Reversing the decay depends on those interactions retaining quantum coherence for both histories to interfere.
The window for the simplest machine lasts roughly one femtosecond before the environment retains too much information. More elaborate machine versions could theoretically reverse longer intervals by introducing vastly more quantum branches.
“The arrow of time enters exactly where the machine stops,” the researchers write.
Post-selection maintains the second law of thermodynamics
The second law of thermodynamics dictates that total entropy in an isolated system can never decrease naturally. Sir Arthur Eddington said that any theory violating this law must collapse in deepest humiliation.
Post-selection provides a loophole where apparent time reversal occurs in only a small fraction of outcomes. Considered together, the overall system still obeys the second law of thermodynamics without increasing global entropy.
How quantum models limit backward time flow
How long can time run backward in a quantum model?
Time can only effectively run backward for about one femtosecond in the simplest model explored by researchers.
What stops time from running backward longer?
The limit is imposed by the environment’s memory of the emitted light rather than time itself.
Does quantum time reversal break thermodynamics?
No, because post-selection applies only to a small fraction of outcomes while the system as a whole obeys the second law of thermodynamics.
The research is available on preprint server arXiv.
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