Physicists Find a Mathematical Limit to Making Time Run Backward

Key Points:

• Physicists Yakir Aharonov and Avshalom C. Elitzur have mathematically explored the possibility of time running backward in quantum mechanics, finding that such reversal can only occur for an extremely brief interval—about one femtosecond.
• Their theoretical quantum time-translation machine relies on quantum superposition and post-selection to make a system behave as if time has reversed, but environmental interactions impose strict limits on how far back time can effectively run.
• The researchers used atomic decay as a test case, showing that reversing decay processes requires maintaining quantum coherence, which environmental "memory" disrupts almost immediately, preventing longer time reversals.
• This quantum time reversal does not violate the second law of thermodynamics because the apparent reversal occurs only in a small subset of outcomes selected post-measurement, preserving overall entropy increase.
• The study sheds light on why time appears to flow unidirectionally in our macroscopic world, suggesting that environmental effects cause the breakdown of quantum time reversal and give rise to the arrow of time we experience.