A QUANTUM ARROW OF TIME
The mathematical laws of physics work just as well for events going forward
or going backward in time. Yet in the real world, hot coffee never unmixes
itself from cold milk. A theorist publishing in the 21 August Physical Review
Letters offers a new explanation for this apparent conflict between the
time-symmetry of the physical laws and the forward "arrow of time" we see
in everyday events. When viewed in quantum terms, events that increase the
entropy of the Universe leave records of themselves in their environment. The
researcher proposes that events that go "backward," reducing entropy,
cannot leave any trace of having occurred, which is equivalent to never
happening.

Lorenzo Maccone, Phys. Rev. Lett. 103, 080401 (2009)
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