## Tuesday, August 18, 2009

A fascinating, yet simple approach to show that under certain circumstances time can flow in both directions, forward and backward:

I show that entropy in a system can both increase and decrease (as time reversal
dictates), but that all entropy-decreasing transformations cannot leave any trace of their having happened. Since no information on them exists, this is indistinguishable from the situation in which such transformations do not happen at all: ‘‘The past exists only insofar as it is recorded in the present’’ [11]. Then the second law is forcefully valid: the only physical evolutions we see in our past, and which can then be studied, are those where entropy has not decreased.

DOI: 10.1103/PhysRevLett.103.080401

Lorenzo Maccone, 2009. Quantum Solution to the Arrow-of-Time Dilemma. PRL 103, 080401 (2009).