  The arrow of time dilemma: the laws of physics are invariant for time
inversion, whereas the familiar phenomena we see everyday are not (i.e. entropy
increases). I show that, within a quantum mechanical framework, all phenomena
which leave a trail of information behind (and hence can be studied by physics)
are those where entropy necessarily increases or remains constant. All
phenomena where the entropy decreases must not leave any information of their
having happened. This situation is completely indistinguishable from their not
having happened at all. In the light of this observation, the second law of
thermodynamics is reduced to a mere tautology: physics cannot study those
processes where entropy has decreased, even if they were commonplace.
