Pmb, I am only saying that fundamental physics does not distinguish past and future.

To imagine how nonintuitive can it be, GRT theoretically allows for non-orientable spacetime (e.g. with wormhole glued like in Klein bottle) - in which there exists a path switching past and future:

Here is discussion:

http://www.thenakedscientists.com/forum/index.php?topic=46648If fundamental physics does not distinguish past and future, why thermodynamics does? Because symmetry can and often is broken in concrete solutions, like when trowing a rock to flat water surface - it breaks the symmetry, starting propagation of events.

So what was "such rock" for our Universe? We call it Big Bang - when everything was well localized, and so had low entropy - the tendency of entropy to grow created our 2nd law of thermodynamics.

All low entropic objects we have come from reason-result relation chain starting in our Big Bang - from our past: through Earth creation, evolution, finally leading for example to an egg, which have tendency to increase entropy: scramble and can do it only toward our future.

However, if there would exist such time-reversing loops and we would send a rocket through it, e.g. an egg inside would come from our future and so would have tendency to scramble toward our past - from our time perspective it would unscramble.

About "action optimizing" I have meant the principle of lest action (

http://en.wikipedia.org/wiki/Principle_of_least_action ) - that in Lagrangian mechanics (like classical mechanics, GRT, QFT) we could fix values in past and future (for example in Big Bang in past and Big Collapse in future) and physics choses the action minimizing solution (like the history of the Universe).

Equivalently we could fix values and derivatives in the past (e.g. Big Bang) and Euler-Lagrange equations (

http://en.wikipedia.org/wiki/Euler-Lagrange ) defines their unique evolution.

Equivalently we could change time sign in Euler-Lagrange equations, what allows to find unique backward evolution for given values and derivatives in the future (e.g. Big Collapse).

About unitarity of quantum evolution, indeed I meant that the evolution operator is unitary and so can be easily changed to time-reversed.

Example of wavefunction collapse (

http://en.wikipedia.org/wiki/Wave_function_collapse ) is decoherence - loosing unitary evolution due to complex interaction with environment. If we would consider complete QM: of the whole Universe, there would be no longer exterior.

I would like to propose a good temporal exercise for this thread:

**what impact to our society would have appearance of technology to send information back in time?**