The Fabric of the Cosmos CH 6

In this reading of The Fabric of the Cosmos, Greene asks the question “Does time have a direction?”  We expect and experience certain things happening in a specific way, but does it have to happen that way?  Is it possible for it to happen another way? For example, if i dropped an egg on the floor, would it be possible for the egg to reform and come back to my hand?  It would seem like that would be impossible, but according to the time-reversal symmetry nothing says that it can not happen.  The time reversal symmetry says that if something happening in the present occurs, then the opposite of that thing can also happen.  An easy way to imagine the time reversal symmetry is to imagine watching a film on a projector, stopping that film, and then reversing the direction of that film.  I think that the concept of time reversal symmetry makes sense, but I do not understand what would be needed for the direction of time to be reversed.


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