The laws of thermodynamics were summed up by C.P. Snow:
1. You cannot win (that is, you cannot get something for nothing, because matter and energy are conserved).


2. You cannot break even (you cannot return to the same energy state, because there is always an increase in disorder; entropy always increases).


3. You cannot get out of the game (because absolute zero is unattainable).

Since you can't create or destroy matter, travelling back in time sure does seem impossible still. If you travelled back in time, your atoms would exist twice in the time period you travelled to!