Re: Why doesn't the 2nd Law of Thermodynamics prohibit time travel?

This is an interesting point. However, the energy required to transport 
something back in time could be enough so as to compensate for the entropy 
loss when they move to the past. Also, perhaps they could have to arrive 
in some place of entropy lower than themselves. On the face of it, it 
would appear that the second law of thermodynamics places limits on time 
travel, but does not prohibit it.
However, the answer to your question is probably somewhat more profound.
When time, as a dimension, is so similar to space, why is it that we only 
see it flowing in one direction? The answer would appear to be connected 
with that same law that is responsible for time-irreversible events: The 
second law of thermodynamics. Violate the arrow of time, and we probably 
would violate this law. However, we don't really know where this law comes 
from, or the full extent of its applicability. It would appear that the 
law is inextricably tied in with the collapse of the quantum wavefunction 
(the only other time asymmetric process). Our understanding of this, 
however, is hazy. Ask a physicist why the super-positioning of states 
collapses and they will tell you "observation." There is no science 
describing this. Perhaps we need a 'theory of everything' before your 
question can be answered with certainty.