X-Message-Number: 31712
From: Daniel Crevier <>
References: <>
Subject: Pizer's time travel
Date: Tue, 02 Jun 2009 11:55:07 -0400

Traveling back in time by running a backwards simulation from the present is 
a good idea, but it suffers from a major weakness: you can't go back more 
than a short period in time without running into the same problems you get 
when trying to predict the weather. The differential equations describing 
the world can be integrated both backwards and forward, but a small error 
accumulates at each time step in both cases. This is due to the so-called 
butterfly effect, whereby small causes can have large consequences. More 
specifically, tiny errors in the  evaluation of the initial state of the 
system get amplified over time, until the calculated state doesn't have any 
relation with reality. In order to preserve accuracy over time, a perfect 
knowledge of the initial state would be needed, which is impossible because 
of quantum uncertainty (you can't know both a particle's speed and position 
with perfect accuracy). This is a fundamental limitation of nature, and a 
galaxy-wide computer wouldn't do you any good.

For this reason, earlier proponents of the idea, like Mike Perry in Forever 
for All, if I understand him correctly, recommend a backwards simulation 
that would be constrained to conform to known historical facts (which would 
be used as 'boundary conditions' in mathematical jargon). Whatever facts the 
simulation provides that are not part of the historical record would then 
constitute at best educated guesses on what really happened, as there is an 
infinite number of historical novels that can be written and fit what is 
known of history. The simulation would just extract the most likely ones.

Of course the task would be easier for historical characters for which many 
facts are know : Napoleon, about whom there are mountains of bibliographies, 
could be reconstructed with a lot less guesswork than an anonymous grunt in 
his army.

So if you want to be brought back, leave as many traces behind as you can. 
And get your loved ones to do the same.

Daniel Crevier, Ph.D.

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