X-Message-Number: 8877
Date:  Sun, 30 Nov 97 16:53:47 
From: Mike Perry <>
Subject: Re: #8868

Andre Robatino, #8868:

>Even if one makes multiple backups, it may not be possible to get
>geometrically decreasing probability of all of them being destroyed. 
>If the probability of one of them being destroyed in a given time
>period is p, it's not true in general that the probability of n
>backups all being destroyed is p^n, since the probabilities are not
>totally independent.  A catastrophe acting over a region encompassing
>all of them can destroy all of them simultaneously, and for large
>enough n this becomes the limiting factor on how safe one's backups
>are.

But you don't need a geometrically decreasing probability of 
destruction to have the possibility of infinite survival--a slower 
convergence to zero will do--though it must not be too slow.
(If for example, the probability decreases like at^-2 with time t, 
a= a constant, that is fast enough; this is much slower than 
exp(-at), i.e. a geometric rate; at^-1 is too slow, however.)
Also, while large-scale catastrophes are 
certainly possible, they are less common than small scale 
catastrophes. By the time that supernova goes off in your 
back yard, then, your backups could be spread so far and wide 
that even this will not wipe any large fraction of your memories.

Mike Perry

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