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<div class="moz-cite-prefix">On 21/06/2013 15:29, Dennis May wrote:<br>
</div>
<blockquote
cite="mid:1371824963.42841.YahooMailNeo@web160701.mail.bf1.yahoo.com"
type="cite">
<div style="color:#000; background-color:#fff; font-family:Courier
New, courier, monaco, monospace, sans-serif;font-size:14pt"><span>Why
aren't we a thousand miles deep in grasshoppers?</span>
<div><span>A: disease, parasites, predators, cannibalism,
resources to survive/replicate.</span></div>
</div>
</blockquote>
<br>
This applies if you have a system where evolution applies. The
standard interstellar replicator scenarios tend to use multiple
local hops, allowing many generations between the start and end
point. Plenty of chance for evolutionary drift or divergence,
although artificial probes can be equipped with error correction
making any accidental diversity as negligible as you want. Stuarts
and mine scenario has two generations: the end-state will not have
had much chance to evolve (and, again, error correction can prevent
it).<br>
<br>
The assumption that given time parasites will evolve is based on the
image that the system is free to evolve. But non-evolving
replicators getting there first can also prevent the appearance of
evolving replicators. If the first seeders didn't want to allow
them, they could do it. We might *like* the concept of evolving
replicators a great deal more than those boring non-evolving, but
the latter can win against the rest if they are programmed to be
through.<br>
<br>
...however. I have a poster at a conference in two weeks (<a
href="http://star-www.st-and.ac.uk/%7Eap22/setinam2013.html">http://star-www.st-and.ac.uk/~ap22/setinam2013.html</a>)
where I do a further analysis of the stability of this kind of
scenario. Basically non-evolving probes preventing the spread of
evolving parasites (that is, other civilisations) must be vigilant
and effective in order to be an explanation of the Fermi question. I
show that even when taking anthropics into account our existence is
a counter-argument, and in addition such systems do not seem to be
stable against invasion - the only way to be truly certain nobody
else can invade is to turn everything into your kind of replicator.
Hence the "deadly probe scenario" is not a likely answer to the
Fermi question. <br>
<br>
<br>
<pre class="moz-signature" cols="72">--
Anders Sandberg,
Future of Humanity Institute
Oxford Martin School
Faculty of Philosophy
Oxford University </pre>
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