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From: edp@jareth.enet.dec.com (Eric Postpischil (Always mount a scratch monkey.))
Newsgroups: rec.puzzles,sci.math,comp.theory.cell-automata
Subject: Re: How Intelligent are the Winning Ways?
Message-ID: <20805@shlump.nac.dec.com>
Date: 6 Mar 91 14:12:17 GMT
References: <1991Mar5.171804.1429@zorch.SF-Bay.ORG> <BAGCHI.91Mar2000950@snarf.eecs.umich.edu> <5539@acorn.co.uk>
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Reply-To: edp@jareth.enet.dec.com (Eric Postpischil (Always mount a scratch monkey.))
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In article <1991Mar5.171804.1429@zorch.SF-Bay.ORG>, xanthian@zorch.SF-Bay.ORG
(Kent Paul Dolan) writes:

[about creating a Turing machine in the universe of Conway's Life]

>You have to be able to reverse direction on the tape.  Rebuilding the
>head one space left rather than one space right is at least feasible,
>but reversing the motion of a tape which is by definition infinite in
>length is not.

What's been written to the tape is never actually infinite; reversing the motion
of the portion that has been written and marking the current ends is sufficient
to achieve the emulation.


				-- edp (Eric Postpischil)
				"Always mount a scratch monkey."
				edp@jareth.enet.dec.com