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From: stekas@hou2g.UUCP (J.STEKAS)
Newsgroups: net.physics
Subject: Re: Non-linear systems.
Message-ID: <389@hou2g.UUCP>
Date: Mon, 14-Jan-85 11:16:21 EST
Article-I.D.: hou2g.389
Posted: Mon Jan 14 11:16:21 1985
Date-Received: Tue, 15-Jan-85 02:07:39 EST
References: <209@talcott.UUCP>, <328@rlgvax.UUCP> <384@hou2g.UUCP>, <273@harvard.ARPA>
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 26

>> The concept of "predicable, in principle" is usefull if and only if 
>> it has some connection with reality.
>
>Sorry, you're wrong. The technical term for "predictable in principle"
>is computable.

You mean "predictable in principle" (i.e. computable) is a useful concept
even when it has NO connection with reality?!  I wasn't aiming at a
definition of "computability" only an understanding of the usefullness
of the concept.  Even so, I would think that most of modern physics is
NOT computable!

The point I was trying to make was that if the universe is governed by
a collection of non-linear equations (of sufficient ugliness) then the
question of whether the universe is predictable may moot because we
might not be able to calculate with them.  So we couldn't verify the
correctness of our theories, or determine the future even if they
were correct.

Is QED a "computable" theory?  QCD?  General Relativity?  Have we verified
them to sufficient accuracy to believe them?  Are they deterministic
theories?  Are they deterministic approximations of a non-deterministic
reality?  Are these questions of more concern to a physicist, philosopher,
or theologian?

						Jim