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Re: Simulations
- From: Tim Gwinn <***>
- Date: Tue, 28 Dec 2004 10:53:15 -0500
Tim,
-snip--
TG: Such programs suffice very well for the
sake of emulating the behavior of an N-body system, but because the
entailment structure is now entirely different from the entailment structure
of the natural system, it is a simulation rather than a model,
in the Rosennean sense of those terms.
HP:
This is a logical or conceptual distinction that I still do not see how to
make empirically. We name the cause of n-body orbits "gravity," but we never
causally model gravity itself. We have many concepts of gravity (curved space,
gravitons, etc.) but we don't know causes except by our models. We understand
this when we compute n-body systems, so we are not misled. Almost all computer
models are simulations in this sense.
We know that there is seldom anything in the program, gates, and memories
with the same physical causal entailments as in the system being modeled, but
as you point out, we always take care of this by our encoding and
interpretation. I would say that only a physical analog like a model airplane
could be a model in Rosen's sense.
Howard,
I assumed since
you understand Rosen's argument you understood what I would mean by "because
that is all the entailment that the computer hardware is capable of". I was
not referring to "We know that there is
seldom anything in the program, gates, and memories with the same physical
causal entailments as in the system being modeled...". Instead, I was referring to Rosen's discussion that the
entailment at the read head in a Turing-machine (and thus, the same entailment
in any realization of a Turing machine) is a predicative entailment only.
In the equations
for the 3-body system - the ones which we cannot solve in closed form -
gravitational mass is assigned the role of a force imposed on an
inertial mass; that is, each gravitational mass is assigned the role of
efficient cause of the motion of the other two inertial masses. So, we do assign
causal roles to gravity in this kind of system. These relations between the
gravitational masses and inertial masses are simultaneous, impredicative
ones. But since a computer can only process predicatively (stepwise,
recursively, sequentially), it cannot embody the impredicative causal
role of gravity. It must instead use some predicative simulacra.
Regards,
Tim