[Martin Taylor 931214 17:25]
(Bill Powers 931209.0600)
I think that people easily forget that
concepts like energy and entropy are not primary physical
concepts; they are derived, they are generalizations from more
detailed physical relationships.
In the sense that all scientific concepts are a long way removed from
the immediate sensations of any particular moment, this is true. But
when I first read it, my immediate thought was "What IS this--is Bill
now denying the physical sciences?"
After a day or two of reflection on Bill's posting, I saw that what he was
doing was EXACTLY what most of my human factors colleagues (and psychologist
colleagues) do when I tell them about PCT. They say, yes, that's all
well and good, but it is much too general, and you can't usefully apply
it in the specific instances. For those, we need the special parameters
and descriptions that we know and love. We certainly believe PCT applies,
but anything so generally applicable can't be useful in any specific
circumstance.
While some very useful
computational shortcuts come out of considering these generalized
concepts, they are not a substitute for the underlying details.
Did anyone suggest they were?
This is easy to see when you ask how a system works. If I answer
by decribing only the energy or entropy relationships, you are
left with no conception of what the system is, much less how it
works or what it does.
Well, "no conception" may be a bit strong, since you can at least
determine what kind of a heat engine you are dealing with, and with
the entropy relationships you may be able to determine whether it
is a control system and if so what the gain is. But basically, the
point is valid.
But if I describe the system in terms of
variables and functions relating them, you can calculate energies
and entropies (etc) from the described relationships, and show
that the more abstract conditions are indeed met. That is how
they were found in the first place.
Now, now! Remember that what happened "in the first place" was a lot
of individual specific experiences. That's how the variables and
functions relating them were determined--or at least how the variables
and functions you learned about that allowed you to discover the
variables and functions that allowed you to discover ...
What makes one particular place in this network of discovery THE correct
stopping place?
I think that this relationships gets turned topsy-Turvey
sometimes; it sometimes seems that the generalized relationships
are given the status of the more fundamental aspects of real
systems, while the actual detailed relationships are relegated to
the status of mere examples -- as if it is the generalized
principles that make the system work. My position is exactly the
opposite: the way real systems work is what makes the generalized
principles true.
Agreed. But I think we also agree that we CANNOT know the way real
systems work. So, we are left with a back-and-forth approximation
procedure. A whole bunch of specific examples can be seen as leading
to a more general (yes, fundamental) description. If that general
description is asserted to hold, then other specific examples should
be observed. They are, repeatedly. (Or are not, in which case we
forget the general assertion). So we take the general description
as a temporary truth. You, for example, take a general description
of "control system" as a truth about the way living systems work, and
find that it produces correct statements (a lot of the time) for specific
instances--new ones that did not lead initially to the generalization.
Energy and entropy have been found to be useful physical concepts in so
many places over the last century or so that it is a brave person who
can assert that they do not apply in some specific instance.
I think this is in agreement with what you say: "[Energy]
is a property of the situation -- it is not 'in' anything, nor is
it any 'where.'" Energy can be reduced to the product of force
and distance, which are measurable aspects of the situation, but
force and distance cannot be reduced to energy.
Energy can be reduced to mass, but not to much else. It can be manifest
in force and distance, in a magnetic field, or in many other ways. So
what? Useful energy is a different matter. It depends on entropic
relationships. When you say "reduced to" I take it as implying an
equivalence relationship, with no one-way connotations. "Reduced to"
simply means "in terms with which I am more comfortable."
ยทยทยท
------------------
I think a vortex is probably much like a marble in a bowl.
It took me a couple of days to see why you might make such a strange
statement, and even now I'm not sure. So I may answer something other
than the point you wanted to make.
A ball in a bowl is moved off centre up the sides of the bowl by a
disturbing force. The energy supplied by the disturbance is slightly
greater than the potential energy difference between the ball's raised
position and its stable position. The stability that is disturbed is
the position of the ball. Energy is provided to the ball by the
disturbance.
I think you threw us off the track by speaking of the general
flow as a power source for the vortex. It is not: it is a
disturbance acting on the vortex, tending to make it spin faster.
The existence of the vortex depends on the energy flow. It is a structural
quality of the energy flow. The flow does not disturb the vortex as the
disturbance moves the ball. A disturbance to the vortex might come from
a wave initiated elsewhere, from a stick thrust into the vortex wall, or
somewhere else. The existence of the vortex depends on an instability
in the flow, in the sense that under the existing conditions of fluid
velocity, viscosity, and so forth, any small disturbance to smooth laminar
flow is augmented. There is positive feedback of such small disturbances,
which builds up until the vortex is created. At some point, the stable
vortex exists, and small disturbances to its shape are resisted. What
can be seen as a positive feedback system leading away from the laminar
flow regime is a negative feedback system around the vortex state.
Now reduce the flow. At some point there will be an insufficient gain
to sustain the vortex against even minor disturbances, and it will
dissipate into heat. It has been dissipating into heat all along, but
the flow rate was enough to supply the dissipated energy and to power
the vortex.
I think we have been overlooking something about "closed loop"
systems. It is not just the closure of the loop that matters.
What matters is that it is a _unidirectional_ closed loop, at
least at one point. Sensing the state of the controlled variable
_does not affect the state of the controlled variable_ (to any
meaningful degree).
You shouldn't overlook this point. It is important. If there were no
imbalance between the energy flow from the error signal to the output and
that from the output to the energy signal, there could be no gain and
no control. This imbalance is essential, and the gain at the sensory
periphery is almost as important (I think). In both cases, one has to
remain aware that the main energy flow is provided NOT by the signal
being amplified, but by an external energy source. It is this external
source that permits the unidirectionality of the signal flow. The marble
in a bowl does not have it; a control system and a vortex (or any self-
organized structure) do.
What is purposiveness? It is the ability to bring a perception to a
reference level, isn't it? The reference level itself is the purpose.
And THAT is what the vortex lacks.
Martin