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disipativo

estructura disipativa

Definicin: Se dice que un sistema es disipativo si su energa se degrada en forma de calor,que en parte no es transformable en otras formas de energa menos degradada. Una

estructura es disipativa en la medida que ayuda a los mecanismos disipativos.

Segun la clsica segunda ley de la termodinmica, un sistema aislado ha de ir perdiendo

(disipando) toda la energa libre que posee con lo cual su entropa se maximiza. Un sistema

en equilibrio trmico ya no disipa ms y se halla en un estado de mxima entropa. Si un

sistema se halla en las cercanas del equilibrio, sus tendencias espontneas e irreversibles lo

son hacia el equilibrio. La fuerza impulsora es la de producir entropa.

Por definicin, en el equilibrio ya no puede producir ms entropa (principio de la mnima

produccin de entropa).

Pero no abundan los sistemas aislados, por lo cual puede haber sistemas alejados del

equilibrio (como el planeta iluminado o el cerebro con nutrimentos) que no pueden llegar a

l - aunque lo buscan espontneamente - porque mientras tanto siguen recibiendo aportes

de energa externa (el sol, la glucosa en sangre).

Con esos aportes las ecuaciones diferenciales descriptivas de la dinmica, ya no son ms

lineales. No estn en el equilibrio sino en el desequilibrio.

Como hay sistemas disipativos con estructuras, es lcito llamarlas, con Prigogine,

"estructuras disipativas", aunque a primera vista su estudio parezca poco interesante. Pero

hay algo muy real: las condiciones "lejos del equilibrio" o "en el desequilibrio" implicanleyes no-lineales.


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Toda la fsica interesante en nuestro universo es no-lineal

Se deduce que lo que recin pareca poco interesante sera una afirmacin apresurada.

La nica regla general para las ecuaciones diferenciales no-lineales es que no hay reglas

generales. El caos es una posibilidad, como tambien la presencia de atractores, repulsores,bifurcaciones, autoorganizaciones.

Lo que afirma Prigogine es que aunque no hall para esta rama de la fsica incorporada a

la mecnica estadstica, una nueva constante universal, por lo menos ha encontrado una

desigualdad matemtica, un "criterio de evolucin universal". As como hay transiciones

de fase en la fsica lineal, con roturas de simetra, muy cercanas al equilibrio (como el hielo

que se funde), tambin las hay en la fsica no-lineal donde las estructuras disipativas se

vuelven inestables y tienden a veces hacia patrones de organizacin coherente que

minimizan la energa libre y disminuyen los grados de libertad.

Prigogine propone que dentro de un sistema complejo no-lineal lejos del equilibrio existensubsistemas fluctuantes. De vez en cuando se combinan y amplifican las fluctuaciones y se

disrrumpe la estructura previa, ocasin en la cual aparece una bifurcacin, un punto de

bifurcacin. La teora no puede predecir, por adelantado, si el resultado ser una

estructura de dinmica catica o una estructura autoorganizada con un orden "superior",

un "orden por fluctuaciones". En este ltimo caso, como la estructura necesita de energa

externa para seguir organizada, es aceptable llamarla "estructura disipativa", puesto que

necesita ms energa externa que la estructura no-disipativa (ms simple) previa

reemplazada. Tiene un lmite para su evolucin y es la falta de capacidad para eliminar

ms y ms calor. Los seres vivientes funcionan como sistemas disipativos, autoorganizados

por fluctuaciones ambientales.

Cabe destacar que no todos los autores aceptan incondicionalmente estas afirmaciones. Por

ejemplo, un crtico de las ideas de irreversibilidad de Prigogine es Jean Bricmon.

estructuras ordenadas de Prigogine

concepto de organizacin, enraizado en el universo fsico

sistemas vivientes como disipadores de gradientes

introduccin a la termodinmica de los procesos irreversibles

concepto de organizacin, enraizado en el universo fsico

glosario de estructuras disipativas

"Science of Chaos or Chaos in Science?" de Jean Bricmon

25.ene.2001

Web Dictionary of Cybernetics and Systems, traducido por CvdB
http://var/www/apps/conversion/tmp/scratch_6/prigesor.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigsten.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigbolt.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigittt.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigsten.htmlhttp://var/www/apps/conversion/tmp/scratch_6/estrdisi.htmlhttp://xxx.lanl.gov/abs/chao-dyn/9603009http://var/www/apps/conversion/tmp/scratch_6/prigesor.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigsten.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigbolt.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigittt.htmlhttp://var/www/apps/conversion/tmp/scratch_6/prigsten.htmlhttp://var/www/apps/conversion/tmp/scratch_6/estrdisi.htmlhttp://xxx.lanl.gov/abs/chao-dyn/9603009


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estructuras ordenadas de Prigogine.De acuerdo con la segunda ley, la tendencia de la evolucin de los sistemas biolgicos en los

niveles qumicos y supramoleculares se puede determinar mediante el estudio del efecto de

la autoorganizacin termodinmica (autoarmado). El criterio para estimar el desarrolloevolucionario de las estructuras supramoleculares de biosistemas (biotejidos) est dado por

la variacin de la funcin de Gibbs especfica para su formacin. Durante los procesos de

ontognesis, filognesis y los de la evolucin biolgica en general, el componente especfico,

supramolecular, de la funcin de Gibbs de un biosistema, tiende a un mnimo relativo. Se

entiende esto en un sistema cuasi cerrado tanto termodinmica como cinticamente. El

valor al que se llega en ese mnimo es una caracterstica conjunta del sistema y del

ambiente dados.

Prigogine ha demostrado que una nueva forma para estructuras ordenadas puede existir

en las recin expresadas condiciones. Les di el nombre de estructuras disipativas. As

enfatiz que solamente pueden existir en conjuncin con ciertos ambientes.

La ms caracterstica de esas estructuras disipativas es la inestabilidad de Bnard.

29.mar.2000

el concepto de organizacin est bien

enraizado en el universo fsico


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Prigogine Ilya & Stengers Isabelle: ORDER OUT OF CHAOS (Bantham, 1984)

This is the english edition of "La Nouvelle Alliance" (1979). Prigogine analyzes the history

of science and scientific thought and derives a new vision of the world.

Classical science (and quantum mechanics) describes a world as a static and reversible

system that undergoes no evolution, whose information is constant in time. On the other

hand the second law of thermodynamics describes the world as evolving from order to

disorder, while biological evolution is about the complex emerging from the simple

(structure, i.e. order, arises from disorder).

Irreversible processes are an essential part of the universe. Conditions far from

equilibrium foster phenomena such as life that classical physics does not cover.

Prigogine focuses on the peculiar properties exhibited by systems far from equilibrium.

Non-equilibrium conditions favor the spontaneous development of self-organizing systems

(i.e., dissipative structures), which maintain their internal organization, regardless of thegeneral increase in entropy, by expelling matter and energy in the environment. Most of

Nature is made of dissipative systems, of systems subject to fluxes of energy and/or matter.

Dissipative systems conserve their identity thanks to the interaction with the external

world.

The concept of organization is deeply rooted in the physical universe.

Prigogine considers living organisms as dissipative structures in states of non-equilibrium.

A system that is not in equilibrium exhibits a variation of entropy which is the sum of the

variations of entropy due to the internal source of entropy plus the variation of entropy due

to the interaction with the external world. The former is positive, but the latter can equally

be negative. Therefore total entropy can decrease.

An organism "lives" because it absorbs energy from the external world and processes it to

generate an internal state of lower entropy. An organism "lives" as long as it can avoid

falling in the equilibrium state.

Probability and irreversibility are closely related. Boltzman had already proved that

entropy grows because probability grows.

introduccin a la termodinmica de los

procesos irreversiblesPrigogine afirma que la cantidad de energa en desequilibrio, ptima para lograr

formacin de patrones ordenadas, nunca debe ser desmesurada, pues resulta

contraproducente ya que en dichos casos el sistema suele no poder ser suficientemente

disipativo.

Prigogine Ilya: INTRODUCTION TO THERMODYNAMICS OF IRREVERSIBLE

PROCESSES (Interscience Publishers, 1961)

Prigogine introduced the minimum entropy principle (stable near- equilibrium dissipative

systems minimize their rate of entropy production) to characterize living organisms.


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Simple dissipative systems ...predictably move to simple minimised states. But the

dynamical systems of animal behaviour...come to ...a detailed balance at many and varied

points....To search behaviour for explanations armed only with optimality is not an

optimum strategy for understanding." (p.221)

The form of expression this self-organization takes is not predictable in advance becausethe very process of self-organization is by catastrophic (in the catastrophe theory sense)

change; it "flips" into new regimes. As noted earlier, one of the characteristics of

catastrophic change is that systems may have several possible behavioural pathways

available at a catastrophe threshold. Which pathway is followed is largely an accident of

circumstances. A reductionist world view, which cannot deal with the reality of emergence

and self-organization in non-equilibrium systems, cannot offer sufficient explanation of

how the world works.

An important observation about systems that exhibit self-organization is that they exist in a

situation where they get enough energy, but not too much. If they do not get sufficient

energy of high enough quality (beyond a minimum threshold level), organized structurescannot be supported and self-organization does not occur. If too much energy is supplied,

chaos ensues in the system, as the energy overwhelms the dissipative ability of the

organized structures and they fall apart. So self-organizing systems exist in a middle

ground of enough, but not too much.

The global weather, wind and ocean circulation patterns are the result of the difference in

heating at the equator relative to the poles. The general meteorological circulation of the

earth, although affected by spatial, coriolis and angular momentum effects, is driven by

gradients and the global system's attempt to dissipate them and come to local equilibrium.

Paltridge (1979) has suggested that the earth-atmosphere, climate system configures itself

into a state of maximum dissipation and that the global distribution of clouds, temperatureand horizontal energy flows are governed by thermodynamic dissipative processes similar

to those described above. We see that the earth-climate system, as well as other dissipative

systems, do not reach a static equilibrium state because they are open thermodynamic

systems constantly receiving a supply of external energy (i.e. from the sun), which drives

them and maintains them in a nonequilibrium organized state.

So far we have focused our discussion on simple physical systems and how thermodynamic

gradients drive self-organization. The literature is replete with similar phenomena in

dynamic chemical systems. Prigogine and the Brussel's School and others have documented

the thermodynamics and behavior of these chemical reaction systems. Chemical gradients

result in dissipative autocatalytic reactions, examples of which are found in simple

inorganic chemical systems, in protein synthesis reactions, or phosphorylation,

polymerization and hydrolytic autocatalytic reactions.

Autocatalytic reactions systems are a form of positive feedback where the activity of the

system or reaction augments itself in the form of self-reinforcing reactions. Consider a

reaction where A catalyzes the formation of B and B accelerates the formation of A; the

overall set of reactions is an autocatalytic or positive feedback cycle. Ulanowicz (1986)

notes that in autocatalysis, the activity of any element in the cycle engenders greater

activity in all the other elements, thus stimulating the aggregate activity of the whole cycle.

Such self-reinforcing catalytic activity is self-organizing and is an important way of

increasing the dissipative capacity of the system. Cycling and autocatalysis is a

fundamental process in nonequilibrium systems.


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The notion of dissipative systems as gradient dissipators holds for nonequilibrium physical

and chemical systems and describes the processes of emergence and development of

complex systems. Not only are the processes of these dissipative systems consistent with the

restated second law, it should be expected that they will exist wherever there are gradients.

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