Eric Chaisson is an astrophysicist and cosmologist who proposes a new metric for the complexity in various cosmological and biological systems. In his 2001 book, Cosmic Evolution: The Rise of Complexity in Nature, Chaisson argues that all organized systems share generic phenomena that characterize their emergence, development, and evolution. Other thinkers have argued for similarities and homologies in all evolving systems, but Chaisson proposes a single "complexity metric" that can quantify the major changes over both cosmological and biological time scales. The Chaisson complexity metric is the "energy rate density," the energy flow per second and per gram of material found in a given system. Thus the Sun generates in its nuclear interior 4 x 1033 ergs/second and has a mass of 2 x 1033 grams, so Chaisson's complexity metric for the Sun is 2 (ergs/sec/gm). Chaisson does not use the mass of planet Earth in his mass-normalized energy-rate metric, for two reasons. First, the heat generated internally by the Earth is a small fraction of the incident solar radiation. Second, the solar energy is deposited into the surface layer of atmosphere and biosphere, from which it gets re-radiated into the dark night sky as the earth turns. The solar energy flux falling on the earth is about 2 x 1024 ergs/second. Chaisson uses the mass of the atmosphere and the layer of water in the oceans as the mass of the relevant complex system, deriving an energy rate density of about 75 (ergs/sec/gm). Chaisson then estimates complexity metrics for plants, animals, human brains, and the product of our brains - society as a whole. Within each of these complex systems, Chaisson has shown that his complexity metric begins small, then evolves, increasing with time.
Information and ComplexityChaisson notes that others have used information as a measure of complexity. He says, "the extent to which a system is ordered can be estimated by appealing to information theory; the more ordered a system, the greater the information content (or negentropy) it possesses." (Cosmic Evolution, p.127) He says that "the necessary (though not necessarily sufficient) condition for the growth of information is guaranteed by the very expansion of the Universe." (p.128)
Early on, when the primordial nuclei began falling out of equilibrium, their reactions still generated entropy increases (mainly by creating new photons and neutrinos, as would later occur in the cores of stars), but these increases were insufficient to reestablish equilibrium. accordingly, the gap between the actual S and the maximum possible S that would have been achieved had equilibrium been restored increased even more, thereby building up free energy, to be released much later when stars formed. Here, then, we gain insight into the origin of free energy; it is not new energy as such, rather newly rearranged energy thereafter available for use in the course of evolution. The cosmologist Layzer (1975) perhaps put it best:But Chaisson remains skeptical about information as the appropriate measure of complexity.Suppose that at some early moment local thermodynamic equilibrium prevailed in the universe. The entropy of any region would then be as large as possible for the prevailing values of the mean temperature and density. As the universe expanded from that hypothetical state the local values of the mean density and temperature would change, and so would the entropy of the region. For the entropy to remain at its maximum value (and thus for equilibrium to be maintained) the distribution of energies allotted to matter and to radiation must change, and so must the concentrations of the various kinds of particles. The physical processes that mediate these changes proceed at finite rates; if these "equilibrium" rates are all much greater than the rate of cosmic expansion, approximate local thermodyanamic equilibrium will be maintained; if they are not, the expansion will give rise to significant local departures from equilibrium. These departures represent macroscopic information; the quantity of macroscopic information generated by the expansion is the difference between the actual value of the entropy and the theoretical maximum entropy at the mean temperature and density.The very expansion of the Universe, then, provides the environmental conditions needed to drive order from chaos; the process of cosmic evolution itself generates information. How that order became manifest specifically in the form of galaxies, stars, planets, and life has not yet been deciphered in detail; that is the subject of many specialized areas of current research. We can nonetheless identify the essence of the development of natural macroscopic systems — ordered physical, biological, and cultural structures able to assimilate and maintain information by means of local reductions in entropy — in a Universe that was previously completely unstructured.
We are converging on an answer to the foremost question at hand: Have the many varied real structures known to exist in the Universe displayed the sort of regular increase in order and complexity during the course of time as suggested by human intuition and implied by theoretical analysis? The answer is yes, and more. value of information...The conceptual idea of information has been useful, qualitatively and heuristically, as an aid to appreciate the growth of order and structure in the Universe, but this term is too vague and subjective to use in quantifying a specific, empirical metric describing a whole range of real-world systems. Nor do we feel any compelling need to treat information as a third, fundamental ingredient of the Universe — after matter and energy — for, to us, information basically is a form of energy, whether flowing, stored, or unrealized.Chaisson prefers to use the more familiar and traditional concept of energy, in particular the rate of energy produced or consumed per gram of the system being described, for his complexity metric. He does not need the new science of Yet how shall we best characterize that order? What single common term might be used to quantify order on all spatial and temporal scales? In appealing to the real world surrounding us, it is perhaps best to avoid use of the term information. When examined on a system-by-system basis, information content can be a slippery concept full of dubious semantics, ambivalent connotations, and subjective interpretations. Especially tricky and controversial is meaningful information, the information.
In the spirit of the opening remarks in the Prologue — namely, that no demonstrably new science likely need be invented to understand cosmic evolution and its attendant rise in complexity — we prefer to return to a steadfast concept of fundamental thermodynamics and to characterize that complexity by using quantifiably straightforward terms. In short, energy and energy flow seem to be more accessible, explicit, and primary quantities, and not just because this is a worldview espoused from an admittedly physical perspective. What is more, the concept of energy remains meaningful for any macroscopic state, obviating the difficulties noted above for entropy or information. More than any other term, energy has a central role to play in each of the physical, biological, and cultural evolutionary parts of the inclusive scenario of cosmic evolution; in short, energy is a common, underlying factor like no other — a "DC baseline" in physicists' lingo — in our search for unity among all material things.
For TeachersEric Chaisson's web site
Wright Center web site
Cosmic Evolution web site Books by Eric Chaisson include: Cosmic Dawn: The Origins of Matter and Life , Atlantic Monthly Press, Little Brown, 1981. Cosmic Evolution: Rise of Complexity in Nature, Harvard Univ. Press, 2001. Epic of Evolution: Seven Ages of the Cosmos, Columbia Univ. Press, 2006. Astronomy Today (co-authored with S. McMillan), Pearson, 6 editions, 1993-2008.
For ScholarsSee Chaisson's research page at Tufts University's Wright Center. Some of his recent research papers include: "Rhythm of the Cosmos: Finding Unity among the Natural Sciences," Tufts Magazine, p 16, Spring, 2001.
"A Unifying Concept for Astrobiology," International Journal of Astrobiology, v 2, pp 91-101, 2003
"Complexity: An Energetics Agenda," Complexity, v 9, pp 14-21, 2004
"Non-equilibrium Thermodynamics in an Energy-rich Universe," In Non-Equilibrium Thermodynamics and Production of Entropy, Kleidon, A. and Lorenz, R., Springer-Verlag, 2005.
"The Great Unifier," New Scientist, v 189, p 36, 2006.
"Long-Term Global Heating from Energy Usage," EoS Transactions American, Geophysical Union, v 89, p 253, 2008.
"Energy Rate Density as a Complexity Metric and Evolutionary Driver," Complexity, v 16, pp 27-40, 2011; DOI: 10.1002/cplx.20323
"Energy Rate Density II: Probing Further a New Complexity Metric," Complexity, in press, 2011.
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