Home > Positioning > Persons > Chaisson

Eric Chaisson

Chaisson set out to tell the history of the universe as one continuous story of rising complexity, from the radiation of the early universe through galaxies, stars, planets and life to human society, and to give that story a single physical measure. He called the subject cosmic evolution and defined it as “the study of the many varied developmental and generational changes in the assembly and composition of radiation, matter, and life throughout all space and across all time”. His measure is energy rate density, the rate at which energy flows through a system per unit of its mass, which he found rising across the whole sequence. He uses “evolution” in the broadest sense on purpose and widens selection to match, and his epochs and energy metric were taken up by big history.

Eric Chaisson took a degree in physics at the University of Massachusetts Lowell in 1968 and a PhD at Harvard in 1972, and served as an officer in the United States Air Force. He taught at Harvard and Johns Hopkins and worked at the Space Telescope Science Institute, then spent twenty years at Tufts University directing the Wright Center for Science Education, before returning to the Center for Astrophysics at Harvard and Smithsonian, where he teaches natural science at Harvard. With Steve McMillan he wrote the textbook Astronomy Today. Cosmic Dawn (1981) won the Phi Beta Kappa Award in Science and Epic of Evolution (2006) the Kistler Book Award; he was elected a fellow of the American Association for the Advancement of Science in 2018.

Key concepts

Cosmic evolution. Chaisson treats the history of the universe in seven epochs, particulate, galactic, stellar, planetary, chemical, biological and cultural, each building on the last, and sees in them a single process: the emergence of ordered, complex systems as the universe expands and cools. The idea has antecedents in Harlow Shapley’s popular cosmology, and Chaisson’s first book belongs with the cosmic histories of the early 1980s, Carl Sagan’s Cosmos among them. Biological evolution, on his account, is “just one, albeit important, subset of broader evolutionary action”, and “what Darwinian change does for plants and animals, cosmic evolution aspires to do for all material systems”. The process has no goal: cosmic evolution, he writes, “is an aimless, meandering process”.

Energy rate density. Complexity needs a flow of energy to build and sustain it, and Chaisson proposed measuring it by the free energy flowing through a system per second per gram of its mass. Across the sequence the figure rises by orders of magnitude: galaxies lowest, then stars, planets, plants, animals, the human brain and modern society. Building on an argument of the astrophysicist David Layzer, he explains that order is possible in an expanding universe because expansion opens a growing gap between the actual entropy of matter and its maximum possible value, so that local order can grow while the total entropy rises. Cosmic Evolution: The Rise of Complexity in Nature (2001) set out the argument in full.

Selection widened. To extend evolution beyond biology Chaisson extends selection with it: “selection can be generally taken to mean preferential interaction of any object with its environment”, and on that definition “selection occurs in the inanimate world as well as among animate objects”. A star or a galaxy is selected in the sense that some configurations persist and others do not. He notes that Darwin never used “evolution” as a noun in the first edition of the Origin and argues that the word “need not be the sole province of biology”.

Where Chaisson stops

The metric has been pressed as a measure of complexity. Ken Solis’s “Reexamining ‘Free Energy Rate Density’ as a Complexity Metric” (Journal of Big History, 2023) argued that Chaisson’s criteria for what counts as complex admit systems that are not complex by usual standards, that the metric is never compared with other measures of complexity and so becomes its own standard, that it slides from measuring the degree of complexity to measuring function and structure, and that the figures reported are often total energy flow, so that a more efficient system can appear less complex. Chaisson has acknowledged that the correlation of energy rate density with complexity is approximate and subject to limits.

Key works

See also: Evolution · Cosmic evolution · Thermodynamics · Hazen