Now one might well ask how the directionality of these ecosystems differs in any qualitative way from, say the billiard ball mentioned in the opening paragraph of this chapter? For one, the direction of the billiard ball is a consequence of the collision with the other ball, the Newtonian law of momentum and the Newtonian-like law of elasticity. The ball itself remains essentially unchanged after the encounter. Furthermore, if the ball is highly elastic, the encounter is considered reversible. That is, if one takes a motion picture of the colliding balls and the movie is shown to a subject with the projector operating in both the forward and reverse modes, the subject is incapable of distinguishing the original take from its reverse. Reversibility is a key attribute of all Newtonian systems, and until the mid-1960s all Newtonian laws were considered strictly reversible. Early in the 20th century, Aemalie Noether (1918) demonstrated how the property of reversibility was fully equivalent to that of conservation, i.e. all reversible systems are conservative. There is no fundamental change in them, either before or after the event in question.
This pair of fundamental assumptions about how objects behaved set the stage for the first challenge to the Newtonian worldview. In 1820 Sadie Carnot (1824) had been observing the performance of early steam engines in pumping water out of mines. He observed how the energy content (caloric) of the steam used to run the engines could never be fully converted into work. Some of it was always lost forever. This meant that the process in question was irreversible. One could not reverse the process, bringing together the work done by the engine with the dispersed heat and create steam of the quality originally used to run the engine. (See also the discussion of the second law of thermodynamics in Chapter 2).
But the steam, the engine, and the water were all material things, made up of very small particles, according to the atomic hypothesis that had recently been formulated. Elementary particles should obey Newtons laws, which always gave rise to reversible behaviors. Whence, then, the irreversibility? This was a conundrum that for a while placed the atomic hypothesis in jeopardy. The enigma occupied the best minds in physics over the next half century. How it was "resolved" demonstrates volumes about common attitudes toward scientific belief.
Ludwig von Boltzmann (1872) considered the elements of what was called an "ideal gas" (i.e. a gas made up of point masses that did not interact with each other) to obey Newton's laws of motion. He then assumed that the distribution of the momenta of the atoms was normally random. This meant that nearby to any configuration of atoms there were always more equivalent distributions (having same mass and momentum) that were more evenly distributed than there were configurations that were less evenly distributed. Any random walk through the distributions would, therefore, would be biased in the direction of the most probable distribution (the maximum of the normal distribution). Ergo, without violating conservation of mass or momentum at the microlevel, the system at the macrolevel was biased to move in the direction of the most even distribution.
This was a most elegant model, later improved by Gibbs (1902). It is worth noting, however that the resolution was a model that was applicable to nature under an exceedingly narrow set of conditions. Nonetheless, it was accepted as validation of the atomic hypothesis and Newtonian reversibility everywhere, and it put an end to the controversy. This rush to consensus was, of course, the very antithesis of what later would be exposited as logical positivism—the notion that laws cannot be verified, only falsified. Laws should be the subject of constant and continual scrutiny; and scientists should always strive to falsify existing laws. But when conservation, reversibility, and atomism were being challenged, the response of the community of scholars was precisely the opposite—discussion was terminated on the basis of a single model that pertained to conditions that, in relation to the full set of conditions in the universe, amounted to "a set of measure zero"!
Such inconsistencies notwithstanding, the second law does indeed provide a direction for time and introduces history into science. The second law serves as a very significant constraint on the activities of living systems and imparts an undeniable directionality to biology (Schneider and Sagan, 2005).
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