The Second Law, Part 1. [11/5/03]
Entropy and Order
A simple thought experiment can illuminate the difference between thermodynamic entropy and order. Suppose you take some coins from your pocket place them in a stack on the table. Then the experiment is repeated by tossing the coins haphazardly on the table. From a thermodynamic point of view the stacked coins may have more potential energy than those that were tossed, but the arrangement of the coins has no further thermodynamic implications. One could say that the stacked coins were more “ordered” or contained more “information” than the tossed ones, but only by reference to an observer applying an interpretive algorithm (a value judgment). The observer need not be conscious or even complex, but merely must possess a bias that discriminates between various patterns of coin arrangements. The point is that one cannot discuss the information content of a system in the absence of the encoding/decoding device which interprets the patterns. As with Maxwell’s demon, any such device requires energy to operate, resulting in an increase in entropy.
But what is the physical relationship between thermodynamic entropy and order? None, that I can see. Does information theory relate to the material world at all, or is it a purely mathematical construct, a model? Or perhaps information is simply a certain variety of potential energy. For instance, a computer "writes" electric or magnetic charges at certain locations, increasing entropy in the process. “Reading” this information converts this potential energy back to kinetic energy, further increasing entropy. In principle, this process may be no different from pumping water into an elevated reservoir, which later can be released to drive a turbine.
Entropy Production and Locality
Another important point regarding entropy is that it must
increase locally, not just in the system (universe) as a whole. This
observation seems to be widely overlooked or ignored.1
So the total entropy of the Earth may be decreasing, but only because the energy input from the Sun more than compensates for the positive entropy generated by each and every process involving energy conversion on Earth. And of course every reaction on the Sun is also creating entropy. While Prigogine (following the lead of Schrödinger3) would characterize the energy input from the Sun as a “negative entropy flow”, I feel that this is a misuse of the term. To my mind, the equation dU = dW + T dS is a process equation, flowing from left to right. Prigogone turns this around to get something like deS = (dU – dW) / T, which looks to me like putting the genie back in the bottle. Nevertheless, his point is clear. There is no place anywhere where entropy is locally decreasing as the result of a local process, which is the idea I tried to convey above.
But what about order? Doesn’t increasing order correspond to decreasing entropy? This is where Jaynes weighs in. In his article “The Minimum Entropy Production Principle”, he enumerates seven different definitions of entropy, two of which he characterizes as “information entropy”. His preferred definition is what he calls “experimental entropy”, where dS = dQ / T. “While the properties of [information entropy] are mathematical theorems, those of [experimental entropy] are summaries of experimental facts.”
While there is certainly overlap among the various entropies, at least by analogy, Jaynes' discussion is clearly a warning against mismatching concepts, particularly by comparing theoretically derived propositions with principles derived from experimental observation.
The above discussion can be summarizes as follows:
1. Entropy increases whenever and wherever energy is transformed. Whether the transformation causes order to increase or decrease is thermodynamically irrelevant.
2. The fact that the term “entropy” has been appropriated to information theory has no logical bearing on its thermodynamic meaning, just as the use of the word “energy” to describe mood or motivation is irrelevant to its meaning in physics.
3. The seeming contradiction of increasing order (complexity) in the face of the 2nd Law is a false paradox. Information theory is a mathematical construct, whereas thermodynamics is based on empirical observation, so the comparison is one of apples and oranges. A similar “paradox” is that which contrasts the time-reversibility of the equations of motion (classical or relativistic) with the directionality of the 2nd Law.4 The ideal cannot logically be compared to the real.
4. If the 2nd Law is irrelevant, how does thermodynamics relate to life? Through the 4th Law, which provides for selection based on differential efficiency.
The 2nd Law, Complexity and Life
At the end of his chapter on the 2nd Law, Prigogine loses it (but gains a Nobel prize) by mistaking “association” for causality: “As we shall see in the following chapters, systems that exchange entropy with their exterior do not simply increase the entropy of the exterior, but may undergo dramatic spontaneous transformations to “self-organization.” The irreversible processes that produce entropy create these organized states. Such self-organized states range from convection patterns in fluids to life. Irreversible processes are the driving force that create this order.” [his italics] The implication is that complexity leads inexorably to life.
First let’s do a reality check: If you look out over your back yard, the ubiquity of life is obvious. However, if you look through a telescope into the rest of the universe, life (as far as we can see) is completely absent. However, there is no shortage of complex patterns to be observed. Looking out from the Earth, one can only conclude that life is extremely rare, while complexity is extremely common. Now if complexity led inevitably (or even occasionally) to life, one would expect to observe a variety of living creatures of various types and scales distributed throughout the cosmos.
One could argue that the reason we can’t detect life is that living organisms are too small to be detected by our sensors. But the argument that complexity generates life has not been qualified (as far as I know) to say that it only operates at certain scales. (I have just such an argument based on the 4th Law, which I will present below.)
Another criticism involves Prigogine’s use of the word “self-organization”, which is an oxymoron in this context. While no one would argue that non-living matter can take a fascinating variety of complicated forms, few scientists would expect any such forms to possess a “self” in the usual sense of the word. This criticism may seem like a semantic quibble, but I feel the consequences of using the word “self” are unavoidably misleading, since we are so psychologically attuned (and properly so) to thinking of a self as having volition. But of course no one is doing any organizing; matter and energy are just falling down entropy gradients in the most efficient manner, which requires no more volition than a light beam “finding its way” through space-time along the shortest path.
Life forms generate a wide array of fractal structures.5 Doesn’t this also imply that life has roots in the complexity of chaos? Not necessarily. Evolution has been able to take advantage of natural algorithms to provide parsimonious means for constructing circulatory systems or light-gathering arrays, for instance. This does not, however, imply that life originated from those algorithms. ( I have not seen anywhere that Mandelbrot has made such a leap of faith, even speculatively.)
I believe that Prigogine, Jaynes and others fall short in their efforts to fashion the 2nd Law into a motive force with the potential to animate matter. In this regard the 2nd Law is necessary but not sufficient. However, the 4th Law provides sufficiency by offering a mechanism for selective optimization.
Which brings me to the question of scale limitations. An important component of 4th Law selection is the existence of channels through which energy and matter can flow. For living organisms, the channel walls are highly impermeable. This feature distinguishes living cells, for instance, from convection cells and most other non-living structures. It is clear that it is difficult or impossible to maintain such barriers beyond a certain temperature. The flow of matter and hence energy is also constrained as temperatures fall. For terrestrial life, the temperature range so defined is roughly between the freezing and boiling points of water. Other forms of life might exist elsewhere in a different temperature range, but it would still presumably be fairly narrow.
In my opinion, almost everyone I’ve read on the subjects of complexity and the origins of life are on the wrong track in seeking the answer in either rejecting or elaborating the 2nd Law. The real problem is that a law is missing, the 4th Law, and it is this “new” law that can provide the answers.
Footnotes and References:
1 Donald Haynie, Biological Thermodynamics,
2 Dilip Kondepudi and Ilya Prigogine, Modern Thermodynamics, John Wiley & Sons, 1998, p. 88-90.
3 Erwin Schrödinger, What Is Life? with Mind and Matter and Autobiographical Sketches, Cambridge University Press, 1967, p. 70-71. First published in 1944.
4 Ilya Prigogine, The End of Certainty, The Free Press, 1997.
5 Benoit Mandelbrot, The Fractal Geometry of Nature, W. H. Freeman and Company, 1983.