Probability and the Second Law of Thermodynamics

© 1996 Frank Steiger; permission granted for retransmission.

The following is a digest of my FAQS article on the creationist thermodynamics argument. Some additional explanatory material has been added. Click on FAQS to review the original article.

In simplest terms, creationists believe that every process in nature goes downhill. One example they give is that water can never flow uphill, because, in doing so it would violate the laws of thermodynamics. They fail to explain how the water got to the top of the hill without violating the laws of thermodynamics!

Creationists promote the falsehood that the second law of thermodynamics does not permit entropy to spontaneously decrease, and therefore evolution could not have happened. According to creationists, entropy can only increase, resulting in a "universal decay" of any and all systems. However, the mathematical laws of thermodynamics make it perfectly clear: it is possible for the entropy of a system to spontaneously decrease, providing the over-all entropy of the system's surroundings increases to a greater degree.

Thermodynamics deals in a quantitative manner with the relationship between heat and work. Therefore thermodynamics applications are limited to man-made devices and chemical changes for which values of thermodynamic quantities (entropy, free energy, etc.) have been determined. Although these parameters have been established for many biochemical reactions, the knowledge of how these thermodynamic functions interact in living cells is far from complete. Nevertheless, it is an incontrovertible axiom of thermodynamics that if energy is available (in this case, from foodstuffs) a spontaneous decrease in cell entropy is possible.

Creationists take advantage of the general lack of specific data for the thermodyanmics of cell growth by postulating a pseudo science explanation for the obvious flaw in their argument: if all systems can only go in the direction of universal decay, then how can one explain the growth of living things, which is just the opposite of universal decay? Creationist propaganda postulates, with no scientific justification whatever, an "energy conversion mechanism" for living things that "overcomes" the laws of thermodynamics. However, in the case of the evolution of living things, this "energy conversion mechanism" is strangely absent!

The controversy can be summed up as follows:

Creationist: The second law of thermodynamics states that entropy can only increase, resulting in a universal decay of all systems.

Evolutionist: But the mathematical laws of thermodynamics state very clearly that entropy can spontaneously decrease!

Creationist: Well, that is technically true for inorganic systems, but it doesn't apply to living systems.

Evolutionist: So you're saying that entropy can not spontaneously decrease for living systems? Doesn't that mean that living things can only undergo universal decay? How then do you explain the fact they grow and reproduce?

Creationist: Well, we believe that there is a special "energy conversion mechanism" that allows living systems to overcome the laws of thermodynamics.

Evolutionist: First you said the laws of thermodynamics were universal, and now you say they are not. Please explain the discrepancy.

Creationist: God can do anything He pleases.

Creationists believe that the second law of thermodynamics does not permit order to arise from disorder, and therefore the macro evolution of complex living things from single-celled ancestors could not have occurred. Regardless of whether or not evolution might have taken place, the mathematics of thermodynamics and observation of the world around us makes it very clear: order can spontaneously arise from disorder. Let me reiterate that the subject here is limited to thermodynamics only. The issue to be resolved is simply this: does thermodynamics permit or does it not permit order to spontaneously arise from disorder? The laws of thermodynamics and observation of the world around us make it very clear: order can and does arise spontaneously from disorder.

Creationists have not been able to refute the fact that thermodynamics does, in fact, permit order to spontaneously arise from disorder. However, instead of conceding defeat, they have attempted to change the laws of thermodynamics by stating that thermodynamics applies only to systems that are completely isolated from their surroundings. Creationist pseudo-thermodynamics, when applied to open systems, are based on the idea that second of law of thermodynamics would not ordinarily apply to open systems, and therefore there must be a "growth directing program" and/or "energy conversion mechanism" which would "supersede" the second law of thermodynamics. Since these postulated mechanisms are completely fictitious, the creationist position is tantamount to saying that the second law does not apply to open systems.

There is a method in this creationist madness: creationists hide the fact that it is only the over-all entropy of a collection of interacting systems that can not spontaneously decrease; the entropy of each of the interacting individual systems can either increase or decrease. However, creationists state flatly that entropy can never spontaneously decrease, and hence order can never spontaneously arise from disorder. Page 40 of Scientific Creationism, published by the Institute for Creation Research, states: "All real processes go with an increase in entropy." This statement is contradicted by the fact that the growth of living things represents order spontaneously arising from disorder. Creationism is forced to concoct a fictitious pseudo-thermodynamics which "supersedes" the Second Law to "explain" the obvious anomaly. On page 43 we read "Now, if one examines closely all such [open] systems to see what it is that enables them to supersede the Second Law locally and temporarily..." The text goes on to postulate "explanations" based on the fictional terms ("growth directing program," "energy conversion mechanism") mentioned in the previous paragraph.

Creationists assert that since living things eventually decay and form simpler compounds, the natural tendency is for order to revert to disorder. This statement is contradicted by the fact that seeds spontaneously grow into flowering plants and eggs spontaneously form living birds.

Another creationist smokescreen is the statement that engineering thermodynamics does not apply to living things. We don't know, on the basis of experimental evidence, that the laws of engineering thermodynamics apply to organic systems. However, if they don't, then the creationist argument that evolution violates the laws of thermodynamics is meaningless.

The creationist argument is also based on their interpretation of the relationship between the probability of change and a thermodynamic property called "entropy." This relationship states that a thermodynamically irreversible change has a high degree of probability and is accompanied by an increase in total entropy. Therefore, creationists assume that a change characterized by a decrease in the local entropy of one of a number of interacting entities can not occur under any circumstances. This is just not so! In fact, a spontaneous entropy decrease in one of a number of interacting entities can and does take place, providing only that the total over-all entropy of the system and its surroundings increases.

The various parts of an isolated system can interact in such a manner that some subsystems gain entropy, while others lose entropy. Nevertheless, the over-all net entropy of the isolated system must increase when a spontaneous change occurs.

The basic definition of entropy S (ie increase in entropy) is heat absorbed divided by absolute temperature T. Due to a misinterpretation of the classic textbook definition of entropy, many persons believe, erroneously, that this definition holds only if the heat is absorbed reversibly. That is not always the case; if the change does not involve the possibility of any energy exchange other than heat flow, then the entropy change in a system is the same, whether the heat transfer is reversible or irreversible. An example of this would be dropping a hot stone into a bucket of cold water. Nevertheless, changes in the thermodynamic properties of entropy, internal energy, and enthalpy are defined on the basis of reversible processes.

It has been claimed that as the universe expands, it is gaining entropy merely because it is expanding. This is not the case. Take the example of a baseball thrown upwards: its kinetic energy is transformed into potential energy when it reaches the top of its trajectory. Just before it strikes the ground, its potential energy is converted once again into kinetic energy. Up to this point there has been no change in entropy. The return velocity at the end of its trajectory is the same as the initial velocity at the start of the trajectory. (based on assuming the same elevation at the start and finish and neglecting air friction) When the ball strikes the ground and finishes bouncing, all of its kinetic energy is converted into heat and the increase in over-all entropy is equal to that amount of heat, divided by the absolute temperature of the surroundings. Since there was no heat flow from the ball itself, there was no decrease in entropy from that subsystem (ie, the ball) and the overall entropy is equal to heat of impact divided by the absolute temperature of the surroundings (assuming, of course, that the small amount of heat generated did not change the temperature of the surroundings) There is no increase in entropy until the ball strikes the ground. If we had a perfectly elastic surface and a perfectly elastic ball and no friction losses, in theory the ball would keep on bouncing forever with no loss of kinetic energy, and the overall entropy change would be zero.

If the ball loses one-half of its kinetic energy on the first bounce, the increase in entropy from the first bounce would be one-half of the original potential energy divided by the absolute temperature of the surroundings.

I suspect that the mistaken belief in "entropy of inter-galactic expansion" is the result of assuming that the expanding universe constitutes a thermodynamic parallel to the entropy change upon free expansion of a perfect gas. There is a fundamental difference involved. The universe is expanding against gravitational attraction, so its decrease in the kinetic energy of expansion is converted into potential energy. In the free expansion of a perfect gas, used as the classical textbook development of the relationship between entropy and probability (see thermo faq), there is no potential energy increase resulting from the free expansion. If a ball is thrown upward, it gains potential energy and a restoring force is created. When a gas undergoes free expansion, no restoring force is created. This is explained in some detail in the thermo faq.

To sum up: the entropy of the universe is not decreasing because of its expansion. It is decreasing because of heat loss from the nuclear reactions taking place in stars. Stars eventually use up their nuclear fuel and explode or become black dwarfs. They can, however, coalesce to form new stars; thermodynamics does not rule out this possibility.

Finally: if the baseball is thrown upward with a velocity exceeding its "escape velocity," it will never return to earth. However, its kinetic + potential energy at any point will always be equal to its original kinetic energy (neglecting friction), and there will be no increase in entropy. If the stars are receding so fast that they exceed the "escape velocity," then the universe will never contract again. The possibility that the universe may never contract does not constitute an entropy increase.

More recently, creationists have resorted to using arcane statistical thermodynamics arguments. Their contention is that there is a good possibility that molecular changes in living cells don't necessarily follow the laws of classical thermodynamics. However, if that be the case, then the creationist argument should be clearly stated in those terms. It is not so stated because that approach is speculative and fails to demonstrate the "impossibility" of evolution.

To reiterate: In thermodynamics, spontaneous changes accompanied by an increase in over-all entropy are described as "irreversible." When we say that a change is irreversible (in the thermodynamics sense) it means only that the change will not spontaneously reverse itself without some change in the surrounding conditions. It does not mean that it can not be reversed by any means at all! Misuse of the term "irreversible" is a basic flaw in the creationist thermodynamics argument. Failure to understand that in thermodynamics the probabilities of change are not fixed entities has led to a misinterpretation that is responsible for the wide- spread and totally false belief that the second law of thermodynamics does not permit order to spontaneously arise from disorder.

In order to provide a definitive answer based on the laws of thermodynamics, the FAQS thermodynamics article goes into considerable mathematical detail to show the relationship between thermodynamic entropy and probability, and to show that the mathematics of thermodynamics does not prevent order from spontaneously arising from disorder. The details are in my original paper in the FAQS file.

A favorite argument of creationists is that the laws of thermodynamics would not allow a junkyard to spontaneously become an airplane. Nevertheless, there is nothing in thermodynamics that would prohibit the formation of said airplane by an array of incredibly sophisticated robotic machines that would require nothing more than the spontaneous flow of electrical energy. There is also nothing in thermodynamics that would prohibit the manufacture of said robotic machines from the energy of other machines. All that is really essential, as far as thermodynamics is concerned, is a source of sufficient energy.

The numerical calculation of entropy changes accompanying physical and chemical changes are very well understood and are the basis of the mathematical determination of free energy, emf characteristics of voltaic cells, equilibrium constants, refrigeration cycles, steam turbine operating parameters, and a host of other parameters. The creationist position would necessarily discard the entire mathematical framework of thermodynamics and would provide no basis for the engineering design of turbines, refrigeration units, industrial pumps, etc. It would do away with the well-developed mathematical relationships of physical chemistry, including the effect of temperature and pressure on equilibrium constants and phase changes.

A reader stated that he was disappointed in not finding information in this web page on the probability of evolution. He is missing the point! The basic purpose of this article is merely to demonstrate some very elementary aspects of thermodynamics in order to refute the fundamental creationist claim that the second law does not permit order to spontaneously arise from disorder. Period. Creationist propaganda about the probability of evolution diverts attention from the one single, basic issue: Does thermodynamics permit order to spontaneously arise from disorder? I went to a great deal of trouble to show that indeed it does.

Creationists have created a smoke screen of propaganda that presents thermodynamics as a fuzzy concept subject to interpretation by the "experts" rather than what it is: a rigorous set of mathematical relationships that deal mostly with things like steam turbines, voltaic cells, and diesel engines. It is not very applicable, in a quantitative sense, to things like cell growth and evolution.

Creationists argue that the probability that evolution took place is comparable to the probability of a junkyard spontaneously changing into an airplane. Whether or not this true, the fact remains that this is merely an intuitive idea that anything that seems unlikely must violate the laws of thermodynamics. There is no way that creationists can demonstrate the validity of their "junkyard to airplane" claim using the mathematical laws of thermodynamics. Creationists are fond of trying to apply entropy and probability to ultimate origins, but succeed only in hazy, non-rigorous speculation that proves nothing.