Wednesday, September 1, 2010

A Short Lesson in Thermodynamics as it Relates to Life

I was recently presented with the statement that “life is the only force that counters entropy”. This can be taken to mean one of two things. The first is that life results in a net decrease in entropy.  The second is that while life may result in a net increase in entropy, entropy of the life form itself is reduced and life is the only such system that can do this. The first argument is nonsense, as it violates the 2nd Law of Thermodynamics.  The second statement is just plain false; there’s nothing thermodynamically unique about life. Let’s look at both situations to understand why.

The first order of business is to define entropy. There are lots of valid definitions and all valid definitions can be shown to be equivalent. One definition of entropy is the amount of energy that cannot be used to do work. Doing work requires energy. For example, if you want to move something, you need to expend energy.  Except for theoretical systems, whenever energy is expended by a system, some of the energy is wasted—not all the energy can be used to do work. For example, when you move an object, you expend energy to flex your muscles, and this is translated into kinetic energy (energy of motion). As your muscles flex, heat is created. This heat is sent into the environment and is not used to move the object. Thus, entropy is increased in an amount related to the wasted energy.

The inefficiency of systems that do work is at the crux of the 2nd Law of Thermodynamics. In a theoretically perfect system, all the energy would go to do work and no heat would be generated. a perfect system can at best use all energy to do work (resulting in no net decrease in entropy). An imperfect system (e.g., a real world system) will have an inefficiency so that some energy is wasted. The wasted energy produces a net increase in entropy. No system can do work to produce more energy than is needed to do the work. This would be a perpetual motion machine and it would generate a net decrease in entropy.

We can now investigate whether life counters entropy. The answer is no, at least not in the global, net sense. Life can do work (e.g., grow, reproduce, move, etc.), but all these processes involve an inefficient expenditure of energy. Life increases entropy. Period.

Now let’s look at the second possible meaning of “life is the only force that counters entropy.” To do this, let’s consider a different, but equivalent measure of entropy. Entropy is also a measure of organization (or disorganization). A system that moves toward more order has a decrease in entropy. The existence of such a system may seem impossible given the discussion above, but it is not. A system can have a decrease in entropy at the expense of its surroundings. The surroundings will suffer a larger increase in entropy than the decrease in the system, resulting in an overall increase in total entropy. There are a multitude of examples of systems in which entropy is decreased.

Life is indeed one such example. Cells and organisms are assembled from a more disorderly system of atoms and molecules. Life takes in nutrients and assembles these into orderly functioning life systems. So, in this sense, one could say that life counters entropy. This of course ignores that life, on the whole, actually increases entropy when life plus its environment is considered. The outstanding question is whether life is the ONLY system to do so. The answer is clearly and resoundingly, “no”.

Well known examples of systems that have a reduction in entropy are air conditioners and refrigerators, crystal formation, and planet formation. The true list of such systems is possibly infinite. Air conditioners produce cold air from hot air. The cold air is slightly more ordered than the hot air. Entropy decreases. But this is only true if the waste heat is neglected. Anyone that has spent even a small amount of time around an air conditioner knows that in order to produce cold air, hot air—much hotter than originally ingested—must be
ejected. So, in net, entropy increases. Entropy is lower in the cold air, but it is higher in the hot air, and the net effect is an overall increase in entropy. Crystals are very orderly. This order represents a reduction of entropy compared to the original unorganized, uncrystalized molecules. But, as in the previous example, the environment suffers a larger increase in entropy in order to allow for crystallization. Cyrstalization generates waste heat and, therefore, an increase in entropy. The net change in entropy is positive. Solar Systems form from a collection of gases and dust. These spiral in and accrete into larger bodies, eventually becoming planets—a more orderly arrangement. But, this process generates heat, and the entropy generated from solar system formation exceeds the reduction in entropy associated with greater organization.

So, there you have it. The first interpretation is wrong and the second interpretation is wrong. Life does not counter entropy, at least in the global sense. Life increases entropy. The second interpretation is wrong even if life is viewed in isolation from its surroundings. In such a situation, life produces a reduction in entropy in the isolated life system, but there is nothing unique about this process. Lots of systems, in absence of their environment, constitute a reduction in entropy. Life is not the “only” system to do so.

Making a statement such as “life is the only force to counter entropy” is at best misleading, because it implies that life has the overall effect of decreasing entropy. It can’t do this, because of the 2nd Law of Thermodynamics. If the proponent of such a statement tries to hide behind the canard of only talking about the system in isolation, they will find themselves once again in the wrong corner, because life is not in any way unique in this regard. In short, there’s nothing thermodynamically special or unique about life.