Sunday, June 04, 2017

Life, complexity, and negative entropy

By Mathew Goldstein

Our universe started in a very low entropy state and evolves toward a very high entropy state.   Any decrease of entropy on earth is more than offset by the increase in entropy on the sun.  Does life resist, or at least slow down, the universal increase in entropy?  Some people claim it does and they may then draw conclusions about human ethics from their belief that it does.  

I am skeptical that life slows down the rate of entropy increase.  I am also very skeptical that the answer to this question has any implications for human ethics.  I am wary of getting into discussions on technical topics like this.  I am not a scientist.  But for whatever it is worth, here is my explanation for my skepticism.

For each visible photon the earth absorbs from the sun the earth radiates back to space about 20 infrared photons (earth converts visible light energy absorbed from the sun to "heat" energy that it radiates back to space).  That is a net twenty fold increase in overall entropy.  The overall energy is unchanged (ignoring changes to atmospheric greenhouse gases, etc.) because the energy of one visible photon is twenty times greater than the energy of one infrared photon.  

The amount of solar radiation that is reflected back to space is referred to as the albedo.  Ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the sun's energy. Deserts, ice, and clouds, however, have high albedos (desert sands get hot but they still reflect more sun light back into space than grasslands). Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space.  Higher albedo reduces the rate that the Earth is contributing to entropy increase.  

Reflected light is a no change in entropy outcome relative to the no earth context and there is little increase in entropy overall because each photon mostly retains its preexisting concentrated energy status (although each photon becomes more separated from other photons).  A photon utilized photosynthetically by plants locally decreases entropy but it increases entropy overall because the photon itself is consumed, more than offsetting the decrease in entropy from the corresponding new plant tissue. In the short term my guess is that the entropy increases more in the latter scenario.

Entropy should not be confused with complexity.  Both low entropy and high entropy conditions are uniform and thus non-complex.  The highly lumpy, highly varied, far from equilibrium, conditions that characterize complexity reach their maximum when entropy is moderate.  Life, because it depends on complexity, is impossible in very low or high entropy conditions.  Moderate entropy is the current condition of our currently complex universe.  So life is consistent with current conditions.

How does low entropy life start given that entropy increases?   One way to try to tackle this question is to focus on metabolism.  The complex chemical pathway, catalyzed by metals such as iron, that converts carbon dioxide to methane, known as serpentinization, resembles the metabolic chemical pathways in some microbial life.  Some people speculate that life may have originated via such a pre-RNA "metabolism first" route.  

Adding hydrogen atoms to carbon is referred to as carbon hydrogenation.  Carbon dioxide molecules (one carbon and two oxygen atoms) have lower entropy than methane molecules (one carbon and four hydrogen atoms).  But all known paths from carbon dioxide to methane molecules have intermediary molecules that are lower entropy than carbon dioxide.   We can depict lower entropy as a higher elevation relative to higher entropy.  This analogy of higher entropy to lower elevation allows us to represent the pull toward higher entropy as being equivalent to the downward pull of gravity.  The overall path from carbon dioxide to methane is downhill.  But an initial uphill push that is offset by an increase in entropy overall is required to reach the peak and start the trip downhill.

No natural process, including metabolism, can occur unless it is accompanied by an increase in the overall entropy of the universe.  Life is not a substance or force.  Life is a process that is sustained by increasing entropy, it is an entropy generating machine. Life contributes to increasing entropy even though life itself is inconsistent with very high (and very low) entropy. A living organism is an open system, exchanging both matter and energy with its environment.

For example, an animal builds cells, tissues, ligaments, etc. This process increases order in the body and thus decreases entropy. This is the local "negative entropy" that characterizes all of life.  Animals also radiate heat into space, consume and break down energy-containing substances (i.e., food), and eliminate waste (e.g., carbon dioxide, water, etc.). When taking all these processes into account, the total entropy of the system (i.e., the animal together with the environment) increases. Although the details relevant to the calculations vary, this same result must also hold for photosynthetic plants and microorganisms.  

Life depends on, and affects, the overall increase in entropy.  Maybe the evolution of life favors a more efficient, and more entropy neutral, metabolism (for example, being sluggish and cold blooded) because that is more environmentally sustainable over the long term.  But I suspect that evolution also favors exploiting entropy increasing opportunities because that provides paths to competitive advantage (for example, active, intelligent, and warm blooded).  The more energy consumed by life the more entropy will increase because there is no possible path for life to utilize more energy without thereby also increasing overall entropy.   Increased efficiency maybe can reduce the entropy increase, but it does not alter the direction of this equation.

The decrease in albedo due to oceans, and the increase due to ice, suggests that physical features of planets, and their relationship with nearby stars, impacts the rate of entropy increase of planets independently of, and potentially more substantially than, any life that may reside on the planets.  It is not clear, at least not to me, that an overall decrease in the rate of entropy increase is an expected result, or a function, of life.  I think not.