Predictability - the link between action and reaction...
We have been examining life on earth, how and even why, it might have come to be. We have not said much about the conditions necessary to support living beings. Just from the life forms that we know to exist on earth we see that life seems to expand to fill all the available environmental conditions, from Death Valley to Antarctica to the depths of the sea where the sun never shines to hot methane springs. On earth the greatest diversity of life occurs in what are called temperate conditions... not too hot, not too cold with plenty of sunshine as a low entropy source of energy. We have no information about life on other planets but given the adaptability of life on earth I would not be surprised to find it in conditions where human beings could not survive for an instant.
On this page I am going to try to get at the most basic requirement for a life supporting environment. I suspect that environment must be reasonably predictable. Reasonable predictability may be taken to mean that the environment does not change wildly within a time comparable to a few lifetimes of living individuals. It takes many lifetimes for evolution to match a life form to a significant environmental change. Individual members of living species may experience a major change in the environment simply by moving. On earth, transporting an under sea methane breathing worm to an oxygen atmosphere would kill that individual in a very short time, as would transporting a canary to any oxygen free environment. Aside from individual dislocations and local cataclysms (a momentous and violent event marked by overwhelming upheaval and demolition) that are rare events in the environment, the universe seems to be a congenial place for the development and maintenance of life.
As well as excluding major rapid changes in the environment, reasonable predictability sets an absolute lower size limit on living beings. Life is delicate. Living beings are built from organic molecules arranged in complicated structures, which on a previous page I called "complicated barely stable aggregations of marginally stable molecules". All the life forms we know of exist in a bath of inorganic molecules, like air or water, The molecules are bouncing around banging into one another and into anything immersed in them as described in the Feynman excerpt. In order for a living being to survive the beating it gets from the environment, it must be large enough that it interacts with a huge number of the environment molecules at once so that the effects of their impacts average out to a slight overall pressure rather than having the living being reacting to collisions with individual environment molecules rather like the life of a ping pong ball in a lottery machine. Such an existence would certainly disrupt the delicate structures life requires and could not be called reasonably predictable.
The smallest examples of (partial) life we know of are viruses. I call viruses partial life because they fail to meet all the characteristics of life listed in "What is Life". They need a host in order to reproduce and lack some other of life's attributes. The average virus is about 100 times smaller than the average single cell living being. It turns out that there are about 600,000,000 atoms of average atomic weight of about 12 in the average virus. The average molecular weight of air is 29 and of water is 18 so the ratio of virus mass to mass of an individual environment molecule is: for air, 600,000,000 X 12/29 = 248,000,000 to 1 and for water 600,000,000 X 12/18 = 400,000,000 to 1. This is certainly mass enough that the virus is not endlessly jerked about by the environment molecules.
For living beings of any appreciable size, in the absence of the odd cataclysm, we seem to be in good shape for finding reasonable predictability in the existing universe as far as the causally closed laws of nature go. There is no room for a certain action producing an unpredictable reaction in Newton's third law. "But wait!" the well read person exclaims. "At the atomic and smaller size Newton's laws do not apply." That is of course correct. There is uncertainty in what particular reaction will result for a given action in quantum mechanics. But the average reaction over a large number of identical actions approaches the Newtonian value as the number of such actions increase. Any organism large enough to smooth out the buffeting from the thermal bombardment of the environment molecules will also smooth out the quantum mechanical uncertainties and achieve Newtonian behavior.
On the "Why is There Life" page I argued that the causally closed laws of nature could not account for the works of mankind and invoked free will, as something outside the laws of nature, to account for those works. What, one wonders, is the impact of mankind on reasonable predictability. There is certainly an element of unpredictability in the behavior of living beings. Even the simplest one cell creature is unpredictable in its detailed response to stimuli. Whether it is unpredictable in principle or only as a practical matter is not known. What is known is that the average response over a large population of the same species is much easier to predict than the response of any individual, sort of like the quantum mechanical atoms.
If we go to the other end of the living being complexity scale where we humans exist, the unpredictability in behavior seems to be greatly amplified. Not only is it hard for an external observer to predict how a person is going to react in a certain situation but we often surprise ourselves. To misquote the poet Scott, Breathes there the man with soul so dead, Who never to himself hath said, "What was I thinking?".
Clearly there are instances of individual reaction to stimuli, which if exercised by the population as a whole would have extinguished the human life form long ago. There are individuals with very little empathy toward others that react in a strongly self centered way. The race could not exist if all the members acted on every selfish impulse regardless of wider consequences. There are other individuals with excessive empathy toward other people, animals, trees, rocks or whatever, whose reaction to threatening situations has such a small self preservation component that a race of such individuals could not exist for long. Since the extremes tend to be self-limiting there is probably a normal distribution between them such that the great majority of the population has a reasonable mix of self preservation and empathy. We could probably build bases other than empathy along which to measure human behavior and find similar extremes with a normal distribution along those axes.
In spite of some deficiencies in predictability the human race is still in existence so evidently our errant free wills taken all together fall within the definition of reasonable predictability. The predictability provided to living beings by their size relative to individual atoms and molecules, the relative stability of the classical physical environment and reasonable predictability of mankind is critical to the development of human beings. In the absence of that predictability in which a given action produces a narrow range of reactions it is hard to imagine how a baby could grow up with the mental capacity to do anything other than be prepared to dodge the slings and arrows of outrageous fortune.
Even including the effect of free will, it seems that except for the occasional cataclysm, some of which are caused by living beings, the universe as it is represented by planet Earth is well suited to the development and maintenance of living beings. This may be a good point to begin wondering about what goes into making the earth such a livable place. On following pages I will try to get at that issue.