There are many elegant ways to state or (often mis)interpret the infamous third law of thermodynamics. Its most precise and current formulation states that as the temperature of a system tends to absolute zero the entropy of a perfect crystal also approaches zero. This statement has broad and far reaching implications that are not immediately obvious. Arthur Eddington poetically referred to the third law of thermodynamics as the arrow of time. According to him time apparently moves in the direction of increasing disorder. As far as experiment has borne out this is a very good approximation of the universe in which we live. Potential energy is converted into work and entropy. Work accomplishes some physical, chemical, nuclear, or even subnuclear process, while entropy is transference of some of that potential energy into random microstates of the system that are unrelated to the process.
Let us examine a process that is not terribly familiar to people on earth, but is integral to our existence: hydrogen fusion. Stars are huge balls of hydrogen and trace amounts of other elements. Gravity pulls these balls tightly together – so tightly in fact that hydrogen moves very close to the speed of light and the proton and the electron separate into a nuclear soup known as plasma. In this superheated state under the pressure at the core of a star, two protons hit each other in just the right manner for fusion to happen every once in a while (many collisions will not result in fusion). They fuse into a helium nucleus. This is not the end of the fusion story, but we’re going to stop here to examine what happens to all the energy that is released. The electromagnetic shockwave and pressure fuel additional fusion reactions and counteract the gravitational collapse of the star. This is the useful work that fusion accomplishes for a star. Much of the energy, however, is given off as electromagnetic radiation (various wavelengths of light). All radiation that escapes the work of staving off the collapse of the star is “wasted” energy, or entropy.
Now, let’s consider the star called the Sun and a lonely rock about one hundred million miles away from it. There is a still a great deal of energy to be extracted from these photons (light radiation) streaming away from the sun. They can be converted to lower energy photons through numerous processes. This particular rock is also brimming with chemicals that could undergo chemical reactions that do just this (through a process called photocatalysis). Chemistry starts happening as a result of this. The third law of thermodynamics is then responsible for the increasing complexity of chemistry on this rock.
This opens up another and more classical way of viewing the third law of thermodynamics. If there is energy in a system, the system will dissipate that energy as efficiently as it can. Now we come to a crucial juncture. It is very important to understand the difference between thermodynamics and kinetics. Many chemical reactions are thermodynamically favorable, but kinetically unfavorable. Remember the point earlier about how not all the proton collisions lead to fusion – that’s kinetics. They have to hit each other at just the right velocities and angles or they just bounce off of each other. All chemical reactions require the reactants to be in the right orientation and moving with a certain minimum velocity relative to one another as per the above illustration.
All of chemistry is, in some sense, the study of molecular pinball. An example of a thermodynamically favorable reaction is the spontaneous decomposition of diamonds into graphite. On earth we never observe this decomposition in standard conditions because the carbon atoms must be heated to an extraordinary temperature before they will vibrate fast enough to break the bonds that make up the diamond and rearrange into graphite. This is an important point because this kinetic barrier is the driver for the formation of life.
Because many chemicals form on this rock called Earth that are not easily broken back down into their constitute parts due to the kinetic barrier, a huge source of potential energy is beginning to accrue. This is the basis for evolution and life. Many people try to argue against evolution using the third law of thermodynamics, but that is because they fail to see the subtleties that can allow for seemingly organized phenomena to arise spontaneously simply because there is a great deal of potential energy present in the system. Let’s backtrack and borrow a simpler example from the late, great Lynn Margulis. Imagine a system of gas at high pressure meets a system of gas at slightly lower pressure and both these systems of gas are close in temperature. The antievolutionist would immediately claim that due to the third law of thermodynamics gas would diffuse from the system of higher pressure to the system of lower pressure and nothing extraordinary would happen. In fact, the following occurs:
Tornados simply mix gases of different pressures more efficiently than simple diffusion. Similarly, once the Earth has stored up a large enough well of potential energy, systems that can exploit that energy start to arise. The way living systems harvest energy that the environment cannot is that they consist of molecules that hold reactants in just the right way so that the kinetic barrier to the reaction is greatly lowered and they then harvest some percentage of the energy available.
It is hard to say what the first self-propagating enzymatic reactors were like. One popular hypothesis holds they were likely enzymatic RNA strands perhaps mediating photocatalyzed reactions of iron oxide or iron chloride. Another hypothesis suggests it started with simple active proteins possibly obtaining energy from the highly exothermic reaction of peptide bonding that would be required to form order proteins. Maybe both occurred and they met up at some point to create the first cells? Life on Earth is like this tornado in that it is efficiently dissipating the potential energy of the Sun.
A great example from modern life is that the conversion from glucose to carbon dioxide and water is hugely energetically favorable yet one can watch a pound of sugar for years and nothing will happen to it. It will not spontaneously convert itself into water and carbon dioxide. But when you eat it a series of enzymatic biological reactions convert it into just those products and quite a bit of usable energy along the way.
To start, life really requires two things: an abundance of similar potential chemical energy and enough time. If there are just trace amounts of many different kinds of chemical potential energy then life may start, but it is likely it will soon end if it doesn’t have sufficient time to evolve mechanisms for taking advantage of a new source of energy. So why isn’t there life on Venus or Mars? We haven’t ruled out the possibility of life on either, but it is unlikely for different reasons. Mars is very cold with almost no atmosphere and constantly bombarded by intense solar radiation since there is not a magnetic field surrounding Mars like the Earth. Most of the surface of Mars sits at the lowest potential energy it can sit at given current conditions. This is why all of the current Mars missions to search for life come equipped with drills – it is quite possible there may be extraterrestrial soil microbes a meter or more below the surface of Mars living in some novel or interesting way on chemicals that cannot be observed without excavation. Data coming back from the rovers has a mixture of equal parts optimism and reality. Soil samples have turned up unexpected chemicals that may be the signature that life once existed on Mars but nothing that points to extant life.
Venus, on the other hand, is extraordinarily hot and covered in molten lead and sulfuric acid. This combination means that most chemicals, once formed will easily break apart again so it will necessarily take Venus a much longer time to create a large reserve of chemical potential energy. Still, this does not rule out life, it just makes it unlikely. That said, humanity would have no idea as to what life in a place like Venus would look like, and if it has evolved to survive on Venus, the relatively chilly and rarified climate of the Earth would surely prove fatal to any organism recovered from Venus (not that we have any way of obtain samples from Venus currently).
Humans tend to focus on liquid water as a criterion for life because the chemistry of somewhat similar life is likely to arise in such an environment. It might be vastly different in many respects, but there is great hope that humanity could, at the very least, recognize it as alive. It is likely that our universe is teaming with life – metabolic tornadoes dissipating available potential energy from stars and sources of which humanity is not even currently aware. It is only in the last thirty years that scientists have become aware of naturally occurring bacteria that metabolize everything from iron sulfides in boiling sea vents to nuclear radiation, both naturally occurring and manmade. The greatest difficulty will be perceiving life so different from that which lives on earth that its timescale for existence or metabolism is millennial or atomic, that its size is molecular or planetary, or that it spontaneously evolved or was constructed by another sentient race (a technology that is currently in its nascence here on Earth).
There is an excellent case for the evolution of life as a consequence of the third law. From the onset of life, increasing complexity is likely to eventually give rise to intelligence. On our own planet, behavioral biologists have observed recently that such divergent creatures as humans, bonobos, parrots, scrub jays, and cetaceans all likely have members whose sentience and intelligence are on similar levels. Humans are not unique as thinking beings even on their home planet so it is very unlikely we are unique in the universe. Still, SETI’s failure to detect anything hints at two statistical likelihoods – one is that we are among the first in our galaxy and the other is that life elsewhere is so unlike us it will choose different means of communication.
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