Hard SF : Aliens : Attempting To Define “Life”
One of the challenges of discovering extraterrestrial life is the ability to distinguish what is living. Identifying intelligent life may not be as hard, as it is likely to be using its intelligence to do things in a way that does not look like natural processes. It would not be wise to assume life elsewhere will use the same chemicals or chemical processes Earth life does. Just as Earth plants exhale oxygen as a waste product, but Earth animals inhale oxygen as an essential of life, some alien life may operate on different foundations. What rules might we use to try to distinguish any kind of alien life from any kind of non-life? Below is a suggested definition of life. As you will see, it may have its shortcomings. However, by considering the strengths and weaknesses of this definition, we may get a better idea of the problems of identifying extraterrestrial life.
Trying to give a definition of “life” that would apply to any form on any world is a tricky business. The only life-forms we have to assist us in considering the alternative paths possible are limited to those we have been able to identify on a single planet, Earth. To the best of our knowledge all the life-forms we have found on Earth are descended from a single origin of life. (At most, known Earth life seems to be descended from several independent origins.) It is true that there are some major differences between various organisms on Earth, but it would be rash to assume life could not begin differently elsewhere and develop according to a different dynamic from that unique beginning.
I will present a tentative definition – making clear it is not meant to be all-inclusive. For instance, it applies most effectively to naturally evolving life-forms. I am clearly not attempting to say what organisms might be consciously designed by scientists. In addition to such clearly acknowledged considerations, there may also be subtle assumptions I am not already aware of.
Life: A form that develops in nature which functions in a manner to achieve one or more of the following objectives:
The participation of the form (in some way) in the function towards the objective is assumed. We might envision rocks as gaining some benefit from weathering, but if the weathering was solely the result of external forces, it would not be evidence of rocks being a life-form. The function requirement assumes the form has some sort of process(es) that can influence either its environment and/or itself. Life must do something. In the abstract, one might suppose anything merely capable of conceiving “I think therefore I am” or other contemplation is “alive”. In the concrete, an organism presumably needs to get and expend energy in order to think. Also, organisms are unlikely to evolve thinking without starting as organisms that don’t think but act (and so the later thinking ones will have some of those characteristics from the earlier non-thinking ones).
A “benefit” or “objective” does not have to meet much of a criteria as far as measurable value or significance. At least from one perspective, the only real driving force of Earth life is the proliferation of DNA molecules with genetic coding that results in the greatest number of copies being made. Nevertheless, for DNA to copy, it needs to acquire raw materials, have an apparatus to split the DNA molecule and assemble the appropriate materials on each half to create two full, separate DNA molecules. Earth life carries out various activities that result in those things happening.
We need to distinguish from is natural processes that aren’t life. For instance:
(1) Running water could be viewed as “active” and causes effects on its environment.
(2) Chemical reactions involve changes in which the chemical substances “participate”. One might argue that when you combine hydrochloric acid (HCl) and sodium hydroxide (NaOH) those molecules are destroyed in order to create water (H2O) and salt (NaCl), and that destroying themselves suggests the molecules aren’t life-forms. Someone else might then argue that the atoms have not been destroyed, so perhaps they are active entities that are alive.
Perhaps, we need to view life as a system. We might consider a DNA molecule in some body of water as not a life form by itself. We could say that only when the DNA is surrounded by a working apparatus to collect raw materials and make copies of the DNA does it become “life”. Insofar as running water and chemical reactions aren’t the result of systems with a working apparatus, they could be excluded. Whether or not life needs to be an enclosed system (for instance, a cell with a cell wall) is a question on which not all would agree. Even if not expressed as a question of whether or not it is enclosed, there are questions on whether viruses should be considered life – because they are dependent on host life forms in order to copy themselves.
One could argue that machines act to perform a function and work towards some objective. These are not objectives of their own, for the benefit of themselves or their community, they are turned on and off by an external force (people) for purposes of the external force. However, these counter-arguments are not so clearly applicable to an AI robot or some other artificial construct that has been made to be self-directing both in ability to figure out how to accomplish something and in not being told what to do by others [at least not in more ways than a free human is]. I assume most people will find it “seems wrong” to categorize an inorganic manufactured item as being a life-form. I’m just not sure what a valid basis for excluding them would be. The fact they are man-made does not seem to justify excluding them, since I would not exclude a man-made organic form with cells, some sort of chemical genetic material, etc. The fact that they are not “organic” may not be a good reason, as naturally evolved life on other worlds may not meet the criteria we currently use to define “organic”.
The definition of life I’ve used doesn’t require reproduction, so an AI robot that can’t reproduce doesn’t violate the above definition (on that basis). Perhaps, the definition should say something about reproduction (maybe not). In any case, something like a Von Neumann machine does reproduce and therefore could meet a definition that required “reproduction”. It may not be “reproduction” in the way amoebas or humans do it, but then again who knows how life on other planets may accomplish reproduction. It would be tricky to define reproduction in a way that would include all forms of naturally evolved life in the universe, but would exclude how any and all manufactured forms might create additional forms similar to themselves.
The definition above excludes any of these machines by saying “develops in nature” – but should it? In the case of non-intelligent machines we find elsewhere, our main interests in them are likely to be learning any new technologies and trying to identify who made them. However, we might find intelligent, self-directing machines fill the shoes of neighbors, colleagues and intellectual partners (or enemies) as well as intelligent organisms.
When we look for extraterrestrial life, what is it that we want?
If we’re looking for evidence we weren’t the only planet with life, finding machines will suggest that some other life probably made the machines.
If we want companions, intelligent machines might meet that need.
If we’re looking for organic stuff to eat, alien organism might be as indigestible as machines.
If we’re looking for planets that have been made habitable by life, perhaps alien organisms are more likely to have done that than machines.