What is life exactly? Humans are alive, and so are trees. What do trees and people have in common that allows them to be subsumed into the category of life? Trees don’t walk or think. They just stand there.
The reasons, of course, are internal. Trees are what we call “organic” – that is made of cells, and having the ability to grow. This is what fundamentally unifies all known life on Earth.
One of the interesting things about “life” is that most of it is very small. The smallest living thing ever discovered is called Nanoarchaeum, and it is only 400 nanometers (billionths of a meter) in diameter. Here is photograph of the creature:
In fact, scientists debate whether or not the thing can actually be called living. At this level of microscopy, the conceptual distinctions between living and non-living become too vague – a telling reminder of how poorly constructed our concept of “life” truly is. Indeed, no line has yet to be conclusively drawn between that which is alive and that which is not. It’s a bizarre fact of science, that a concept so fundamental to our nature as “life” is, as of yet, undefinable.
There is, however, one thing that distinguishes life, and this is it’s unusual relationship with energy.
All matter and movement in the universe is predicated by the laws of thermodynamics. The second of those laws states that matter in a closed system cannot increase in entropy without an outside energy source. In other words, the level of chaotic kinetic energy in a closed system will remain in equilibrium unless acted upon. Life is unique, in this respect, as it seems ostensibly to violate this, the second of Newton’s laws. Of course, this is because “living things” tend to increase in entropy, rather than remain stable, or indeed, decrease.
The reason for this is that Earth is not a closed system; it recieves it’s energy from the Sun. Thus, the whole notion of “life” is built around the complex relationships between matter and energy that is created by the Earth and Sun. Really, it’s not that far-out of an idea; after all, there is clearly fundamental correlation between solar activity and this phenomenon that we have called “life.” Where does sunlight stop being cosmic rays and start being “nutrition” to some living thing? Living matter is no different than all other matter, except it needs to absorb energy, in order to increase it’s entropy. From photosynthesis to sunday dinner, you prove it every time you lift the fork to your mouth.
“Life” would be better defined as a universal cosmic tendency towards increased entropy (and thus, increased complexity of the physical organization of matter).
In light of this realization, one can conclude that the question of “life on other planets” is often addressed incorrectly: as if life on other planets would be anything like life on Earth. I doubt very much that we have the imaginative faculties necessary to fathom what strange patterns between matter and energy occur on other worlds – worlds whose particular solar circumstances are drastically different than our own. I believe that there are, floating out in the depths of space, depths of consciousness beyond the narrow constructions of the Earthbound Ape, and beyond our limited concept of “life.”