The meaning of life

What is the meaning of life? That turns out to be more than a question for philosophers and theologians.

Will we recognize life if we find it on other planets? We're pretty good at recognizing life as we know it, but the whole reason for looking for life on other planets is to find life other than as we know it. And the literature of science fiction is filled with stories of life other than as we know it.

Silicon based life is old hat, though silicon won't function as a substitute for carbon. Real silicon based life would have to be very different from carbon based life.

Larry Niven's "Outsiders" thrive at extremely low temperatures, pay a rental fee to Earth for space on the moons of Neptune, and "feed" off of a form of photoelectric energy and trace minerals in their liquid helium broth.

James Hogan wrote a story about robotic life which had survived the original carbon-based builders, and evolved a whole ecosystem.

Robert Forward imagined life, made of compressed matter, living on the surface of a neutron star. These creatures' metabolism was based on nuclear reactions rather than chemical ones.

And then there are the weird ones.

But even in the real world, the question of "what is 'life'" is far from cut and dried.

No matter what characteristic we specify to separate living from nonliving matter on Earth, we can always find an example that blurs or erases this distinction. Some or all living creatures grow, move, or decay, but so too do objects that we would never call alive. Does life reproduce itself? So does fire. Does life evolve to produce new forms? So do certain crystals that grow in watery solutions. We can certainly say that you can tell some forms of life when you see them -- who could fail to see life in a salmon or an eagle?-- but anyone familiar with life in its diverse forms on Earth will admit many creatures will remain entirely undetected until the luck of time and the skill of an expert reveal their living nature. Since life is short, we must press onward with a rough-and-ready, generally appropriate criterion for life. Here it is: Life consists of sets of objects that can both reproduce and evolve. We shall not call a group of objects alive simply because they make more of themselves. To qualify as life, they must also evolve into new forms as time passes. This definition therefore eliminates the possibility that any single object can be judged to be alive. Instead, we must examine a range of objects in space and follow them through time. This definition of life may yet prove too restrictive, but for now we shall employ it.

It's hard to imagine any form of life that does not reproduce and evolve, but that may be a failure of imagination. The universe is not restricted to only those things I can imagine.