Wednesday, February 09, 2005

What is life?

It is notoriously difficult to find a satisfactory definition of life. No matter what definition you come up with you can always find a counter-example. I've come to believe that the reason is that you can't separate a life form from its environment. To go even a little further, I don't think life can be separated from the underlying dynamical laws from which it arises. Thus, the fact that macroscopic life forms on earth tend to be discrete and identifiable is incidental. We can come up with ad hoc rules to define them but there will always be exceptions. One could imagine an ocean of self-catalyzing molecules (i.e. RNA world) that reproduce, exchange information and behave as a giant life form. Morris Hirsch and Freeman Dyson both have suggested that you could have electromagnetic life forms living in a plasma cloud of charged particles.

So my definition (which I'm sure is still flawed), is that life is a universe comprising of a set of dynamical laws acting on a set of constituent particles in which entropy can decrease in a localized region that is much larger than the scale of the particles and for timescales longer than the average interaction time of these particles. Implicit in this definition is that the localized region is in a state of quasi-equilibrium so that entropy can be defined. Thus a set of non-interacting particles would not be alive because entropy lowering fluctuations only last as long as the particle interaction time (or particle transit time in this case).

I have no idea what would be a minimal set of laws or rules that could support life. Maybe life-possible rules fall into Steve Wolfram's select class of cellular automata (CA) that exhibit nontrivial complexity. Wolfram's argument is that you cannot predict a priori what a set of CA rules will do. There is no short cut to deducing what patterns can arise without actually iterating the rules. Thus, life itself is the ultimate grand experiment. I've always found great solace in Andrei Linde's multiverse idea. Universes with different physical laws pop in and out of existence, and no one can predict if life is possible in a given universe before it exists.

9 comments:

Anonymous said...

Thank you so much, Carson. Suddenly I can read rapidly again :) Now to consider the post.

Anne

Anonymous said...

"So my definition (which I'm sure is still flawed), is that life is a universe comprising of a set of dynamical laws acting on a set of constituent particles in which entropy can decrease in a localized region that is much larger than the scale of the particles and for timescales longer than the average interaction time of these particles."

Extrememly clever. I have been playing with this all afternoon.

Anne

Anonymous said...

Now the question must be further focused or narrowed.

Anne

Carson Chow said...

Any suggestions?

Anonymous said...

Working backwards; what is irreducible about life? What is it about life that can not be reduced to parts? A bird is not so many feathers, nor an amoeba so many cilia. I am thinking.

Anne

Carson Chow said...

It's not just that life cannot be reduced to parts, I don't think life can be reduced at all. Life forms are states in the system like waves in the water or patterns in the sand.

Anonymous said...

"It's not just that life cannot be reduced to parts, I don't think life can be reduced at all. Life forms are states in the system like waves in the water or patterns in the sand."

There, you have added wonderfully to your definition and helped me understand what was troubling me about Oliver, my conure. Oliver cannot be reduced to parts. There is thus a distinctiveness to a living thing that I must now think about. Very nice, as usual, Carson.

What do I mean by living things begin distinct? All living things? Some? Oliver is distinct, but possibly not a bacterium. Thinking...

Anonymous said...

That was distinctive me :)

Anne

Anonymous said...

http://www.nytimes.com/2005/02/01/science/01bird.html?ei=5070&en=0ed10634f3d9b33b&ex=1108789200&pagewanted=all&position=

February 1, 2005

Minds of Their Own: Birds Gain Respect
By SANDRA BLAKESLEE

Birdbrain has long been a colloquial term of ridicule. The common notion is that birds' brains are simple, or so scientists thought and taught for many years. But that notion has increasingly been called into question as crows and parrots, among other birds, have shown what appears to be behavior as intelligent as that of chimpanzees.

The clash of simple brain and complex behavior has led some neuroscientists to create a new map of the avian brain.

Today, in the journal Nature Neuroscience Reviews, an international group of avian experts is issuing what amounts to a manifesto. Nearly everything written in anatomy textbooks about the brains of birds is wrong, they say. The avian brain is as complex, flexible and inventive as any mammalian brain, they argue, and it is time to adopt a more accurate nomenclature that reflects a new understanding of the anatomies of bird and mammal brains.

"Names have a powerful influence on the experiments we do and the way we think," said Dr. Erich D. Jarvis, a neuroscientist at Duke University and a leader of the Avian Brain Nomenclature Consortium. "Old terminology has hindered scientific progress."

The consortium of 29 scientists from six countries met for seven years to develop new, more accurate names for structures in both avian and mammalian brains. For example, the bird's seat of intelligence or its higher brain is now termed the pallium.

"The correction of terms is a great advance," said Dr. Jon Kaas, a leading expert in neuroanatomy at Vanderbilt University in Nashville who did not participate in the consortium. "It's hard to get scientists to agree about anything."

Scientists have come to agree that birds are indeed smart, but those who study avian intelligence differ on how birds got that way. Experts, including those in the consortium, are split into two warring camps. One holds that birds' brains make the same kinds of internal connections as do mammalian brains and that intelligence in both groups arises from these connections. The other holds that bird intelligence evolved through expanding an old part of the mammal brain and using it in new ways, and it questions how developed that intelligence is.

"There are still puzzles to be solved," said Dr. Peter Marler, a leading authority on bird behavior at the University of California, Davis, who is not part of the consortium. But the realization that one can study mammal brains by using bird brains, he said, "is a revolution."

"I think that birds are going to replace the white rat as the favored subject for studying functional neuroanatomy," he added....

Anne