John Wilkins at Evolving Thoughts has a great post about the development of the modern definition of “Life” (which, one strongly suspects, is by no means fully developed). Once we break free of the most parochial definitions involving carbon-based chemistry, we’re left with the general ideas that life is something complex, something that processes information, something that can evolve, something that takes advantage of local entropy gradients to make records and build structures. (Probably quantum computation does not play a crucial role, but who knows?) One of the first people to think in these physical terms was none other than Erwin Schrödinger, who was mostly famous for other things, but did write an influential little book called What Is Life? that explored the connections between life and thermodynamics.
Searching for a definition of “Life” is a great reminder of the crucial lesson that we do not find definitions lying out there in the world; we find stuff out there in the world, and it’s our job to choose definitions that help us make sense of it, carving up the world into useful categories. When it comes to life, it’s not so easy to find a definition that includes everything that we would like to think of as living, but excludes the things we don’t.
For example: is the Milky Way galaxy alive? Probably not, so find a good definition that unambiguously excludes it. Keep in mind that the Milky Way, like any good galaxy, metabolizes raw materials (turning hydrogen and helium into heavier elements) and creates complexity out of simplicity, and does so by taking advantage of a dramatic departure from thermal equilibrium (of which CV readers are well aware) to build organization via an entropy gradient.
Update: Unbeknownst to me, Carl Zimmer had just written about this exact topic in Seed. Hat tip to 3QD.
Does the Milky Way reproduce?
No, but neither do a lot of people.
Not to brag, (too much), but I’ve been saying this for a very long time before Lenny every latched onto it, and Wilikins has known about this for just about as long:
Course… I was naive back then as my terminology betrays, and thought that an entropic anthropic principle would be welcomed by science… lol@me.
http://www.lns.cornell.edu/spr/2003-10/msg0055522.html
Jame Kay, Eric Schneider, Dorion Sagan, and Scott Sampson have also made the local connection to this:
http://www.taipeitimes.com/News/edit/archives/2004/09/30/2003204990
This is Sagan and Kay’s book:
http://www.amazon.com/exec/obidos/ASIN/0226739368/sciencewriter-20?creative=327641&camp=14573&link_code=as1
Does the Milky Way reproduce?
Does the universe evolve?
Mayhap:
http://www.lns.cornell.edu/spr/2006-02/msg0073320.html
Sure, but most have the capability, and they’re part of a group (Homo sapiens) in which the prototypical member is able and willing. I’m just saying that ability to reproduce is a pretty common part of the definition of “life”, but you ignored it in this post.
I’m having trouble with the “make records and build structures” part, which precludes anything but intelligent life (unless you include fossil records and stuctures like external shells or coral colonies). Surely the presence of something like bacteria found elsewhere in the solar system would be regarded as “life” under most definitions. And I have to agree with lylebot about the reproduction part. No life without some kind of reproduction. I’ll even give straight-up replication the nod. Ya gotta kick against entropy somehow.
I’d say the ability to generate a reasonable fascimile of oneself and evolve over generations of replications are probably THE only universal qualities of life anyone will be able to agree upon. Those attributes generally require the ability to extract energy and raw materials from the evironment self-sufficiently, but entities like viruses make even those criteria controversial to this day.
The trick here is “self-regulation”:
http://en.wikipedia.org/wiki/Gaia_hypothesis
http://arxiv.org/pdf/astro-ph/0110694
The self-evident prediction here is that the structure of the “eco”-balanced universe is also “self-regulating”.
“i” said:
The self-evident prediction here is that the structure of the “eco”-balanced universe is also “self-regulating”.
Rats I forgot to include that Eddington also thought that the cosmological constant version of the general-relativistic field equation expressed the property that the universe was “self-gauging”.
That’s just a “minor little detail” thought, and everybody thinks that Eddington was nuts anyway, so no big deal… 😉
Lyle-bot:
What about non-breeding worker ants?
“Reproduction” is not at all a crucial aspect of what we think of as “life.” If a self-aware golem spontaneously generated out of the ooze, walked around scratching itself, played checkers, wept at La Boheme, and gently passed away without ever reproducing, it would be a useless definition indeed that refused to admit that such a being was alive.
I’ve always thought that a good definition would be as follows:
The entity in question can be classified as a member of a group of entities whose evolution through time can be accurately described through the theory of evolution. This requires that at least some members of the group in question are capable of reproduction, and all members of the group were produced via reproduction (that is, produced by other members of the same group). It also requires that there be some sort of selection process that makes it so that certain members of the population are more likely to have successful offspring than others, based upon variation within the population (this means, for example, that your typical computer virus is not alive, because though it reproduces, there is no variation introduced by the process of replication for selection to act upon).
This definition handily removes stars, planets, and all other astronomical objects from being alive. It would include viruses (unlike some definitions of life), and would also include any computer program that has the requisite properties (frequently used in evolutionary computation).
Self-Replicators play a much more fundamental role in physics than most people realize.
http://arxiv.org/abs/0706.3379
three words: replicating entropy pumps.
can anyone do it in two words?
Larry Moran brings up an interesting historical tidbit:
Waiting for the paradox
In the mid-20th century a number of physicists became involved in biology, hoping to discover the new physical laws that made “life” work. Eventually, to their great surprise, it became clear that “life” does not require new physics or new chemistry.
I think that “life as we know it” can be distiguished from other layers of similar energy forms by the fact that life as we know it metabolizes raw materials (turning hydrogen and helium into heavier elements) and creates complexity out of simplicity, and does so by taking advantage of a dramatic departure from thermal equilibrium… more efficiently than any other level of comparison.
As this applies to intelligent life… among other things, we make matter/antimatter pairs much more efficeintly than black holes and supernovae do as we more “naturally” maximize work.
I agree with Island #8 and Sean #11. What we mean by a living organism is, in general, some sort of machine that takes care of itself (tends to maintain homeostasis).
A star can perhaps also be considered as a very simple machine (it regulates its energy production). So, I guess its just a matter of degree. There exists a spectrum of machines ranging from very simple systems (galaxies, stars) to very complex systems (humans).
Of course, a star when described exactly is not at all simple. But considered as a machine it is simple.
A version of ‘What Is Life?’ by Erwin Shrödinger is available on the web.
http://bill.srnr.arizona.edu/classes/182/Lecture%202007-01/What%20is%20Life/What%20is%20Life.pdf
There may be a subtle difference between:
What is life?
and
What is existence?
Both, probably, may be treated as mathematical objects through mathematical game theory?
Who needs von Neumann Algebras when there is a von Neumann Minimax Theorem?
Golems are the product of intelligent design and/or miracles, neither of which seem to occur in nature. The only way known to get from single cells to opera fans is through evolution, which requires slightly-less-than-perfect reproduction. The only hypothesis I’m aware of that doesn’t resort to the miraculous to explain how we got from molecules to cells in the first place was the spontaneous generation of the first pre-biotic replicators. Perhaps those were single molecules that could catalyze their own reproduction. No one really knows. It’s hypothesized that the pre-biotic replicator or replicators increased in complexity through reproduction, mutation, and natural selection. Molecular evolution, if you will. Were those putative molecular self-replicators alive? Depends on who you ask at this point. They might satisfy the minimum criteria, according to some.
I think the idea of synthetic organisms does throw a spanner in the game of defining life, though. We’ve been watching the evolution of simulated organisms for decades now, and studying them is an important part of research into evolutionary theory. Are those little cellular automata and their ilk alive? Does answering “no” render the “evolving replicator” definition useless? Does it matter that it’s a snippet of computer code instead of a “natural phenomenon”? Tough call to make.
I suppose it’s conceivable that some day we will make a machine that is completely self-sufficient, and perhaps even self-aware. Would it be alive? It’s hard for me to have an opinion at this point since nothing of the sort is known to exist, and we don’t know if it ever will.
Right now, though, given what we know of how things actually happened in the “real world”, the way life arises at all seems to be though the spontaneous creation of a self-replicating entity that evolved greater complexity, speciated, etc. Theoretically, reproduction and evolution are at the very foundation, and without them you don’t get past square one.
Once life has been adequately defined, someone will inevitably attempt to explain it’s meaning. Oddly enough, the question of meaning goes back even further than the question of definition – perhaps all the way back to the first creature in the universe with enough sentience to climb to top of a small mound, survey it’s surroundings, and think to itself, “what’s all this, then?”
I’ve always thought that a good _general_ definition of life is anything capable of undergoing Darwinian evolution. Of course I have to say general, because that definition excludes things like worker ants, mules, and childless humans. I quite like the extended version Jason wrote above.
Computer viruses, or programs specifically intended to imitate life, are tricky because of the environment in which they exist. They don’t so much exist inside a computer or on a hard drive as within a theoretical mathematical matrix and set of algorithms, of which the computer is a model. A particular magnetic domain cannot be said to be part of a piece of code the way a cell or even a carbon atom is a part of a conventional life form, because the code itself can freely move across bits. The magnetic landscape is more like an environment than parts of an organism. So what is a computer virus actually MADE of? It’s just information, isn’t it? Do we also have to add corporeality to the definition of life? An idea can evolve in a Darwinian fashion too, but we call that a meme rather than another form of life.
Bear in mind that information is all that any life is: we are nothing more than a transient configuration of atoms. The specific atoms that make up our own bodies are entirely replaced every few years. What we are is not the matter that makes us up, but rather the information that tells that matter which structures it is to take.
That being the case, I have no problem calling the right kind of computer program alive. It’s just a different medium than our own carbon-based life.
Bear in mind that your typical, run-of-the-mill computer virus is not alive, because all it does is replicate itself. There is no variation introduced in this replication upon which natural selection can act. But people have been working with simulated life for some time, and it turns out that evolutionary algorithms can be very useful in finding good solutions to problems. Personally I wouldn’t hesitate to call such a program alive, as it would be difficult to come up with a functional definition that would exclude such things.
In theory, a computer could be programmed to reproduce itself and the program running on it.
But the idea that life could evolve as a program, or the universe is a “computer simulation” would seem to violate the “principle of least action”, as stated by Maupertuis:
“Le Grand principe que la nature, dans la production de ses effets, agit toujours par les voies les plus simples.”
I suspect that anything that bypassed nucleosynthesis and the evolution of replicant organic molecules would have led to a much more rapid increase in the entropy of the universe.
So ulitmately, the localised low entropy that made the “computer life” possible would break down faster than its ability to replicate itself.
In order to be adaptive and self-sustaining, it would need to evolve towards more intelligence, but that evolution would go towards infinity in any computer based on floating point arithmetic (continuum number)
So there’s almost certainly a cut-off point in the evolution of intelligence.
You have to think about the entropy of a closed expanding system that has an increasing negative pressure component that is inceasing at an accelerating rate, but is most apparently being counterbalanced/regulated by in equally effective increase in gravity.
You don’t like it, and you want to project a one-sided feature into this coin, but that’s not what the geometry most apparently indicates without assumptions that aren’t necessarily justified, and since there is physics that does justify the process for what is observed… there is a most peferred theory, until proven otherwise by a complete theory of quantum gravity that doesn’t require some modification to the negative energy soloutions and the vacuum state.
LotsaLuck
As Sean has pointed out Mules are alive but cannot reproduce
one of the drawbacks or benefits of ‘hybrids’ – depending on your preference
Are sterile women (and men) not ‘alive’
Are women on contraception not ‘alive’
Are men who’ve had the snip not ‘alive’
and what did monkeys evolve from?