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.
I am wondering whether one can come up with a sensible definition without employing any scales at all? Imagine a living system that ‘operates’ (acts?) on timescales such that we don’t take notice of it? That might de facto be impossible, but the reason for this would be a scale dependence of certain features, like formation of structures, stability, equilibration times etc.
Besides this, the problem with the notion of ‘life’ is that in absence of a definition we might not all understand the same under it. E.g. read the comments above and the issue of ‘reproduction’. Sean (#11) thinks reproduction isn’t a crucial requirement as long as the ‘living thing’ can scratch itself, whereas Lee (#33) includes it. Whether or not it’s a requirement to be included in what Sean calls the ‘best’ definition is not a question one can really answer with yes or no, since a definition is exactly what one is looking for. One or the other definition might be more or less in agreement with ones intuition, or ‘better’ depending on what we think it should cover and be useful for. At least for me, my understanding if ‘life’ is influence by the 7 points I learned at school (see Wikipedia: Life, that includes reproduction).
@ # 27,28 Calling on ‘awareness’ doesn’t help either, since it’s just another word that lacks an appropriate definition.
I’d put my hopes on complexity that could turn out to be a useful characterization of essential features, and one that could be properly definable.
Occasionally I can’t avoid having the impression that life is characterized by the urge to do completely useless things, such like writing comments on other people’s blog or books about the meaning of liff. But more seriously, how would evolution work without trial and error? Or is that just an illusion and part of the ‘program’ we’re running on?
There are different levels of structures that form part of “life” systems, as Sean state the milky way may not be alive?..but it certainly contains living organisms. Those living organisms contained within a standard milky way, also contain living organism systems ( bacteria inside humans, or head-lice outside humans ).
The chemical/molecular structures are evidently more complex than we can label them, can we catagorize the source of “life” as being:
http://en.wikipedia.org/wiki/RNA_world_hypothesis
Running a fetch-execute cycle of stored instructions doesn’t seem to be the way living things operate and nor is it proven that the universe (as a whole) is algorithmic.
But that’s not the point. Lots of things aren’t what they seem to be, and Turing machines are equivalent to many things that don’t look like them at first glance.
The real question is: Is there any property above and beyond a theoretical Turing machine that is required for “life”, whether or not the universe has this property or not? If the answer is no, then software life is possible. If the answer is yes, then you need to identify that special property.
Well, I guess I’d like to provide an argument as to why reproduction should be an essential component of the definition of life (not the reproduction of the individual, naturally, but of at least some members of the group at large).
First, let’s consider the purpose for generalizing the definition of life. Our foundational purpose, I claim, would be to see if we should behave towards these potential organisms as we behave towards living things here on Earth. This is the foundational purpose behind forming categories, after all: so that we can simplify our interactions with the world around us, to better understand it and better interact with it.
If you accept this proposition, then it follows that the best general definition would necessarily be a functional definition. As such, I claim, we should be concerned not with what life is, but rather with what life does. And the fundamental behavior that all life on Earth follows is properly described by the theory of evolution. One might also add a few other things, such as the “entropy pump” definition, but I suspect that they would be redundant. One addition that may not be redundant is that life is capable of affecting its own environment (this would likely eliminate the evolutionary programs I mentioned previously).
If we make use of another definition than a functional definition, we might end up including something in the category of life which behaves in a fundamentally different manner from the life we recognize here on Earth, which, in turn, makes the inclusion in the category rather unhelpful.
Sean #11
You mean like Hoyle’s Black Cloud
Of course his Black Could and Sean’s Golem have the same improbability as Hoyle and Wickramasinghe’s “tornado in a junkyard” created 747. They meant this as an argument against abiogenesis not evolution. However this is naive, in that it ignores that abiogenesis is itself a stepwise process dependant on selective processes. Firstly chemical evolution which is not dependant on a replicator but is instead a secular evolution dependant on competing kinetic systems resulting in complex polymers. This provides a basis for the evolution of small self-replicating molecules which start the evolution of replicators.
My point is really, that for any form of life to evolve anywhere with any chemistry (OK here I refer to chemistry, but life forms not based on chemistry using other physical processes instead, where the energy is to high for the electromagnetic force produce molecules may be possible) there must be a replicator. So the formation life is dependant on a replicator to come into being.
From a modal realist point of view it is possible to argue that Hoyle’s Black Cloud and Sean’s Golem exist somewhere in the multiverse, however they are so highly improbable that the probability that they or anything like them exists in our universe is vanishingly remote. Therefore any life form we are likely to encounter must be the result of evolution and therefore a replicator.
I think that capacity for awareness is something found only in “life”, but not necessarily the converse (not all life is aware.) I know, “awareness” is hard to define, but so oddly enough is “stuff” as I have pointed out here before (and check my link if you’ve forgotten.)
“As such, I claim, we should be concerned not with what life is, but rather with what life does.”
I think this is superbly put. Because what life “is” is a collection of matter. So is a rock. It’s properly distinguishing the two that is the confounding problem, and how that collection of matter comes to be distinguishable at all is of paramount importance. Without resorting to vitalism, what have we got? To me talking about something living without reproduction and evolution seems a bit like talking about something living without there being “life”.
I belive the ability to replicate a similar version should be part of the operational definition of life. I believe there should be a distinction between a complex information processing object, and an object that extracts energy from the environment and within its boundary the second law of thermodynamic is locally violated. I think there is an important but subtle distinction.
Elliot
How can such little creatures as we know if the
galaxies are alive or not? Do the bacteria in
our intestines know they are part of a much larger
living system?
Galaxies interact in ways that appear to be alive.
Carl Sagan once said that intelligent beings were
a way for the galaxy to know itself. I wonder if
little collections of tiny creatures on scattered
balls of rock are enough to do the trick?
Maybe in those energy fields surrounding the really
big black holes in galactic centers are more “worthy”
of such massive entities as galaxies. See Gregory
Benford’s SF novel Eater for a related concept.
We are too buried in the “trees” called stars to
know enough about the “forest” called the Milky
Way galaxy. Living galaxies – would not surprise
me one bit.
Ping.
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.
You just previously admonished us that our definitions are supposed to make sense out of the stuff that is there, not the stuff that we imagine but doesn’t exist.
If such a golem could exist, or angels or God we’d have a problem too.
So it seems to me that you’re not doing what you’re saying.
Wasn’t one of the crucial advances in biology the experimental refutation of spontaneous generation? So it became clear that biology did not have to deal with these kinds of golems. Now you’re saying biology is useless unless it can deal with these kind of impossibilities.
Living galaxies is a really interesting idea, and not theoretically impossible either. If galaxies as a whole are able to undergo Darwinian evolution, then I would say that yes, they are certainly alive. The problem is with the evolution.
First, are galaxies able to produce copies of themselves with high copying fidelity, but some slight random changes to their properties (the changes themselves able to be passed down to the next ‘generation’)? I don’t think they are. There is a theory about black holes being able to evolve (I think I read it in Hawking… not sure) but I can’t think of a mechanism for galaxies to pass on their individual properties. Doesn’t mean it can’t be done though.
Second, there would have to be some means of competition. The above paragraph is about the heritable random mutations, the other side of evolution is natural selection which requires competition for scarce resources. Again, I can’t think of how this could play out among galaxies, and even if it could, they are simply too remote to interact. Galaxies so rarely come close enough to one another to be able to have any effect, competition and thus evolution and thus life (ignoring the golem or black cloud as philosophical considerations, all natural life surely requires evolution to form).
Finally, and related to the remoteness in space of galaxies, there has not been enough time. Life on earth has been ticking along for billions of years to get to where it is. For much (most?) of that time, each generation (from birth to reproduction) was on the scale of minutes or hours, and even now on the scale of a few dozen years for the longest generations. If galaxies were able to evolve, surely each generation would take them millions if not billions of years, and they’ve only had about 14 billion in total. Not enough for any real ‘life’ to form from galactic ‘nonlife’.
If there is a way for galaxies to satisfy any definition of life without having evolved I’d be extremely interested to hear it.
What about a “boundary” or membrane? Does there not need to be a clearly defined surface separating something that is alive from the rest of the cosmos? I believe there does. But does it need to be a solid boundary or can a gravitationally bound glass cloud potentially be living.
Elliot
another related definition by Stuart Kaufmann who has deeply investigated this area.
“An autonomous agent is something that can both reproduce itself and do at least one thermodynamic work cycle.”
are all autonomous agents alive? open question
is everything alive an autonomous agent? probably
Elliot
Here:
http://biology.clc.uc.edu/courses/bio114/spontgen.htm
Or else we could say that since physics doesn’t cover many events in Harry Potter stories, it is quite a useless science.
If the measure of life is the ability to produce life
then the (whole) Universe or multiverse is alive.
Even if biological life became extinct in the Universe
that would not automatically make the Universe dead
The Universe exists before & beyond biological life.
correction to #63 above “gas” cloud not “glass” cloud.
e.
Maybe it’s time to remind everyone: words and the boundaries of definitions are a snare: there are all sorts of things with all of the properties we ascribe to “life”, some that have some properties in various portions but not others (like the sterile living beings), etc. That what things are, and we shouldn’t fall under the territory of names.
Neil,
Agreed but there should be some precise “test” determining if a particular thing alive or not alive. Deconstructing the components should in principle help in identifying what that test is. It should be a bit more scientific than for example the story about legal definition of pornography where someone said to paraphrase “I can’t give you a definition but I know it when I see it”.
Elliot
I can see fuzzy boundaries and definitions within the category of “life”. For instance, the boundary of ‘species’ is definitely not rigid. You could very easily have a broad spectrum of creatures between chimps and humans – in fact such a spectrum has existed, just not all at once. It’s an accident of nature (not exactly an accident, but by no means logically necessary) that species are usually so separate from one another.
But the boundary between life and non-life surely must be clear, mustn’t it? To say that “life is that from which life comes” and then to declare the entire universe alive is skirting the issue a bit. It’s skewing definitions and bordering on metaphysics. Surely we can come to a satisfying definition that includes all life and excludes all non life without getting too mystical.
“If the measure of life is the ability to produce life”
One more comment on this: it’s circular reasoning. You can’t include the concept of life in its own definition, you’ll end up with gobbledygook like “the universe is alive”.
“You can’t include the concept of life in its own definition”
lol – no wonder you call yourself the jolly blogger.
If there is no life in it – we tend to call it DEAD and/or inanimate.
“If the measure of life is the ability to produce life …”
@71/72 The criterion X can reproduce X isn’t an empty statement. The non-circularity lies in ‘reproduce’ (F ‘reproduces’ F under derivation is a perfectly useful definition for the exponential function). It would be a circular reasoning if you’d say something like ‘life is what is aware’ and vice versa define ‘awareness is what characterizes life’.
Whats wrong with “replicates”
Everything that does “replicate” is life and everything that doesn’t “replicate” is not.
Making “man” = life and “a virus” = life and “the universe” = not life
If you can think of any reason why that very simple definition is not ok then mail me
alan2here@gmail.com