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We don't even really have an agreed-upon definition of life, so the question is basically non-sensical. Alive are the things we say are alive, which is mostly s

by t8sr 4y ago

We don't even really have an agreed-upon definition of life, so the question is basically non-sensical. Alive are the things we say are alive, which is mostly stuff like us: some type of metabolism, growth, response to stimuli. By that definition we would certainly recognize it, at least up close. If we're willing to expand the definition of life, e.g. to accept some type of self-aware computer as alive, then we'd have to tweak the definition, such as it is, and then we're no longer talking about "life" as we think of the word today. I guess people often wonder about other biochemistries. As far as I know, from a semester of undergrad astrobiology, we don't currently consider those very likely. Silicate chemistries were speculated for decades, but the chemical dynamics just don't tend to work out. The other main idea was ammonia as solvent, but that'd be hardly exotic, and the current thinking is that biochemistry would be entirely too slow at the temperatures where ammonia is liquid. The wilder ideas, like high energy chemistries on the surface of neutron stars only ever appear in science fiction. (Excepting the odd paper from the seventies, but that was a strange time.) Maybe people will accuse me of a lack of imagination, but I think it's telling that these threads are almost always mainly the home of comments shouting the word "quantum" like it means something, or getting their information from Mass Effect. I do think it's an interesting question whether we'd be able to recognize other types of intelligence, but I am pretty sure we'd be able to recognize other types of life. (By the way, the article seems to be mostly about whether we'd recognize alien life seen through a telescope, but that hasn't deterred speculation about other kinds of life in the thread.) Source: I actually studied this in college, for a bit.

yamasanama 4y ago
> We don't even really have an agreed-upon definition of life, so the question is basically non-sensical. ... > I am pretty sure we'd be able to recognize other types of life. ... > Source: I actually studied this in college, for a bit. So, let me summarize. You find the question non-sensical, then you are sure you know the answer, and finally you appeal to authority (your own) to elevate your opinion over those of us mere common-sense mortals. Sorry, I'd have liked to give more weight to your angle, but you kinda made that hard.
- t8sr 4y ago
  Yeah, I didn't express myself very clearly, sorry. Let me try again: I think our various definitions of life are not the actual definition that most astronomers have in their head. I think the actual definition people believe is "I know it when I see it." Two illustrative examples: Let's say we find a system of chemical reactions going on somewhere that meets most definitions: self-regulating, initial chemical seeds as a basis for heredity, some chain of triggers that causes it to look like it's responding to a stimulus.... This is isn't far-fetched, reactions like this exist in a lab. I don't think we'd call that thing alive - we don't currently. There would be arguments, but most people would probably agree it's a technicality. Now let's say we find something totally way out there crazy from a science fiction book, like a naturally occurring "computer" running an alien civilization on it. It doesn't have homeostasis - would we call it alive? Most people probably would. I guess I am prposing that the prevailing actual definition of life that lives in our heads is probably closer to "would I feel bad about killing it?" By both this and the more technical definition, we'd almost certainly recognize it, at least when studied up close. (But probably not through a telescope.) Is that more sensible? > you appeal to authority (your own) to elevate your opinion over those of us mere common-sense mortals. I mean, I'm actually kind of arguing in favor of a common-sense definition, and it's a comment on the internet, which would take me three times as long to type if I sourced it. I would do that for a blog post, but I don't even know if anyone is going to read this :) But, in fact, this is all so abstract that I wouldn't know what to cite. I still think having an idea of biochemistries, the energy scales, etc. makes someone in the field more qualified to speculate than completely uninformed internet comments.
retrac 4y ago
> we don't even really have an agreed-upon definition of life Regulated metabolism. Homeostasis. I think that's close to capturing the idea we're after. A self-aware computer still has to store its bits somewhere, and get energy and material to do that, presuming it wants to stay alive. That process might as well be called metabolism. Unless it is just totally alien (heh) physics, there would be some kind of metabolism, and localized regions which create an environment suitable for that metabolism to place. That would, if nothing else, produce heat and unusual compounds.
- _Algernon_ 4y ago
  Are viruses alive? They aren't commonly considered to be. They don't have a regulated metabolism without parasitising on the cells of other organisms. But then again, I doubt humans would have homeostasis for long without their microflora. How is that different? Isn't a virus just a more extreme case of symbiosis? What about less extreme obligate parasites? Life defies categorization. At best you can find common markers (in addition to the ones you've mentioned, the traits required for natural selection to occur - i.e. reproduction, variation in trait, and variance in offspring - may also be of interest), but in the end it boils down to the anthropocentric notion of "things like us", which isn't very useful outside earth.
  - retrac 4y ago
    It's possible a life-like system might have parts that, on their own, don't seem like-life. Human economic activity would fall under the category of "metabolism" that I used, for example. Cars are not alive either; but they, are, in the loosest sense, part of a living system. Fossil fuels for shipping food around are a part of the larger system of metabolism, ultimately enabling viruses to replicate in their human hosts. Viruses are dependent on metabolism, too; even if they can't do it themselves.
troops_h8r 4y ago
>We don't even really have an agreed-upon definition of life, so the question is basically non-sensical. I disagree. The most common definition used by working astrobiologists goes along the line of "chemical system that can self-sustain and can undergo darwinian evolution". Our definition of life is intricately tied with our theories about life's origins, and the counter-examples to that definition are fictional and their premise falls apart on closer examination. Let's take the self-aware computer you mentioned. How would such a computer come to being, exactly? It could have been built by something that does satisfy the above definition (in which case you could consider the computer not alive, but a biosignature). Or it could have been built through some darwinian process, and you're still firmly within the above definition. Steve Benner has a nice paper about this. DOI: 10.1089/ast.2010.0524 The article doesn't actually address the problems with the 'evolutionary definition', probably because it would mean trying to chip away at a theory that a lot of people spent a lot of time trying to disprove. > Alive are the things we say are alive, which is mostly stuff like us: some type of metabolism, growth, response to stimuli. By that definition we would certainly recognize it, at least up close. What about fire?
- mongol 4y ago
  "chemical system that can self-sustain and can undergo darwinian evolution" Is not also some kind of metabolism also assumed?
  - troops_h8r 4y ago
    metabolism is just a means to self-sustaining
- t8sr 4y ago
  I don't think I expressed myself very clearly, but I actually agree with you on most counts. The definition you listed is also the one I mentioned + evolution. > What about fire? I actually rambled about fire, and then deleted the paragraph because the comment was already all over the place. Fire, to me, is a good example of the "good definition" problem I mentioned. I guess we'd all agree it doesn't respond to stimuli, and a necessary property of having a metabolism would be some kind of homeostasis. It also doesn't undergo evolution. But rejecting it on those grounds doesn't feel complete. Let's say we find some chemical reaction that does self-regulate. This exists in a lab or computer modeling (e.g. https://doi.org/10.1073/pnas.2022987118 https://doi.org/10.1073/pnas.2022987118). Let's say its properties are determined by some initial chemical markers: a basis for heredity. Would we now call it alive? I don't think we would. I think our definition of life is "I know it when I see it" and the more formal definitions we've tried on in astrobiology are basically just retrofitted to the examples we know or can think of.
_Algernon_ 4y ago
Sources are supposed to be previously published, so that they can be independently verified. This "source: trust me bro" format is getting old.

by t8sr 4y ago

yamasanama 4y ago

> We don't even really have an agreed-upon definition of life, so the question is basically non-sensical. ... > I am pretty sure we'd be able to recognize other types of life. ... > Source: I actually studied this in college, for a bit. So, let me summarize. You find the question non-sensical, then you are sure you know the answer, and finally you appeal to authority (your own) to elevate your opinion over those of us mere common-sense mortals. Sorry, I'd have liked to give more weight to your angle, but you kinda made that hard.

t8sr 4y ago
Yeah, I didn't express myself very clearly, sorry. Let me try again: I think our various definitions of life are not the actual definition that most astronomers have in their head. I think the actual definition people believe is "I know it when I see it." Two illustrative examples: Let's say we find a system of chemical reactions going on somewhere that meets most definitions: self-regulating, initial chemical seeds as a basis for heredity, some chain of triggers that causes it to look like it's responding to a stimulus.... This is isn't far-fetched, reactions like this exist in a lab. I don't think we'd call that thing alive - we don't currently. There would be arguments, but most people would probably agree it's a technicality. Now let's say we find something totally way out there crazy from a science fiction book, like a naturally occurring "computer" running an alien civilization on it. It doesn't have homeostasis - would we call it alive? Most people probably would. I guess I am prposing that the prevailing actual definition of life that lives in our heads is probably closer to "would I feel bad about killing it?" By both this and the more technical definition, we'd almost certainly recognize it, at least when studied up close. (But probably not through a telescope.) Is that more sensible? > you appeal to authority (your own) to elevate your opinion over those of us mere common-sense mortals. I mean, I'm actually kind of arguing in favor of a common-sense definition, and it's a comment on the internet, which would take me three times as long to type if I sourced it. I would do that for a blog post, but I don't even know if anyone is going to read this :) But, in fact, this is all so abstract that I wouldn't know what to cite. I still think having an idea of biochemistries, the energy scales, etc. makes someone in the field more qualified to speculate than completely uninformed internet comments.

retrac 4y ago

> we don't even really have an agreed-upon definition of life Regulated metabolism. Homeostasis. I think that's close to capturing the idea we're after. A self-aware computer still has to store its bits somewhere, and get energy and material to do that, presuming it wants to stay alive. That process might as well be called metabolism. Unless it is just totally alien (heh) physics, there would be some kind of metabolism, and localized regions which create an environment suitable for that metabolism to place. That would, if nothing else, produce heat and unusual compounds.

_Algernon_ 4y ago
Are viruses alive? They aren't commonly considered to be. They don't have a regulated metabolism without parasitising on the cells of other organisms. But then again, I doubt humans would have homeostasis for long without their microflora. How is that different? Isn't a virus just a more extreme case of symbiosis? What about less extreme obligate parasites? Life defies categorization. At best you can find common markers (in addition to the ones you've mentioned, the traits required for natural selection to occur - i.e. reproduction, variation in trait, and variance in offspring - may also be of interest), but in the end it boils down to the anthropocentric notion of "things like us", which isn't very useful outside earth.
- retrac 4y ago
  It's possible a life-like system might have parts that, on their own, don't seem like-life. Human economic activity would fall under the category of "metabolism" that I used, for example. Cars are not alive either; but they, are, in the loosest sense, part of a living system. Fossil fuels for shipping food around are a part of the larger system of metabolism, ultimately enabling viruses to replicate in their human hosts. Viruses are dependent on metabolism, too; even if they can't do it themselves.

troops_h8r 4y ago

>We don't even really have an agreed-upon definition of life, so the question is basically non-sensical. I disagree. The most common definition used by working astrobiologists goes along the line of "chemical system that can self-sustain and can undergo darwinian evolution". Our definition of life is intricately tied with our theories about life's origins, and the counter-examples to that definition are fictional and their premise falls apart on closer examination. Let's take the self-aware computer you mentioned. How would such a computer come to being, exactly? It could have been built by something that does satisfy the above definition (in which case you could consider the computer not alive, but a biosignature). Or it could have been built through some darwinian process, and you're still firmly within the above definition. Steve Benner has a nice paper about this. DOI: 10.1089/ast.2010.0524 The article doesn't actually address the problems with the 'evolutionary definition', probably because it would mean trying to chip away at a theory that a lot of people spent a lot of time trying to disprove. > Alive are the things we say are alive, which is mostly stuff like us: some type of metabolism, growth, response to stimuli. By that definition we would certainly recognize it, at least up close. What about fire?

mongol 4y ago
"chemical system that can self-sustain and can undergo darwinian evolution" Is not also some kind of metabolism also assumed?
- troops_h8r 4y ago
  metabolism is just a means to self-sustaining
t8sr 4y ago
I don't think I expressed myself very clearly, but I actually agree with you on most counts. The definition you listed is also the one I mentioned + evolution. > What about fire? I actually rambled about fire, and then deleted the paragraph because the comment was already all over the place. Fire, to me, is a good example of the "good definition" problem I mentioned. I guess we'd all agree it doesn't respond to stimuli, and a necessary property of having a metabolism would be some kind of homeostasis. It also doesn't undergo evolution. But rejecting it on those grounds doesn't feel complete. Let's say we find some chemical reaction that does self-regulate. This exists in a lab or computer modeling (e.g. https://doi.org/10.1073/pnas.2022987118 https://doi.org/10.1073/pnas.2022987118). Let's say its properties are determined by some initial chemical markers: a basis for heredity. Would we now call it alive? I don't think we would. I think our definition of life is "I know it when I see it" and the more formal definitions we've tried on in astrobiology are basically just retrofitted to the examples we know or can think of.

_Algernon_ 4y ago

Sources are supposed to be previously published, so that they can be independently verified. This "source: trust me bro" format is getting old.