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Proteins (Or the ability to make them) is not requisite for life, nor are nerve impulses, nor is evolution. If we encountered an immortal (Does not evolve), sterile (Does not reproduce), silicon-based alien (No proteins), that would most certainly be heralded as finding extraterrestrial life.

Reasoning about much large systems than our own is difficult. Imagine a bacteria attempting to reason about a human - the existence of fluid transport and centralized heating may seem a bit odd, as bacteria have developed to solve an entirely different class of problems at the organism level.

I'm not necessarily disagreeing, but what's the definition of life you're saying that would satisfy?

The "I'll know it when I see it" variety.

I don't have one. The normal definitions of life are useful approximations of some property, but we should always be reminded that they are just that, approximations.

What if a stable exotic location exist, where chemisty is able to form boolean processes.. is that live, if it computes and thinks, and is still just a huge state machine.

There are animals on Earth with a decentralised nervous systems (most famously the octopus) and there isn't any confusion about whether they're classed as one animal or multiple ones.

Octopus does have the "central brain" though - which, I guess, may account for making it "one animal". On the other hand, an animal with a truly decentralized nervous system often can regenerate from its pieces, which makes the question of it being "one animal" in the first place a bit fuzzy.

There was an article just yesterday on HN about such a creature, with a decentralized nervous system:

> Ctenophores have no brain or central nervous system, but instead have a nerve net (rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth ... Ctenophore nerve cells and nervous system evolved separately from other animals and have a different biochemistry.

A starfish might be a better example.

There are also a lot of living beings with no nervous system at all, and there's no confusion either about whether they're classed as living beings or not.

I don't get why nervous systems should be brought into the discussion. Most living beings on Earth do not have any kind of nervous system, and I don't see any obvious reason for why fungi or plant-like life forms could not be as big as galaxies.

Maybe you should expand your definition of life to not require proteins.

From Wikipedia:

> From a physics perspective, living beings are thermodynamic systems with an organized molecular structure that can reproduce itself and evolve as survival dictates. ... A major strength of this definition is that it distinguishes life by the evolutionary process rather than its chemical composition.

Is a sterile human not alive by this definition? I find that life needing to reproduce is a questionable condition. If we say reproduction is not requisite, then neither is evolution as evolution requires generations.

"Evolve as survival dictates" is a bit pointless, as if you don't change to survive (Assuming the change is requisite) you do not survive. Survival is defined as still being alive, so if you don't survive you are no longer alive and thus not living. We've then defined a state where a living being must continue living to be living, well sure - I agree.

If you remove reproduction, remove evolution, what else is left?

Evolution doesn't necessarily involve reproduction. A sufficiently advanced human (or computer) could enhance itself technologically, continuously replace it's organs (Ghost in the Shell), even change it's form and this would count in my book as a form of (non-biological) evolution. You could say the same thing about culture.

I think that definition is more for lower level life forms, where is not clear (see viruses).

Higher forms of life could be defined differently - agency, ability to sustain/repair itself, opposition towards environment. But then you might encounter difficulties with classifying a self repairing (non-sentient) robot as "alive".

> Evolution doesn't necessarily involve reproduction.

According to Darwin, it does. A giraffe got its long neck not by stretching it but by dying if the neck was not long enough to survive and letting the ones with (accidentally) better genes to live on and - importantly - reproduce.

My apologies, I was being a bit pedantic - Evolution is formally traits changing over generations, while adaption is changing traits over an organism's lifetime, though I believe you are using them interchangeably as "something changing".

I don't know what is left, but I have a hard time using such an embattled definition.

Well, your sterile human appears to be a strawman. Obviously, a person may be sterile, but the organism cannot stay alive if the continual process of reproduction (of cells, their organelles, and molecules) inside it ever stops.

A multi-cell organism is an emergent phenomenon that helps protein-based molecular structures to do their thing ("life").

No, it isn't. Imagine a hypothetical sterile human that was single celled - suddenly we've reached the same conclusion without the sub-organism reproduction issue. Making reproduction a core premise of being alive really drives to the idea of mortality, as reproduction is a way of overcoming our mortal coils. Yet, it is truly reasonable to classify reproduction as the only way? It also assumes a lineage, which is not required for synthetically created organisms.

I've said this elsewhere, but proteins are not required, in any regard, for life. They are an emergent phenomenon of the Earth's chemistry, not a rule for life. I mean, what is a cell even - "A fluid filled, membrane enclosed thing"? The reason cells are structured as they are is because of water, so a non-water based organisms would likely look much, much different.

Somehow I am having hard time imagining a single-celled, synthetically created, non-water-based, sterile human being...

I am not sure I understand the drive to redefine life beyond the life as we know it (i.e. what it actually is, as a fact), the drive that leaves one in what can only be seen as the domain of fiction at best (and pseudo-science at worst).

Do you believe its possible for a computer, more specifically an Artificial General Intelligence, to be "alive" (if one such thing ever gets built)?

Conscious? Yes (possibly). Alive? No.

I'd say It differently. Intelligent? Yes Conscious? Probably not.

Does intelligence imply life? I'd say no.

The drive was that I don't believe the original question, "Can a living creature be as big as a galaxy" is reasonable for organisms we are familiar with. If we hope to answer such big hypothetical questions, we may need to find ourselves in foreign lands as well.

We are discussing hypothetical life forms, different to the ones on Earth. Obviously we can not use a definition of life which only applies to earth-based life: we need to generalize more.

There are a few things that are wrong with Wikipedia's (re)definition of life; one thing I would like to mention is that what is missing from it is that a "living being" cannot exist outside of a pretty rich (and sufficiently dense) material environment, which 'space' as such is not.

Incidentally, a galaxy-size dense environment (and the creature itself) would collapse into a bunch of stars and black holes "in no time".

> a "living being" cannot exist outside of a pretty rich (and sufficiently dense) material environment

What makes you say that, though? I don't see any reason for it. Some ethereal galaxy-scale lifeform might be thinking as well that "life cannot exist in the extremely hot, hyperdense environment of a planet" for example. This is pure speculation from your part.

You may be right about ethereal lifeforms, but real ones need to eat, breathe, etc.; they are also subject to the laws of thermodynamics: they need energy, and also in order to maintain low entropy inside they must increase the entropy of the environment - which would be hard if there was no environment to speak of.

> Another one is that nerve impulses travel slower than the speed of light, and so, again, it would take forever for a signal to reach the central nervous system.

Signals in the human body do not arrive instantaneously. A life form at the galactic scale would either be decentralized as others have suggested, or it would operate at a much longer time scale.