[at_a_remove]

Numerous other ideas exist for alien life, but most of them have their shortcomings. It all boils down to chemical reactions and physics.

Imagine a universe of only helium. No valence electrons would be readily available, so chances are you are not going to see helium molecules. Lithium, sure, as a metal. Continue examining various smaller (and more plentiful elements). Fluorine? Well, only one slot available, so you can make molecules with two atoms, but no chains. If you have two slots open, you can make a chain, but little else. Although try making a chain of oxygen -- even the three-link ozone is not particularly stable. Carbon can have four different bonds going at once, which is pretty crazy, and so you want something with three or four slots available.

But then you say, well, silicon could have four. And it could ... but for the fact that it is so huge by comparison to carbon (that whole other "electron shell") that it rarely has analogues to organic (carbon-based) compounds. Also, it doesn't like to do much until you get it fairly hot. So you climb back up a row and it looks like carbon, nitrogen, boron ... those are going to do your heavy lifting when you need multiple bonds, with hydrogen, oxygen, and the halogens or alkali metals somewhere on the outside. Hence carbon chauvinism. Carbon is something like the fourth most common element in the universe after all.

Then you start talking about solvents -- all of this stuff has to slosh around in something after all, your chemistry experiments take place in liquids -- and the field is a bit wider but dang, water has some crazy properties that make it quite a catch when it comes to solvents.

You've got temperature ranges: down in the single digits of Kelvin you're not going to have much chemistry happening, and up in the thousands of Kelvins even iron boils and then again, no more structure.

Once you start looking for these various "sweet spots" it all comes down to finding a place where you can have liquids (your solvent), solids (for structure), and gases (even if they are dissolved in liquids). Combine that with the more common elements (especially those that are friendly to complex chemistry) and you have something not entirely un-Earthlike.

There's tons of Wikipedia articles on it, but ... the restrictions of chemistry are the bulk of the culprit, I'm afraid.

[henearkr]

You only considered chemistry-based life. Cannot hold it against you of course. But some hard SF writers like Baxter also imagined life based on electromagnetic interactions, nuclear reactions (life in the mantle of a neutron star), fluid vortices and turbulences (life in liquid or gaseous planets), etc. Basically he included a lot of substrates that could be used to implement complexity.

Also, a big drawback of searching only chemistry-based life is that it limits a lot the range of viable temperatures. Would be too slow and simplistic at low temperatures, and a complete uncontrollable chaos at high temperatures.

In particular, finding a brine on an ice dwarf like Ceres is not very exciting, as the temperature is too low, and no interesting chemistry can happen.

Whereas other substrates could be totally fine at different temperature ranges.

[Balgair]

That neutron star story is wild. Hard recommendation to read.

One point I have with Baxter is that his premises are just bananas, but the people acting in them are pretty recognizable. To be fair, if the characters were also nutter-butter, no one would read it, as it wouldn't be a story anymore, just a strangely formatted research paper.

https://en.wikipedia.org/wiki/Flux_(novel)

[gizmo686]

For another life on neutron star: Dragons Egg, Robert L. Forward

[henearkr]

I don't know if you read the other books from the Xeelee series, but the neutron star is probably not even the strangest life form Baxter considered :) I don't want to spoil too much though.

[Balgair]

The Xeelee Sequence is totally crazy.

Not many authors can go the distance from the beginning of the universe, to the end, and then back around again.

They Hyperion Cantos is also good reading. It starts with a re-write of the Canterbury Tales in the far future, and has the pilgrims making their way, not to Canterbury, but to face off with a six-limbed red-eyed spike-covered murder beast that exists outside of time. It ends with hyper-evolved humans and the second-coming facing off against god-like AIs.

Trust, me, it actually works really well.

[thom]

I can never forget reading Titan for the first time. Extremely bleak in many ways but worryingly plausible.

[simonh]

Where I think most of the proposed alternate forms of life in SF fail is they focus on complexity but disregard the need to maintain order. Vortices are a good example, theres plenty of complexity there, but also a huge amount of entropy and no real way to maintain ordered, purposeful changes in structure, or isolation of function. Complexity just isn't enough, so until some of those ideas get a lot more clearly developed, and solve some fundamental issues I just don't find them compelling. Great for fiction and speculation, but not really relevant in real world investigations.

Yes, I know that makes me sound like the boring, hide bound characters in such novels that are incapable of understanding this new amazing form of life, but frankly in the real world those guys would be absolutely right. The protagonists who contact the amazing new life forms just make far too many fundamental errors and intellectual shortcuts to really be credible.

[henearkr]

I agree those are mere plausible ideas with no real foundations, but the aim is to open our imagination to the realm of the plausible. Even those phenomena that are improbable and barely plausible, there are some of them we will encounter, and thanks to the work of sf writers we will not be too much caught off guard.

Specifically for your point on entropy and stability of vortices, I don't disagree totally though I'd like to call your attention on what is known with magnetic skyrmions. They are stable because to change spin the particles would need to overcome huge energy barriers. Then, the only difference with fluid vortices is that in a fluid there is friction that can bring the fluid elements to gradually shed their angular momentum at the contact of other elements with a different momentum. But in superfluid states that you could encounter at very low temperatures (like on some distant planet etc), it is plausible that there would exist analogous energy barriers preventing the fluid elements to directly shed their momentum! And even though, you can of course imagine fluidic circuits and rotational computations, which would have some stability thanks to the superfluid state.

Of course all of that is baseless speculation, but the point of SF is precisely to appreciate the vastness of the world of plausible phenomena...

[Razengan]

"Life" could also be AI machines.

If we seeded a planet with sufficiently advanced robots, that can mine and build and learn and make copies of themselves on their own, and a few centuries later they have a civilization, does it matter what they're made of?

For all intents and purposes, it would be life.

Perhaps at some point we should/will have to stop saying "life" at some point and use terms like "agent" instead, to include disembodied intelligences shaping the universe through their decisions and stuff like that. :)

[fsloth]

I don't think you should confuse life and agency. I fail to see how a bacteria would have any agency.

[bryanrasmussen]

Well then if you found a place with bacteria but nothing advanced you would put in your captain's log that "the fundamental requirements for agency to arise were found, but as yet agency had not been achieved" or some such phrasing.

[ajuc]

> I fail to see how a bacteria would have any agency.

It does stuff on its own. That's agency.

[Razengan]

> I fail to see how a bacteria would have any agency.

Why would it not?

[at_a_remove]

Oh, I remember the convection cell Qax from Baxter, read Dragon's Egg when it came out, or Brin's Sundiver for solar dwellers, or go even further back for the Piers Anthony's OX, even the frozen lattices of light from 2001. I could try speculating on some pretty strange forms of life, and have, but ultimately life -- and as fuzzy as that term is -- might well be construed as interlocking sets of cycles, self-perpetuating at the expense of various resources, always with the second law coming up from behind, waiting to knock it all out of balance, pluck teeth from the gears.

I could talk about how the various conditions at the surface of a neutron star probably cannot support the kind of interactions that would lead to a nuclear pseudo-chemistry (the elements aren't as important so much as the kind of bonds, and therefore structure supported, are, with the other physical properties being less important), but I would like to take a different tack and think about life arising. How would all of those little cycles, circadian down to Krebs, get started?

My intuition? guess? is that just as we are big bags of internalized seawater, life would have hijacked, replicated, and then encapsulated already existing cycles, multiple cycles. Just once cycle wouldn't cut it, probably not even two. I think for us it was all about warm little tidepools, water comes in, then the day-night cycle keeps changing the concentrations. Another tide comes, more water and something, perhaps waste, is washed out. Enough times, enough luck, maybe something sticks, holds something back in case the next splash isn't as big. The next big trick is a membrane, something to isolate the cycles. Separates the outside from the inside. Now there's something to separate.

Overall, though? You're still going to need matter that supports both structure (at least for a membrane, a boundary between in and out) and fluidity (for those cycles). I've thought a lot about what the bare minimum would be, but the Earth chemistry is just a shorthand for "what bonds, and therefore structure, can be maintained?" and "how will it flow?" I am pretty confident in saying it would have to obey Fermi statistics, instead of Bose, because bosons love to play follow the leader, but fermions need to stand out (and apart) enough that they can at least stand. The convection cell entities were interesting to think about but would require an almost unparalleled level of stability and uniformity across large expanses of matter, sort of a large-scale "lab conditions" environment, free of fluctuation and interference, somewhere relatively calm. I have considered something not too far away from that, life existing in an environment where the average energy levels were constant, but the entropy wasn't: life optimized to hunt not for calories but for pockets of order to bolster its own.

Non-chemical life is fun to speculate about but ... I keep coming back to structure and fluidity. It's difficult to find phases of matter that support both that aren't in the chemical domain.

[henearkr]

Nice to see that I speak with a peer SF lover! ^^

I agree, I would be surprised if we discover first alien life in a non-chemical form. Somehow though I would love to be surprised!

There are so many possible environments and substrates, and we cannot think of all of them. The multiplicity of the places where things could happen, and the scale and age of the universe, also increase the possibilities that there are places with the right conditions or stability. The conditions on Earth were kind of lab-grade perfect too! Any nearby supernova, ill-directed solar flare, etc. could have nipped life in the bud. Only the huge number of Earth-like environment in the universe, and the large interval of time allotted, can justify that we have had the chance to arise in these conditions.

PS: also, to be fair, a lot of life forms in Baxter are actually engineered. But our First Contact (or First Observation) could be as well with a "natural" form as with an artifact! And that would not matter that much.

[imhoguy]

Reminds me Solaris written by Lem. https://en.m.wikipedia.org/wiki/Solaris_(novel)

[Johnjonjoan]

This is so non esoteric. I can't remember who said it but I'm reminded of the old saying: "if you can't explain something to a high school student then you don't understand it". You certainly understand it and I certainly appreciate you sharing your understanding in such an accessible way, thanks.

[marcodiego]

I think it was Feynman: https://kottke.org/17/06/if-you-cant-explain-something-in-si...

[jacobkania]

This was fascinating to read, thanks for sharing! I hadn’t thought about how much harder it would be to create life out of other elements due to chemistry. Had previously believed the chances were small that other life would be similar to us, given so many options. You’ve convinced me that it seems more likely that other life would be carbon/water based as well due to those limitations.

[throwaway2048]

Another problem with silicon is that its basic oxide (Silicon Dioxide) is a solid that is non-reactive with virtually everything.

If you assume water based life, there is almost certainly going to be oxides involved, such as how CO2 is involved with earth life.

And I think water based life chemistry is an extremely good bet, because alternative liquids based off of light, common elements are either only liquid at very low temps, (ammonia, methane) extremely reactive ( Hydrogen fluoride, hydrogen sulfide), or both.

Another consideration with silicon is that the earth contains vastly more silicon than it does carbon (in many ways life on earth is fairly carbon starved), yet life based on carbon arose, not silicon.

[thaumasiotes]

> Another problem with silicon is that its basic oxide (Silicon Dioxide) is a solid that is non-reactive with virtually everything.

I was amused once by reading an MSDS for silicon dioxide.

As I presume is required, they do list some hazards of exposure, but if you read the sheets for other substances the contrast is pretty striking. Look at the health effects listed on https://fscimage.fishersci.com/msds/09890.htm :

> Eye: Dust may cause mechanical irritation.

> Skin: Dust may cause mechanical irritation.

> Ingestion: May cause irritation of the digestive tract.

> Inhalation: Dust is irritating to the respiratory tract.

> Chronic: May cause cancer in humans. Prolonged exposure to respirable crystalline quartz may cause delayed lung injury/fibrosis (silicosis).

> NFPA Rating: (estimated) Health: 1; Flammability: 0; Instability: 0

So yeah, as long as you're not inhaling sand, you're fine.

[erik_seaberg]

I'm reminded of

> [Chlorine trifluoride is] also a stronger oxidizing agent than oxygen itself, which also puts it into rare territory. That means that it can potentially go on to “burn” things that you would normally consider already burnt to hell and gone, and a practical consequence of that is that it’ll start roaring reactions with things like bricks and asbestos tile. It’s been used in the semiconductor industry to clean oxides off of surfaces, at which activity it no doubt excels.

—Derek Lowe, “Sand Won’t Save You This Time”

[thaumasiotes]

And from an MSDS for that...

> Hazardous combustion products: None that are more toxic than the product itself.

( https://www.boconline.ie/en/images/chlorine_trifluoride_tcm6... )

It's a fun read.

[erik_seaberg]

Material Safety: None. The End.

> Contains no other components or impurities which will influence the classification of the product.

In the unlikely event you survive adding something, you won’t make it any better. You certainly can’t make it worse.

> Unsuitable extinguishing media: Water, Foam, Halons

> Specific methods: Move away from the container and cool with water from a protected position.

Um.

> May react violently with combustible materials.

Specifically, everything.

> Seek medical advice before using product.

“Don’t.”

> Ensure vehicle driver is aware of the potential hazards of the load

Driving is a strong indication that he is not aware.

> and knows what to do in the event of an accident or an emergency.

“Run.”

[zaarn]

Similar fun would likely be the safety sheet for FOOF (Dioxygen diflouride), a compound that you can't really get to room temperature without said compound tearing itself and it's container apart. I do recall it reacts "vigourosly" (I believe the paper I read on this used those specific words) with Chlorine triflouride at 90K (-180C, -300F).

[thaumasiotes]

To be fair, most stuff involving fluorine is like this. Compare https://fscimage.fishersci.com/msds/11171.htm (for extra chlorine trifluoride fun, this is one of the things you get if you react it with... water. Hence the entry in "unsuitable extinguishing media".):

> Danger! May be fatal if inhaled, absorbed through the skin or swallowed. Both liquid and vapor can cause severe burns to all parts of the body. Specialized medical treatment is required for any exposure... can cause metabolic imbalances with irregular heartbeat, nausea, dizziness, vomiting and seizures. Long-term exposure may cause bone and joint changes. Will attack glass and any silicon-containing material. Corrosive to metal.

> Potential Health Effects

> Eye: Contact with liquid or vapor causes severe burns and possible irreversible eye damage.

> Skin: May be fatal if absorbed through the skin. Causes severe burns with delayed tissue destruction. Substance is rapidly absorbed through the skin. Penetration may continue for several days. Causes severe tissue necrosis and bone destruction.

> Ingestion: Causes severe digestive tract burns with abdominal pain, vomiting, and possible death.

> First Aid Measures

> Eyes: Do NOT allow victim to rub eyes or keep eyes closed. SPEEDY ACTION IS CRITICAL! GET MEDICAL ATTENTION IMMEDIATELY!

> Skin: Discard contaminated clothing in a manner which limits further exposure. Destroy contaminated shoes. Spills should be flushed until medical attention arrives. SPEEDY ACTION IS CRITICAL! GET MEDICAL ATTENTION IMMEDIATELY.

> Ingestion: Get medical aid immediately. SPEED IS ESSENTIAL. A DOCTOR MUST BE NOTIFIED AT ONCE.

> Inhalation: SPEED IS ESSENTIAL, OBTAIN MEDICAL AID IMMEDIATELY. POISON material. If inhaled, get medical aid immediately.

(ALL CAPS are original to the MSDS.)

> Wear chemical splash goggles and face shield.

> Wear butyl rubber gloves, apron, and/or clothing.

> Wear appropriate protective clothing to prevent skin exposure.

> Wear a NIOSH/MSHA or European Standard EN 149 approved full-facepiece airline respirator in the positive pressure mode with emergency escape provisions.

> Never work alone with this chemical.

> Substance is noncombustible

[zaarn]

The statement that it produces no chemicals more toxic than itself is true but misleading because the combustion products will largely consist of chlorine gas, hydroflouric gas (not the acid, the gas) and a variety of other flourine compounds waiting to make your internal metabolism incompatible with life.

I would also argue that CO2 and dry powder isn't an suitable extinguishing media for this, as CF3 doesn't give much of a shit when it's got it's own hyperactive oxidizer on board.

[raverbashing]

SiO2's "toxicity" is more mechanical than chemical (it's coarse and rough, etc)

But it's pretty much inert chemically (for human levels of chemical reactions)

[Simon_says]

Sure. It's just sand, and it gets everywhere.

[int_19h]

> common elements are either only liquid at very low temps, (ammonia, methane)

But they are very low temps on our scale, which is centered around water.. There are many places, even in our own solar system, where these are liquid all the time, and water is solid all the time.

[throwaway2048]

Sure but the problem is that chemical reactions are MUCH slower at such low temps, perhaps much too slow for life to exist.

[philipswood]

Essay by Isaac Asimov considering viable chemistries for life: "Not as we know it. The chemistry of life"

http://www.bigear.org/CSMO/HTML/CS09/cs09p05.htm

I was surprised to realise (that besides being N author) that he was a professor of biochemistry.

[philipswood]

He ends up with a "list of life chemistries, spanning the temperature range from near red heat down to near absolute zero:

1. fluorosilicone in fluorosilicone 2. fluorocarbon in sulfur 3.*nucleic acid/protein (O) in water 4. nucleic acid/protein (N) in ammonia 5. lipid in methane 6. lipid in hydrogen Of this half dozen, the third only is life-as-we-know-it. Lest you miss it, I've marked it with an asterisk."

[RandyRanderson]

Also water has one of the highest heat capacities [0] of elements that are common so this makes a great solvent that is more resistant to freezing and boiling. For this fact and the ones above, I see it as very unlikely that complex life exists outside any region that doesn't have liquid h20 almost all the time.

[0] https://en.wikipedia.org/wiki/Heat_capacity

[IgorPartola]

Thank you for that write up.

What about non-chemical life. What I mean by that is something like a giant CPU with traces literally the size of freight trains? I recently finished the first Dark Tower book and it’s got me thinking about scale.

[hoseja]

Then you run into speed of light issues. In one nanosecond, that is one 1GHz cycle, light travels only ~30cm.

[IgorPartola]

Hah, good point. The authors of our simulation really put a damper on things, huh?

[frogperson]

This is an outstanding comment. You have made my day.

[qppo]

Are there conditions in the universe where the chemistry required for life can occur with other compounds, that don't exist on earth? For example, extreme temperature, pressure, gravity, electro-magnetic field intensity, etc.

[Dragonai]

Terrific comment. Thank you so much for the excellent explanation!