Does life actually locally *increase* entropy?
post by tailcalled · 2024-09-16T20:30:33.148Z · LW · GW · 7 commentsThis is a question post.
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Answers 10 quetzal_rainbow 7 Dmitry Vaintrob 2 DaemonicSigil 1 apnea None 7 comments
The usual materialist story of life I've heard is that life acts like an entropy pump, creating local reductions of entropy within the organism but increasing the entropy outside of the organism. (I think I've even seen that in The Sequences somewhere? But couldn't find it, feel encouraged to link it.) But I've come to think that might actually be wrong and life might increase entropy both inside and outside the organism.
Here's a rough account:
- We ought to expect entropy to increase, so a priori life is much more feasible if it increases entropy rather than decreasing entropy.
- Living matter is built mainly out of carbon and hydrogen, which is extracted from CO2 and H2O, leaving O2 as a result. Entropy breakdown:
- The O2 left over from breaking up CO2 ought to have somewhat lower entropy than the original CO2.
- The O2 left over from breaking up the original H2O ought to have... higher entropy because it's a gas now?
- The hydrocarbons don't have much entropy because they stick together into big chunks that therefore heavily constrain their DOFs, but they do have some entropy for various reasons, and they are much more tightly packed than air, so per volume they oughta have orders of magnitude more entropy density. (Claude estimates around 200x.)
- Organic matter also traps a lot of water which has a high entropy density.
- Usually you don't talk about entropy density rather than absolute entropy, but it's unclear to me what it means for organisms to "locally" increase/decrease entropy if not by density.
- Oxygen + hydrocarbons = lots of free energy, while water + carbon dioxide = not so much free energy. We usually associate free energy with low entropy, but that's relative to the burned state where the free energy has been released into thermal energy. In this case, we should instead think relative to an unlit state where the energy hasn't been collected at all. Less energy generally correlates to lower entropy.
Am I missing something?
Answers
- We do not expect increasing entropy a priori, because Second Law is true only in closed systems. Open systems in general case have arbitrary entropy production. Under some nice conditions, Prigogine's theorem shows that in open systems entropy production is minimal. And the Earth, thanks to the Sun, is open system.
- You analyze wrong components of life. The main low entropy components are membranes, active transport, excretory system, ionic gradients, constant acidity levels, etc. Oxygen is far down the list, because oxygen is actually a toxic waste from photosynthesis.
↑ comment by tailcalled · 2024-09-16T21:17:01.127Z · LW(p) · GW(p)
- We do not expect increasing entropy a priori, because Second Law is true only in closed systems. Open systems in general case have arbitrary entropy production. Under some nice conditions, Prigogine's theorem shows that in open systems entropy production is minimal. And the Earth, thanks to the Sun, is open system.
Entropy production is not the same as entropy, though. I think entropy production can be minimized by maximizing local entropy, since then there's no more space for entropy? I.e. since most of the CO2 has been broken up into carbon, there's not much more photosynthesis that can be done.
- You analyze wrong components of life. The main low entropy components are membranes, active transport, excretory system, ionic gradients, constant acidity levels, etc. Oxygen is far down the list, because oxygen is actually a toxic waste from photosynthesis.
They are all very dense, so they have high local entropy.
Replies from: quetzal_rainbow↑ comment by quetzal_rainbow · 2024-09-17T06:47:55.324Z · LW(p) · GW(p)
- When I say "arbitrary" I mean "including negative values".
- I think your notion of life as decreasing entropy density is clearly wrong, because black holes are maxentropy objects, black hole volume is proportional to cube of mass, but entropy is additive, i.e., proportional to mass, so density of entropy is decreasing with growth of black hole and black holes are certainly not alive under any reasonable definition of life. Or, you can take black holes in very far future, where they consist the most of the matter, and increasing-entropy evolution of the universe results in black hole evaporation, which decreases density of entropy to almost-zero.
↑ comment by tailcalled · 2024-09-17T07:20:11.684Z · LW(p) · GW(p)
I think your notion of life as decreasing entropy density is clearly wrong,
My notion wasn't that life decreases entropy, my notion was that life increases entropy.
because black holes are maxentropy objects, black hole volume is proportional to cube of mass, but entropy is additive, i.e., proportional to mass, so density of entropy is decreasing with growth of black hole and black holes are certainly not alive under any reasonable definition of life.
Black holes seem like a suboptimal hypothetical since we don't really know what's going on inside them. Their entropy especially seems paradoxical.
Under my model, density of entropy ought to increase with the growth of life.
When I say "arbitrary" I mean "including negative values".
I see. Though, what would that look like for Earth, using free energy to sort all the resources into separate bins? Which I suppose is something a utility maximizer might want. But are we really anywhere close to that? Maybe the theorem just doesn't apply yet, since it's only supposed to apply to a steady state.
I think one shouldn't think of entropy as fundamentally preferred or fundamentally associated with a particular process. Note that it isn't even a well-defined parameter unless you posit some macrostate information and define entropy as a property of a system + the information we have about it.
In particular, life can either increase or decrease appropriate local measurements of entropy. We can burn the hydrocarbons or decay the uranium to increase entropy or we can locally decrease entropy by changing reflectivity properties of earth's atmosphere, etc.
The more fundamental statement, as jessicata explains, is that life uses engines. Engines are trying to locally produce energy that does work rather than just heat, i.e., that has lower entropy compared to what one would expect from a black body. This means that they have to use free energy, which corresponds tapping into aspects of the surrounding environment where entropy has not yet been maximized (i.e., which are fundamentally thermodynamic rather than thermostatic), and they also have to generate work which is not just heat (i.e., they can't just locally maximize the entropy). Life on earth mostly does this by using the fact that solar radiation is much higher-frequency than black-body radiation associated to temperatures on Earth, thus contains free energy (that can be released by breaking it down).
↑ comment by Dmitry Vaintrob (dmitry-vaintrob) · 2024-09-16T23:22:21.167Z · LW(p) · GW(p)
Maybe I'll add two addenda:
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It's easy to confuse entropy with free energy. Since energy is conserved, globally the two measure the same thing. But locally, the two decouple, and free energy is the more relevant parameter here. Living processes often need to use extra free energy to prevent the work they are interested in doing from getting converted into heat (e.g. when moving we're constantly fighting friction); in this way we're in some sense locally increasing free energy.
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I think a reasonable (though imperfect) analogy here is with potential energy. Systems tend to reduce their potential energy, and thus you can make a story that, in order to avoid just melting into a puddle on the ground, life needs to constantly fight the tendency of gravitational potential energy to be converted to kinetic energy (and ultimately heat). And indeed, when we walk upright, fly, build skyscrapers, use hydro power, we're slowing down or modifying the tendency of potential energy to become kinetic. But this is in no sense the fundamental or defining property of life, whether we're looking globally at all matter or locally at living beings. We sometimes burrow into the earth, flatten mountains, etc. While life both (a), can use potential energy of other stuff to power its engines and (b), needs to at least somewhat fight the tendency of gravitational kinetic energy to turn it into a puddle of matter without any internal structure, this is just one of many physical stories about life and isn't "the whole story".
The amount of entropy in a given organism stays about the same, though I guess you could argue it increases as the organism grows in size. Reason: The organism isn't mutating over time to become made of increasingly high entropy stuff, nor is it heating up. The entropy has to stay within an upper and lower bound. So over time the organism will increase entropy external to itself, while the internal entropy doesn't change very much, maybe just fluctuates within the bounds a bit.
It's probably better to talk about entropy per unit mass, rather than entropy density. Though mass conservation isn't an exact physical law, it's approximately true for the kinds to stuff that usually happens on Earth. Whereas volume isn't even approximately conserved. And in those terms, 1kg of gas should have more entropy than 1kg of condensed matter.
↑ comment by tailcalled · 2024-09-17T10:28:19.735Z · LW(p) · GW(p)
The organism isn't mutating over time to become made of increasingly high entropy stuff,
I mean, actually it is. Plus accumulation of various kinds of damage, experiences, etc. which makes it differ from other organisms.
nor is it heating up.
Looking it up, apparently people drop very slightly in temperature when they age, which I guess might dominate the entropy considerations (though I guess that is due to slowly dying, so it also seems compatible with entropy being related to life if reduction in life is related to reduction in entropy).
The amount of entropy in a given organism stays about the same, though I guess you could argue it increases as the organism grows in size.
Couldn't it be reasonable to say that entropy increases as a sign of increased vitality associated with growing up to adulthood, and then afters has a mixture of an infinitesimal increasing effect from life experience and a moderate associated wirh vitality breakdown?
It's probably better to talk about entropy per unit mass, rather than entropy density. Though mass conservation isn't an exact physical law, it's approximately true for the kinds to stuff that usually happens on Earth. Whereas volume isn't even approximately conserved. And in those terms, 1kg of gas should have more entropy than 1kg of condensed matter.
But if we go by unit mass, shouldn't we count both the entropy in the air and the entropy in the organic matter, since they're both related to the original mass that goes into life, meaning therefore life still increases entropy?
↑ comment by quetzal_rainbow · 2024-09-17T10:27:57.807Z · LW(p) · GW(p)
I think the correct unit is "per particle" or "per mole".
Replies from: tailcalled↑ comment by tailcalled · 2024-09-17T10:28:51.220Z · LW(p) · GW(p)
Of atoms or of molecules?
Replies from: quetzal_rainbow↑ comment by quetzal_rainbow · 2024-09-17T10:33:11.889Z · LW(p) · GW(p)
If we go there, I guess the best unit is "per degree of freedom".
I think the confusion may arise from this concept of 'entropy density'?
To compare density levels, we need to look at a specific volume or amount of matter (recall the units of entropy are energy over temperature, no reference to space or mass). In this closed system the 2nd law tells us that overall entropy only goes up, but it does not help us to differentiate between different areas of density. It also tells us that, overall, the density will increase over time, which is not intuitive.
Considering open systems makes things easier. Energy and matter can flow in and out. You can still sample your 'entropy density' in defined volumes, and you may indeed find that 'life has increased entropy locally'. But, through thermodynamic coupling, a greater amount of entropy has been exported to the environment. This coupling is the 'life engines' Dimitry and jessicata refer to.
In summary, the 'entropy density' concept needs to be considered carefully in local vs. global terms.
↑ comment by tailcalled · 2024-09-17T17:35:13.996Z · LW(p) · GW(p)
If you were to analyze which places life increases entropy and which places life decreases entropy, what would you decomposition look like?
Replies from: apnea↑ comment by apnea · 2024-09-18T17:18:08.457Z · LW(p) · GW(p)
I think you're flogging a dead horse with this line of questioning. Or perhaps its the teleological language you choose to employ. There are a bunch of good replies. What is it you actually want to know? To get better answers you need to ask better questions.
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comment by jessicata (jessica.liu.taylor) · 2024-09-16T21:03:59.854Z · LW(p) · GW(p)
Efficient heat engines locally slow entropy increase. If they could reverse entropy, they would (to get more energy out of things). They can also export high entropy (e.g. medium-temperature water) while intaking low entropy (e.g. un-mixed high and low temperature water) to locally reduce entropy. Entropy is waste from the perspective of a heat engine. Likewise, animals intake low-entropy food and excrete high-entropy waste.
comment by tailcalled · 2024-09-16T20:33:31.411Z · LW(p) · GW(p)
Also, we typically think of utility-maximizers as wanting to decrease entropy. There's various attempts at formalizing this, e.g. Utility Maximization = Description Length Minimization [LW · GW]. Could this account for some of the difficulty in alignment? Like if life wants to maximize entropy, and utility maximizers want to minimize entropy, that seems like a natural conflict. But there's a bunch of technical details that makes me unsure whether this logical actually works out.
comment by Morpheus · 2024-09-16T20:37:28.130Z · LW(p) · GW(p)
Photosynthesis? Most of the carbon is bound from CO2 by using sun exergy.
Replies from: tailcalled, Morpheus↑ comment by tailcalled · 2024-09-16T20:43:09.117Z · LW(p) · GW(p)
Yes, much of what I'm talking about is photosynthesis. I don't understand your comment.
↑ comment by Morpheus · 2024-09-16T20:40:37.464Z · LW(p) · GW(p)
Also you should halt and reevaluate your intuitions if they lead you to believe there is a perpetual motion machine.
Replies from: tailcalled↑ comment by tailcalled · 2024-09-16T20:43:32.908Z · LW(p) · GW(p)
Increasing entropy is perfectly physically allowed, it doesn't lead to perpetual motion.
Replies from: Morpheus