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This question is a restatement of Death by entropy.

Question: Does aging, as we understand it nowadays (shortening of telomeres etc.) goes hand in hand with the increase of entropy? If so and an organism reaches a certain level of entropy, is it the cause of his death? Hence, would holding entropy under a certain level hold the organism alive?

I would expect that even though entropy goes hand in hand with aging, it is not its cause.

How should one think about this?

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Entropy in physics has little to do with ageing.

Physical entropy deals with how many microstates are compatible with a given macrostate. Commonly we say that disordered states have high entropy since many microstates are compatible with the macrostate we observe, while in a highly ordered state the number of microstates is relatively low.

Ageing is a gradual loss of homeostasis and adaptability of an organism, in the end caused by the lack of evolutionary selection for long-term maintenance far beyond reproductive age. Various systems lose their efficiency because of build-up of substances that cannot be broken down, mitochondria and immune cells that do not work effectively, there is DNA damage and so on.

But note that the bad things about ageing are not due to high entropy. The entropy of an old and a young body of the same mass are more or less equal, and mostly due to the degrees of freedom of water molecules. Entropy in physics does not well describe the macroscopic disorder of bodies or messy rooms.

Most importantly, bodies are open systems that persist through time by having a flow of energy and material pass through them. This allows them to reduce their entropy in principle arbitrarily. The fact that this ability fails is a separate issue. There are non-ageing species: had entropy applied strictly, then they would have been impossible.

One can argue that we can describe the gradual disorganisation of the functions of the body due to ageing as entropy but within the abstract system of bodily states. I am sceptical that this approach is likely to produce any fruitful results: the functional physiological states are also an open system. There is no shortage of papers playing around with analogies of entropy and ageing, but few, if any of them have produced anything of lasting value in biogerontology as far as I know.

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    $\begingroup$ Entropy in physics does not well describe the macroscopic disorder of...messy rooms. And yet this is the analogy always used to explain entropy to the common person. $\endgroup$ – Aaron Stevens Feb 11 at 15:58
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    $\begingroup$ @AaronStevens Unfortunately, this is a misconception still used to explain entropy to the common person, but it is just folklore without a sound link to physical concepts. Basically it holds only for a perfect gas, i.e. non-interacting systems. Real world does interact! $\endgroup$ – GiorgioP Feb 11 at 16:10
  • $\begingroup$ Aaron, Giorgio - Yes, my mind was mildly blown too by the linked paper criticising the sock analogy. I don't think the problem is the interaction (entropy makes sense for crystals and liquids too) but just the lack of degrees of freedom. In the room I am in right now there is probably a few thousand objects I could mess up. Yet in one mole there is $10^{23}$ molecules that can be messed up. $\endgroup$ – Anders Sandberg Feb 11 at 16:35
  • $\begingroup$ Re, "lack of selection for long-term maintenance." I think you are missing something: Death is an advantage for sexually reproducing species. Sex shuffles our genes to create endless variation with very few fatal mutations. But it only works if the older generation dies off so as not to compete with its own offspring. Individuals in the animal kingdom generally live long enough to produce and raise an average of one successful offspring each, and then they die. In the case of humans, "raise successful offpring" includes living long enough to teach our kids how to raise their kids. $\endgroup$ – Solomon Slow Feb 11 at 16:52
  • $\begingroup$ @SolomonSlow - The death advantage only works under restrictive ecological conditions (it is somewhat similar to Hamiltonian group selection); it is not particularly popular in biogerontology compared to the Kirkwood selection theory. If it was a solid reason one would see genetic programs like in the pacific salmon everywhere. $\endgroup$ – Anders Sandberg Feb 11 at 20:19

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