Q: Why doesn’t life and evolution violate the second law of thermodynamics? Don’t living things reverse entropy?

Physicist: In very short: nope.

The second law of thermodynamics is sometimes (too succinctly) stated as “disorder increases over time”.  That statements seems to hold true, what with all of the mountains wearing down, machines breaking, and the inevitable, crushing march of time.  But living things seem to be an exception.  Plants can turn dirt (disordered) into more plants (order), and on a larger scale life has evolved from individual cells (fairly ordered) to big complicated critters (very ordered).

However, there are a couple things missing from the statement “disorder increases over time”, such as a solid definition of “disorder” (it’s entropy) and the often-dropped stipulation that the second law of thermodynamics only applies to closed systems.

Creatures, both in the context of growing and reproducing, and in the context of evolution are definitely not closed systems.  Doing all of that certainly involves an increase in order, but at the expense of a much greater increase in disorder elsewhere.  Specifically, we eat food which, with all of its carbohydrates and proteins, is fairly ordered, and produce lots of heat, sweat, and… whatnot.  Food, and air, and whatnot are what make living things “open systems”.



If a creature could take, say, a kilogram of non-living, highly disordered material and turn it into a kilogram of highly ordered creature, then that would certainly be a big violation of the second law of thermodynamics.  However, people (for example) consume along the lines of about 30 to 50 tons of food during the course of a lifetime.  Some of that goes into building a fine and foxy body, but most of it goes into powering that body and fighting degradation (blood and skin and really everything wears out and needs to be replaced).  So, about 0.15% (give or take) of that food matter is used to build a body, and 99.85% is used for power and to fight the entropy drop involved in body construction and temporarily holding back the horrifying ravages of time.

When compared to the entropy involved with turning food into the many, many bodies that make up a species, evolution is barely an afterthought.  In fact, the entropy (as used/defined in thermodynamics) of most animals (by weight) is all about the same.  A person and a mountain lion have about the same entropy as each other, simply because we weight about the same.

The big exception is photosynthesizing plants.  They really can turn a kilogram of inert, high-disorder dirt, air, and water into a kilogram of low-disorder plant matter.  But, again, they’re working with a bigger system than just the “plant/dirt/air/water system”.

There's a huge drop in entropy between the incoming sunlight

There’s a huge increase in entropy between the incoming sunlight and the outgoing heat that’s radiated away from the Earth.

Sunlight is a bunch of high-energy photons coming from one direction, which involves relatively little entropy.  A little later that energy is re-radiated from the Earth as heat, which is the same amount energy spread over substantially more photons and involves a lot more entropy (relatively).  This huge increase in entropy, between the incoming sunlight and the outgoing heat, is the “entropy sink” that makes all life on Earth possible (with just a handful of exceptions).  In particular, green plants take a tiny amount of the sunlight that hits the Earth and turns some of the energy into sugars and other useful plant-ey material.  It all eventually turns into heat and radiates away, but instead of doing it all at once it does it through a few links in the food chain.

You can think of this huge sunlight-to-re-radiated-heat increase in entropy like water going over a waterfall, and life as being like a hydro-electric dam.  It all ends up at the bottom of the falls, but sometimes it can do some interesting stuff (life and other useful mechanical work) on the way.

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57 Responses to Q: Why doesn’t life and evolution violate the second law of thermodynamics? Don’t living things reverse entropy?

  1. Dark Star says:


    If religiosity promoted health then sub-Saharan Africa would be the healthiest place on Earth. Residents of highly secular Sweden (life expectancy of 81 years) and Japan (life expectancy of 82 years) outlive the more religious Americans (78.5 years). And, by the way, the life expectancy in China is 75.2 years now. The problem with your numbers and these numbers is that this has nothing to do with religiosity. You are committing the sin of bearing false witness and the logical fallacy of Post hoc ergo propter hoc (aka correlation is not causation).

    itsnobody wrote: “I don’t see any explosions decreasing entropy, do you?”

    As a matter of fact, yes I do. When Hydrogen oxidizes (explosively) there is a vast drop in the entropy of the atoms with the heat being released into the air molecules. So yes, I absolutely see “explosions decreasing entropy” locally. If you are as educated as you claim how can you possibly miss this?

    And we can calculate the exact change in entropy in the micro-explosions such as the process of ATP -> ADP hydrolysis: http://chemwiki.ucdavis.edu/index.php?title=Biological_Chemistry/Biochemical_Energy/ATP%2F%2FADP

    This is elementary science.

    itsnobody wrote: “I don’t see any engineers in any engineering field going with the crackpot evolutionist idea of “it’s an open system the 2LOT doesn’t apply”.”

    Never heard of an air conditioner? Or the Peltier effect? Or the entropy of chemical reactions? NOBODY says 2nd Law doesn’t apply – what they are telling you is that you have to apply it to the WHOLE system — not just one tiny piece of the system.

    I think it is clear that you haven’t studied thermodynamics at all.

  2. Thomas Gray says:

    @Dark Star “The entropy, which is the level of disorder, of ADP is greater than that of ATP. Therefore, due to thermodynamics, the reaction spontaneously occurs because it wants to be at a higher entropy level.” Going from ATP to ADP is a spontaneous increase of entropy, which means that it’s a micro explosion increasing entropy. Hydrogen reacting with oxygen also is an increase in entropy for the products.

    The 2LOT is much more complex than just saying “entropy increases”. The statement “entropy always increases” applies only to closed systems, and that is only a part of the 2LOT. Furthermore, entropy is not always accurately represented as disorder. In classical thermodynamics, which I studied, entropy change is defined as the change of total energy minus the change in energy that is available for work.

    It doesn’t sound to me as if anyone posting here has studied Thermodynamics to a great extent. That is not a slam, because thermodynamics is an arduous study program. its nobody seems to have talked to engineers who have and his comments are basically correct. I recommend that you all read the comment at the end of the chapter on the Second Law of Thermodynamics in the textbook of Classical Thermodynamics by Gordon Van Wylen and Richard Sonntag. I am not at home so I can’t give you the quote or the exact book title from my copy at this time.

  3. Dark Star says:

    That page I linked and then you quoted is from UC Davis that looks at the exact entropy change and yes, considering entropy as ‘disorder’ doesn’t tell the whole story but neither is it entirely inaccurate (nor does ‘unavailable for work’ tell the whole story, the MATH tells the story). And yes, we all know it applies only to a “closed system” which is exactly what the theists posting here are missing — that’s what we’re pointing out to them.

    You missed where I stressed there is a LOCAL decrease in entropy — while at the same time the TOTAL entropy increases.

    So nothing you said really addresses anything I stated.

    I think that if you re-read it more carefully you will see that what I said and the linked information is both accurate and in agreement with thermodynamic principles.

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  6. Angel says:


    According to your own arguments, life shouldn’t be possible either since it would violate 2LOT. According to your arguments, 2LOT cannot be true. You also speak about plants having pre-set system. One that doesn’t exist, by the way.

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