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.

This entry was posted in -- By the Physicist, Biology, Entropy/Information, Evolution. Bookmark the permalink.

61 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.

  4. Pingback: “Mammal-like Reptile” is simply wrong…Extinctions may be more rare than you think and marine “reptiles” may not be reptiles either… | Digging up the future...

  5. Pingback: Candidates misunderstand laws of Thermodynamics « KaiserScience

  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.

  7. Benson says:

    Can someone give me a clear and accurate definition of entropy? I haven’t been able to find one yet, and it seems very important for understanding the laws of thermodynamics.

  8. Non Credenti says:

    @Benson that isn’t an easy question to answer, because it is very context-sensitive, but I’ll give it a shot. I think it’s easiest to boil down the various way of thinking about entropy into two quick answers:
    1) Entropy measures the “disorderliness” of an object or conglomeration of objects.
    2) Entropy measures energy that is lost to a system and no is longer available to do work.

    I think these two definitions cover most of the ways we non-engineers use the term. To expand on the definitions a little:
    1) An example is an egg. There are countless ways the individual bits of an egg can be configured that, on a macroscopic view, we would call “ordered.” For example, you could swap two bits of shell with each other, or two bits of yolk, and it’s still an ordered egg. But there are many, many, many more ways to configure it that are “disordered.” Take that example to the extreme, and it might be easier to visualize–if you take the billions of bits of egg and configure them in random ways, you’ll see that there are relatively very few configurations that we would call an ordered egg. (For example, any configuration that doesn’t have shell as the outermost part of the egg is disordered.)

    So as the universe goes on its merry way, with bits of it interacting with other bits, there are many more ways that the resulting interactions yields less order. That’s entropy, and a simple way of envisioning why it seems that entropy either stays the same or increases, but doesn’t decrease. (The definition and egg example (though I’ve changed it up) are from Sean Carroll’s book, From Eternity to Here.)

    2) I’m not sure there’s much I can do to expand on this, but there are two important things to remember, in the context of the discussion of 2LOT and evolution:
    a) 2LOT addresses the entropy of an entire system, and life on earth is part of a system which includes the sun (because this is the source of energy for plantlife, without with there would be no animal life). It isn’t accurate to say, “In a growing plant order is increasing (entropy is decreasing) so this is a violation of 2LOT–checkmate atheists!” This ignores that the sun is part of the system feeding the plants, and the energy that radiates away from earth (no longer able to do the “work” of growing plants) is wayyyyy more than the energy captured by the plants to do that growing. So 2LOT is not violated because the entropy of the *entire system* has increased, just like 2LOT says.

    b)2LOT does NOT say that entropy will ALWAYS increase in EVERY part of a system; it only says that the OVERALL, or NET entropy will remain the same or increase. So you can have *pockets* of decreased entropy (a growing plant) in a *system* of increasing entropy (the plant-earth-sun system).

  9. 23rds says:

    Darkstar: “Japan (life expectancy of 82 years) outlive the more religious Americans (78.5 years).” “If religiosity promoted health then sub-Saharan Africa would be the healthiest place on Earth. ”

    The longest living people in the entire united states are the 7th Day Adventists. So some religions, DO, promote health. And I’m pretty sure people are sick and tired of unthinking ‘I hate all religion’ people out there.

  10. Jason Shanley says:

    Arguments of entropy are compelling…really made me think. In a cosmological sense the law of entropy is preserved. However, Life on earth seems to counter the entropy argument. Simple systems combining into complex forms. While mathematicians can make arguments how this in thermo dynamically not a big deal on the Macro level…I find it interesting on a molecular level especially the thermo dynamics of DNA and cellular physiology…thoughts?

Leave a Reply

Your email address will not be published. Required fields are marked *