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

  1. Only a small percentage of a plant comes from dirt. The whole point is that they take carbon out of the atmosphere.

  2. Russ Abbott says:

    You wrote that “99.85% [of the energy we "consume"/degrade] is used for power and to fight the entropy drop involved in body construction and temporarily holding back the horrifying ravages of time.” Can you say where that statistic comes from?

    One of the distinctive features of living entities is the way they (we) store and use energy functionally–in ways that non-biological entities don’t. For example, biological organisms use energy to move themselves against an energy field, e.g,. uphill. Or we use energy to write and speak, i.e., to produce sounds and symbols for the purpose of communicating–again, which isn’t seen among non-biological entities.

    Are there statistics about the proportion of energy that biological organisms use in this functional manner? By functional I mean something other than just maintaining the machine, i.e., different from just fighting entropy. This may be a difficult line to draw. I would include building a house/nest/burrow/dam as functional but translating DNA into proteins as fighting entropy. (I’d include sex on the functional side.)

  3. Matthew Gill says:

    “Plants can turn dirt (disordered) into more plants (order)”

    Wouldn’t it be more accurate (and more interesting) to say that plants turn water and air into more plants? If I remember correctly, soil nutrients only provide a very small part of a plant’s mass.

  4. The Physicist The Physicist says:

    Wow, that was a huge brain-fart on my part. Fixed!

  5. G says:

    Continuing on the theme of this post, there is a good article HERE explaining that, in the search for extraterrestrial life, we should be looking in places where there is a steep entropy gradient, as this provides the necessary chaos-into-order conditions to create and evolve life.

    Life and evolution are processes that seem to require the input of low entropy (order) and the output and disposal of high entropy (disorder).

    And now for something completely different: I am not myself a believer, atheist or agnostic (I am in a fourth category that does not encompass any of these three), but I came across a fascinating account of a young woman who had a near death experience — the usual NDE story that you hear: died for some minutes on the operating table, but brought back to life by her doctors a while later. Bear with me on this, because it relates to entropy gradients.

    While this woman was dead, she had a typical near death experience: communicated with deceased relatives and saw the white light that is frequently reported in NDEs. Being a curious soul, she asked her relatives whether that white light was God. A relative replied, and said: “No, that white light is not God; the light is the breath of God; God lives by breathing in our memories, and breathing out nothing”.

    Immediately on reading this odd statement, it occurred to me that what she described was a God who is living creature dependent on an entropy gradient for life, just like us. Why? Because human memories are specific states of information (a memory is a definite arrangement of bits, in information theory terms), and information theory will tell you that a specific state of information has a higher state of order (lower entropy) that a state of no information (randomness or nothing).

    So although his woman had no scientific knowledge, the way she described God’s respiration was perfectly consistent with what thermodynamics tells us about energy flow: God obtains energy to live by “breathing in” low entropy information (human memories), and “breathing out” of high entropy information (randomness or nothing).

    I know that all this sounds like terrible pseudoscience, and of course it certainly cannot be classed as any empirical evidence; but mention it as I was so astounded by the way that, as she described it, God is a creature like us, that needs energy, and lives on an entropy gradient.

  6. A different G says:

    G it is a very interesting story, a deeply theological one. From a Christian theology perspective, do not forget that by Genesis, the entropy was maximum in the beginning (the seas and the earth were one.) Genesis as creation decreased the entropy by bringing order. We should not confuse information entropy with physical. Information entropy assumes order at least from the receiver. From the story you recite, memories do not seem to disappear (the relatives remember the NDE witness and the witness herself is drawn to memories.) Therefore, this “energy” is not consumed but rather persisted. One can say that our memories under the light of God’s breath are the eternal paradise or hell. The eternity comes from the immutability of the memory. Just some thoughts, even though we take this article in a totally different direction.

  7. Mehrdad says:

    Let’s not forget the sunlight itself is produced at the cost of huge amounts of entropy in the sun.
    Question: Is life going to die out eventually as a result of the entropy pollution it produces? Should we worry about it after going green and reusable?

  8. Anthony Carnasis says:

    Just a casual observation. the whole open/closed system of thermodynamics is flawed. The only time it is an ‘open’ system is if ‘life’ is present; otherwise, it is a closed system.

    Space-Time dimensions have their own attributes which are measured and affect each other, hence why they are considered dimensions. I propose that life is another of those dimensions which affects space-time, a 5th dimension (or 11th dimension) in which the 2nd Law of thermodynamics does not apply to. The physics of life (should it be a dimension) would interact with the other dimensions, and in the case of ‘Life’ force it’s special forces into them such as disorder brought into order, or in the case of sentient life, extreme disorder brought into extreme order.

    John Archibald Wheeler, a renown physicist, proposed in a self-aware universe in which life is intrinsic to the universe’s design, bringing forth sentient life, which further produces order to the universe by simple observation by this sentient. There are leagues of physicists who have similar theories, and quantum physics actually supports such.

    So I propose LIFE is it’s own dimension, with it’s own special physics which can be seen to affect the space-time continuum and hence why the 2nd Law of Thermodynamics does not apply.

  9. SM says:

    I’m a little confused. I thought that the physics text books regard the “closed system” as theoretical and not actually existing in the universe, although some have proposed that the entire universe might be considered a closed system – that is if there does not exist a multiverse. In other words, there is a constant and continuous interaction between stuff in the universe, especially from the quantum physics view point, but from the more classical view points as well. So that ultimately no thing within the cosmos is isolated from all else.
    Also, @Mehrdad citing the increase in entropy within a star in the creation of light, wouldn’t the nuclear fusion reactions that occur within the star be regarded as a decrease in entropy (hydrogen atoms fused to make helium… and so on)?

  10. Joseph McCard says:

    If the total energy of the universe was decreasing, does that mean a child born 5,000 years ago would be more energetic than a child born today?

  11. Mikkel R S says:

    What do you know about this, really? Do you have any special tools to measure the complexity-level of the entire universe, at specific points in time (say, before the existence of life compared to right now)?

    If not, then you cannot answer this question. But you can say that you believe, that life cannot violate the second law of thermodynamics – which can’t really come as a surprise to anyone, seeing as you are a formally educated physicist.

  12. Triple High Fives to Anthony from me. Everyone else had great input also so a high five for each of you. So I was reading and I couldn’t grasp how the 2nd law was basically amended in relation to life there to make it all almost fit the explanation. It didn’t flow right for me, something had me cringing for fear that it just wasn’t correct. Because I think agreeing with the question makes much more sense than changing the law and going the route you did. It just feels more right. We can’t just theorize missing information to add onto the 2nd LAW of thermodynamics and say oh well now life and evolution are in line. Basically you’ve helped me to agree with the question with your disagreement of it. You’ve said besides life entropy is correct. With life, entropy, the 2nd law, are also correct BUT not as things are stated currently. I think you said they left a couple things out which you told us about with the rest of the explanation. So if entropy is decreased order over time then you can’t fit life, especially human life snuggly into that equation. Because thoughts from when I was a baby or child to now have definitely not increased in disorder, neither has my body. Now depending on how long I live that may change direction again. But that doesn’t extinguish life just because I’m gone. Hopefully I’ve done my part so that human life becomes less and less disorder for more and more people. That’s why some of us plant trees, knowing the shade from them will be enjoyed one day, not by us but by a someone probably not even in existence yet. I find that amazing, the fact that I’m gone, yet an action I did will have a benefit and definitely not fall in line with the 2nd law of thermodynamics. Another crazy thing is that I can to this day still feel the love of people who don’t walk the earth anymore, but never felt hate or anger from someone who’s passed on. Just a thought. But you are an inspiration and great teacher Mr. Physicist. Thanks. You could be right. Just this scenario didn’t feel like it was to me is all. I’m out like Baby Ruth trying to steal second. Hah. Just made that up on the fly.

  13. Steven Colatrella says:

    Why doesn’t iced coffee defy the Second Law of Thermodynamics? Ok, I know nothing does or can – but having just poured some hot coffee over a big mug full of ice cubes, then stirred it (admittedly an external source of energy expended, but that should work at both sides of the equation: the hot and cold side no?). The hot coffee got cold instead of the ice melting and the coffee remaining warm.

    Is ice coffee a form of dark matter/dark energy? Seriously, just wondering what the everyday physics explanation is, given that a typical popularization for non-scientists like myself as to how the 2nd law works is that energy doesn’t go from hot to cold without an equivalent expenditure of energy somewhere else intervening in the process. Though, those of us who drink it know that if there WERE one thing that might be able to resist the second law of thermodynamics, it would be iced coffee.

  14. Frank Ferris says:

    Iced coffee certainly does not violate the 2nd law of thermodynamics. When you pour hot coffee on ice which we can assume is at 0 degrees C, the ice most certainly does gradually melt (even with no stirring) and dilutes the coffee in the process. The melting process requires the absorption of heat energy from the surroundings but any remaining ice stays at 0 degrees until it too turns to water. The heat required to melt the ice comes from the hot coffee and as this heat is taken from the coffee, its temperature decreases. Eventually the coffee reaches a temperature of 0 degrees and this hopefully happens before all of the ice has melted. Now the ice and cold coffee will stay at a temperature of 0 degrees until all of the remaining ice has melted. The heat needed to melt this remaining ice must come from outside the cup (eg. from your hot little hands). If you quickly drink the cold diluted coffee, you will notice that the amount of ice left in your cup is less than the amount you started with. The 2nd law of thermodynamics says that the entropy (amount of disorder) of a closed system can only increase as various interactions occur within that system. Even though the slow molecules of cold coffee have less entropy than the faster molecules of hot coffee, the liquid form of water at its melting point (with its randomly moving molecules) represents more entropy (disorder) than the solid form of water (with its ordered array of molecules) at that same temperature. The increase in entropy due the melting of the ice exceeds the decrease in entropy due to the temperature decrease of the coffee. Therefore, there is a net increase in the entropy of the contents of your cup and the 2nd law of thermodynamics has not been violated!

  15. Re: second law of thermaldynamics question and creation. The 2nd law of thermaldynamics does not apply. The celestial entities and spot horses absorb heat by hovering the photosphere, through getting into flares and from the chromosphere, After that absortion they move into the Corona and help with the exorbitant heat to form within the corona.

  16. Mark My Word says:

    So the plasma of creation cooled and coalesced and eventually formed into galaxies, but somehow all the intricacies we see are less orderly than that soup at the beginning? Sorry, but I lack the necessary faith in science to accept that.

  17. Tim P says:

    @Mark My Word. I don’t mean to be rude, but the whole, “I don’t have enough faith to be a [fill in the blank atheist/evolutionist/whatever]” almost always actually means,”I don’t have enough understanding…”

    The key is that the entropy as a whole increases, but that doesn’t mean that entropy can’t decrease in parts of the system. The article gave the example of plants. If we take the sun/earth system as an ‘entropy system’ (not technically closed, but close enough for illustrative purposes), the entropy of the entire system increases, while it decreases in little pockets when plants grow.

  18. Frank Ferris says:

    To: Mark My Word. If someone says they “lack the necessary faith in science” to accept its various concepts, explanations, theories, etc. (all based on evidence, reproducibility, experimental controls, and unbiased peer review), it seems somewhat hypocritical to me that they would also not lack the necessary faith to believe in supernatural concepts, powers, explanations, beings, etc. (where no acceptable evidence, reproducibility, experimental control, or unbiased peer review exists)! As a wise person once said “the good thing about science is that it’s generally true whether you believe in it or not”.

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