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2nd law of thermodynamics

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Thermodynamics is the study of the conversion of heat energy into other forms of energy.  In our lesson on thermodynamics we learned the three laws of thermodynamics. They are not always obvious or intuitive: without careful study it is easy to misunderstand their implications.

In particular, many people have misconceptions about the 2nd law of thermodynamics. One common question is “Does the Big Bang theory violate the second law of thermodynamics?”; click the link for a discussion on that topic.

Another question which often arises is “Does evolution violate the 2nd law of thermodynamics?”

We’ll address that question here, with a discussion from Five Major Misconceptions about Evolution, © 1995-1997 Mark Isaak

This shows more a misconception about thermodynamics than about evolution. The second law of thermodynamics says, “No process is possible in which the sole result is the transfer of energy from a cooler to a hotter body.” [Atkins, 1984, The Second Law, pg. 25]

Now you may be scratching your head wondering what this has to do with evolution. The confusion arises when the 2nd law is phrased in another equivalent way, “The entropy of a closed system cannot decrease.”

Entropy is an indication of unusable energy and often (but not always!) corresponds to intuitive notions of disorder or randomness. Creationists thus misinterpret the 2nd law to say that things invariably progress from order to disorder.

However, they neglect the fact that life is not a closed system. The sun provides more than enough energy to drive things. If a mature tomato plant can have more usable energy than the seed it grew from, why should anyone expect that the next generation of tomatoes can’t have more usable energy still?

Creationists sometimes try to get around this by claiming that the information carried by living things lets them create order. However, not only is life irrelevant to the 2nd law, but order from disorder is common in nonliving systems, too:

* Snowflakes
* sand dunes
* tornadoes
* stalactites
* graded river beds
* and lightning

are just a few examples of order coming from disorder in nature; none require an intelligent program to achieve that order.

In any nontrivial system with lots of energy flowing through it, you are almost certain to find order arising somewhere in the system.

If order from disorder is supposed to violate the 2nd law of thermodynamics, why is it ubiquitous in nature?

The Essential Link Between Life and the Second Law of Thermodynamics

The overall direction of change in the universe is from less probable (more organized) states to more probable (less organized) states. Life does not “oppose” this but rather makes use of it. The “downhill” movement can be used to raise things “uphill” (just as water flowing downhill through a water wheel can be used to raise a weight). There is, however, always a net loss of organization in the process.

For life on earth, the dissipation of energy from the sun is the downhill movement. Photosynthesis creates “uphill” molecules which in turn can be used in cellular respiration to create additional “uphill” molecules from which, in turn, all of the “uphill” organization of life and culture derive.

All of biological and human organization represents a state of improbability very much less than that of the concentration of energy in the sun, and one which would quickly dissipate if the sun ceased shining (or there was some disturbance in the chain of water wheels which link the sun to biological and cultural organization).


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

Posted on March 24, 2013 by The Physicist


Physicist: In very short: nope.

The second law of thermodynamics is sometimes (too succinctly) stated as “disorder increases over time”.

Slideshare on the 2nd law of thermodynamics:

Energy, Entrophy, the 2nd Law of Thermodynamics and how it relates to the Environment

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.

Organisms, 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”.

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.
from Why doesn’t life and evolution violate the second law of thermodynamics?

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