# The second law of thermodynamics and life on Earth [closed]

The second law of thermodynamics says that order always decreases in a system. The appearance of life seems to contradict that law, but if we invoke the Sun, everything falls into place again. But what if humans create order (which is obvious as we take a look around), what does that mean for the Natural order (biodiversity)? If we consider only the closed system of the Earth, this means that the increase of human-made order must be compensated by a decrease of order in Nature, which means a decrease of biodiversity. Does what I write makes sense?

• The second law of thermodynamics says that entropy cannot decrease in a closed system. The relation to "biodiversity" or the claim that "humans create order" is entirely non-obvious. Jan 29, 2016 at 18:47
• A person tidying their room may feel like they're making the universe more orderly, but the high-entropy waste heat given off by their body tells a different story. Any "order" created by humans is far outweighed by the amount of neg-entropy we burn to do so. There are accessible explanations of this concept on the internet, such as this blog post, which might help you understand. But if you want to really understand, that involves learning physics and doing the math. Jan 29, 2016 at 19:00
• It's a very simple experiment, really. Get into an airtight box of 2x2x2m and a puzzle with 1000 pieces. We will open the box and see if you have been able to decrease the entropy of the puzzle a week from now. Are you willing to do this simple experiment? Why not? Jan 29, 2016 at 21:16
• related question : Entropy is…disorder?
– user46925
Jan 30, 2016 at 16:00
• @CraigGidney- You write: "Any "order" created by humans is far outweighed by the amount of neg-entropy we burn to do so." This holds also for the order we create with machines. The order we create with them is far outweighed by the disorder created by the machines. A disorder that is conveyed to Natural Biodiversity. Feb 27, 2019 at 23:22

## 3 Answers

As mentioned in a comment, entropy cannot decrease in a closed system. When you are considered life on Earth, it is not a closed system. Energy comes into the earth from the sun and radiates away from the earth as heat.

In thermodynamics, we normally consider mostly engines because they are really simple in comparison to other systems. If we ignore the fact that we had to put in energy Q_h and exhaust head Q_c then yes it seems like we not only violate the second law, but also the first law. This approach to engines is analogous to trying to understand the Earth as a closed system.

• Welcome to Physics.SE, tdoughty84!
– rob
Jan 29, 2016 at 19:29

The second law of thermodynamics says that order always decreases in a system.

This is incorrect. My air conditioner alone proves that this statement of the second law of thermodynamics is incorrect. I live in Houston, TX; I rely on my air conditioner for about six months of the year. My air conditioner does not violate the second law of thermodynamics. Nor does life, nor does humans building roads and bridges.

There are two things with this statement. One is removing the caveat isolated that qualifies the systems for which this simplified version of the second law of thermodynamics applies. The other is the equating entropy and disorder. Defining what "order" vs "disorder" means is hard. Quantifying it is harder yet. On the other hand, whether energy is concentrated or dispersed is quantifiable. Entropy is a measure of whether energy is concentrated (low entropy) or widely dispersed (high entropy).

With these corrections, the second law of thermodynamics in its simplest form says that entropy cannot decrease in an isolated system.

Some take this to mean that the second law of thermodynamics only applies to isolated systems. Were that the case, the second law of thermodynamics would be rather useless. The only known isolated system is the universe as a whole. However, the second law can be recast as various mathematical expressions that apply to closed systems (systems that exchange energy but not mass with their surroundings), open systems (systems that exchange energy and mass with their surroundings), and even systems that are far removed from thermodynamic equilibrium.

The second law of thermodynamics places constraints on how efficiently systems can operate. In the billions of years that Earth has supported life, not one biological organism has yet managed to violate the second law of thermodynamics. Evolution is a powerful mechanism. If the second law was not valid, life would have found a way to violate it.

The second law also places constraints on humanity's construction of apparently orderly roads, buildings, dams, etc. Building such constructs requires a good amount of energy. That consumed energy is shortly dispersed to the surroundings, and then radiated out into space. After accounting for that dispersed energy, building a building does not decrease the entropy of the universe. It increases it.

Of course the order in an open system always decreases. Locally, like on the Earth, beautifull orderly structures can evolve (wich wouldn´t be possible if the Earth always had one side directed to the Sun), and nonetheless the total order of the system Sun-Earth would decrease. But human activity, by wich I mean the changing of the surface of the Earth by humans, doesn´t involve the Sun. We generate energy and that energy gets converted to order, wich is not so difficult to see if we look around us. And that increase in order can only be compensated by an bigger amount of loss in order in the Natural world (decrease in biodiversity, wich is also very obvious to see) to compensate for the man-made order. It would have been better for the bidiversity if we just would have let all the fossil fuels burn, than making use of that energy and built the Earth full of human structures.