Consequences of the Second Law of Thermodynamics

FROM THE LECTURE SERIES: The Joy of Science

By Robert Hazen, Ph.D., George Mason University

The second law of thermodynamics has three consequences. These include the ‘heat death’ of the universe, the nature of time, and the central biological concept of trophic levels. How do these consequences affect human lives?

Image showing the second law of thermodynamics.
The second law of thermodynamics states that the heat in the universe must eventually spread out. (Image: BrainCityArts/Shutterstock)

The Heat in the Universe

One of the more sobering consequences of the second law of thermodynamics is the idea that the heat in the universe must eventually spread out. Life on Earth is possible because the Sun provides a steady stream of heat, radiant energy to the Earth’s surface. The Earth also holds a vast reservoir of internal heat.

In the 1890s, people began to realize that that heat in the Earth is still radiating out into the coldness of space and the Sun’s energy is radiating out into the coldness of space. Eventually, billions of years down the road that heat energy is going to disperse and distribute evenly. The ultimate fate of the universe, therefore, must be for the heat to spread out uniformly, as in the second law. No energy transfer would be possible, as all the energy is taken up in matter at the exact same temperature.

Learn more about the first law of thermodynamics.

The Second Law of Thermodynamics and the Nature of Time

People experience time as one of the major variables in their lives. They try to deduce historical aspects of things and time becomes an important parameter in those kinds of discussions. Time is considered an intangible physical quantity.

However, what is the nature of this quantity, time? The second law of thermodynamics gives some tantalizing insights. Take the instance of a movie being played backward. Many scenes are going to appear perfectly normal, but some forms of motion would look very strange. For example, people diving out of a swimming pool, unbreaking an egg, unshooting an arrow, and so on.

It all ties with this tendency of entropy to increase. The tendency of entropy to increase rather than decrease defines the arrow of time. It defines the direction in which time moves in people’s everyday experiences. Only the second law of thermodynamics incorporates this arrow of time.

Somehow, the second law defines the direction of time. Whether that is a cause or effect, or whether people are merely describing some aspect of the natural world, scientists who think about energy also think about time.

Learn more about the second law of thermodynamics.

The Limitations of the Laws of Thermodynamics

Every organism has to compete for a limited supply of energy. Plants are in this first trophic level because they get their energy directly from the radiant energy of the Sun.

In the second trophic level, are herbivores—animals that eat plants. But almost 90 percent of the energy that is contained in plants’ chemical substance cannot be used. While plants get about 10 percent of the Sun’s energy, herbivores only get about 10 percent of a plant’s energy.

Finally, there are the carnivores—animals that eat other animals—in the third trophic level, and carnivores only get about 10 percent of the energy contained in the herbivores, in the meat that they eat.

Image showing the trophic levels.
Every organism on Earth has to compete for a limited supply of energy. (Image: VectorMine/Shutterstock)

Each trophic level has 10 percent efficiency. This explains why there are so few large carnivores compared to the number of herbivores as one goes up the food chain.

Compared to the total biomass of all the different plants, as a natural consequence of the second law because one is passing energy through a system, but it is not an efficient passing of energy. It takes 10 herbivores for every carnivore. And if there is a large carnivore that eats other carnivores, those are really going to be rare.

A Misunderstood Natural Law

The second law of thermodynamics is often invoked by creationists to prove that life could not possibly have evolved on Earth. They say living things are exceptionally ordered states.

Every single cell in people’s bodies has trillions of atoms that are precisely and intricately ordered. If certain atoms are moved out of position, the cell does not work anymore. If those cells do not work anymore, the body does not work anymore. So, life is probably the most ordered system one can imagine.

The human brain is probably the most ordered and intricate object one can conceive of. Given the second law, it seems since systems tend to become more disordered with time. It is inconceivable that such an ordered system could have evolved within the context of the second law.

This is a transcript from the video series The Joy of Science. Watch it now on The Great Courses Plus.

Reshuffling of Energy

Every time a plant grows, every time a person gets a cut and it heals, there is a sort of overcoming the second law because energy is used to fix the damage.

The so-called violations of the second law are just a reshuffling of energy, its systems have learned how to use energy and collect energy from their surroundings. Their surroundings have the entropy that is increasing, but the individual objects then become more ordered, and that is how one can explain the origin of life.

Systems with Large Numbers of Components

There is a seemingly universal tendency for systems with large numbers of components to display complex behavior that one wouldn’t normally see in the individual components. There are several examples, such as atoms form planets, and then life forms from those planets. Then, consciousness forms from life, and there are these emergent, complex processes that seem to arise spontaneously out of local order in an otherwise large system in which the entropy is increasing.

In each of these cases, one can see local states of extremely high order arising from a large collection of small particles, whether they be cells or atoms, or so forth. These are called emergent properties. Therefore, by defining the sequence of possible events, the second law of thermodynamics provides a definition of the arrow of time.

Common Questions about the Consequences of the Second Law of Thermodynamics

Q: What is the meaning of ‘heat death’ of the universe in the context of the Second Law of Thermodynamics?

The ‘heat death’ of the universe is one of the consequences of the second law of thermodynamics that states that the heat in the universe must eventually spread out.

Q: How much efficiency does each trophic level—plants, herbivores, and carnivores—have?

Each trophic level—plants, herbivores, and carnivores—has 10 percent efficiency.

Q: Why is the second law of thermodynamics often invoked by creationists?

The second law of thermodynamics is often invoked by creationists to prove that life could not possibly have evolved on Earth.

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