The Entropic Definition of Sentient Life

Scientists and philosophers alike have long pondered life. What exactly is life? How can we define what sentient life is and what makes it unique? There are many ideas floating around about these questions. One really interesting idea that I was recently exposed to while reading Sean Carroll’s excellent book From Eternity to Here defines life in terms of entropy. I wanted to briefly share some of these insights I gathered from thinking about this definition and it means to us.

So what is entropy? The way that it is usually described is as a measure of disorder of a system. Entropy is part of the second law of thermodynamics, which states:

The entropy of an isolated system never decreases, because isolated systems always evolve toward thermodynamic equilibrium, a state with maximum entropy.

Just describing entropy as a measure of disorder is overly simple and can cause some misconceptions about the nature of the universe, as I have discussed in past blog posts. Instead, we should look at the definition given to us by Ludwig Boltzman, the father of thermodynamics. He stated:

Entropy is a measure of the number of particular microscopic arrangements of atoms that appear indistinguishable from a macroscopic perspective.

Ok. That is a little more complex. What does that even mean?

Consider the following example, taken from Sean Carroll’s book. A hard-boiled egg is an example of low entropy object. If we cut a hard-boiled egg in half, we see that it has two definite parts to it: a yellowish yolk and the egg white.


Say we started to switch the positions of the yolk atoms and the egg white atoms. Eventually we would noticed a difference. The yolk would be more evenly distributed throughout the egg whites. There would not be a very clear line between them.  Eventually if we kept switching atoms around, the yolk and the egg whites would be indistinguishable. Entropy would have increased. In this example there are few microscopic arrangements of atoms that would still look like our original hard-boiled egg from our macroscopic perspective. Therefore, the egg is a low entropy object.

An example of a higher entropy object would be one of my personal favorite examples: a delicious bottle of Coca-Cola


For this example we will be only focusing on the liquid within the bottle, the system does not include the glass bottle.

Coca-Cola has a lot of ingredients: water, high fructose syrups, caffeine, etc.. They are mixed together in the liquid state. Now if start switching out water atoms with syrup atoms what happens? Do we see any difference? Not really. Not for a long time at least. We can keep switching around the positions of the ingredients of the drink and it still would look exactly the same to us from our macroscopic perspective. Eventually, given enough switching around, we would be able to separate the ingredients, but that would take far too long and would need too fine of manipulation to even be possible in our life times. For all intents and purposes, the individual arrangements of the elements of Coca-Cola are indistinguishable from our macroscopic perspective. Coca-Cola is a high entropy object.

Those two examples are admittedly sloppy examples (not some that you would use on a term paper) but they can give us a feel for what entropy is. With this definition of entropy we can begin to ask ourselves what it has to do with life.

Entropy inherently deals with energy. This is what the definition of the second law of thermodynamics tells us. In any closed system, entropy (or the amount of microscopic configurations of an object that are indistinguishable) increases over time. But that only applies to a closed system.

A closed system is one that does not have any energy being put into it by an outside force. Our hard-boiled egg is a closed system (not a perfect closed system, but for our purposes we can consider it as such). Over time, it will begin to decay and we will be left with a gooey mass of rotten egg stuff were the egg yolk and egg white will be indistinguishable from each other. The egg will have increased in entropy over time. The egg will also have lost an amount of usable energy.


Usable energy is a key concept to understanding entropy. Any reaction or interaction between objects requires energy. When a machine works it is able to convert one form of energy to another. However, not all the energy is converted. In the example of a steam engine, some of the available energy is lost in forms or heat and noise. Therefore we do not get as much energy out of the machine as we put into it. This is a key part of entropy, and one of the reasons that perpetual motion machines are impossible.

Living things have reactions occurring within them at all the time. Left on their own, the reactions within our bodies would eventually begin to wind down and our bodies would begin to evolve towards a high entropy state as we lose usable energy. Fortunately though, we are an open system.

An open system is one that receives energy from an outside source. Plants are great examples. Plants are able to stave off entropy by converting the sun’s rays into usable food. They are an open system and are able to work against the tendency of their system to increase towards a high entropy state.

Now what about humans, are we an open or closed system?

Experience tells us that we are an open system. We take in food and are able to use that energy to keep our bodies from evolving towards a high entropy state. And this is the key part of the definition of sentient life.



It seems that a key aspect of sentient life is the ability to choose to consume energy to stave off entropy. Humans are sentient and we chose to eat or not to eat. When we eat, our body is able to convert the food to energy and keep our open system (our bodies) functioning without evolving towards a high entropy state.

Think about when a person dies. The body is put under the ground and begins to decay. It no longer is consuming enough energy to keep up a low entropy state. The chemical processes in the body are begin to lose usable energy. When you lose energy, entropy increases. Eventually, the body will decompose to the point that the individual parts will be indistinguishable from each other.

But if we are able to stave off entropy by consuming energy, why do people die anyways?

This is where the idea of usable energy mentioned above comes in to play. When we eat something, not all of the energy in the food is usable. In the act of eating, some of the energy is lost in heat and sound. Thus, not all 100% of the possible energy of the food that you ate is converted to usable metabolic energy.

Even though you are able to stop entropy from having its way for a time, eventually your systems lose energy and succumb to the forces of nature. When the body can no longer choose to stave off entropy, we call this death. The body is now at complete mercy of the laws of physics.


This idea brings up some interesting points. It seems that one of the key factors of our sentience is the fact that we can choose to stave off entropy by consuming energy in the form of food product. Is this a valid physics definition of sentient life? I think so, because it leads to the idea of free will. A person can choose not to eat (if they can ignore their body’s natural ways of getting us to eat) and if they do they will die. Most of us choose to eat though, and thus we choose to fight against entropy. Most of us also choose to avoid dangerous activities that will compromise our ability to fight against entropy by damaging our brains or other bodily systems. It seems as though our natural evolutionary traits are specifically designed to fight off entropy for as long as we can by the exercising of free will.

Deep down, the defining characteristic of our life may just be an ability to fight against the laws of physics.


One response to “The Entropic Definition of Sentient Life

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