What does entropy mean? And why does it never decrease? And what does it have to do with disorder or order? Entropy is simply explained to you here.
Bob: Hey Alice, what is the definition of entropy? And what does it have to do with physics?
Alice: Entropy is a measure of the number of possibilities.
Bob: What kind of possibilities?
Alice: Imagine a box whose bottom is completely covered with red and blue marbles. Now you arrange the marbles so that exactly the right half is blue and the left half is red. How many possibilities are there to arrange the marbles this way assuming that marbles having the same color are identical?
Bob: Only one, right?
Alice: Right. For this arrangement there is only one possibility. The entropy is low, in this case even zero. Now you shake the box and wait for the marbles to cover the ground again. What do you think happens?
Bob: Well, the marbles will mix. Now I have a random complicated pattern of blue and red balls, right?
Alice: Correct. How many possible arrangements of the balls are there to make a complicated pattern of red and blue balls?
Bob: A lot, I suppose.
Alice: Yes. The entropy of this arrangement is high because there are many possibilities to arrive at a random pattern.
Bob: I understand. Many possibilities for a certain result means high entropy and few possibilities we call low entropy.
Alice: Right. Since there’s only one way that the marbles can perfectly sort themselves out in color, the probability that this happens by shaking is extremely low. And since there are many ways that the balls will mix, the probability that this will happen is very high.
If there are many balls, these probabilities are so extremely different that the balls never sort themselves spontaneously by shaking. They will always mix, because there are more possibilities for this to happen. In other words: entropy never decreases. This is the famous second law of thermodynamics.
Bob: Cool! That’s totally logical. Sure, what happens is what has the most possibilities. But what if I now sort the balls by hand? Then entropy decreases!
Alice: Only if you look at the box in isolation. But to sort the marbles, you need energy. Your body burns calories for that. For those calories, your intestines turn vegan quinoa avocado salad into a giant pile of shit.
There are fewer ways to arrange atoms to make a quinoa-avocado salad and many ways to make a pile of shit. So a pile of shit has more entropy than a quinoa-avocado salad.
This means that as you decrease the entropy of the marbles, the entropy in your body increases! Now the second law of thermodynamics says that after you put the marbles in order the total entropy still increases, even though the entropy of the box alone might decrease.
Bob: Ah okay. I see. Can you apply that to life? There are few ways that my life can go the way I want it to, and many ways that my life falls appart. Does that mean that if I don’t use the energy from my food to order my life, according to the second law of thermodynamics, it automatically gets worse and worse?
Alice: Ha ha, I like that.
Bob: But I don’t quite understand it yet. Where does my food get its low entropy from? Wasn’t it all earth, air and water with high entropy at some point?
Alice: Excellent question! The solution is the sun. The light that arrives from the sun is mainly in the visible range. Plants make photosynthesis with this light and split the carbon dioxide (CO2) in the air into carbon (C) and oxygen (O2). In that process, some energy (photons) get absorbed. Since these photons have a rather high energy, they are relatively few and therefore have low entropy.
Bob: Why are there fewer photons if they have a lot of energy?
Alice: Because the total energy is the number of photons times the energy of a single photon. If each photon has a lot of energy, you need fewer photons to reach the total energy.
Bob: I see. And fewer photons means fewer possibilities to arrange them, means less entropy. That means plants use the low entropy of sunlight to get low entropy for themselves? That’s pretty clever.
Alice: That’s right. Now we can eat the plants or burn them. In both cases the energy is converted into heat by the plants. This heat radiates photons again. These photons are mostly infrared.
Infrared photons have relatively little energy, so they are a lot of them and therefore they have high entropy. So the system sun plus earth constantly increases its entropy, because the earth converts few photons into many photons. And this is the reason why life can exist on earth within the second law of thermodynamics.
Bob: So entropy is always increasing in the universe? Then the universe must have had extremely low entropy in the very beginning, at the big bang. How did that happen?
Alice: That’s actually an unanswered question. Nobody knows.
Alice: Yes, and very interesting. The second law of thermodynamics also tells us in which direction the arrow of time is pointing. It’s the only physical law that makes a difference between the past and the future.
Bob: How is that?
Alice: Imagine an egg that you throw on the ground and it breaks. There are many possibilities for broken eggs, but few possibilities for whole eggs.
When you break an egg, entropy increases. If you film the whole thing and play the movie backwards, you can immediately see that the movie was played backwards. You know immediately in which order the events must have happened. From few possibilities to many possibilities and not vice versa.
Bob: Wow. Okay. Anything else?
Alice: Yes! We constantly use the second law of thermodynamics in baking and cooking! If we want ingredients to mix evenly, we just have to stir. Because there are many ways that your ingredients mix and very few ways that suddenly all your flour is in just one corner of your bowl. Next time you cook, remember that! 😊
Conclusion: Entropy is a measure for the number of possibilities. Something with many possibilities has a higher probability to happen. Therefore entropy always increases.
What experiences with entropy have you had in your life? Do you have any questions? Share them with us!