Why is the sky blue? When I ask you this question, I’m guessing you either have no idea or you think you already know the answer. This classic question has been asked and answered many times by many people going to varying degrees of depth and breadth. Some stop with the term “Rayleigh scattering” while others may go into detail about the emission spectrum from the sun and the cones in our eyes.
How far does your understanding go? Do you have a well informed intuition for why the sky is blue, or have you just memorized a textbook answer? The purpose of this post is to illuminate the difference between simply knowing a fact and really understanding something. Understanding a concept at a deep level is what constitutes real, satisfying knowledge.
A Real Understanding
Just because you know the textbook answer to a question, that doesn’t mean you understand it. Let me give you an example. Imagine you are a student and you are listening to your teacher give a long and confusing lecture about shnargalids. You have no idea what shnargalids are, but you take good notes and manage to write down the following facts:
Shnargalids are made by trawindingers in blorthigies. Blorthigies are common in Drastilshinia. The shnargalids provide frinipication to the trawindingers.
At the end of the lecture, the teacher gives you a quiz to see if you were paying attention. See if you can give the correct answers to the following questions:
- What makes shnargalids?
- Where are shnargalids made?
- How are shnargalids useful?
I’m willing to bet that you aced the quiz, but I’m also one hundred percent certain that you still don’t know what shnargalids are.
In order to gain a real understanding of anything, you have to go deeper. You have to try to comprehend and visualize the nature of the shnargalids. If you manage to do this, then the answers to the questions should seem to follow intuitively. You should be able to easily deduce the answers without memorizing anything.
Let me show you how it feels when you actually understand the content that you are being tested on. Change “shnargalids” to “beaver dams”, “trawindingers” to ” beavers”, “blorthigies” to “rivers,” and “frinipication” to “shelter.” Now try answering the questions again. This time, you will notice that you have real knowledge instead of just memorized facts. You might even feel confident enough to disagree with the teacher on certain points of the lecture.
The example above illustrated the difference between knowing shallow facts and really understanding a concept at a deep level. A typical person’s understanding of why the sky is blue is probably very shallow. The same is likely true for our understanding of most things. The way to get to a deeper understanding is to ask “why” and “how” repeatedly to dig down through the many layers of explanation.
The Human Layer
The question of why the sky is blue is more precisely phrased as “why am I perceiving the color blue in the sky?” The color blue really only exists in your mind after all. For now, we’ll try not to think about the unanswerable philosophical question of why blue looks blue to us. Instead, we’ll focus on the physical question of why our brain is receiving a signal consistent with what we usually interpret as the color blue.
The sky is blue because your brain interprets the electrical signals transmitted through the optic nerve as the color blue, meaning they are consistent with other optical signals that the brain has learned to identify as blue. Those electrical signals are generated by the “blue cones” in the back of your eye. The blue cones are biological light detectors that are strongly activated when they absorb light signals of certain frequencies. Light in the frequency range that activates the blue cones most effectively is called blue light, for obvious reasons.
There are many questions that we can ask about this to gain a deeper understanding of what is going on. How does the brain organize and remember signals? What is the physical nature of these signals? How do these signals in the brain exhibit themselves as actual thoughts? How does the electrical signal travel from the eye to the brain? What part of the brain interprets it? What are the chemical reactions involved in this transmission and interpretation of stimuli? Why do the blue cones respond better to blue frequencies than other frequencies? What is the physical mechanism in the cone that allows it to detect light and emit an electrical signal?
There is so much to be understood in the human part of the answer to the question of why the sky is blue. The details are far too many to be described in this post. But this is only one part of the explanation. Why is the light from the sky coming at us in the blue frequency range in the first place?
The Atmospheric Layer
The next layer of understanding the answer to this question comes from knowing what kind of light is hitting the atmosphere and what the atmosphere does to that light. Almost all the light in the visible spectrum that hits Earth’s atmosphere comes from the sun. The visible light from the sun is white when seen from space. White light is a mixture of all frequencies. Why does it turn blue after hitting the atmosphere?
If we take a sample of Earth’s atmosphere, we can discover that it consists of about 80% nitrogen and 20% oxygen. Due to the structure of the electron orbitals in these molecules, they tend to absorb and re-emit incident photons of light in the high bluish frequencies more efficiently than they do in the low reddish frequencies. This means that red light tends to pass through the atmosphere mostly undisturbed, while blue light is strongly scattered by molecules in the sky. Green light, which lies between red and blue on the frequency spectrum, will be more scattered than red, but still much less scattered than blue. This phenomenon is known as Rayleigh scattering.
If we decompose the white light from the sun into red, green, and blue components, we can understand why the sky turns blue and the sun turns yellow. The blue light is scattered throughout the atmosphere to the point where every part of the sky ends up looking like a source of blue light. Red and green light, on the other hand, do not experience that level of scattering, and instead tend to travel straight down from the sun to the ground. Red and green together look like yellow to our brain, and thus we see the yellow sun in the blue sky.
Once again, we are only scratching the surface of the physical explanation of this process. I have left many questions unanswered. Why do the molecules in our atmosphere reflect blue light more than red light? Would different molecules produce different kinds of reflection and transmission effects? Why is our atmosphere made of mostly nitrogen instead of some other kind of matter? Would the color be different if our atmosphere were thicker or thinner? Why is the light from the sun white? What are the laws of physics behind the absorption and re-emission of photons by molecules, and why do those laws exist?
The Solar Layer
Let’s move on to the question of why the light from the sun is white, or in other words, a combination of all frequencies. The sun almost perfectly matches a concept in physics called a blackbody, which is a kind of physical object that emits light entirely from an internal thermal energy source rather than reflecting light from external sources. The spectrum of frequencies emitted by a blackbody is distributed over all frequencies, with a peak determined by the temperature of the body.
The energy that the sun emits is generated by fusion reactions within the core. This is where the highest energy photons are first emitted. From that point, photons are absorbed and re-emitted repeatedly by the charged particles that make up the mass of the sun. This process is akin to shuffling a deck of cards, randomizing the distribution of energy throughout the different radiation modes. This randomization is typical of thermal processes, and this randomization combined with the quantum nature of light is what generates the blackbody radiation spectrum.
We have now reached the origin of the light that eventually becomes the blue sky. But once again we have only scratched the surface. What are the laws of physics behind nuclear fusion, and why do they exist? How does it cause an emission of light? Why does nature obey quantum mechanics? Why was the sun there in the first place? How does it have enough gravitational force to contain fusion reactions? Why does gravity exist in the first place?
The Anthropic Layer
So far we have tried to understand the physical processes that lead to the development of the blue sky on Earth, but there is still another layer to this question. Why did this particular setup occur instead of a different one? For example, if we lived on Mars right now, the sky would usually have a reddish color, not blue. Is there any reason why we happen to live on a planet with a blue sky rather than some other color?
Perhaps humans could not have developed on any other type of world. Maybe the chemistry necessary to create intelligent life requires the elements that make up a blue sky. I must admit that I don’t really know if this is true, and so I don’t yet fully understand why the sky is blue. It’s possible that life exists on other worlds with skies of different colors, which means the fact that our sky is blue could simply be the result of pure chance. In other words, we just happened to be born on the planet with the blue sky.
Conclusion
We have journeyed through the various layers of explanation of why the sky is blue, and our understanding has improved but is not yet complete, and probably never will be. The same is probably true for any other question we can ask. There is always so much more to learn and understand than what we might initially think.
Personally, I believe it’s worth the effort to dig deeper and gradually increase our knowledge of all things. But is it really? That’s a tough question to answer objectively and definitively. I’ll have to look into it more deeply. I find a special kind of joy in understanding nature, an internal joy that is independent of all external circumstances, and that, for me, is what makes it worthwhile.
Truth Reconciled 