You don’t know why the sky is blue.

Photo by Darius Krause

If you have ever encountered a magnet and thought about it for a few minutes you will have undoubtedly wondered how the damn things work. Why, when I bring my souvenir photo of Yosemite National Park, encased in plastic, near a fridge does it decide to stick? How does it know to move in that direction? What information is communicated by a seemingly inert piece of rock and metal that should make them move towards each other? And why sometimes do they want to move apart?

Before we answer that, here’s another question: why is the sky blue? Easy, you might say. Rayleigh scattering. Next question. I read that on Wikipedia years ago.

Whilst technically correct, the two-word answer “Rayleigh scattering” does omit a few minor details. That being said, it’s a cool-sounding answer which, last time I checked, is what physics is largely all about. However, the persistent amongst you might follow up with “well, how does Rayleigh Scattering work?” Another good question.

There are two important factors that make the sky blue: sunlight and air. Sunlight comprises lots of different colours of light which, combined together, give the white colour we perceive when we look at the sun (disclaimer: don’t look at the sun). We’re able to see these colours when we hold a prism up to sunlight or there’s a rainbow.

When some of this coloured light — a single photon of light, to be precise — encounters an atom of air, it can be deflected. This process is called scattering. Scattering, as the name suggests, causes light to go from moving in one direction to moving in a new, totally random direction. Incidentally, the same process takes place when I have to read a map.

However, the magnitude of scattering is not uniform across the colours of the rainbow. In fact, light at the bluer end of the spectrum scatters far more than at the redder end. This means that blue and violet photons end up being flung into random directions far more than red and orange.

Whilst standing at an arbitrary point on the Earth relative to the Sun, you are more likely to see the bluer light because it is more likely to have scattered and hence be travelling in whatever random direction is required for it to hit your eyes. More of this light arrives at your eye than photons of red, yellow and so on. The sky appears blue. The sky is blue.

Naturally, you might wonder why, since shorter wavelength photons scatter more, the sky is not violet. The reason for this is partly because the sun releases more blue light than violet and partly because our eyes have evolved to see blue more clearly.

That seems like a good answer. But, as Randall Monroe articulates it in his XKCD comic, when we ask about why anything else is a colour we usually just respond with: “because it is that colour.”

Why are oranges orange? Because they’re orange! That’s also a perfectly acceptable answer. Why overcomplicate things when it comes to the sky? You might argue it is somehow different, but it’s really just another coloured thing. Blue light bounces off it and goes into your eye. No need to get all quantum mechanics-y.

But then, you might ask, why does blue light scatter more than red?

Why have we evolved to see blue light better?

Why is sunlight made up of coloured light in the first place, and why is there more blue than violet?

And even if you answer those questions, you might find yourself wondering how an atom actually interacts with a photon. How is that process possible? And what actually is a photon in the first place?

This ad infinitum spiral of questioning is reminiscent of some of the world’s greatest scientists, philosophers and plenty of five-year-old kids who have just learned to use the word why. You can decide which group you fit best.

It’s a pretty profound idea. You can just keep asking questions. And nobody can stop you. Some might be satiated by an intermediate answer in which you assume that the universe, your existence and a few other details are a given and, in return, get to dabble in a bit of light (in both senses) quantum mechanics and some cool-sounding words. But in truth, any answer will only open more doors than it closes.

So, what about the magnets? This whole blog post was inspired by a video of the great Richard Feynman responding to a question about how magnets work. His response was similar to the above line of reasoning, albeit phrased in a suaver way. You might say that there’s a magnetic force there. You might choose to explain the domains that exist inside a magnet and how they all line up when the conditions are right. You might zoom in to the level of electron spin and explain how this gives rise to a magnetic field. You might throw in Einstein’s special relativity to explain the duality between the electro- and magnetic- parts of an electromagnetic wave. You might realise that you first need to clarify what the nature of electron spin is mathematically, but to do that it might be helpful to define a few terms. Like what actually is a number? What is an empty set? And so on…

Alternatively, you might concede that any of those answers explains magnets to some extent, but it will never tell you everything. There will always be more questions.

To some, the weight of uncertainty might be enough to send you into an existential spiral, leaving you refusing to believe anything. This is not unlike the famous ancient Greek sceptic Pyrrho, who famously was so sceptical of everything that he found himself regularly walking up to the edges of cliffs in sheer (excuse the pun) disbelief that anything could happen, only to be pulled away by his less sceptical and more (or less) rational followers.

However, having more questions might inspire you, like those scientists, philosophers and pesky children who came before. The universe is a never-ending collection of questions and we are question-answering machines.

Maybe our job is to have a go at answering them whilst we’re here — whether that be through science, philosophy, parenthood, art, music, politics, spirituality or something else. Life is a never-ending discovery. A constant state of moving from not-knowing to knowing. Whether that be finding the meaning of life, discovering the origins of the universe or standing in the street, looking up at the sky, giving a cunning smile and whispering “Rayleigh Scattering” as passers-by frown and cross the road to avoid you.

Keep looking for answers and, more importantly, keep asking questions.

Hi all — just a note to say that, if this article looks familiar, it’s because it is! It’s a repost of one of the first articles I wrote.

I have been writing blog posts for a little over a year and I’m always looking for feedback to help me improve my writing, suggestions for posts you’d like to see and opportunities to branch out, collaborate and write for something/someone. With that in mind, comments are greatly appreciated and, as always, thank you for reading!



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