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In The Cuckoo's Egg Cliff Stoll recounts an episode from the oral defense of his astrophysics PhD thesis. A bunch of people ask questions but one prof holds back until...

""" “I’ve got just one question, Cliff,” he says, carving his way through the Eberhard-Faber. “Why is the sky blue?”

My mind is absolutely, profoundly blank. I have no idea. I look out the window at the sky with the primitive, uncomprehending wonder of a Neanderthal contemplating fire. I force myself to say something—anything. “Scattered light,” I reply. “Uh, yeah, scattered sunlight.”

“Could you be more specific?”

Well, words came from somewhere, out of some deep instinct of self-preservation. I babbled about the spectrum of sunlight, the upper atmosphere, and how light interacts with molecules of air.

“Could you be more specific?”

I’m describing how air molecules have dipole moments, the wave-particle duality of light, scribbling equations on the blackboard, and . . .

“Could you be more specific?”

An hour later, I’m sweating hard. His simple question—a five-year-old’s question—has drawn together oscillator theory, electricity and magnetism, thermodynamics, even quantum mechanics. Even in my miserable writhing, I admired the guy… """

It also needs a bit of biology. Our eyes don't have a flat response over frequency, they're more sensitive to blue than violet. Violet gets scattered even more than blue, and the violet light does shift our perception of the color. But it does so less than it would if we had photoreceptors more sensitive to violet, so the resulting perceptual color depends not just on the intensity of the light at different frequencies but also on our particular biology. People with tritanopia (blue-yellow color blindness) don't have blue-sensitive cones (S cones) and thus to them there is no perceived blue. Not to mention the linguistic history of the word "blue" and why English uses "blue" instead of "青" or some other word, the questions around qualia & what it means to perceive color, etc.

The real question is, is the sky blue for everyone? Some creatures can see ultraviolet. Some lack color at all…

"Could you be more specific" is a great question to find out more what the person knows and how they thing. You give an answer that, just due to the nature of knowledge and the limitation of language, has some black boxes. And "could you be more specific" is basically asking to go through the black boxes.

Its like asking how does Java work or something like that? You can go from "The JVM interprets java byte code" to quite a lot of depth on how various parts work if you have enough knowledge.

i used something like this in unstructured technical interviews all the time.

"you type a phrase into google search, you press enter, get some results. tell me, in technical detail, what happened in that chain of actions"

the diversity of replies is fascinating, you learn a lot about a "full stack" candidate this way.

Feynman's classic "Why?" chain: https://www.youtube.com/watch?v=36GT2zI8lVA

I'd probably spend at least 20 minutes just to get through how the keyboard works, much more if it's a USB-HID device.

Hah - that is exactly what I did. Someone asked me this question and after 5 minutes in the weeds of the debounce on the mouse click they said "look all we wanted was to find out if you'd ever heard of DNS, let's move on, that was great".

the good ones would usually follow up with, "how much detail do you _really_ want ;D"

I am positively excited about the upcoming first generation of humans who will have all their questions answered, correctly and in the way they can best understand, and as often and many of them as they want – and what that is going to enable.

The same anticipation of great things happening preceded the arrival of widely available internet, but all we really got was cat videos initially, and doomscrolling more recently. I don’t have much hope for great things anymore.

We got more than that. We got 24/7 surveillance.

Me too but I don't think these sorts of Solved Society endgames are likely to show up. Basically presents the same issue with a utopia.

Progression and regression are always going to be at war with each other. There will always be humans that want to hurt instead of help, there will always be humans who TRY to help but ultimately hurt. There will always be misinformation, there will always be lies, and there will always be liars.

The good news is there will also always be people trying to pull humanity forwards, to help other people, to save lives, to eradicate disease, educate, and expose the truth.

I don't think society will ever be solved in the way you're saying because there will always be hurtful people, but there will also always be good people to keep up the fight.

... and due to that, people will not appreciate all the knowledge, we will take it as air - invisible but cut the access in a myriad ways and its a catastrophe.

We value what we achieve with effort, I would say proportionally to energy put in (certainly true for me, thus I like harder efforts in activities and ie sport climbing).

Really cool article! Tangential:

> “Scattering” is the scientific term of art for molecules deflecting photons. Linguistically, it’s used somewhat inconsistently. You’ll hear both “blue light scatters more” (the subject is the light) and “atmospheric molecules scatter blue light more” (the subject is the molecule). In any case, they means the same thing

There's nothing ambiguous or inconsistent about this. In English a verb is transitive if it takes one or more objects in addition to the subject. In "Anna carries a book", "carries" is transitive. A verb is intransivite if it takes no object as with "jumps" in "The frog jumps.".

Many verbs in English are "ambitransitive" where they can either take an object or not, and the meaning often shifts depending on how it's used. There is a whole category of verbs called "labile verbs" where the subject of the intransitive form becomes the object of the transitive form:

* Intransitive: The bell rang.

* Transitive: John rang the bell.

"Scatter" is simply a labile verb:

* Intransitive: Blue light scatters.

* Transitive: Atmospheric molecules scatter blue light more.

There are many verbs like this, and English is somewhat open toward using verbs that way, or becoming so.

Did English speakers say "this novel reads well" two, three hundred years ago?

Labile verbs is a source of ambiguity of natural languages (only western ones?) that we are all accustomed to.

The bell rang should become The bell was rung, either way it means The bell rang another bell.

"the bell was rung" illustrates a cause (and introduces a question: who rang the bell?)

"the bell rang" illustrates an effect (the vibration and sound of the bell as it rings).

i think this is more an illustration of the ambiguity of the root word "ring", which can be an action by a subject upon an object, or to describe the behavior of the object itself.

I have always wondered about this. The verb for the first person is to 'see'. To a third person you 'show'

For the first person there is 'listen' (or 'hear'). Does English not have a corresponding word for the third person ?

What about Germanaic or Nordic languages ? Do they have a third person analogue of 'listen' ?

AFAIK listen used to be used therefor[sic] but it has fallen out of use nowadays. From wiktionary:

> Listen the watchman’s cry upon the wall.

Edit: formatting

'Hear the watchman’s cry upon the wall' works the same way, no ?

I have clarified what I am looking for in a cousin comment.

"tell"?

Ah! That's not bad but it's not the same thing. Good nevertheless.

I can 'show' (or point someone to a) a sight that I am not myself creating in anyway. The word I am looking for would mean to 'make you hear' in the same may to show is to make you see.

I showed him the distant tower.

I ??? him the faint sound.

play?

I played him the faint sound.

TIL!

Debates whether to update the sidenote with an explainer on ambitransitive and labile verbs

Now do clam steamers and shrimp fried rice.

Interesting here is: Actually, for most blue butterflies, it’s not even a pigment-it’s just a trick of the light. Since blue is so rare in the biological world (hardly any plants or animals can produce real blue chemicals), they evolved structural colors. Their wings have these microscopic ridges that reflect blue light while canceling out other colors.

It’s basically the same reason the sky looks blue, just built into a wing. If you were to look at the wings from a different angle or get them wet, the blue often disappears because you're messing with that physical structure

Not just butterflies, birds too! But what selection pressure drove the evolution of these structural colors? Presumably signaling, the opposite of muted, camouflaging colors.

Also, as many might know, blue eyes are the result of a lack of pigment (eumelanin). The iris is translucent, but Rayleigh scattering preferentially backscatters blue photons. Green eyes have some pigment, making them a mix of brown and blue.

Also the blood veins that you see as bluish through the skin are blue for the same reason, due to light scattered in their walls.

I thought they are green.

Definitely more blue/purple.

It's also the trick employed by Iridigm, which Qualcomm acquired in late 2004 (i was there then).

https://en.wikipedia.org/wiki/Interferometric_modulator_disp...

I'm curious how they were able to patent a technique invented by nature millions of years ago.

the displays have an array of switchable mirrors individually addressable, unlike nature in this case.

(but sort of like chromophores in an octopus or cuttlefish, perhaps).

I see, but those MEMs mirrors were already invented.

Inventions can be useful recombinations or applications of other inventions. They don't need to be wholly unique unto themselves. Indeed, the vast majority of them are not wholly unique.

I think we can simplify the answer to this question for most audience and say "the air is blue".

If they say, the air appears to be clear when I stare at something other than sky, the answer is you need more of air to be able to see its blue-ness, in much the same way that a small amount of murky water in your palm appears clear, but a lot of it does not.

If they ask, why don't I see that blue-ness at dawn or dusk, the answer is that the light source is at a different angle. The color of most objects changes when the light source is at a flat angle. And sun lights hits at a flat angle at dawn and dusk.

If they ask, what exactly is the inside phenomenon to see the sky color to be blue, then explanations like this blog are relevant.

If they ask, what exactly is a color, the answer is that it is a fiction made up by our brain.

This level if geekiness is amazing. I hope more, a lot more, Americans can get into STEMS with this level of passion. It's sad that in the past few decades more and more people seemed to forget that STEM is a pillar of the modern civilization that we enjoy.

It's also not just why the setting or rising sun is red, but why it's yellow when high in the sky. The sun doesn't look yellow when viewed from outside the atmospheric veil.

For the sunset example then, a natural question (for me) is then why isn't the sky green in the transition from blue sky to red sunset sky?

Your intuition isn’t far off; there is an angle where the weight of green relative to the sum over wavelengths sees a local maximum. But it doesn’t dominate. In that transition zone, there is still an overlapping, transitioning abundance of redder and bluer wavelengths, adding with the green. Consequently, you see red, going into a red+green transition (== oranges, yellows), go into into a green+blue transition (== cyan), which already has few photons relative to the red and yellow zones, so it’s a dark/weak cyan, before it blends into the darker blue of the night sky.

Because the color of the sky is determined by a shifting mixture of wavelengths, not a single shifting wavelength.

Basically, the scattering process that "remove" blue from the spectrum also removes green, albeit to a lesser extent. There are some greenish and yellowish wavelengths in the sunset sky, but they're dominated by red, so the overall color appears red or orange.

In order for the sky to look noticeably green, there would have to be something that scattered reds and blues, without significantly absorbing green.

If you try to interpolate between sky-blue and orange using graphics software, the result depends on what "color space" you're using. If your software interpolates based on hue, you might see green (or purple) in the middle. But that's not physically realistic.

A realistic model is to interpolate each wavelength of the continuous spectrum separately. Interpolating in RGB color space is a crude approximation to this. And if you try the experiment, you'll see that the midpoint between sky-blue and orange is a kind of muddy brown, not green.

You won't get a green sky, but at least there is a meteorological optical phenomenon called the green flash around sunset. To see it, I think, you have to know what you're looking for - and you need good conditions.

Blue + Green + Red = White

Green + Red = Yellow

Red = Red

That is the natural transition from overhead sun to sunset as each higher energy wavelength gets cut off more and more. When blue is mostly gone and green starts to fade we call it the Golden Hour.

It can be - https://en.wikipedia.org/wiki/Green_flash

That's refraction not scattering though.

I have a related but deeper question about sun and colors:

Sunlight in space is considered white. When it reaches earth surface, it's considered a warmer color. Why human eyes that never (during evolution) saw sunlight without the atmosphere, consider it true white, and not colder color?

I think at this point you need to consider how the human eye see color. It's not like each wavelength gets picked up and then communicated perfectly.

(I'm going to skip over some basic stuff, and use some generalities)

Each Cone in the eye responds to a range of frequencies. This means that things that unless it's on the extreme low, or high, end of the frequencies that the human eye can discern you are going to have two, or all three, Cone types responding. The strength of those responses is what your brain uses to interpret the color that you see.

The real problem is that out in space there is no attenuation of sunlight, it's bright. Super crazy bright. It basically overloads all of your Cones, and Rods, all at once, there is no way for your brain to find a signal of "oh there's more higher wavelengths here so interpret bluer than normal" because all of the signals got maxed out. If you max out all of the signals, you get white. It doesn't matter that in absolute terms there's more blue, the lower and mid frequencies are also maxed out.

Is it considered a warmer color on the surface?

Mid-day sun in a clear sky is very white, in the 5k-6k color temperature range. It's hard to get a sense of how white it is because of how bright it is. In fact, the color temperature on the surface can be even higher than in outer-space!

Compare this to a "warm" light bulb, which is around 2.5K. Sunrise/sunset is also around that range.

Perhaps the "warm color" sun mindset comes from the only times that people can look directly at it. That is to say, around sunrise or sunset.

Implementing an atmospheric shader in three.js is a fun way get an understanding of the interaction of the different scattering components, light, and observer’s position. Plus you get a pretty cool effect to play around with after you’re done.

The explanation that made it click for me a while ago was by someone who implemented a shader https://www.alanzucconi.com/2017/10/10/atmospheric-scatterin... — the explanations that don't end up producing an image all seemed to skip over one detail or another.

I always loved this question when I played the 'Why' game with my kids: They ask why, and I'd ELI5. Then they'd ask why, and the process continued until I could excitedly say "We don't know for sure!! We think it might be XYZ, but we're still exploring that frontier."

So are there animals that see a violet sky?

This was great as it went farther than Rayleigh scattering. On this topic you have to watch this fantastic undergrad physics lecture demonstration: https://www.youtube.com/watch?v=sJG-rXBbmCc&t=1674s

So, does that mean, and bear with me here, that… air is blue?

In terms of "qualia", its the other way round probably? Like the way we see colours would have evolved (within the available environment of wavelengths and scatterings and the possibilities with rods and cones) so that the things we want to see are more likely to stand out. So we see the sky as blue because leaves are green and berries are red.

https://www.youtube.com/watch?v=yV-KiTAAcrY

Great article! I have to admit I had also heard of "Rayleigh scattering", but didn't really know more than that, until today.

Actually, I liked it so much that I went to the homepage of the blog, only to find out that this is the only article. Oh well... I hope there will be more to come!

There will be! Requests welcome!

(I will almost certainly do one on quantum mechanics, but that's such a big explanation that I want to do some simpler ones first)

This is a really great piece, the bit at the end showing why IR works in smokey environments and guessing the planet's composition based on color was really good.

Okay, why does visible light have that range of frequencies?

because they are the frequencies that pass though water most readily, and we are made of mostly water

that's interesting. I thought it was because our sun's spectrum has the most energy in visible light band - therefore we evolved to see the light which can give us the highest SNR.

Can you be more specific?

Anyone else immediately think of this commercial?

https://www.youtube.com/watch?v=PbKsC4GCT5k

*Since blue is the shortest wave length...*

Back in my youth, after the Internet became common but before Wikipedia, I tried to discover the answer to this and came away disappointed again and again. Every article I could find simply stated "because light scattering", and barely much more.

How does scattering work? Why does light scatter? _What does scattering even mean in the context of light?_

Yes! This is exactly why I wrote this article :)

Any other questions give you the same disappointment?

How can light "bounce off" something if it doesn't have mass?

Funniest memory re: Rayleigh scattering: in anime show Aldnoah Zero, the uber-genius protagonist mansplains about it to a high profile girl, basically completely out of blue. An impostor of the girl later appears on an in-universe pirate broadcast, making an agitating environmentalism talking point using a technically incorrect explanation of the phenomenon that isn't consistent with the fact. The ever-right protagonist immediately notices it, having enlightened the girl previously on that exact topic, and it leads to actions.

Like, dude, as if anyone would care about such a highly technical point, like eg some React framework quirk or race condition mitigation for specific generation of Intel procesdor or a semi-well known edge cases with btrfs inode behavior, even if I had been on that exact camp.

Brilliant explanation and beautifully presented. I wish I had a technical writer who could write up our business case this well!

I'm the writer of the article, and happy to chat. Email is my username at gmail.

Your blog layout, particularly on desktop, is brilliant.

My day job is UI design, so I especially appreciate this

(Is there something in particular you're referring to? I feel like sticky nav and sidenotes aren't particularly unusual?)

The same reason it's polarized.

Here is a wonderful lecture with real-world demonstrations of the effect:

https://www.youtube.com/watch?v=4a0FbQdH3dY

https://en.wikipedia.org/wiki/Rayleigh_scattering

I do have a question though.

The article says:

> blue and violet have the closest frequencies to a “resonant frequency” of nitrogen and oxygen molecules’s electron clouds

I thought it was more to do with the photon frequency matching the physical size of the air molecules? Or is that the same as its resonant frequency?

Air molecules are much smaller than the wavelength of visible light, by several orders of magnitude. This is why you can't resolve individual molecules in an optical microscope, and why photolithography with visible light doesn't go down to molecular feature sizes.

https://www.youtube.com/watch?v=4a0FbQdH3dY&t=2038

Direct link to timestamp 33:56

Fs is the frequency at which whatever your measuring is most efficient at vibrating

So it’s a combination of the composition of the thing and the environmental coupling with other vibrating things

Size and material composition are the primary factors

So for this case, the photon spectrum interact with nitrogen-oxygen mixture most efficiently at the frequency that reflects blue

I mostly studied sound frequency mixing with static objects (matching or cancelling the fs of room/space with the fs of a driver) but the principles of resonance hold across media

If you think about it "because air is blue when you look at it from the side" is about all the explanation we'd require if the sky was some normal object like an apple. Nobody asks "why is wood brown?" as if it's some deep question, but "why is the sky blue?" is somehow given greater gravitas, as if the reason is more mystical. I guess because the sky is so big and uniform?

It could also be this sorta thought:

"There's air in my room, it appears transparent. The sky is made of air, it appears blue. Why the difference"

didn’t cv raman prove just that via his raman-effect for which he got the noble prize ?

Brilliant, thank you

The sky isn’t blue. It’s transparent. That’s why you can see stars that aren’t blue at night. When struck by sunlight at the right angles it appears blue, but saying it is blue is like saying the ocean is green when a bucket of it clearly isn’t.

Going to be that guy, even though I think this is a really nice work overall...

But the winking and "cool guy" emojis are so grating. In general, technical explanations that apologize for themselves with constant reassurances like "don't worry" and "it's actually simple" undermine their own aim.

Your job -- if you're making content for people with double digit ages -- is to make the explanation as clear as you can, not to patronize and emotionally hand-hold the reader.

No, your job is to help your reader get to the end of the text. That means writing in a way that most of your audience finds compelling, readable, and not intimidating.

Not all readers are the same, so you will fail at your job for some readers.

But few readers are emotionless automatons that need nothing but dry technical content, unless it’s a topic they are very motivated to understand.

> That means writing in a way that most of your audience finds compelling, readable, and not intimidating.

I would agree with that. And I think emojis and unnecessary reassurances subvert that goal. It's fluff, it's more to read, and if the writing isn't already clear, they don't fix the problem.

> But few readers are emotionless automatons that need nothing but dry technical content

Nothing in my post argues for dry technical content.

Bartosz Ciechanowski's superb work, which may have inspired the author, gets the balance just right without any hand-holding asides:

https://ciechanow.ski/mechanical-watch/

Some of the demonstrations are not working correctly, at least on my machine (Windows + MS Edge). Any demo with a "reference image" is not correctly updating the reference.

Nitrogen.

Very well explained. I love the in-depthness of the article.

Not discussed but should be:

Prior to the great oxygenation event, Earth's sky was not blue; it was likely red-orange, carbon dioxide and methane being primary components.

Air is mostly nitrogen. Nitrogen gas is blue.

There.

Oxygen is blue actually. That only contributes to the sky blueness a little tho.

Go watch a video of nitrogen gas evaporating from liquid nitrogen ... tell me what colour you see?

Obligatory xkcd: "Sky Color" https://m.xkcd.com/1145/

Obligatory xkcd[2]: "Rayleigh Scattering" https://m.xkcd.com/1818/

Others?

The "Rayleigh Scattering" comic is really spot on.

Air is blue. The reason air is blue is blah blah blah physics, see the article we're all commenting on, but at the end of the day air is blue. We don't demand the same elaborate physics questions for why a ripe banana peel is yellow.

But a banana is yellow for a very different reason (and a much easier to explain reason) than why the sky is blue. And air isn’t blue, because it’s red at the end of the day?

Though is some cases it is a very interesting question, like why gold and copper the color they are instead of being boring and silvery like all the other metals?

Not really. If the explanation was "air is blue" then the naive expectation would be that sun would appear blue against blackish background, basically the image of sun is being filtered through the atmosphere; if sun is white and air is blue then white filtered through blue should be blue? But sun appears yellowish against blue background. So clearly something different is going on.

to piss me off, goddammit

It’s not. It’s raining here.

[flagged]

HN is in fact quite receptive to humorous comments. The bar on what's considered humorous is just higher than on Reddit. It's about the signal/noise ratio.

complaining in a self reply makes me downvote more

Let's be real. The sky is blue because God thought it was a pretty color, simple as. All this stuff about wavelengths and resonant frequencies and human color perception got retconned into the physics engine at some point in the past millennium, that's why all these epicycles are needed.

Our lord Zeus always thinks of everything

His noodly appendage touches all.

> thought it was a pretty color

So was blue intrinsically pretty and thus made into the sky, or considered pretty and thus imprinted in the minds of humans that way?