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I've heard it said that white light contains all the colors in it. Our sun produces white light and that the visible spectrum is just the range we evolved to see because our atmosphere is transparent to it.

Every planet with an atmosphere in orbit around a star will have it's own visible spectrum for the life evolving on it's surface, potentially causing those lifeforms to have their own concept of "white" which may be the color their star emits, or maybe the collection of all colors in their visible spectrum. In other words, they may call red (Or whatever color their star is) white and say it is the collection of all colors.

Some other planets may even have atmospheres where the visible spectrum is too dark, or doesn't reflect off of common materials in a way that is useful for eyesight. On such planets, vision may not even be a trait that evolves in the first place.

My question is: how special is our situation? How many outcomes are there to the life-atmosphere-starlight equation and how likely is each to occur?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch May 16 at 3:29
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    $\begingroup$ How many humans call the sun's light "white?" I've called it yellow all my life. I've never heard anyone refer to it as white. Note that "white" isn't simply all colors, it's also a word we use to refer to "too much light," meaning our rods/cones are overwhelmed. A light source could be specifically one wavelength, let's call it blue, and if the amplitude is high enough, all we'll see is white. $\endgroup$ – JBH May 20 at 6:47

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Obviously, you don't literally mean "will they use the phonemes we put into the English word 'white' to describe the color they see?"

Because not even terrestrial humans do that, apart from some of the minority who speak English.

So I'm guessing you mean "will an alien that can see color always come to consider the light that they get from their sun to be a 'neutral' color?"

It depends. Obviously, if their atmosphere filters a lot of the light, then their eyes will not be adapted to the light of the sun above the atmosphere, but tot he filtered light.

If there's a brighter or more constant light source than the sun, then their eyes will be adapted to that.


An aside about vision.

Most of us see colors in basically the same way: we have three receptors for reddish (low frequency, long wavelength), greenish, and blueish (high frequency, short wavelength) in our eyes. The color we see is caused by a mix of those colors.

So if we see a color between red and green, we see yellow.

If we see a color between yellow and blue, we see cyan.

And if we see a color between red and blue... well, that's the color inbetween, so we'd see green. But if we see TWO colors, red and blue, mixed, then we see magenta. Magenta has no wavelength. It's not a "real" color at all.

Not everyone sees color this way. There exist people with more than three types of receptor: tetrachromats. There exist colorblind people, with fewer. Unless they have no color receptors at all, they have their own "magenta"s, made of no real color.

There's a whole bunch of light from the sun in the UV and IR ranges, which we do not see, and which do not make up part of our definition of "white". Our vision goes from roughly 380-740nm. Some fish can see as far into the UV as 100nm, and snakes can see as far into the IR as 5μm, allowing pit vipers to detect vulnerable areas to strike.

So it is arguably false to say we call sunlight white. What we call "white" is a very small subset of the Sun's electromagnetic radiation.

And at sunrise and sunset, we might call the light from the sun "red" or "orange".

We are also more sensitive to some colors (red) than others (blue). So it takes less of them put together to make up what we'd call "pure white" where our color (but not brightness) receptors are flooded.


So would an alien know the colors they could percieve, as "white"? Well.... yeees.

As others have said, it's a slippery word.

Look through night vision goggles:

Night vision goggles: creative commons

That car is clearly "white". But it's clearly also "green".

So what we ourselves mean by "white" is complicated. We mean something more like "reflecting all the incident visible light in the current scene, but not coherently like a mirror".

It's likely that any alien capable of seeing color will be able to express a similar concept, even if their color perception is different to ours.

After all, colorblind people do, and so do tetrachromats.

And, if the wavelengths they consider "visible" are significantly different to ours, something they describe as white we might call black (some black t-shirts glow white in IR, for instance).

The wavelengths they consider visible are likely to be some subset of the wavelengths available to them, so they likely would consider their own sun "white" unless some other light source was a more major part of their evolution (perhaps they live at the bottom of an ocean and only ever see phosphorescent bacteria).


So would they call our sun "white"?

Eh. If they're from a red dwarf, probably not. They would likely consider it "yellowy", if they had the concept of "yellow". "Averaging a little more in the short-wavelength part of the spectrum compared to what we'd call white", they'd say.


Edit: I should also add that there are some basic constraints on the range of radiation that can be "seen".

Seeing much farther into the far infra-red than animals see requires cooler eyeballs, for example, or the warmth you are seeing is swamped by the warmth of your eyeball.

UV light is very energetic, so interacts readily with the eyes that are viewing it (UV is why our lenses harden, etc), as well as damaging the receptors.

Seeing into the UV range requires smaller eyes, as the material of the eye absorbs UV. We are in fact capable of viewing UV with our blue receptors, but our eyes (particularly the lenses) are efficient blockers of UV, blocking all but 5% of it, possibly to prevent damage to our receptors. Removing the lens (an old cure for cataracts, undergone by Monet in one eye).

Past a certain point, radiation just can't be focused by structures anything like our eyes, but we know that the sun broadcasts in other wavelengths, too, and we have instruments to measure and record them. How useful they would be for "seeing" is debatable, though - on the other side of IR is radio, and long-wave radio probably wouldn't be much use on any scale that didn't require you to see around the planet. On the other side of UV is high-energy radiation, which again might be something that you would want to shield against rather than perceive.

So one could imagine aliens having other materials or structures for eyes which would allow a far wider set of wavelengths to be seen: we are just victims of our heritage, which gave us eyes based around the "blob of water" architecture.

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    $\begingroup$ This answer understands what I'm asking vastly more than any other so far. $\endgroup$ – Muuski May 13 at 22:36
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    $\begingroup$ The TL;DR is: "white" is either an extremely bright source which overloads all color-discriminant sensors or is so dim that only non-discriminant sensors respond at all. (Not to take anything away from an excellent discussion of color and neurobiology) $\endgroup$ – Carl Witthoft May 14 at 13:28
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    $\begingroup$ Don't forget recalibration, our brain changes what we see as white constantly, look at the world through tinted glasses long enough and your brain adjusts "white" to correct for the tint. $\endgroup$ – John May 14 at 13:41
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    $\begingroup$ Oooh, a cliffhanger! Removing the lens (an old cure for cataracts, undergone by Monet in one eye). > What happens if a lens is removed? :) Awesome answer otherwise! $\endgroup$ – Tinkeringbell May 15 at 8:41
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    $\begingroup$ @Tinkeringbell - If you remove the lens, then your ability to focus is shot (OK, you probably guessed that!), but far more UV makes it to the retina, so you see UV. This apparently looks like bright blue (since it mainly triggers the blue cones), or white if it swamps all the photoreceptors (since all are somewhat sensitive to UV). $\endgroup$ – Dewi Morgan May 15 at 15:53
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They won't call it white, but they will likely see white, -ish.

First, 'white' is just a word it has no intrinsic meaning.

Second Our colors or lack thereof is an artifact of how we see light, which wavelength sensitivities we have. And more importantly how our brains interpret available light. Our star is a white star but through the atmosphere it has a red shifted hue due to the atmosphere. So time of day, angle and elevation all effect what color the sun looks like.

In general Will light from their sun look white, probably.

Not for any intrinsic reason but because of evolution. Eyes tend to evolve for the spectrum of light available, if their star emits no blue their is no reason to evolve blue receptors. If they have color vision it will likely see naked sunlight on their planets surface as the default (white) or close to it, since color is based on differential absorption of the light from that source. Whatever "color" of light that reaches the surface that is what they will see as white. If you have color vision seeing any combination of colors other than what your star emits as white AKA the baseline, defeats the function of color vision. The only way they would not is if they orbit a dual star system (or have a non transparent atmosphere), in which case they will see the average light that reaches the surface as white, but each star will have a tint. So even if we would not see their daylight white they will. Likewise our sun will not look white unless their star is very similar in color, at least at first.

In detail, will they see white, sort of

White is also highly situation and subjective, two random humans may not see the exact same spectral profile as white. Note that our color vision recalibrates itself constantly. Look through red tinted lenses for days and your brain adjusts so everything looks white again, take them off and everything looks blue even if you are looking a white surface. It's not perfect but it does means over time we would see ANY star as white if we spent enough time below it. The brain has no objective way to decide what color something is, it has to adjust based on context. Not just our eyes but every animals with eyes we have studied works this ways including bees. This works with white as well our eyes reconstruct shade and color. I will use a few optical illusions to illustrate.

All these chess pieces are the same exact color and shade. Source and relevant paper. But our brain interprets based on context.

enter image description here

Or stare at this image than look at the below black and white one and suddenly see colors that are not there. because your brain has already started recalibrating itself.

enter image description here

enter image description here

Human and mammals in general have vision with all kinds of problems anyway, we see a much narrower band of the spectrum than say a bird, to a bird the sky looks violet not blue. You also have the problem of time of day, the lower the sun is on the horizon the more red shifted the light reaching your eye is. So the color of their atmosphere will also matter. If their atmosphere shifts the color sufficiently and they ever get to see their sun without it it may have a strong color.

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  • $\begingroup$ Compare with Dewi Morgan's answer: you will see you've made some errors. $\endgroup$ – Carl Witthoft May 14 at 13:29
  • $\begingroup$ @CarlWitthoft Can you give an example? $\endgroup$ – John May 15 at 12:22
  • $\begingroup$ John, they are not going to view daytime starlight as "white" based on spectral output any more than we do. Our sun is "white" because of extreme retinal overload. Attenuate sufficiently with a neutral filter and you will see a colored sun. $\endgroup$ – Carl Witthoft May 15 at 12:34
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    $\begingroup$ @CarlWitthoft No it is actually white, the sun is a white star, The sun appears slightly yellow on earth at sea level due to differential light absorption of the atmosphere. Uncolored images of the sun like this one are white. upload.wikimedia.org/wikipedia/commons/thumb/5/54/Sun_white.jpg/… $\endgroup$ – John May 15 at 12:51
  • $\begingroup$ again, "white" is a subjective term, but in any case it makes no sense to discuss exoatmospheric color since our proposed inhabitants of Planet X are at ground level. $\endgroup$ – Carl Witthoft May 15 at 14:26
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Let's look at the likely milestones for a species to understand light:

  1. The species evolves a sensory organ which detects the presence or absence of light.
  2. (a) The light-sensing organ evolves to measure variable quantities of light. An all-or-nothing response in no longer sufficient; the level of light provides an advantage.

    (b) Perception of light evolves to become comparative to ambient light, rather than absolute levels. If the species' planet has a day-night cycle, then there will be periods of very high ambient light and periods of low ambient light. The light-sensing organ needs to evolve to accept this wide dynamic range of light. (In humans, this is handled by the iris and the retinal response.) In addition, one would want the same reaction to a situation, whether it was day or night. Thus, perception should be relative to the level of the ambient light, instead of to absolute levels of light. This effect is seen in humans: an object's colors look the same to us whether in bright or moderate light.

  3. (Optional) The light-sensing organ evolves to process images. The species can now distinguish light coming from different directions, and create a mental model of their surroundings which is advantageous to the organism. In humans, this is due to our lenses, retinas, and visual cortex.

  4. The species evolves color vision. The vision apparatus has multiple sensors, some with a different chemical composition which react differently to various wavelengths of light. This provides more information which may be advantageous to the organism. In most humans, this is due to the red, green, and blue cones in our eyes.

  5. The species uses language to give colors names. This could be spoken words, written symbols, etc.

  6. The species develops tools to measure light. If the species has evolved 2(b), then it needs an artificial way to meaure absolute levels of light. It may also need a way to separate light into its various wavelengths.

  7. The species discovers the physics of light. They learn about the electromagnetic spectrum, radiation and absorption, blackbody radiation, etc. They are able to apply these concepts to their own experiences. They study the light from other stars and predict how species on other planets may perceive color.

I would argue that these milestones would happen mostly in the order I have given above. Milestones 2/3/4 can be shuffled in any order, although 3 does help 4 to evolve. Some milestones are optional.

In particular, notice how giving colors names (5) requires color vision (4), which in turn requires detecting the presence or absence of light (1). Thus, it is reasonable to assume that a species who names colors will have an "everything color" and a "nothing color", as the sensation of lightness and darkness would be a primitive concept.

It's possible for a species to never evolve color vision (4), yet still develop tools to measure light (6), discover the physics of light (7), and then later go back and give the colors names (5). Even in that case, one would expect that they would still have a concept for an "everything color" and a "nothing color". An extreme case would be a species that invents the tools (6) and discovers the physics (7) of light, yet never evolves vision. However, even such a species would likely have the concepts of nothing and everything.

It should also be noted that the names we humans give colors is rather qualitative and comparative. For example, many colors can be described by the word "blue". This is because of 2(a): our eyes really compare what we see to ambient light, rather than to arbitrary absolute light levels. We are driven to make vague names. So our perception of what is "white" depends on the ambient light, as could also be expected of an alien.

What we call "white" is probably going to be a different combination of wavelengths than what an alien calls their "everything color". This is because what we label white (5) is influenced by the particular cone cells that we evolved (4). A species that evolves a different set of primary colors will have a different set of wavelengths as their "white".

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  • $\begingroup$ I'd argue about having a concept of colours if the vision is colourless. We don't see colours beyond the visible light and while those wavelengths still are differentiated, we did not produce any colours because we simply don't perceive them. Of course we create ranges (UV-A, UV-B, UV-C for example) but given that the visible light range is far narrower than any other range, we should have gazillions of colour names to cover all ranges. We don't, instead we name the ranges based on physical properties only. $\endgroup$ – Ister May 14 at 16:34
  • $\begingroup$ @Ister: Are you commenting about the paragraph that starts "It's possible for a species to never evolve color vision"? Notice I said that it's possible, not that it's likely. Nor did I mean to imply that they would choose the colors we use. But the fact that we have names for unseen colors (infrared, ultraviolet) suggests that it is possible, for a civilization that understands the physics of light. $\endgroup$ – DrSheldon May 14 at 16:57
  • $\begingroup$ Yes, to that one. I fully agree that such colourless-vision creatures can understand the physics of light and that they will name ranges based on some properties of light. What I argue is the last point - that they will name some reasonably narrow ranges so that we can name that still "colours". Yes, we do have name for ranges, but it's like saying "visible light" is a colour. We also know that there are other "colours" perceived by some animals (e.g. bees) as we were able to see that flower petals have UV-colours that we cannot see but that apparently attract those bees.... $\endgroup$ – Ister May 14 at 17:01
  • $\begingroup$ contd. Yet it is nowhere within our range of vision (and by no means cover a large part of UV-A spectrum, and finally can be even different colours to bees) and even though we do have a concept of colours, we do not name them. Actually colours are very tightly related to our vision (and to a degree to a rainbow effect). $\endgroup$ – Ister May 14 at 17:03
  • $\begingroup$ Agreed that such creatures would name ranges that are significant to them, that such ranges would be few and fairly broad, and that it is unlikely such ranges would match our choices. If my use of the word "color" in that case bothers you, I will try to use other terms. Back to the question, a scientifically advanced society would eventually discover that there is a particular range of wavelengths in starlight (with variance between stars), and would probably have a name for such range. $\endgroup$ – DrSheldon May 14 at 17:18
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I follow your logic. Generally. I would agree. The main problem is the hand waving we need to do to talk about perception. Some would argue about whether or not we all see the same "red". I don't think those arguments are interesting or useful as they're not falsifiable.

Now here's some hair splitting idea: Consider the Mantis Shrimp and its 12 types of photo receptors and solar spectrum gaps. I could imagine an organism evolving a system if vision detailed enough to see the variations in the spectra between stars. Then it would be able to tell the different between true white-full-spectrum light and the light from the sun.

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A mathematicians answer!

"White" typically is taken to mean radiation whose intensity spreads uniformly across the spectrum in a given frequency band. (For reasons of energy it is not possible to radiate with equal intensities across all frequencies.) If the radiation of a star does not follow that distribution, it would not be called "white" or the local equivalent of it. Put differently, radiation of the "colour" white appears "white" however narrow we make the visible range. On the other hand, some radiation may appear white to a limited range of frequencies but may be seen as non-white once the visible band is expanded.

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    $\begingroup$ No, white is about colour perception. The frequencies aren't uniform, and neither are photon energies. However, the response of the photoreceptors also isn't uniform or linear. White just means that all the photoreceptors are stimulated the same (taking colour adaptation into account too, of course - white paper still looks white under incandescent light bulbs, even though they emit very different light from the Sun's; and let's not even get into LED lights). $\endgroup$ – Luaan May 14 at 8:28
  • $\begingroup$ @Luaan "White" in the scientific means exactly that. While the question is about perception the science plays a role in determining what those perceptions may be like in different species. $\endgroup$ – ANone May 17 at 16:18
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There is a very good answer by Dewi Morgan that suggests a good candidate for 'whiteness' that might be very universal: "even emission across [their] visible spectra". I am inclined to agree this may well be a recurring them but what about exceptions? Is it universal?.

There I think not:

We see in a very narrow band in the grand scheme of things and we detect light fairly evenly across that spectrum. If the spectrum was wider or we had more fidelity within the range we already see that concept might loose its meaning. In fact, we don't really need to hypothesis here. We already have quite a good example: hearing.

While it lacks the directional fidelity of sight, we hear in a much greater spectral range than we see. We don't have a name for a noise that's even across all the range of tones we hear. It would be difficult to imagine one too. That phenomenon is really very rare and subjective.

Its also worth noting that only one species talks about colour here on earth and the range of light doesn't change much between members the population or for any given member of the population. That might also be a factor in how a society views colour.

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  • $\begingroup$ Welcome to the Worldbuilding Stack Exchange! Might I suggest you refer to the answer you touch upon with the username just to clear that up? $\endgroup$ – A Lambent Eye May 15 at 15:23
  • $\begingroup$ @ALambentEye edited. $\endgroup$ – ANone May 15 at 15:41
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    $\begingroup$ Imagine a spiecies that has lots of color receptors. Then it is possible that the sunlight does not fill the entire spectrum of their vision so clearly that isnt the brightest possible light. Same hapens if the spoeciew is actually equipped with a spectrometer. In anycase language of color becomes quite tedious in these cases..humans have relatively "easy" visual systeem. Even though it is hideusly complicated, with 3 sensors we can fool ourselves to think of color as a 2D circle of kinds. If we had more sensors this becomes impossible, and really really hard to understand. $\endgroup$ – joojaa May 15 at 20:27
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    $\begingroup$ @DewiMorgan Thanks. I thought about the irony of white-noise being such a close analogy, but it isn't in the sense I interpreted the question. White noise is not a single noise, a type of noise characterised by its range of frequencies. We don't have a special term (to my knowledge) of one that is specifically range of tones we-as-humans hear. Maybe if you generalised in this way there would be some notion of "smooth" colours, that have a lot of frequency components and "sharp colours" with specific peaks. $\endgroup$ – ANone May 17 at 15:55
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    $\begingroup$ @DewiMorgan This is getting a bit log, maybe it should be added to an answer but: I think whats missing is: we only have one perception of white. Hence it is a useful concept, hence it's universality. However we hear white noise all the time, but we never think of it as white, we think of it as low pitch or high pitch depending on the range. White noise is a scientific term, and it is named after white-light. If we didn't have sight and didnt have a name for white that way, I doubt the general population would be familiar with whatever term we came to to talk about spread-spectum-signals. $\endgroup$ – ANone May 17 at 16:29
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  1. It is empirically untrue that the light coming from the Sun is always called /hwaɪt/.

    • For example, in Romanian we say /alb/, and in Russian they say /ˈbʲelɨj/.

    • Even in English, light coming from the Sun is not always called /hwait/. For example, light coming from the Sun at dawn or at dusk is called /red/.

  2. In their 1969 book Basic Color Terms: Their Universality and Evolution, Brent Berlin (an anthropologist) and Paul Kay (a linguist) put forth a widely respected conjecture that color naming evolves in stages:

    • Stage I: All languages have a term for dark-cool and one for light-warm. In later stages, the term for dark-cool may be repurposed for "black", and the term for "light-warm" may be repurposed for "white", but they may be repurposed for other purposes and new terms created for "black" and "white".
    • Stage II: Most languages have at least three color terms, the third term being for "red".
    • Stage III: If a fourth color name is present it is "green" or "yellow".
    • Stage IV: When five color terms are present, the language has names for both "green" and "yellow". (Actual ancient) Latin is somewhere between this stage and the next -- there is no way to say generic "blue" in Latin.)
    • Stage V: When a sixth basic color term is present, it names "blue". Most modern languages are at this stage, although some languages, e.g., Russian, still do not have a word for generic "blue". (Russian has goluboy (blue like the sky) and siniy (blue like the sea), but no "blue in general".)
    • Stage VI: Brown.
    • Stage VII: Purple, pink, orange, or gray.
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    $\begingroup$ All that is fine from the Earth perspective and shows human languages evolution. Yet if bees had languages that could be seriously different, since they see can see colours we don't see so the naming conventions will be different. Also it may depend which colours are most critical e.g. in relation to nutrition. If for example most edible flowers are white or yellow, it may be likely that those colours will be named earlier than others (there might be a third term which is roughly meaning "neither white nor yellow").. $\endgroup$ – Ister May 15 at 6:27
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TL;DR

Most probably the species will have a name for the light consisting of all wavelengths they can perceive, all of them fully saturated. This, respective to our standards is something we call white. For them the Sun will be just as "white" as it is for us and their star will be for us as white as the Sun is.

Atmosphere can significantly impact colours. We can perceive the colour of star at their ground level as violet while they can perceive ours as "orange" (in their terms). It may even happen that stars will be totally invisible for them on our planet and for us on theirs even though both we can see them on Earth and they will see it on their planet.

It is also possible, that they will use that very same name for the electromagnetic pulse saturated in full spectrum of electromagnetic wavelength of some range just like we do, but they may as well have some totally different name for it.

It's all just not so simple.


Full answer

How we see

To understand why we see colours we need to understand how is it possible that we actually "see". There are 3 layers responsible for that:

  1. Physics (what reaches our eyes)
  2. Physiology (how the light is converted into neural signals)
  3. Psychology (how our brain interprets the signals that reach it)

Let's go one by one.

1. Physics

What we call light is actually an electromagnetic pulse in a certain range of wavelengths that (depending on the accuracy of definition) either humans are in general capable of perceiving through their sense of vision (a more common term) or physicians have defined as light (a scientific definition). The term light as we usually use it is the first one and all of this "light" is included in the second one (with plenty of additions).

Whenever we look at an object, what we actually see depends on the electromagnetic pulse that comes from the direction of that object (and can consist of pulse produced by the object, reflected by the object and that went through the object). I will get back to that later.

The Sun and pretty much all stars emit electromagnetic pulses that consists of the wide spectrum of wavelengths, from IR (or even radio) to X-Rays and Gamma. There are some (very small) bits missing and some of them are stronger than others but all in all you can "see" Sun if you look in "visible" light, UV, IR and other frequencies.

These are photos of the Sun taken in various frequencies (taken from this page). Sun in various parts of the spectrum

For comparison, this are the spectres of other stars (as observed on the Earth - remember about the Doppler effect that shifts the frequencies, taken from space.fm) Spectres of various stars

As you can see, most wavelengths are covered which essentially means - those stars are to our eyes white.

2. Physiology

Essentially humans see through their eyes. To be more specific - those are the organs that capture light, focus it and change it into the electric signals that can be transmitted through a nerve system to a brain. Let me focus on that last role of eyes. Retina of a human eye is covered with two main types of light receptors - cones and rodes. Cones are further split into three subtypes. Each of those types responds to a different range of an electromagnetic wavelength (and within the range where it reacts the strength of reaction is different).

Here is a representation how human rods and cones are sensitive to specific wavelengths (from this page): enter image description here

Let me add that rods and cones cannot work at the same time. There is a simple mechanism that "turns on and off" rods when respectively there is not enough or sufficiently enough light for cones to operate. Let me just focus on cones as they are responsible for our colourful vision. When a specific area of an eye is exposed to some visible light (in common understanding - visible to us), the cones of that eye are "activated" if this light consists electromagnetic within specific wavelength. As a result it send impulses to brain. A strength of such depends on strength of the light of this wavelength and how close the wavelength is to the peak of the sensitivity are wavelength. If we look at something green, yellow and orange, in each case both M cone and L cone will be activated, but with each of the colours the strength of M cone reaction will lessen while L increase. Additionally at green colour also S cone will be slightly activated. All that brains receives are signals from those 3 cones.

Now the brain cannot verify what exactly triggered cones. It only knows that those three signals combined tell about a colour of the thing on which we're looking. If it was just a very narrow "beam" (in terms of consisted wavelengths) or the contrary - a broad one, a brain has no way to determine. As a result the same set of "results" from all three cones produce the same "colour" for the brain.

This phenomenon is actually used to artificially produce colourful images, especially on screens (TVs, monitors). Rather than trying to emit all colours, all that the screen has to produce is the three colours to which our cones respond and changing the intensity of those colours it can trigger (almost) any composition of signals from the cones, effectively producing for a human brain (almost) any colour visible to a human.

Unfortunately that's still not that simple as here we step into

3. Psychology

To make it short (errmm...), the brain uses its previous experience to get results while interpreting the environment. As someone has described it, a brain is not really experiencing the environment. It's like a general in a bunker. It sits in a dark, humid closed small space and gets information from the outside world through "liaison soldiers" - neurons. Then based on those messages it builds its own interpretation of the world outside.

We will focus on the part responsible for vision. In general, each of wavelengths (a single one) produces a signal on cones specific for itself. We "learn" those reactions when observing a rainbow - either natural (created on droplets of water) or an artificial (created through a prism). Now our brain can compare reaction of mixed colours to those of a single wavelength and distinguish similar colours. It can also notice that there are colours others than those in a rainbow so it has to present us those in a different way. This way we get colours like white, black, purple, brown, grey and others that require mixing various wavelengths.

But that is not all. Our brain also "learns" how different things look like in various lighting. If we have a white sheet of paper and we look at it in various coloured lights it will be changing its colours accordingly into the colour of the light in which it is now. But our brain knows that it is white so despite those changed lighting conditions it will still suggest its white. Now if we do not have other prior experience and to someone who already has seen white paper in a red light we give in a room with red light a red sheet of paper, the brain of such person will assume that it is a known paper and since most sheets of paper are white it will suggest that this specific sheet of paper is also white. Only changing the lighting will make brain change its mind (eventually someone with authority stating that the sheet of paper is now red). The problem is human brain fills in the missing information and it actually captures only a very small part of the surrounding. Similarly when we look at tree needles after the dusk we kind of see them greenish even though what we can see is just grey. Moreover it may turn out in a broad light that they are brown rather than green (this is my personal experience actually)

So in general our brain will adapt the picture according to the lighting. If it cannot determine the lighting for sure it will make a best guess. As a result you have this kind of picture, with question - what is the colour of this dress - white and golden in a deep shade or blue and black in a very strong light? The Dress

If you don't know that yet, make your guess and then check e.g. on The New York Times and read more about it on Wikipedia (though probably everyone know by now).


How other species can see

Having said all that...

When saying "white" we can express one of three things:

  • something that we can observe in a way that the range of wavelengths coming from a direction of that object (be it emitted, reflected or passed through) triggers a more or less uniform response of all cone types (that's the most common and natural understanding) - physiology
  • something that we know from our experience that if put in the typical white light (in reality - Sun light passed through our atmosphere) is known to produce the wavelengths as described above even if at the moment we are observing it in a different light so the actual cones response is different than expected for white objects - psychology
  • something that produces wavelengths in (at least and almost) full range of what we call visible light - physics.

If we consider a different species that developed in different lighting conditions we may safely assume that it will have a differently developed sense of vision (if any). If it has a sense of vision it may or may not be colourful. It may also perceive light in a totally different way, e.g. the way we perceive warmth on our skin or smell scents. We may safely assume that if the species cannot perceive colours at all, it will not have names for the colours, however it will still have a name for existence of light or lack of it (it might be gradual). Even more interestingly if the species does not have vision and can only perceive electromagnetic wave through instruments, the naming will be very technical, focusing on ranges of the spectrum, most probably based on some specific properties, in no way related to colours as we know them. So depending on those circumstances we may consider those names that a species might give as matching to some of our definitions of white1 for the species. In other words it may have a name for a light covering such ranges of electromagnetic wavelengths that all light receptors of the species are triggered at their maximum. It may have a name for an object emitting electromagnetic wave in some huge pre-defined range. And it can have a name for objects that are capable of reflecting/passing through the first light from this list if put in the right lighting. Those names may be the same or may be different. And those names may not mean the same range of wavelengths as what we call white.

Let's have a look at some on-earth examples just to give a hint. Then I will give even more extreme options.

Note - I know neither the psychology nor the physiology of other Earth species not to mention extraterrestrial ones (just some fun facts about them) ;-).

Bees

Bees are capable of seeing shifted light spectrum compared to us. They don't see the most red part of the spectrum but instead they do see at least some UV (graphics from Bee Culture:

bee vision spectrum

As a result they can see flower patterns that are invisible to us. Note that those pictures have UV filter applied and has the colours adapted to show the additional patterns but of course the "colours" seen by bees are... well, we don't really know how do they see it (BTW we don't even know if brains of different humans present colours in the same way).

Bee vision

While the white as from the Sun will be "white" for both us and bees, if we consider the minimum white range, it will be missing the red part so it will be somewhere between green and blue.

Snakes

Snake use infrared for vision. As a result they actually "see" temperature of other bodies. On the other hand the colours we see are virtually invisible to them.

Snake vision spectrum

Again a white light will be white for both us and snakes. But both our "minimal white" will be black to snakes and their "minimal white" will be black to us. To be more specific, our vision spectrums slightly overlap but for the sake of example I'll ignore that part as rather irrelevant part.

This is how supposedly snakes see:

Snake vision

Moles

Moles have very poor, monochromatic vision. I couldn't find the spectrum, but in general they see light. A speaking mole will probably have a single word that will be a combination of what we call "white" and "bright". It will not understand what you're talking about when you refer to colours unless you use physical properties. For such mole any colour we see can (if intense enough) be perceived as "white/bright".

Mantis shrimp

Well, this case is closest to your expectation. Mantis shrimp live underwater, where the light is limited to mostly blue-green as the rest is quite well absorbed by water. Still those shrimps have 12 photoreceptors which allow their brain to do far less visual processing. They probably also see a broader range of colours.

While for humans "white" requires as little as 3 wavelengths, for shrimps it will require an outstanding 12 different wavelengths. So something that we can already call white will be some colour for a shrimp. On the other hand shrimp's white is white for us as well.

Extreme extraterrestrial forms

I would say sky is the limit. Let me just give you two extreme examples.

First will be a species which developed various photosensors that are combined with other types of senses. These work a bit more like a shrimp - i.e. they are specialised not to see as we understand it but rather to quickly recognise various opportunities and dangers. Combination of specific colours and scents produces a signal "food" to a brain of this species while combination of specific colours and heat produces a signal "danger" (fire). For such organism there will be no such terms as colours. If it does not perceive light as a separate stimuli (due to evolution from mantis-like perception into even more specialised, with recognition of signal meaning moved to receptors and brain able to quickly react to it, but no separate vision and only later development of cognition) it will not be able to communicate in terms of colours or light as we know it. Assuming it will grasp eventually a concept of electromagnetic wave it will have some sort of scientific, technical word describing what we may resemble as white (with either broader or narrower range), however I don't know if there exist a single word for a beam covering all X-Ray wavelengths in any human language so it might not be a word, but rather a description of the observed/potentially possible physical phenomenon.

On the other hand they will have a very complex structure of terms for various things related to perception completely impossible to understand for us. It will be like saying "I perceive a light morning breeze full of tasty aromas incoming" in just two or three words.

Second example will have a human vision similar to us in terms of way the perception is done but due to various conditions developed in radio frequencies. Those aliens will be reading our radio communication by seeing it as colourful patterns changing quickly. But what we consider white light will be completely impossible to "see" for them. On the other hand if they present us their "white light" it will be invisible to us but we will hear a more or less uniform noise in our radio receivers. Yet they will probably have wording developed in somewhat similar manner to ours, just meaning completely different wavelengths of electromagnetic spectrum.


1 It's worth adding that the name black can have meaning so far from our natural understanding of it as "lack of white/(almost) no (visible) light coming from the direction" that the Sun in astronomy is considered almost a black body. See this question for more details.


Sorry, it came as a very long answer but I hope it gives a bit better understanding of the problem.

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  • $\begingroup$ This is an incredibly good answer! I haven't had the time to read every response enough to accept an answer but this one is a sure candidate. $\endgroup$ – Muuski May 16 at 23:05
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Other answers having established that "white" is contextual, so let me add some more potential context.

As an astronomer, the first thing that occurs to me is that the wavelength that stars appear brightest is dependent on their mass. The biggest stars burn faster (1-10M years) and bluer (peak in UV), and the smallest stars burn slower (100B years) and redder (well into IR). If your life got used to one of these extremes and then suddenly/occasionally saw the rest of the universe, it would look shifted the other way. For more, see Hertzsprung-Russell diagram.

Another idea I don't see mentioned above is if there were a filter that was pervasive but possible to change depth within it. Examples would be an underwater species (water absorbs red more strongly than blue, hence using a red filter to make SCUBA footage match your perception) or a fog/smog layer like Titan's methane haze. Swimming to the surface or climbing mountains or flying above the layer would result in a sudden change in perception of the night sky and the rest of the world above.

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I would say the answer is no (probably - as this is only hypothetical until other intelligent life is discovered).

Reasoning: even we as humans don't absolutely always see out sunlight as "white". During daytime, we may perhaps see it as "bluish" white (or "cool white"), early or late in the day it has a more pronounced "yellowish" tinge (or "warm white"). We are very aware of what an "ideal" white looks like (snow, some clouds, paper and other bleached substances ...) and can compare that to the sunlight we see.

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    $\begingroup$ The white of snow or clouds is also dependent on the colour of the incident light, though. It's not like snow emits perfectly white light - it just reflects light very well across a wide range of visible light frequencies. If aliens have colour sight in any way comparable to ours, they'll have the same meaning of white - it's essentially the colour you calibrate all the rest to (within certain boundaries). It's pretty obvious when you compare different light sources incident on something like white paper - after your eyes adapt, it's all the same white to us. $\endgroup$ – Luaan May 14 at 8:31

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