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Consider the sharingan, a power in the anime Naruto that strengthens with the hatred you have (yes, the Uchihas are like ninja Sith Lords XD). The sharingan is 'awakened' after the person experiences strong emotions, and becomes stronger if the person has more hatred. A stronger sharingan glows more.

My question would be : How to explain that realistically ? How to explain that more hatred means your eyes glow more ? (No need to explain the other powers of the sharingan, I'm just interested in how it makes your eyes glow more red.)

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Hatred is tied closely to stress. It is well-known that adrenaline is triggered by stress and changes bodily events, but it also affects genes. Over a period of time, stress will change our DNA. Usually, this is limited to things like cancer due to the DNA breaking down, but stress can affect gene expression. So this could happen either as new expression of a previously-unexpressed gene, or a change to an existing gene.

Onto the eyes, there are actually people with red eyes. Albinism causes a strong lack of melanin, or human pigment, in the eyes causing light to bounce off blood vessels, leading people to see the color of the blood vessels as iris color.

If you were to go this route, you could have the eyes naturally glow in dark light, since this very light eye color is highly reflective. You could also have your character possess a tapetum lucidum, a reflective layer of cells sitting behind the retina that bounces light back so the retina can take in more information. This is what causes glowing eyes of cats and dogs in camera photos. Humans don't currently have tapetum lucidum, but it wouldn't be impossible, especially over time, for this to occur in humans.

It also wouldn't be far-fetched to have a human pigment that causes red eye color. There are people with varying amounts of red hair, and there are plenty of other pigments found in animals, like foxes (and tomatoes too, but that's a stretch), that wouldn't be impossible for the body to synthesize. Unlike the Sharingan, this would only affect the iris and not the sclera, or white 'background' of the eyes.

For bioluminescence, the principal pigment is the light-emitting luciferin, aided by the enzyme luciferase. This is usually the most potent form of bioluminescence, and most animals can't synthesize it on their own and must attain it through their diets, so a character would most likely need to inject themselves, therefore making the glowing eyes partially intentional, or a side effect from something else.

However, through studying luciferin and bioluminescence, it was revealed that human cellular and chemical reactions also produce minimal amounts of light, so you could stretch plausibility a little bit and add chemicals to these reactions that naturally emit more light, etc. Stress could also cause adrenaline release, making these reactions emit more light than normally.

The best explanation would probably be a mix of some of these.

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  • $\begingroup$ This is very good, you get a +1 $\endgroup$ – King of Snakes Jul 31 '16 at 11:34
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Sharingan : Basically an eye that changes ''shape'' under emotive stress to grant new abilities.

Changing shape of the eye like having 4 pupils would result in a distorted sight or in going blind. So, instead of change shape the eye simply changes color and glow to warn enemies.

Eye of sight

High stress will active an organ that senses energy-chackra-electricity like the movements from heartbeats or any other vibration caused by biologic beings. (many animals already have this ability)

The user under the effect of special hormones will process sight faster, have increased memory speed and the brain will try to predict the future with fast probability calculations based on experiences , like many modern computers do.

This allows the sharingan user to somehow see the possible futures, read lips and mimic things instantly.

The eye has many hidden membranes that can cover the eye to increase the focus power and achieve the ability to see at ''cellular level''.

Eye of Hypnosis

A sharingan user can choose to release special ''spores'' in the air that can reach the brain of other organisms through the respiratory system.

Those spores can control neurons of the victim and make them do a set of actions they were created for. But, when the set of actions is finished the ''spores'' will stop working and be dissolved to leave no prints or proofs, so it will need new spores with different sets of actions.

these special ''spores'' can also cause blindness or loss of other senses on the victims like the Mangekyō Sharingan evolution.

Another more realistic but less precise system would be releasing a special pheromone in the air or an hormone that makes victims do what you tell, but they have to understand your language, so both hormones would work only with humans or a specific animal while the spores are almost universal.

Rinnegan

Our brain already stores a bit of stamina cells to create new neurons that will serve to control more muscles, that's why bodybuilders have bigger brains. to make rinnegan possible the user would just need to use those stamina cells to heal the eyes instead of building new neurons, maybe by having more reserves of stamina cells than normal people have.

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    $\begingroup$ This is very good , but i only asked for the changing colours . Still for explaining everything, I gave a +1 $\endgroup$ – King of Snakes Jul 30 '16 at 12:00
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Okay, I'll take a shot - with simple undiscovered chemistry, no handwaivium at all.

It is (mostly) reasonably conceivable that a chemical, molecule or other period chart real compound exists that is sensitive to emotion ( no, not mood rings) but not all that different in concept from 'mood rings', a chemical compound responds to external energy source, changes color, reflectivity, bioluminescence... you get the idea. it is far fetched, but I have see chameleons become stressed and change color according to their mood.... this can happen. This chemical should exist in the biology of the population you describe, only certain training can 'activate' it...however you feel works for you...

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  • $\begingroup$ This chemical could react with hormones in our body. Emotions are mostly regulated by hormones. $\endgroup$ – Annonymus Jul 30 '16 at 7:23
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I can explain some things, as I rage-scienced the sharangan and byakugan last week because Kishimoto kept committing science crimes and it fueled the latest spite-researching spree.

SPOILER WARNING

The sharangan is the result of aliens interbreeding with humans, just like the byakugan, we can get away with fun things that the rest of the humans don't have (so long as we ignore the fact that you can't have hybridisim exist like that unless there was some serious genetic tampering from the aliens that visited so it was stable and makes Uchihas and Hyuugas a subspecies of human I guess) and we can pretend it's a dominant or co-dominant genetic trait so it doesn't die out.

The sharangan has several features-

  • Color changing from a very dark brown or grey that appears black to a cherry red

  • Glowing

  • Dots called tomoe that appear in the iris

  • Swirling movement of the tomoe in the eye

  • Photographic memory and high processing speed

  • Magicy things

  • Mood swings and unstable or violent behavior (justified trait due to all canon examples shown displaying them and the talking of the whole 'curse of hatred')

  • Mangekyou or however you spell it

Of these features, I Cannot explain the magic. It's magic. I've personally decided dragons and electromagnetic telepathy shenanigans as my convoluted crossover explanation but it cannot be scienced away. I also cannot find a way that the eyes would spin. It's just.... not a thing that biologically works. So you can just... not have that in whatever fan project you are doing if you want science.

Color changing can be like an octopus, little balloons of pigment that swell and contract. They can activate automatically whenever enough adrenaline hits the system and controlled however octopi do it (as science doesn't actually know from what I've read). With this they will have always been able to change the color, but we can just have the magic only work after the first adrenaline activation. Magic can tell because magic is nice like that. Don't worry about it too much. Naruto is magic ninjas. Magics gonna happen unless you alternate universe it so handwaving can work there. With this, the color of the eyes is a very very dark red that then contracts to the cherry color when activated. If it activated the first time with adrenaline, it should activate during all times that the body is flooded with adrenaline. EDIT: This is something that can be done in gradients. To have a darker or brighter sharangan would be determined by the adrenaline of anger making the color be brighter than normal. Wasnt sure if that was clear enough in the above paragraph.

The next thing in order is the tomoe. They are either extra holes, or they are color spots. If they are color spots then it's not a big deal, just some clusters of inflated color that didn't retract with the color changing mechanism. In the hard science version, there wouldnt be power levels by the amount of dots, and there probably wouldnt be three large ones only. Jeweling- a pattern in eyes where a bunch of small dots of darker pigment are in the iris is a Real Thing you can base it off of. Especially if you dont want the color change bit.

The hard part is when the tomoe are actual holes in the iris. If you want to use this, it would be polycoria. Which looks kind of horrific and im going to let you google that one on your own. It usually results in being unable to dilate and undilate your eyes, blindness, double vision, etc. This is counterproductive to what the sharangan does, so maybe don't use this one though. Unless you want to go the extra magic route and make them sense everything like Toph does (making it a byakugan recolor).

The photographic memory will have to be a thing at all times or the result of an adrenaline activation only. Then you just have all Uchiha brains wired to remember all aspects of traumatic events instead of repressing them into a blur because Alien Brain Residue or whatever. The high processing would have to be an all the time thing or like a trauma tunnel vision thing or magic. You can't rewire your brain and back again. Either they have the same processing speed or it's all the time or it's only for the adrenaline activations or it's magic.

The mood swings and such are the result of unstable or divergent brain chemistry because they are alien hybrids. It's likely that the clan has a large problem with mental disorders on top of just plain being psychologically wired differently. This gets unpredictable and on top of the fact that most everyone with a sharangan has been traumatized to get it, it's no small wonder that they all exhibit varying degrees of mood swings, suicidal behavior, delusions, dissociation, codependency, low or dysfunctional empathy, violent episodes, and panic attacks over the course of the show.

The glowing is as said above phosphorescent bits or it's magic or a mix of both. It uses the same cues as the color changing and cannot be activated separately as just a glow on black eyes. The glowing is very faint when intentionally turned on. The adrenaline response activation would be stronger. The glowing can be like the color sacs, but is determined by the chemicals that make up anger as well. The higher the chemicals for anger the more it glows. It cannot be consciously increased unless you are actually angry. As chakara is canonically a thing that is connected to physical aspects, it could be a 'chakara is sensed by the glowing organs and the glowing organs then generate the glow accordingly' if you want to magic it.

I also want to add that you could do the refracting thing that cats and dogs have going on with their eyes to simulate glow if you want them to not be really obnoxiously glowy with no obvious source. Its more hard science-ey then hypothetical phosphorescent organs being crammed into an already very complicated eye.

The Mangekyou is like,,, 80% magic but you could use stress induced polycoria as it makes the user go blind and makes your iris warp. Or you could just have the extremely high levels of hormones cause the color changing bits to freak out and display different patterns. Eventually one goes blind either because the eyes got too stressed from the magic flowing through them or the polycoria making it impossible to focus and dilate correctly. Whichever. Eternal Mangekyou Sharangan wouldn't be a thing I don't think. If the eye degrades over time then all Mangekyous would degrade.

There. That's the best I've got.

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"Chakra" is hardly realistic.

Since it has a mental component and can affect the physical world, the short answer would be "magic". It makes as much sense as chakra manifesting solid shapes without a physical medium.

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    $\begingroup$ I thought I said I don't want the other stuff explained (the powers of the sharingan). I only want to know how to explain how their eyes glow more red. nothing else $\endgroup$ – King of Snakes Jul 30 '16 at 5:34
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    $\begingroup$ If it can manifest solid shapes, ...correction, solid, glowing shapes, merely changing eye colour and light emission is a cinch $\endgroup$ – nzaman Jul 30 '16 at 5:37
  • $\begingroup$ That is fine. No need to explain the increase in tomoi, just the eye color. $\endgroup$ – King of Snakes Jul 30 '16 at 5:40
  • $\begingroup$ Well, if it's pseudoscience you want: Chakra in the iris causes light to bend into the pupil. The blue part of the spectrum bends most and almost all of it enters the pupil; the red part doesn't bend quite so much, so much of it is reflected off the iris, creating a red appearance. $\endgroup$ – nzaman Jul 30 '16 at 5:54

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