The key to the byakugan!
And now, to prove my insanity, I present to you thorough explanations of how Byakugan and Sharingan might work if they were real. Mind you, this is just my opinion as a scientist/otaku, so don’t take this too seriously- this is supposed to be as entertaining as it is informative, not a term paper.
Now, Byakugan is the Hyuuga’s bloodline limit, which consists of an all-white eye that enables the user to see 360º (except for one blind spot at the back of the neck), see through solid matter (obviously, as 360º vision would require seeing out of the back of one’s skull), and see Chakra, which is for the most part usually invisible.
Alright, now it took a lot of research and spending two hours talking to people smarter than me, I’ve decided that Byakugan IS in fact, theoretically possible. Theoretically.
First of all, let’s begin with the ability to see through things. After much deliberation, I actually found a way for it to maybe be somewhere close to a little bit conceivably possible. That is, Byakugan enables its user to see so precisely one can perceive matter on a quantum scale. Specifically, to be able to see not only individual atoms but the space between them- to the point that with enough focus one can see straight through the very matter an object is composed of.
It’s actually not all that unreasonable, seeing as the eye can detect a single photon of light, I bet with the application of chakra that could be amplified to subatomic levels.
One can assume that this is possible only due to the flexibility of chakra, which I unfortunately cannot gauge the limitations of, as it does not exist scientifically and therefore cannot be measured. Let’s just hope that it works this way, shall we?
Now, you would think that with that out of the way, seeing 360° would be easy to explain, but its actually what took so much time and effort. Now, in order for an eye to be able to see 360°, the pupil would have to take up almost the entirety of the eye, with just a small portion being reserved for the retina as well as where the optic nerves are located.
This would also be the actual blind spot, whereas in the manga it states that the blind spot is behind the first thoracic vertebrae. Minor technicality. Now, since the pupil is what takes in light, for it to extend to the majority of the eye would mean it takes in a lot more light (obviously), which would make the eye extremely photosensitive. This isn’t actually a good thing, as anyone who’s looked directly into the sun can testify. Now, one possible way to overcome this is with a second eyelid, like cats have. But, the eyelid can only cover half of the eye, plus the question of whether the visual information received through the back of the head would be photosensitive. So, we can probably rule out the eyelid theory. The only other way I can think of is if the chakra being focused in the eyes during the activation of Byakugan protects the eye, perhaps as a shield, as well as expands the pupil to the level required for 360° vision.
Also, the eye would have to be perfectly round, as opposed to the normal eye’s egg-shape.