Some thoughts on the fundamental magical issues in A Certain Magical Sunset · 2:08am Jan 12th, 2023
I first wrote these notes down as a series of ideas back in August 2022 while I was working on completing the first book of this crossover's trilogy, a tale that encompasses the Shimmering Hearts EqG stories within the Universe of A Certain Magical Index and A Certain Scientific Railgun.
While writing the crossover it was clear there were inconsistencies in the magical systems of the two stories. However, with a little bit of reflection as outlined here, these apparent inconsistencies can be cleared up without resorting to plot armor or other theatrical gimmickry.
In the Universe of A Certain Magical Index, Magic users cannot use ESPer powers because once ESPer powers are mastered, the incorporation of the personal reality required to use ESP causes the use of normal Magic to physically damage the casting ESPer's body. The effects of this are so pronounced in cannon that many attempting to perform even simple magical cantrips end up perishing for their efforts.
Despite this fact, in story both Twilight (SciTwi) and Sunset regularly use both their innate magic and ESPer powers once they are accepted into Academy City. So the obvious question is how the heck is that even possible?
The answer has more to do with the types of magic involved than the use of magic itself. No, it's not because the girls have invincible plot armor, though I am sure Sunset wishes it as she dies twice in the course of this story. Simply put, the magic common in the Human realm of A Certain Scientific Railgun and A Certain Magical Index is Sympathetic Magic.
Sympathetic magic works in sympathy with the caster and is powered by their own life force. Sympathetic magic is Harmonically neutral, being based on the sympathies of the creature casting the magic and fueled by the essence of their own soul.
ESPer power is fueled by the caster's personal reality, making ESPer powers a form of sympathetic magic as well.
Unfortunately, although both are types of sympathetic magics, it is well established in cannon that they interact extremely negatively with each other. I suspect this may be due to the imaginary nature of the ESPer magic versus the direct parasitic nature of pure sympathetic magic. In cannon the magics react violently, altering the physical reality of the caster and weakening their life force to the point they cannot survive.
Magic in the Beast Realm, which includes Equus, is by nature Harmonic. It is based on the relationships of the caster to the world around them when they are casting, rather than distinctly on their own sympathies. It is not powered by their own souls essence, but instead by the energy created by the relationships between all living (and to a lesser extent non-living) things in creation. So even when casting the Discordant form of Harmonic magic, so called Dark Magic, there is no actual diminishment of the essence of the caster. Note that the use of Dark Magic, like all Harmonic Magics, does indeed leave a stain, good or bad, upon the soul of the caster, as is true of all actions a soul takes in its lifetime.
Because of this, the use of Harmonic magic does not cause the physical breakdown of an ESPers being, as it does not interact with the caster internally as Sympathetic magic does.
The fundamental reason the natures of magic are different between the two Realms is because of the different development of the two Realms. The so called Human Realm is a Fallen Realm, distinguished in that at a point early on in its creation, all life and essence turned towards itself, viewing itself as the source of its being instead of acknowledging its Maker as its source. This predisposed creation in this realm to the Sympathetic version of magical expression with its known limitations.
The Beast Realm is a so-called Pure Realm, meaning it never turned from its fundamental outlook of the Maker as the source of its being, predisposing creation there to the Harmonic version of magical expression.
As Twilight and Sunset only use Harmonic magic, they are unaffected by the so called magical curse that normally afflicts ESPer power users.
Of course, thoughts and comments welcome. 
It feels like a huge exposition dump above and maybe can be narrowed down in some places where it repeats. Still, I kept thinking, as I read this, that the place this kind of explanation belongs to is in the story itself. Requiring all readers to sidetrack themselves to a blog post to help explain what seems like a pothole in the story isn't a wise strategy. At the very least this blog post needs to be a link to the author's note of one of the chapters. The note itself may provide an abbreviated explanation of the above while a link to this post provides the full explanation.
But I think the ingredients to provide this explanation in the story in character is somewhat likely at some point because both Sunset, and especially SciTwi, have a scientific mindset and, as such, they are likely to ponder this phenomenon too. With some proper hypothesis followed by good old fashion scientific testing, it's not unreasonable to conclude that the characters within the story could reach the same conclusions. it could even turn plot-relevant who else discovered this information along with them. For instance, if this info happened to land in the hands of one of the villains because there was a hidden camera in the lab where these two tested their theories, it could affect the story in a meaningful way.
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Thanks Scroll!
You're not wrong on the expository nature of this blog. Walks like a duck, quacks like a duck, it's a duck.
That said I wasn't intending this to be a critical piece of information for readers of the story, more like a little bit of minutiae showing some of the thoughts going on behind the scenes in trying to keep the story consistent between the two disparate universes. Of course, they way I just waded into the exposition really didn't help clarify that. I guess I was to happy to geek out about the crossover universe. 
You're suggestion on Sunny and Twi unwinding this for themselves is a good one. I can see where they would both be interested in the differences between the realms magics, and the realms themselves, as well as how that knowledge could be beneficial to them in story eventually. I may try to work that into one of the next few arcs, if I can make it happen organically, and not through a forced expository dump. Celestia knows I have enough of those already.
Thanks for all the editing and keeping me sane and on track.