"Mwahahaha!" the human twirled his mustache. "You can't hurt me, Twilight. I am immune to magic!"
"Wait," Twilight frowned. "How is that possible?"
"Because I'm from another universe!" the human laughed. "And we don't have magic where I'm from, so of course I'm immune to magic! Why would you even think otherwise?"
"But that doesn't make sense!" Twilight stomped her hoof in frustration. "We don't have nuclear bombs in Equestria. If I went to your universe, would I be immune to nuclear bombs exploding on my head? Would I get stuck in elevators because we don't have them Equestria so they'd refuse to work for me?"
"I don't make the rules, Twilight," the human shrugged. "I'm just exploiting them to conquer Equestria. Now, submit! There's absolutely nothing a unicorn like you can do to stop me!"
"Ok," Twilight gently rested her face in her hoof and took a deep breath. "I like science. So let's do science. How does this work?"
"If you must know," the human rolled his eyes. "It depends on the fic. Sometimes it's just me that's immune, but not things I bring with me. For example, if you blast me with magic nothing happens, but you can use magic to recharge my cellphone, because that's super convenient for me. Other times, I carry a physics bubble around with me, and even standing near me can turn magic off."
Twilight let loose with a magic beam to his face. It vanished the moment it hit him. Or...wait, did it vanish a few inches before it him?
"Mwahahaha," he laughed. "Do you see?!?"
Her scientfiic curiosity piqued, Twilight lit her horn and began investigating the astral space surrounding him. Sure enough, there was a bubble, and beyond its threshold she received no feedback from her spell of any kind.
"So, this bubble," Twilight prompted. "Works according to the physics of your universe?"
"Yes! And there's no magic where I'm from, so it can't work in my bubble. You're completely helpless! Mwahahaha!"
"And," Twilight continued, "is energy conserved in your universe?"
"I'm not sure why it matters," the human tried to roll his eyes again, but found it interrupted by a few drops of sweat rolling down from his forehead. "But yes, conservation of energy, mass/energy equivalence, e=mc^2, all that's pretty standard where I'm from. Why do you ask?"
"Just curious," Twilight shrugged, her horn now glowing a little brighter. "How do you feel?"
It was a strange question. But now that he thought about it, it suddenly seemed very hot for some reason.
"Is it just me, or it really hot here?" the human asked from the very safe confines of his physics bubble.
"No, it's totally normal here," Twilight explained. "It's hot where you are, because the magic I'm applying can't function inside your physics bubble, but its energy isn't being properly returned to Equestria's aether, because the physics of your universe doesn't allow for it. And since energy can't be destroyed in your universe, it's being converted to waste heat."
"Oh, that's...umm, nice?" the human began panting heavily and frantically fanning his face with his hands. "Now hurry up and give up already."
"Which makes me wonder," Twilight pondered, then suddenly her tone became cheerful. "Hey! Would you be willing to participate in Equestria's first cross-dimensional physics experiment? I'll be sure to add your name as a supporting author on the paper."
Twilight poured more energy into her spell, and over the next few seconds, the human in the complete safety of his physics bubble burst into flames as the air around him turned to super-heated plasma.
"Oops," Twilight sighed. "Didn't get his name."
Here's a name for him, Twilight: Owned.
You keep owning him, so why not?
Oh gods, why haven’t I ever found this before.
It’s so good.
Also, that is totally a problem that I have never seen before in magic nullification fanfics, so good job.
Well now, that's an inconvenient difficulty isn't it?
I wonder if it would have hit that problem through passive accumulation?
Inertia is a cruel mistress and all ponies are capable of at least some form of 'launching of large rocks' or utilizing pressure waves that a human wouldn't be able to handle.
Let's add a new take to the mix, Zecora with a blowpipe and a tranquilizing chemical!
Zecora: You think yourself so mighty from being magically immune, all I needed is a bit of bamboo and some chemicals to make you swoon.
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And I was refering to a memory of an old fic (forget which one) where Flitter and Cloud Chaser are trying to seduce a human with just as much creepiness as a Lyra. I remember the terriible innuendo Flitter used by hinting Cloud Chaser was into mountain climbing to hint they don't mind the size difference...
Anti-magic human with horrible fashion sense vs. Rarity.
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I assure you that Rarity will have her moment to shine.
this right here, this is gold
I have a problem with this one. Just because the air around him can heat... doesn't mean the heat can't just radiate back out into the surrounding particles. If anything, the 'magic' would splash around the barrier, since the bubble would theoretically just reject the foreign energy type in the first place, not convert it into something conventional. That's not to say you can't cook him using fire/plasma in the first place, just pumping raw magical energy into the bubble wouldn't really work.
That would also mean 'pure' magic has the same energy potential as *the sun* if it flash heated the air into a plasma. It may not take much energy to sustain a plasma... but lighting an area like that ablaze is suspect. The amount of space a light bulb ignites is VERY small, and mostly exists purely due to excited electrons from the electricity.
And this goes back to my other point of radiating out of the bubble as soon as it goes in. A light bulb is a confined space... a metaphysical bubble that still allows breathable air flow is not. The raw energy potential needed to flash heat the area is really too much for me, because the heat/air can still escape, so it'd NEED to be a flash heat.
I side with the human on this one, raw energy doesn't act this way. Especially not with all the other variables in play, since it's clear it's an anti magic barrier, and ONLY anti magic. He'd be protected from straight magical manipulation, and raw magical blasts. They would splash off his barrier or be turned into VERY fast flashes of heated air depending on how much energy and how spread out said energy was.
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The simple answer
Insulation solves the problem you're describing, and it should obviously be trivial for a unicorn like Twilight to accomplish that.
The less simple answer
Humans have a tendancy to think in terms of things. It might be better to think of magic in terms of flow. For example, imagine you want to pick up a paperclip. So you reach out with your fingers and pick it up. You're using a thing (your fingers) to push on another thing (the paperclip.) But magic is not..."a thing" in the sense that your finger is.
Now imagine you use a magnet to pcick up that same paperclip. Yes, the magnet is a thing. But the magnet itself is not "pushing" on the paperclip. Rather, there's field of flow around the magnet, and the paperclip is carried by that flow. Similarly, gravity isn't a "thing" that "pushes" you on. It's more like an area of directional flow, and if you're in that flow, you tend to be carried by it.
Instead of imagining Twilight casting the spell as being like "throwing a thing" that "hits" the barrier...try imaginging it as directing the flow around his field. After all, his bubble is flow too, or rather the lack of flow. But it's not a physical thing "pushing" back against magic "hitting" it, in the same way that a man in a big plush mascot costsume might be protceted from a baseball.
Think of it more like an astrophysics bow shock, as happens when the solar wind meets the Earth's magnetosphere. Sure, a lot of that energy is carried away by the "free flow" away from our atmosphere, but nevertheless a lot of energy is absorbed by it too.
That's not really a problem. For example, Twilight Sparkle's feat in season 1, floating the water tower in her fight against the Ursa, according to math from here, she was exerting at a rate of ~15 megawatts, and other estimates put her output rougly on par with a nuclear supercarrier. Even a common household blowtorch can create a localized plasma despite breathable airflow. There's no way she wouldn't be able to do this.
Good old impalement should work well enough