hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdf
Here you go senpai. A translation of Einsteins original paper, which can be used to find e=mc^2.
I'm pretty pleb-tier at physics, but I've read some stuff on SR, GR, and Einstein, so I'll give a lay persons description, but I may get things wrong.
Essentially, if light is a wave, it needs a medium to travel through. This was called the luminiferous aether.
Experiments failed to prove this aether, since light travelled at c (in their respective medium) in every direction.
Einstein (unaware of the relevant work of Lorentz at the time) took the speed of light as a constant in every frame.
By assuming light travels at a constant speed, regardless of reference frame, you can write equations which allow distance, time, and mass, to be different in two reference frames that have relative velocity.
Through some sweet math tricks, like using pythagorean theorem (a^2 +b^2 = c^2) you can show that "a" is relative velocity, "b" is the speed of light, to find how time is different in each reference frame. This trick results in a c^2 term, that you see in e=mc^2.
Now, even further, the m and the e are found when looking at kinetic energy of a particle based on reference frame. Since time is different based on that c^2 and v^2 thing, then you find that mass appears different based on the frame, m1 is from one frame, m2 i the other, and the m in e=mc^2 is the difference between m1 and m2. When you write an equation to express this, you can cancel out a bunch of terms and get e=mc^2
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