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In the later chapters, energy and energy sources in the world of One Piece were discussed. Lilith mentions that Egghead is powered by fire, because fire can be converted into many different types of energy, and if there was everlasting fire, they could create their own sun. Shaka also mentions that the ancient kingdom had an unknown and very advanced source of energy.
It’s possible that the powers behind Devil Fruits and even Haki are based on this: Electromagnetic waves.
First of all, if we look at the parallels between the world of One Piece and the real world, the “old” energy would be the burning of fossil fuels, like what happens in Egghead, and the “new” energy would be the renewable energies (eg sun, wind, waves) or GREEN energy.
We believe that the Old Kingdom knew how to directly harness the energy of the Sun in the form of electromagnetic waves, solar panels in fact, but in several ways.
Assuming Devil Fruits actually come from the Old Kingdom (what we are not sure about) and that Devil Fruits aren’t just magical tools that create powers out of thin air and there’s a scientific basis behind it, so maybe Devil Fruits are also powered by the sun. Being able to change one’s body into different materials, create items out of thin air, and change the user themselves into animals and transform back requires energy. And this energy comes from the sun. Here are two points to support this:
SMILES
SMILES are incomplete and inferior Devil Fruit, only zoan-type, which permanently alter the user’s body and the user does not have the ability to change shape like a real Devil Fruit. The reason why there are no SMILES paramecia/house is that SMILES do not harness energy from the sun and cannot create energy/elements or do other effects that require energy/power. SMILES are only a one-time attempt to mutate the user’s lineage/DNA factor, so the changes are permanent.
Black beard
Blackbeard’s ability to disable Devil Fruit powers when he touches them. Blackbeard mentioned that Darkness is “Gravity”, and like light which is a type of electromagnetic waves, not all electromagnetic waves can escape from a black hole and it is possible that Beard’s ability Noire essentially prevents the Devil Fruit powers from working by absorbing the electromagnetic waves needed to power the Devil Fruit powers.
Finally, it would be possible that Haki is also based on electromagnetic waves, and more precisely on electricity. The first time we saw Haki being used was during Skypiea, where we were introduced to Mantra/Observation Haki. Enel himself said he could extend the reach of his mantra/haki through the powers of his electricity fruit. Haki itself has been shown to be able to physically manifest as lightning.
So, Observation Haki would work like radio waves with the ability for the user to read them.
This would be a basis of explanation for Armament Haki. The arrangement of an object’s electrons determines whether it is solid/liquid/gaseous.
It could also be that the electromagnetic waves of the haki disturb the electromagnetic waves of the devil fruit.
Another point we’d like to add is that Chopper once mentioned that his Rumble Ball worked by disrupting the “frequency” of the Devil Fruit, so…