Orochimaru blinked and considered what to ask.
Then, he asked a very basic question.
"What is chakra?"
It has been over 4 months since he has learnt how to mould chakra.
But, it isn't really clear to him what 'combining Physical energy and Mental energy' means.
He just, does it.
"Chakra is a combination of Physical and Mental Energies.
But you are actually curious about how these two energies combine, right?
Just doing the handsign and 'HMMMMM!' 'RAAAAHHH!' or whatever gives you some chakra. You can't consciously identity the two separate energies and combine them, right?"
"Yes."
"Alright. I am going to tell you something that you should keep in mind. It is very basic, but it will be very helpful in creating high level jutsus and even weapons in the future.
Energy, and matter, are same."
"HUH!?"
Tsunade, Orochimaru and Sakumo gasped. Duy gasped after immitating them.
What Akame was telling wasn't actually something 'basic'. In her mother's world, this thing was proven very late, and actually led to the development of atomic bombs.
In her mother's previous world, Energy was equal to the product of mass and square of the speed of light.
But, it doesn't necessarily hold true in this world.
Still, the existence of pure chakra beings like tailed beast having very physical bodies and soul means that chakra and matter being connected is true.
"Alright. Let me explain it. Do you know what tailed beasts are?"
They shook their head.
"Tailed Beasts, are living beings made entirely of chakra.
And they exist in two states.
One, as any of us. Just living on the earth. Despite being made of entirely chakra, they have physical bodies, soul, and even have weight.
Now, here is a question. If you seal a kunai in a sealing scroll, does the weight of scroll change, or the weight of kunai adds up?"
Orochimaru answered, "It adds up."
Akame nodded. It's true for the normal sealing scrolls that majority of the world uses.
But for advanced sealing jutsus that actually store things in a different destination like the summoning jutsu, and tools like the Gourd, one of the Sage of Six Paths' tool, the weight doesn't add up in the seal carrying medium itself.
Well, that's too advanced. For now,
"A Tailed Beast with huge weight, when sealed inside a human, or anything that can be a suitable vessel, doesn't increase the weight of the vessel. Despite all of the chakra now inside the vessel.
Moreover, the flesh, bones, blood, everything of the tailed beast reverts into the chakra form."
Orochimaru's eyes shined, and asked, "So, we can also become pure chakra?"
"We don't know how, but it is definitely possible. Though, I think there are some prerequisites. There must be a minimum amount of mass, or energy. This will be a really complex matter of Chakra Physics and Maths. And I can't give you a definite answer.
Anyway, now I am going to answer your question.
Why you can't actually use mental energy and physical energy separately, and then combine them to form chakra like the theory says.
Simply because we aren't capable of using mental and physical energies separately.
There is Yin release, which just uses chakra with very ratio of mental energy to physical energy.
My Sharingan is an example. While it is mostly Yin, it still is an organ, containing physical energy.
Other examples are Nara and Yamanaka clans' special jutsus, genjutsu, Yin-based medical ninjutsu, and other.
Likewise, Yang release is the same. You can't perform anything without having used some mental energy. Yang release is mostly physical energy, but it consists of mental energy as well.
Some examples are Akimichi clan's special jutsu, Wood Release, and Yang-based medical ninjutsu.
Compared to Yin release though, we have less advancements in Yang release.
Orochimaru, if you want to actively use Physical and Mental Energies separately to form chakra, you will have to go backwards.
Learn Yin release, and Yang release. And decrease the smaller energy in these types of chakras to a very, very low point. Low to the point that the new Yin release is just mental energy, and Yang release is just physical energy.
Though, there will still be very, very small amount of the opposite energies present, which you can ignore.
There is nothing perfect and pure in this world.
And if there is, perhaps only Gods are capable of that.
Like, the Sage of Six Paths. It is said that he had such a mastery over chakra, especially what you want to do, Yin-Yang release, that he could create just about anything he wanted as long as he had chakra."
Still, without Rinnegan, it will be difficult.
Mangekyo Sharingan already allows to directly manipulate someone's mental energy by bypassing the need to manipulate the chakra. Still, it needs a lot of mastery. She wonders how much more powerful a Rinnegan will be.
Anyway, in Mangekyo Sharingan, such a Yin heavy Yin release actually hurts the eyes and body.
That's why replacing the eye with another Mangekyo Sharingan that shows least rejection starts healing the nerve damages which is non-healable. The additional Yin and some extra, foreign Yang sends signals to start healing the eye nerves.
She doesn't know why this happens.
And Hashirama cells also heals the nerves, and even strengthen them than normal.
Because, Hashirama cells.
If she had to rank Mangekyo Sharingans, it will be like this:
MS < EMS < HMS (Hashirama Amped) < HEMS (Hashirama Amped)
If someone is lucky to have Indra's chakra, the HEMS will evolve into Rinnegan. Indra chakra HMS will probably evolve into Rinnegan too, but maybe will take more time than HEMS.
Wait...
Her mother was Uchiha clan's top healer during Warring Clans Era. Surely she must have some of Madara's blood, right?
Madara's cells probably aren't as active like Hashirama even after his death, to the point of being still alive... those cells must still have very strong mental energy to survive despite the cells failing...
Perhaps, it will be better to say that Hashirama cells are just immortal-like mindless tiny bodies.
So, Madara's cells may be, mind-full and full of will, but failing tiny bodies.
If Madara's cells probably don't hold chakra, but mental energy in the cell's brain, nucleus, and combined medically with Hashirama cells which has fully functional cells but not the 'soul thing' to actually tell the cells to breath, but not needing breathing because the life is just so ridiculously long...
Can...she...cultivate a lots of... Rinnegan?