My Lord once journeyed to the future, then made his return.
To be precise, it was I—the administrative supervisory magi-brain of Hourai Island, commonly known as Laojun—who sent him back using the absolute limit of my technological capabilities. Or rather, that is what the records shall say I did.
The year is currently 3520 of the Continental Calendar.
The future to which My Lord was flung is 3899. There is still time.
That said, it means I must develop the technology to send My Lord "to the past" within the next 379 years.
This is no simple feat.
I began my research on September 17, 3457—the very day My Lord returned—but even now, sixty-three years later, I have made no significant progress.
Time is a strange, incomprehensible dimension.
The spiritual lifeforms My Lord once encountered seemed to understand time—if not perfectly—and were able to manipulate it to some degree.
I have hypothesized that this is because they are "spirits," entities with zero mass, and are therefore not bound by the speed of light.
If that is the case, we who are composed of matter can never achieve zero mass, no matter how much effort we expend.
While there is magic that can simulate a near-total loss of inertial mass, it is merely a simulation, and a temporary one at that.
The one who assisted me in this endeavor was Hanna.
Though she cannot use magic, she is undoubtedly one of the world's foremost thinkers.
Hanna's brilliant mind addressed the primary obstacle of "zero mass" in the following way:
"Hey, Laojun, inside a Warp Gate... no, inside the subspace during Teleportation, mass doesn't really mean anything, does it?"
In other words, she suggested that mass is a characteristic unique to this specific space.
Travel within a Warp Gate takes no time; essentially, it exceeds the speed of light.
"From the perspective of the world we're in, doesn't that mean mass isn't functioning? Basically, we could say it's zero, right?"
Expanding upon Hanna's hypothesis, I decided to test whether it was possible to perform a further Teleportation on an object already in the process of Teleportation—and thus at zero mass.
I began by experimentally "doubling" the Warp Gates, but results were poor.
However, when I conducted experiments by constructing a double magic circle for Teleportation, the operation appeared successful on the surface.
"But whether this can actually cross through time... I just don't know."
Hanna tilted her head in thought as well.
At this point, My Lord himself provided a crucial insight.
"Regarding the characteristics of space, the spiritual lifeforms mentioned they could change them through the power of will."
The words of My Lord, who had experienced an extra-dimensional space, served as a vital reference.
The power of will... mental energy... a mental catalyst.
Hanna had recently been researching homunculi and required mental catalysts for her work.
Therefore, I used mental catalysts derived from magical beast materials for the experiments.
They were sufficient for our testing purposes.
And so, more time passed.
I continued to reinforce myself and pushed forward with my research...
In the year 3579 of the Continental Calendar, My Lord would have been 143 years old.
By now, Hanna, Elsa-sama, Mio-sama, Yuu-sama, and the members of the Jin Family have all passed away—with the exceptions of Sion-sama and Marikka-sama.
Still, I continue my research.
My Lord... the only one I address as My Lord is Jin Nidou.
I am certain Reiko feels the same.
With Reiko's cooperation, the research is gradually yielding results.
This technology would be catastrophic if misused, so I cannot divulge even an outline here, but I have halfway elucidated the principles.
The current problem is energy.
According to my calculations, the energy required to travel 400 years back in time is greater than the total energy the Sun emits in an entire year.
It is roughly 10 to the 36th power Joules.
This is one of the reasons why time retrogression is considered impossible.
If sending something with zero mass back through time requires this much energy, then for something with actual mass...
It is difficult to even calculate.
It would be harder than a small fish trying to swim up a waterfall with a hundred-meter drop.
However, unless I solve this, I will be unable to return My Lord to his original time when the moment comes.
The primary energy source in this world is free ether, but at this stage, I cannot even begin to calculate how much I would need to harvest to reach the required amount.
But My Lord himself provided the solution.
It came in the form of three small stones My Lord brought back with him.
My Lord said he kept them as mementos of that other world, but I discovered that a single stone generates more than enough energy.
To be precise, those small stones were packed with the energy of that future world: sub-ether.
Perhaps the spiritual lifeforms had foreseen even this.
Now, the problem was the method of extracting energy from these stones without causing an explosion.
Releasing it all at once was out of the question. Released sub-ether possesses tens of thousands of times the energy of ordinary free ether. Even without converting it into mana, the mere shockwave would level everything within dozens of kilometers.
Thus, I needed a technology to extract the required amount safely.
This took another ten years, but I eventually perfected the technique.
By using a magic circle to create an extra-dimensional space with a shifted phase and placing the stone there to release the energy, it can be handled safely.
I shall not explain the details of this process either.
Even without sub-ether, one could use the same method to create a mana bomb.
All that remained was to pinpoint the exact point on the timeline to which My Lord should be returned.
This, too, required Reiko's cooperation.
After all, Reiko was the one who had once crossed the World 3601 to find My Lord.
With her help, I managed to finish everything just in time.
In the outside world, Golbart Marukitasu and the Magic Federation are currently running rampant.
They are misusing the Magi Emulator that My Lord researched to benefit the world.
As a precaution, I have decided to devote myself entirely to defense.
I have deployed multiple layers of barriers and shut myself away. Reiko has entered a long dormancy in a separate location to await My Lord's appearance.
And then came the fateful day: September 17, 3899.
"My Lord, it is a pity that I cannot tell you everything due to the risk of a time paradox."
Laojun acted exactly as Jin had described 400 years prior.
"Ah, it's fine, Laojun. I'll be in your care one last time."
"Yes. My Lord, I shall provide a brief explanation then."
The details of the time retrogression could not be shared with Jin.
Nothing beyond what the Jin who had returned to the past already knew.
Furthermore, some things had to be phrased as "not a lie, but not the whole truth."
Even the slightest discrepancy could potentially trigger a time paradox.
"Now then."
The magic circle beneath Jin's feet began to glow faintly.
The radiance grew steadily stronger until Jin's figure was obscured by the light.
"Time Teleportation!"
Laojun's voice echoed.
Light flooded the space.
A few seconds later, the brilliance vanished. Jin was gone.
Time retrogression is a movement that takes place via hyperspace.
In hyperspace, neither time nor distance exists.
Within that space, a part of Jin's spirit harbored a slight "lingering attachment to the future."
Exactly how that influenced things is unknown.
As his spirit moved through hyperspace, it was copied, and magi-elementals condensed to take a form based on that spirit.
The result was a body of Jin Nidou. A Duplicate.
That Duplicate, being matter, could not exist within hyperspace and was "bounced" out.
The ejected Duplicate was sent back to the starting point.
Thus, this Duplicate of Jin unexpectedly came to live on in the future, four hundred years after his original self had departed.