The nature of time travel is not completely explained, however one can deduce its partial nature through dialogue and revelations by characters. Time travel is primarily achieved through technological means such as the TPDD by time travelers or through data manipulation, in the case of Nagato Yuki.
The theories of time travel were invented by Haruhi Suzumiya and the Bespectacled Boy in the future. Ironically, the latter was marked for death by a faction of time travelers (represented by "Fujiwara") before he could contribute to time travel theories.
The nature of time in "Haruhiverse"Edit
Mikuru Asahina (Small) described time as a deterministic 'picture-book'. Many if not all things are predetermined events. One could think of it as a traditional 2D animation: changing one frame won't magically change any other ones.
Time traveling is simply moving in a 4-dimensional direction through the collection of 3D stills. Think of it as somehow diving through a stack of personal photos, the deeper you go, the farther you travel back in time.
How the literature addresses more fundamental problems such as causality, inertia and paradoxes that arise from the nature of time and time travel in the novels and anime is unknown. One is free to speculate.
Questions to be addressed:
- If time is not continuous, how can changes in the past affect the present?
- Does causality even matter? Or has it been done away with as a plot hindrance?
- What are timequakes and how do they affect the timeline?
Time travel by humansEdit
Time travel by humans is achieved using a device called the TPDD or Time Plane Destruction Device. It does not appear to be an external device, rather it is somehow implanted into the heads of the time traveling agents of the future human faction, such as Asahina Mikuru. Time travel by this means is implied to be inaccurate, or generate a certain amount of noise. To specify a destination time and location, space-time coordinates must be inputted into the device. The TPDD is also known to cause disorientation and nausea to those who travel by it, or at least those in-experienced with the device.
Time travel by data entitiesEdit
Data entities such as Nagato Yuki appear to have a much finer capacity for control over space and time. While the theoretical framework of time remains the same for both humans and data entities, the method by which they travel is implied to be completely different. Not much is explained about the devices or processes data entities use to time travel.
Nagato implied that she, personally, could not travel through time, but did demonstrate to "freeze" a local 'region' of time, allowing the entities trapped inside to remain in stasis until the region is unfrozen. This is how Asahina Mikuru and Kyon made their way back to present day from the Tanabata festival three years prior. She also knows how to program a TPDD.
Her "emergency escape program", created when she stole Haruhi Suzumiya's powers, was able to send Kyon from an alternate world more than three years into his past.
Yuki Nagato on the matter:
- "Axiomatic set theory cannot prove the antinomies within itself without antinomies."
Or in layman's terms, the answer to these questions in the Haruhiverse are impossible to answer consistently, so you're better off not asking.
- See also: timeline
Paradoxes and Apparent ContradictionsEdit
Throughout the series are various paradoxes and circumstances that seem to go against the Haruhiverse's time mechanics. The paradoxes are very easy to explain: because the overall outcome of things is predetermined, any time travel and the effects thereof are also predetermined and thus paradoxes are a common occurrence. In novels 9 through 11, however, there seems to be an event that contradicts the nature of Haruhiverse time-travel: an alternate reality forms. However, the two eventually join together, further strengthening the idea that everything is still pre-determined, and the alternate realities were likely created to prevent any potential harm to the Haruhiverse's timeline.
STC Data and The DisappearanceEdit
Another contradictory event occurs in The Disappearance, in which Mikuru says that the Future "felt the effects" of Nagato altering everyone's memories of the past. Since it is unknown what Mikuru meant, it cannot be determined what exactly the contradiction is, or if it even exists at all. It is possible that Nagato's interference created a space-time distortion similar to the one Haruhi created, and it interfered with time travel in some way, in which case little to no contradiction exists.
When Yuki Nagato alters the world in The Disappearance, Mikuru mentions that the STC data (that is, the Space-Time Continuum Data, also called Sensitive Time Control Data in the Japanese version) was rewritten. In other words, the very fabric of space-time was changed. However, when Alternate Yuki tells her memory about Kyon helping her out at the library, Kyon's phone shows Haruhi's name on it, indicating that only the present state of things and people's memories were changed. This means that when the data correction program is activated, it normalizes the changes previously made while leaving all events in The Disappearance intact, effectively preventing a paradox and returning Haruhi's powers.
In The Intrigues Itsuki Koizumi would explain this to Kyon. He called the timeline before the change "X". He said the timeline alternation started on the dawn of December the 18th when Nagato changed the world. Kyon had spent three days in the different world, then traveled back to Tanabata in year -3 through the use of Nagato's emergency escape program. If Nagato hadn't changed the world, Haruhi would not have vanished, Koizumi would not have been rendered a "normal person" and Kyon would have had no reason to go back and change things. Kyon called this an ontological paradox.
However, the "proper" world required Kyon to travel back in time to fix it, otherwise the world would have gone on the "wrong" path and the time Kyon and Koizumi were in now would not exist.
Koizumi said that if Nagato had reverted time when Kyon returned from year -3 on December the 18th, everything would have been fine. However, Nagato hadn't been able to do so, because Ryoko Asakura had attacked at that point. (A future version of Kyon, plus another version of Nagato and Asahina (Small) had interrupted, saving his life.)
Koizumi said that in the memory of Haruhi and himself, Kyon fell down the stairs and lost consciousness, not waking up until December 21st.
The time after "X" Koizumi called "X*". Without an X, there was no X*. The original X did not disappear, but instead became superimposed. The old data was overwritten with new data. The first X ran for a while, but this was then covered up by X*. He said the original X was still there.
He said the memories that he and Haruhi had of Kyon's three days of unconsciousness might not really exist.
In Bamboo Leaf Rhapsody, Mikuru Asahina took Kyon to year -3 in order to influence Haruhi's history. Kyon assisted her in creating large diagrams at her school and unwittingly convinced her to go to North High.
In The Disappearance, the alternate version of Haruhi told Kyon that someone had given her a message. Kyon knew it must be him but couldn't recall doing so. When he met Asahina (big) again, she took him to that point in time to deliver the message.
In The Disappearance Kyon saved his own life through time traveling. He realized he would have to time travel again to accomplish this, which he did early in The Intrigues.
In The Melancholy of Mikuru Asahina, Asahina (Small) convinced Kyon to save the life of the Bespectacled Boy (aka "Hakase") from the machinations of Fujiwara. The child would grow up to become an inventor of time travel.
In The Intrigues, Mikuru Asahina (big) convinces Kyon to alter the present in order to preserve her future.
Kyon's actions include "pranking" a man, causing him to injure his foot. It was later revealed he would meet his future wife at the hospital, and this was a requirement for Asahina's future. He was also asked to throw a turtle into water in front of the Bespectacled Boy; the pattern of ripples would inspire his later contributions to time travel technology.