1. There is only ONE timeline. There are NO multiple timelines/branches.

2. The currently accepted temporal theory that “everything that should happen, already happened” is not applicable in this show, since they showed us how newspapers and events changed.

3. As of episode 4, the only applicable temporal theory is the “time bubble” theory (or “quantum bubble” as some call it).

Meaning, the time period from the origin time period to the target period is in total flux. Nothing is concrete yet.

• origin time period = 2021
• target time period = 1987

4. Since the time machine broke, it is possible the time bubble no longer exist. If it will be fixed later, then 2021 and 1987 will be connected again, which will force everything in between to be in flux (nothing concrete/fixed - everything can change at any time).

* A time/quantum bubble is created because time travellers are changing things; and the time traveller causing changes can still travel through time. (At least that's the theory.)

** Also, a time/quantum bubble usually exist when there is only ONE timeline. It can exist in the “Multiple Worlds Interpretation” (a.k.a. multiple timelines) theory, but only in rare cases (usually seen in DC comics).

I am not familiar with time travel theory, so please enlighten me or excuse me if I am wrong. Following my own logic, this is what I think about the time-travel in the show. However, some things are still weird to me.

There has to be a different branch from the first one, otherwise their work would remove the existence of the branch they came from, making it impossible for them to arrive in the past in the first place. However, I think the time machine only allows for travel in their current timeline, and branches can be formed by altering this timeline. They also can only go back or forth by an exact amount of years. I don't see any evidence to reject the multiple branches/timelines (even them existing at the same time), however we can be sure the main leads only travel from and to the timeline they are currently in.

Alternatively, we can see it as a single timeline where people just hop back and forth, but this would create really weird paradoxes. This is probably what you mean by time bubble, where the time traveller does not cease to exist after changing the timeline. Which is perhaps also the reason why I struggle with the next part.

The thing which I find the weirdest is the time travel back to the future. What happens to the Yae Hoon and Yoon Yong that grew up in the altered timeline? It seems to me, they just get killed off to make place for the ones of the original timeline... All the memories they made in the new timeline just got forgotten by them, only seeing the effects after they returned. Story wise, this makes sense because they worked so hard to achieve this story, but I don't like the thought of a person living out their lives needing to make a place for them.

I like that it is a time travel story where the travellers themselves benefit from the changes they worked hard for. Especially because of the relations that get build in the show, it wouldn't make sense. Probably, the writers mainly focussed on making it a well-written story, and didn't want to make it unnecessarily difficult by making it exactly follow a certain theory of time travel.