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I'm just keeping my fingers crossed that they are keeping their rules consistent. I'm kinda impressed that they are actually setting a time travel story that would satisfy the rules of physics so far, i.e. going with parallel universes to avoid time travel paradox.
Also the way they are using actual physics theorems and hypothesis are pretty consistent so far (as long as I ignore the formulas/maths written in this drama and don't scrutinize it too much)

lol. Hope I don't bore you with my long post!
About Tae Yi, I'm currently thinking that 2020 TaeYi is a different parallel version to 2050 TaeYi, since 2050 Tae Yi is more adept in martial arts.
Maybe at some point before 2020, TaeYi universe is split into 2 creating the 2020 TaeYi who is a physics professor, and 2050 TaeYi who works for Alice and can time travel. They both seem to be around the same age. Or 2050 TaeYi is probably younger than 2020 TaeYi when she went to 1992.
I keep saying that we probably can't probe another parallel universe, but perhaps the rules in this drama are you can travel to any universe at any timeline. Meet your parallel version and kill them or do whatever.
While for Oh Won, there are 2 possibilities now:
- someone from the future told him about what happened
- he already knew 2050 TaeYi/JG's mom before the murder, hence he knew what would happen from reading the Book of Prophecy

imho it's not showing the butterfly effects because we are only shown one universe/one observer (Jin Gyum) so far.
The many universes hypothesis is based on the possibilities/outcomes of all the different actions that you might take in one universe. We just couldn't see it as a viewer because it doesn't affect the universe that we're watching.
i.e. taking an example of the kidnapping mother... there are several points in the drama where she probably had created > 4 parallel universes. The number of universes will grow exponentially, similar to what is called the "butterfly effects"

yeah, that is what still not clear to me yet. And they haven't addressed that yet. They might keep that a secret until the last 2 episodes... :D
I think 1992 Tae Yi (the little kid - I know the drama hasn't established that, but it's kinda obvious at this point) is the past version of both Tae Yi. 2050 Tae Yi and 2020 Tae Yi might be a different parallel version. The thing is they haven't revealed yet if it's possible to travel between the alternate universes, because so far, the time-traveling that has been shown/explained is the one within the original universe (in this case, JG's universe)

They're going for what is called a "many-universe interpretation".
I'll include the different principles mentioned in the drama, and omit the maths, so feel free to read more about it if you're interested/ask me questions.
In a (very large) nutshell, everything in the universe is very fuzzy at the quantum level (Heisenberg Uncertainty), you can't make a precise measurement of say a position or a momentum of a particle. This is because there are many states particle can take on at this level. When we make a measurement (when we disturb/or interact with the quantum system), we are forcing these many states to collapse into one state (Copenhagen Interpretation). And when we repeat the measurement, we again forcing it to collapse into another state (could be the same as the previous state or different).
This interpretation is used to explain the state of things at the quantum level. But since this is physics, there were arguments that what happens at the quantum level should also happen at the macroscopic level. But things get really weird when you think of it that way, which is where the Schrodinger's Cat experiment was posed: A cat is put in a box with radioactive stuff that has a 50:50 chance of decaying and kills the cat. The question is will the cat be dead/alive? Using Copenhagen interpretation, we will only know that the cat is alive/dead when the box is opened (when we are making measurement/interact with the cat and radioactive system). This makes sense if we only limit the observation to the person who opens the box. But this doesn't make sense when we also consider the cat as the observer because before the box is opened, then the cat is in a limbo between being dead and alive -- which is weird.
This thought experiment puzzled the physics community at that time (and still is), and another theorist then tried to solve this Schrodinger's Cat paradox by proposing the Many-universe interpretation. He proposed that instead of forcing the many states of a particle to collapse into just one state, the universe is actually split into different possibilities at the time of the measurement to accommodate all the possible outcomes of the measurement. i.e. now just before the box is opened, the universe is split into a universe where the radioactive decayed and kills the cat, and a universe where the radioactive didn't decay and the cat is still alive.
For the time-travel part which is now is now the science-fiction part of the drama, I think the rules that they set: when you time travel and interact with someone in the past, you are then creating parallel universes based on the outcomes of that interaction.
And judging from the ending of episode 6, they are probably going to tell us the rules on how to make time travel happens through wormhole using electromagnetic radiation. Tae Yi also mentioned the Dirac Sea and using negative energy to create the wormhole - but I don't know much about it. All I know is Dirac Sea was a theory that was proposed to explain the negative energy spectrum left by positron/anti-electron in the Dirac equation before it was discovered some years later. But perhaps some physicists are looking into that model again to explain wormhole, etc. since there are some physics papers in https://arxiv.org/ related to that.
Ok was hoping that my post is not gonna be this long but it's hard to make a concise summary without including some of the important details.

it's time travel and parallel universe combined. It's just that when you travel to the past and change the outcome of something, it will then create a parallel universe with the said change. But if you return to the future, you will return into your original universe. In the case of the kidnapping mother, she will go back to the future where the daughter is still dead. But now, there will exist a new parallel universe where the daughter is probably not going to die that soon in her future, but she will grow up without her mom (because she kill her past self).
The only thing that's still not clear to me is how 2050 TaeYi/Jin Gyeom's mom is connected to 2020 TaeYi. I don't really think she's the older version of TaeYi from the same universe. First because of her age when she had Jin Gyeom, and second she is far more adept in martial art than 2020 Tae Yi. Unless TaeYi has been hiding her "qi" all this time, then it's hard to imagine that the 2050 TaeYi is the future her.

My 2 cents is it might not be just as simple as wanting time travel to stop. The real reasons are probably grounded beyond something philosophical like going against the natural course of the universe, as what Oh Won said in the past episode. It could be that they want to keep the time travel technology for themselves for their own corrupt motives or to prevent Alice from having the technology and use it to make profits for themselves. So far that's the way I think of Alice, it's just some large corporation who uses the time travel technology to make profits from their clients *shrugs*

yeah I'll take quantum physics and general relativity over EM class any days. But perhaps because we used Jackson as textbook, and man that book is too concise for its own good. I barely had any weekends during the time I took the class, because there were not enough time in weekdays to solve those homework problems. It felt like I wrote everything in my blood at the time XD

yeay, a fellow physicist! This is OOT but I always wonder if other physicist would feel the same way as I did watching that ending scene of episode 6, with the EM radiation equation written in blood on the wall. It weirdly reminded me of the painful memory of EM class. :D

TY shows interests though on episode 6, as subtle as it might be. She didn't spell it out, but the way she asked Jin Gyeom about if he'll know if someone has feelings for him, talking about him to her friends, and also the way she keeps checking her phone to see if Jin Gyeom is calling/texting him, hinted that. It might be just a little crush and played for comedic effects, which could have worked fine if not for the way their scenes have been directed/edited so far. Remove all the romantic ballad and slow motion shot, and we get just a regular scene of two persons bonding on platonic level, or a scene of a son just wanting to spend more time with "his mom".

I see. My understanding of the phrase time is fluid means that you cannot stop time. You can go against the flow of the time (travel back in time) or go twice the speed of the flow of time (travel forward in time). Creating a parallel universe doesn't stop time, because time still flows forward in the same direction in both instances. In this sense, we have no control over time, but we can create tools to manipulate where we are at a certain point in time. Just like how we use a car to bring us back from point B to A after traveling a distance of B-A.
Imho, I don't necessarily think that not being able to access different parallel universe contradicts the notion that time is fluid. I think there are 2 ways to access the different universes. In the case of the kidnapping mom, she either has to travel back in time in her original universe at the point of time where she hasn't killed her past self, then stays in that universe until her kid grows up to see how the future looks like in that universe *over time* (because time will continue to flow). Or creates a wormhole that connects one parallel universe to another parallel universe without time travel. i.e. going from universe 1 at time t to universe 2 at time t. However, this second method or the rules that could give this second method have not been completely explained in the drama, so it's all up to speculation. But if time is fluid, it would be impossible to arrive at universe 2 at the exact time as when you leave universe 1, because you still have to take into account the time required to travel through the wormhole, no matter how fast it was.

yeah - that's true. But this is also the man who pointed a gun at her the first time they met. If we replace Min Hyuk with any other random guy sent to kill her, I think we as the audience would think that she needs to call for help, not that he wouldn't harm her. This 2020 Tae Yi has no knowledge of Min Hyuk, so I think it's natural for her to be scared of him when she met him again the second time? Also, it's natural for her to be in shock the first time. I would be too if someone just barged into my office pointing a gun at me. I don't think that just because she teaches physics we should expect her to behave differently. She is knowledgeable in physics, but it doesn't mean that she will know exactly what to do in a situation where she's taken as a hostage or any other situations that don't require her knowledge as a physicist.

But having said that, I agree with wanting her to be less "naive" in future episodes. I hope the writer would be able to write a different, somewhat smarter reactions given what she already faced in these last 2 episodes.

@ABETTERTOMORROW @Aishwarya Nazre - yes the DNA part is why I said this is still f-up, but at least Jin Gyeom won't be involved with a woman whom he know as his mom (or a version of his mom) all his life. Less disturbing than what they are playing it out to be right now.

that is what I think as well. We are just disturbed by the soft-romantic music that plays in the background while they're sharing longing stares when they're together

and we're back to Dark situation again :D so TaeYi and Jin Gyum could be mom/dad at the same time but not lovers. Still f'd-up but less disturbing that having both mom and son romantically involved with each other

I'm always on the fence about the definition of strong FL. Sure I like it when my girls can be badass and fight for themselves, but in reality, only some portion of females who can do this when put in a situation like what Tae Yi faced ( again I'm not saying that females are weak, any female can be strong as any male as long as they train hard). Also, she is not portrayed to be a warrior like character here, she's a physics professor. Her muscle strength prolly comes from writing down a really long derivation of some physics formula. But we know that she's an intelligent woman who is smarter than the male characters in the show, and not the typical damsel whose only strength is her beauty.

I feel like we tend to associate strong FL by bringing hyper-masculinity to the equation, i.e. female character can only be a strong character if she can fight for herself and beat all the bad guys by themselves. But when I see Tae Yi, I feel like I can relate to her. Not drop-dead gorgeous as she is or even as smart as she is, but being someone who stayed in school to study and do research, I can testify that I barely have enough time to sleep or exercise, never mind picking up any self-defense class (yet I always had time to watch dramas, my priorities are questionable). I can be so wrapped up in the work that I do, that I don't really think much about other things outside of that, and can be absent-minded about several things. So if anyone wants to kill me for the research that I did, then yeah I'd be another damsel in distress and I definitely need help from the cops or someone stronger than me. And I totally think that that is fine and instead that's what I need to do -- ask for help for something I don't have the capability to do?

Also, she's in this situation more often in the last 2 episodes because now more people know that she has the knowledge/data to use for the time travel, so more bad guys will go after her. The only option her character has is to defend herself until help arrives, because her character definitely doesn't have the muscle power to fight off the bad guys, and hopefully not getting killed before then.

The writer definitely writes some plot conveniences to move the plot along, i.e. letting Tae Yi entered the house when she knew the door was already opened and right after she knew that her friend was killed because of her research, but the damsel-in-distress situation is a result of that plot conveniences. Having said that though, I would love to see Tae Yi saving Jin Gyeom in future episodes by using her smart brain.

nevertheless she is still a version of his mother. Even if she's not directly giving birth to him, I found it disturbing for them to have relationship because she looks exactly/the exact DNA copy of his mom.

I wish they stop playing slow-romantic music every time they're together. It's super disturbing. At this point, I think Tae Yi has feelings for him (before knowing that he is her future son), but I think Jin Gyeom only thinks of her as his mother. I hope it stays that way until the end

Yes, I agree. I think it's because Jin Gyeom has traveled as a baby through the wormhole that's why he doesn't need the departure gates to time travel (it's hinted in the drama).
I think if we remove the possibility that one parallel world can communicate with the other parallel world, the concept should be plausible. The people who travel from the future would never know how the future of the other world as a result of their actions unless they choose to stay in that dimension. I don't think it's being selective, but it's a concept that a time loop is unique to a universe (just my two cents)

I don't think the reference to parallel worlds should contradict how 2010 Oh Won would know about Jin Gyeom. I was saying in another thread that the rules they have here are different to Dark, but now that I think more on this, time loop can also exist in parallel dimensions, as long as the result of your time travel is your current future in that universe. i.e. the result of 2050 Tae Yi time travel is a part of the timeline in this universe we're watching the characters in. As long as they act according to the course of actions that should take place to create a future where Alice exists and allows time travel to happen, the time loop can exist. And people from the past can have knowledge of people from the future because their interactions are precisely the set of things that must have taken place in that timeline of that universe.

As for your first point (I feel like I'm time-traveling with addressing your points in the reverse order :D ). I think the rules that they set here is that you can time travel through a wormhole. Traveling with the departure gates, probably allows you to pick and choose the time in the past or the future where the wormhole will be created/existing again. So even though you stay in the past for a year, when you return to the future, you can pick the time where the next worm hole will be created, e.g. the next few days. While Jin Gyeom doesn't really have the luxury to pick and choose the time on where he would arrive. Also, this makes me think that the wormhole one can time travel in should be unique, i.e. the wormhole you use to return to the present can't be the same as the wormhole you used to depart the present to the past in the first place. But well, until the show gives us the complete rules, I can only speculate.