Please explain time travel and the realm of sleep in Kingdom Hearts 3D

Question

To start, I have to say, this is by far the most confusing plot I have ever experienced. I have played all the other KH games and understood everything just fine, with perhaps a few question marks over my head here and there, but this is the first one that has left me entirely perplexed as to what even happened. That said, the reason I am asking about the dream worlds and time travel in the same question is that, as far as I can surmise, the two concepts seem to be fairly intertwined. But of course, I could be entirely wrong.

So, as far as I can gather, Sora and Riku have been sent to the realm of sleep, where they are to unlock the sleeping keyholes and awaken the worlds from their sleep. The worlds that they experience are the worlds' dreams. But the Xehanort trio (young Xehanort, Xehanort's Heartless, and Xemnas) managed to enter the dream world along with them, sabotage their mission, and set their plan into motion.

But here's what I don't get.

1. Goofy implied that Sora and Riku jumped through time in order to enter the realm of sleep. If that's the case, what exactly is the realm of sleep, and why did they have to travel through time in order to enter it?

2. How did the Xehanort trio enter the dream world along with them? Goofy says that he traveled through time in the the same way as Sora and Riku, and Master Yen Sid and Mickey say that he was able to do it because Xehanort's Heartless was controlling Riku. Why does that allow him to enter the dream world??

3. Finally, Master Yen Sid said that you have to abandon your body to travel through time. But if that's the case, how does teenage Xehanort travel through time freely with his body still perfectly intact?

I apologize if my questions sound all over the place. But that's really a reflection of how perplexed I am by the game's plot. If one or more of my questions needs to be clarified, please let me know in the comments and I'll edit accordingly.

Answer 1

Here's my understanding, gathered as a result of listening to people who really think about the series, read developer interviews and managed to predict a few elements of this game pre-trailer.

1:

To enter the Realm of Sleep, Sora and Riku needed to 'ride' a world as it fell to darkness. Since no worlds were falling into darkness at the time they needed to enter the sleeping realm, Yen Sid sent them back to Destiny Islands just before it fell to darkness in the first game. There, Riku and Sora stole the raft their past selves had built with Kairi, and rode out to the ocean. Then, after battling Ursula, the world finally started to fall to darkness. This allowed Sora and Riku to enter the 'Realm of Sleep', their Keyblades automatically unlocking the first Keyhole (thus allowing Destiny Islands to return to its original state at the end of the game). However, only Destiny Islands was in the past- the rest of the worlds in the Sleeping Realm are happening at the 'same time' as when Yen Sid originally told them what to do.

2:

I'm not entirely sure on this one, but I do know that the whole reason Ansem was on Destiny Islands in the first game (talking to Sora about darkness before the world fell) was to wait for Sora and Riku's future selves, and to follow them into the Sleeping Realm. The time travel Xehanort uses requires there to be a version of himself present- that way, he can travel anywhere he already exists in some way. Youngnort traveled back to the point where DI was about to fall to darkness, followed Sora and Riku, and as they opened the door to the Sleeping Realms he slipped in behind them before it closed.

3:

This one didn't seem to be mentioned in the game, but it's generally said that Xehanort's Heartless transferred his time travel capabilities to Youngnort when he visited him on Destiny Islands, allowing him to cheat that particular rule.

Incidentally...

the method of time travel used by Sora and Riku is different from the method used by Xehanort. Sora and Riku don't lose their bodies to travel, much like how the doors in KH2's Timeless River worked. I suspect spellcasters as powerful as Merlin and Yen Sid can use different forms of time travel, while Xehanort is restricted to his own rule-laden one.

Answer 2

To add to everyone's account, it is implied that Sora and Riku did lose their bodies when they travelled back to Destiny Island. That is why even though Sora was in Sleep his body was in the Tower. They were just hearts in the Realm of Sleep. Yen Sid probably somehow found a way to preserve their bodies. Them being just hearts would explain why Riku became a Dreameater, which are the Heartless equivalent in the Sleeping worlds.

Answer 3

1. Basically, Sora and Riku were sent to Destiny Islands before it went to sleep so that they could enter The Realm of Sleep. Because sealing the Door to Darkness locked every world's Keyhole (and thus, no world could fall to darkness ever) they had no choice but to utilize a past event of a world being swallowed by darkness to enter there. Once they unloked the first Sleeping Keyhole, that's when they entered The Realm of Sleep although Sleeping DI remained asleep till the end of KH1.

2. Ansem possessed Riku so that at the point DI becomes a Sleeping World, the Xehanort's could travel there. You see, Sleeping Worlds have one special property: they are essentially "Timeless". As explained by Yen Sid during the cutscene right after finishing Prankster's Paradise with Sora, the events occuring in a Sleeping World loop infinitely until the world wakes up. One rule of Time Travel is that you can only go towards the future once displaced. However, since there is no sense of time in a Sleeping World, Xehanort could travel to Sleeping DI with Ansem and Xemnas. Presumably, they utilized dark corridors to traverse The Realm of Sleep once they reached Sleeping DI.

3. You must abandon your body to gain the ability to Time Travel. Afterwards, you can go reclaim your body, you'd still have the ability. Young Xehanort can time travel with his body intact because of Terranort giving up his body, which created Ansem. It's like an Amusement Park ticket: you have one ticket and you can bring your 5 family members along. Same idea. Sora and Riku also utilized this same method of time travel in the beginning. They can time travel because both of them have given up their body(Sora turned the Keyblade of Heart on himself to free Kairi's Heart, turning him into a Heartless, while Riku's Heart got ejected by his own Body once Ansem took over, landing himself in the Realm of Darkness)