Even though the main explanation is consistent and ties the whole movie together there are still a few parts of the movie that are not explained and have to rely on theory. Here are some of those questions with possible explanations and some common mistakes people make.
What Creates The Tangent Universe
Why exactly does a Tangent Universe (TU) occur? Are they a natural phenomenon or are they artificially created. There is one theory that scientists in the future build a machine that somehow causes a rip in time back in the past. Another theory is it could have been caused by Alien involvement.
Quite a common thought is Frank saving Donnie in the beginning is the cause of the TU. Donnie was meant to die and by surviving he altered things which caused the corruption. This is not true though, we are already within the TU when Frank wakes Donnie up. Frank's special powers only work within the TU so he would only be able to wake Donnie after the TU had started.
The same goes for the Artifact not being the cause of the corruption. The jet engine appears several minutes after the TU had started. The Philosophy of Time Travel states that the Artifact is the first sign of evidence that a TU has occurred.
The First Jet Engine
One major question is where does the jet engine in the beginning of the movie come from originally? If it came from the future where exactly and what day did it come from?
A common answer to this question is it's the same engine Donnie sends through the portal from the plane his mother and sister were on. The engine travels through time and lands on Donnie's house. It's exactly the same event as the end of the film only the first time round Donnie survives.
This is extremely unlikely though as the portals are just a gateway between the two Universes. The first engine lands in the Tangent Universe (TU) so if it came through a portal it would have to come from the Primary Universe (PU). The engine Donnie rips of the plane at the end is from the TU and travels through a portal into the PU so there is no way it can be the same event repeating.
If it did come from a future point in the PU once the corruption is fixed by Donnie that future event won't happen now so we will never find out exactly how it occurred.
Another theory is the Tangent Universe is a copy of the Primary Universe but as it's being formed there is a glitch or copying error. This error produces a duplicate engine that is then dumped into Donnie's bedroom. This anomaly makes the TU even more unstable and it must be sent into the PU.
Roberta Sparrow: The Philosophy Of Time Travel
Roberta Sparrow writes the book "The Philosophy Of Time Travel" which helps guide Donnie but how exactly did she know all this information? There are a few plausible explanations for this. It seems like she had a similar experience herself so either she was a Living Receiver or one of the Manipulated Dead in a previous Tangent Universe.
Both options would in theory give her all the information required. If she was a Living Receiver she would obviously have known what to do and the Manipulated Dead have a large understanding of what is happening within a Tangent Universe. It's possible she may have somehow retained some of the information from her experience once the Primary Universe started up again.
Does Donnie Have To Die
This subject is one of the most debated parts of the movie. A lot of people make the mistake of thinking Donnie waking up and cheating death in the beginning is what caused the Tangent Universe (TU) to occur. The assumption is then once the Primary Universe is restored Donnie stays in bed otherwise he risks causing another TU to start and the whole cycle will repeat. This theory is not correct though, Donnie and Frank have nothing to do with the creation of the TU.
We hear about other Living Receivers in The Philosophy of Time Travel who were killed by the Artifact they returned. If we include Donnie then it seems the Living Receiver always dies once the Primary Universe is restored. The question is what would happen if Donnie got out of bed again and survived? The moment we see Donnie laughing in bed at the end the Primary Universe is already restored. If Donnie got out of bed and dodged the second engine after that then in theory nothing would change as the corruption is already fixed.
Another important point is whether Donnie chooses to die. The Donnie we see in bed at the end is not the same Donnie from the last 28 days. How much knowledge he has of what happened in the TU is very questionable and it’s quite unlikely he knows he is about to die.
The question of destiny plays quite a big part here, Donnie was meant to die but Frank saved him. It’s possible he was chosen as the Living Receiver because his life is now expendable; he is going to die anyway. Once back in the real world it's Donnie's destiny to die.
Mixed up my notation. Should have wrote:
If you can understand the geometry, then the solution is this: The first jet engine (the one that lands in the TU at time t' = 1) fell from the PU at t = 0 (as TU would measure if able to look at both universes simultaneously) The 2nd Jet Engine (the one that lands in the PU at t = 1) feel from the TU at t' = 0 (as PU would measure if able to look at both universes simultaneously).
The moment the engine falls in one universe is the moment it lands in the other. It is inversely symmetrical, and each event is separated by 28 days, 6 hours, 42 minutes and 12 seconds as measured by the OTHER universe. Like if the time lines were parallel but shifted by that amount from each other, and then at the finish point of one draw a line to the beginning point of the other.
Regarding the Jet Engine: how it could be the "same" one, but from a different time.
Assuming we have a TU, then the PU and TU must be skew in spacetime; the PU and TU are rays in which the beginning of the TU is at a point on the PU. Since they AREN'T the same universe, they CANNOT be in the identical spacetime line.
For the PU we'll use t = 0 for the fall of the Jet Engine and t = 1 for the landing of the Jet Engine. For the TU we'll use t' = 0 for the fall of the Jet Engine and t' = 1 for the landing of the Jet Engine.
If you can understand the geometry, then the solution is this: The first jet engine (the one that lands in the TU at time t' = 0) fell from the PU at t = 1. The 2nd Jet Engine (the one that lands in the PU at t = 0) feel from the TU at t' = 1.
So what you have is a symmetric CRISS-CROSS of artifacts, each falling from each universe's respective future into the other's past.
Donnie Darko switches them back at the precise moments in question for each universe, causing the TU to safely collapse.
So basically, what we have is that the jet engine DID come from the future, but it came from the OTHER future (and vice versa for each universe).
yeah, but the guy in the red jumpsuit was seen first by the GF and that is why she didn't want to kiss him.
If the jet engine was a result from a glitch, does that mean that there can be TU's that don't have an artifact if the glitch doesn't occur? And if so, would there be any difference between the TU and the PU?
grandma death had to have been a living receiver because she new about the "wormholes that showed where everyone was going information only the living receiver had
To answer a previous question, I believe the man in the red jumpsuit is Gretchen's step dad - he was stalking her and Donnie in the woods and then he shows up again the night Gretchen's mom disappears.
i think some part of the movie that was very crucial but is ignored is the one in which grandma death whispers into his ear that everyone who lives dies alone .This doesn't seem to be involved in any of the explanations we have discussed. Since , so many crucial characters die at the end of the TU ,i guess one factor could be that some of them like gretchin was meant to die along with donnie in the TU ,so that when the TU ends, they don't come back to the PU as everyone might think but instead they have gone into "something much more" as donnie mentioned while a scene of passing through the time tunnel was shown ,which could be some other reality where they are together forever.Which is why he looks so happily at gretchin when the engine is moved from TU to the PU, because he knows that now they will also travel through the portal after the engine is gone into some place where "life looks beautiful",a line said by gretchin when they were about to kiss each other in another scene.
i think some part of the movie that was very crucial but is ignored is the one in which grandma whispers into her ear that everyone who lives dies alone .This doesn't seem to be involved in any of the explanations we have discussed. Since , so many crucial characters die at the end of the TU ,i guess one factor could be that some of them like gretchin was meant to die along with donnie in the TU ,so that when they travel through the time portal they don't come back to the PU as everyone might think but instead they have gone into "something much more" as donnie mentioned while a scene of passing through the time tunnel wa shown ,which could be some other reality where they are together forever.Which is why he looks so happily at gretchin when the engine is moved from TU to the PU, because he knows that now they will also travel through the portal after the engine is gone.
I can't remember if it's in the original or just the director's cut, but there is a very small scene between Monnitoff and Pomeroy (the two main teachers) in the staff room, when Monnitoff simply says "Donnie Darko..." and Pomeroy just replies "I know," and they laugh. I've always wondered what they had found out/knew about Donnie at that point...
I also don't understand where that 1st jet engine came from? Donnie couldn't have transported the engine from a Tuniverse to another Tuniverse, the portals or paths, only transport TU to PU and PU to TU. Is these an answer to this?